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Trane TRG-TRC016-EN User's Manual

1. The coil and the air being conditioned experience no difference in the cooling effect of using a two way versus a three way valve The chilled water system however sees a great difference Recall that with a three way valve the terminal water flow water through the coil plus the water bypassing the coil is constant at all loads With a two way valve the terminal water flow varies proportionately with the load Because there is no mixing of coil and bypassed water the temperature of the water leaving the load terminal remains relatively constant at all conditions In fact this return water temperature may actually rise slightly as the load decreases due to coil heat transfer characteristics Systems that use two way valves have the following characteristics E The temperature of the water returning from the system is constant or increases as the cooling load decreases This increases the effectiveness of options such as heat recovery free cooling and base loading which will be discussed further in Period Three m The water flow through each load terminal varies proportionately to the load resulting in pump energy savings at part load Em A variable flow system is less sensitive to water balance than most constant flow systems A variable flow chilled water distribution system however may require another method to provide constant water flow though the chillers or else the chillers must be equipped to handle variable water f
2. Air Conditioning Clinic Chilled Water Systems One of the Systems Series TRG TRC016 EN BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE THE TRANE COMPANY Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE THE TRANE COMPANY Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES Comment Card We want to ensure that our educational materials meet your ever changing resource development needs Please take a moment to comment on the effectiveness of this Air Conditioning Clinic Chilled Water Systems Level of detail circle one Too basic Just right Too difficult One of the Systems Series Rate this clinic from 1 Needs Improvement to 10 Excellent TRG TRCO16 EN Content 1 2 3 4 5 6 7 8 9 10 Booklet usefulness 1 2 3 4 5 6 7 8 9 10 Slides illustrations 1 2 3 4 5 6 7 8 9 10 Presenter s ability 1 2 3 4 5 6 Z 8 9 10 Training environment 1 2 3 4 5 6 7 8 9 10 Other comments About me Type of business Job function Optional name phone address Give the completed card to the presenter or drop it in the mail Thank you The Trane Company Worldwide Applied Syst
3. 6 7 C 37 8 C Su y 5 C 4 t Ea NN amp Ea OSAN y 85 F ie 85 F 29 4 C ma Ji 29 4 C Pm 54 F 57 F 12 2 C 13 9 C ARI conditions low flow conditions evaporator 2 4 gpm ton evaporator 1 5 gpm ton flow rate 0 043 L s kW flow rate 0 027 L s kW condenser 3 0 gpm ton condenser 2 0 gpm ton flow rate 0 054 L s kW flowrate 0 036 L s kW In the example system shown in Figure 25 the chilled water is cooled from 57 F 13 9 C to 41 F 5 C for a 16 F 8 9 C AT This reduces the water flow rate required from 2 4 gpm ton 0 043 L s kW to 1 5 gpm ton 0 027 L s kW Reducing water flow rates either 1 lowers system installed costs by reducing pipe pump valve and cooling tower sizes or 2 lowers system operating costs by using smaller pumps and smaller cooling tower fans In some cases both installed and operating costs can be saved Low flow systems will be discussed in more detail in Period Three The two ARI rating standards mentioned previously as well as ASHRAE IESNA Standard 90 1 1999 the energy standard allow reduced chilled water temperatures and flow rates System design engineers should examine the use of reduced flow rates to offer value to building owners TRG TRCO16 EN TRG TRCO16 EN S TRANE period one Types of Water Chillers ASHRAFJ IESNA Standard 90 1 1999 a Energy Standard i Building design and materials Minimum equipment efficiencies
4. It keeps the large chillers loaded in their peak efficiency range and operates with the fewest and smallest pieces of ancillary equipment pumps and cooling towers at any system load A common concern is to prevent the swing chiller from cycling too frequently which could shorten the life of the equipment In large chilled water systems however the changes in building load typically occur slowly enough that this is not a problem A final asymmetric design option is to use one high efficiency chiller and one or more standard efficiency chillers In this type of system the high efficiency chiller should be preferentially loaded to minimize system energy consumption 74 TRG TRCO16 EN S TRANE period three System Variations Free Cooling 4 Airside economizer 4 Waterside economizer Strainer cycle Plate and frame heat exchanger Refrigerant migration Free Cooling There are a number of methods that use cool outdoor conditions to reduce cooling energy costs They are often referred to as free cooling because they reduce or eliminate the energy consumed by the compressor They are not truly free but really reduced cost cooling options The most prevalent method is the use of an airside economizer When the temperature or enthalpy of the outdoor air is low enough the outdoor air and return air dampers in the air handler are modulated and the cooler outdoor air is used to reduce the temperature of air
5. Modulation of a control valve in the bypass pipe is commonly used to ensure minimum flow rates through the chillers Control the rate at which the system flow rate changes to ensure that it does not change more rapidly than the chillers can adapt This is especially critical when turning on additional chillers Adequate time must be spent designing the control sequence and commissioning the system after installation to ensure proper operation of a variable primary flow system TRG TRCO16 EN 89 amp ranw period four Chiller Plant Control System Failure Recovery 4 Maintain flow of chilled water a Keep it simple Lock out failed equipment Turn on the next chiller in the sequence 4 Notify the operator 4 Allow the operator to intervene Failure Recovery and Contingency Planning In addition to normal chiller sequencing the chiller plant control system must react when a chiller or another piece of associated equipment fails Failure recovery or ensuring the reliable supply of chilled water is a very important part of the chiller plant control system and is an area where many systems have fallen short This is especially true in field programmed systems because of the difficulty of thorough debugging During periods of equipment malfunction it is important to focus on the primary goal of the system which is to provide the required flow of chilled water to the system at the proper temperature It seems reasonable that
6. calculating the chiller and system efficiency Here is the important part This becomes increasingly important with multiple chiller systems because individual chillers operating within multiple chiller systems are more heavily loaded than single chillers within single chiller systems TRG TRCO16 EN 19 S TRANE 20 period one Types of Water Chillers Standard Rating Conditions evaporator condenser rating chiller type flow rate flow rate standard vapor compression reciprocating e scroll 2 4 gpm ton 3 0 gpm ton rere 1998 helical rotary 0 043 L s kW 0 054 L s kW g centrifugal absorption single effect 2 4 gpm ton 3 6 gpm ton e double effect indirect fired e double effect direct fired 0 043 L s kW 0 065 L s kW 2 4 gpm ton 0 043 L s kW 4 0 gpm ton ARI 0 072 L s kW 560 1992 4 5 gpm ton 0 081 L s kW water leaving evaporator 44 F 6 7 C water entering condenser 85 F 29 4 C The standard rating conditions used for ARI certification represent a particular set of design temperatures and flow rates for which water cooled and air cooled systems may be designed They are not suggestions for good design practice for a given system they simply define a common rating point to aid comparisons In fact concerns toward improved humidity control and energy efficiency have changed some of the design trends for specific applications More commonly chilled water
7. conditions seldom achievable in real installations Also the equipment typically requires regular calibration For these reasons direct measurement of load has not been used as much as the simple and reliable methods discussed previously An alternate way to monitor chiller load is by measuring the current draw of the chiller motor By itself this does not provide an adequate control indicator but when used in conjunction with other information such as system supply water temperature it can be effective System supply water temperature is used to determine when to turn an additional chiller on and operating chiller compressor motor current draw is used to determine when a chiller can be turned off The most effective load indicator for any chilled water system is dependent on the design of that system Creative designers have used the control strategies as described here and in various combinations to effectively control a wide variety of chiller plants It is highly recommended that one of the first tasks undertaken in the design process is to create a simplified flow diagram and a load model of the system that allows for the evaluation of various control strategies and sensor placements This will help to ensure that effective chiller plant control can be implemented TRG TRC016 EN 85 amp ranw period four Chiller Plant Control Chiller Rotation a Pa equally capacity chillers Ss pe When the system has determined
8. 54 6 F 12 6 C reducing the load on the operating chillers Some excess flow is normal in the operation of a primary secondary system The amount of excess flow is almost always less than the flow of one production pump The energy consumed by pumping this excess water through the production loop is typically very low because the production pump only needs to produce enough head to push the water through the chiller evaporator and the bypass pipe If a pump and chiller pair is turned off as soon as this excess flow condition occurs deficit flow will result and the pump and chiller will be turned on again To prevent this from happening a production pump and its respective chiller are not turned off until the excess bypass flow exceeds the capacity of the next production pump that is to be turned off Some systems are designed with variable flow also in the production loop Although this minimizes excess flow in the bypass pipe and further reduces production energy consumption it results in a significantly more complex control system This type of system will be discussed in Period Three 55 S TRANE period two Chilled Water System Design Control of Primary Secondary System condition response deficit flow for start another specified period of chiller and pump time excess flow greater turn off next chiller than 110 to 115 of and pump next pump to turn off neither do nothing Starting and stopping of pump and
9. Air cooled condensers have the ability to operate in below freezing weather and can do so without the problems associated with operating the cooling tower in these conditions Cooling towers may require special control sequences basin heaters or even an indoor sump for safe operation in freezing weather For process applications such as computer centers that require cooling year round this ability alone often dictates the use of air cooled chillers TRG TRC016 EN TRG TRCO16 EN S TRANE period one Types of Water Chillers air cooled or water cooled Efficiency dry bulb S LATIN eT 3 12 12 12 midnight noon midnight Water cooled chillers are typically more energy efficient than air cooled chillers The refrigerant condensing temperature in an air cooled chiller is dependent on the ambient dry bulb temperature The condensing temperature in a water cooled chiller is dependent on the condenser water temperature which is dependent on the ambient wet bulb temperature Since the wet bulb temperature is often significantly lower than the dry bulb temperature the refrigerant condensing temperature and pressure in a water cooled chiller can be lower than in an air cooled chiller For example at an outdoor design condition of 95 F 35 C dry bulb temperature 78 F 25 6 C wet bulb temperature a cooling tower delivers 85 F 29 4 C water to the water cooled condenser This results in a refrigerant condensing temperature of
10. C 80 gpmat 80 F f 5Us at 26 7 C process load Application Outside the Operating Range of the Chiller Some process load applications involve either temperatures or flow rates that are outside the capabilities of any chiller This may include high return water temperatures high or low fluid flow rates or high or low system ATs By using special piping arrangements a standard chiller can still be used to satisfy the requirements of the process load Figure 86 shows a system in which the water flow requirement of the process load is below the minimum flow rate for a chiller with the required capacity The system is designed like a primary secondary system but the production loop has a higher design flow rate than the distribution loop This allows the water flow rate and AT through the chiller to be within the acceptable limits while the water flow rate and AT through the process meet its requirements Alternatively in a smaller system with a single chiller a single pump on the chiller side of the bypass and a diverting valve to maintain the proper flow through the process load can achieve the same result TRG TRCO16 EN TRG TRCO16 EN S TRANE period four Chiller Plant Control Chilled Water Systems period four Chiller Plant Control It is important to understand that no matter how good the system design is adequate controls are necessary for all the components to operate properly as a system It is equally importa
11. In a system with a heat recovery chiller preferentially loading the heat recovery chiller maximizes the amount of heat recovered thus reducing the overall building energy consumption 67 S TRANE 68 period three System Variations E In a system with an alternative fuel chiller such as an absorption chiller preferentially loading the alternative fuel chiller during times of high electricity costs minimizes system energy cost Heat Recovery Chiller Heat Recovery Heat recovery is the process of capturing the heat that is normally rejected from the chiller condenser and using it for space heating domestic water heating or another process requirement Heat recovery has been successfully applied in virtually all types of buildings including hotels schools manufacturing plants and office buildings It typically provides an attractive return on investment for building owners The use of heat recovery should be considered in any building with simultaneous heating and cooling requirements or in facilities where the heat can be stored and used at a later time Buildings with high year round internal cooling loads are excellent opportunities for heat recovery Additionally ASHRAE IESNA Standard 90 1 1999 Section 6 3 2 includes restrictions on the amount of reheat that can be performed in an HVAC system unless it is recovered heat It is therefore likely that heat recovery will be used more in the future Heat recovery can b
12. TRG TRCO16 EN S TRANE period two Chilled Water System Design Three Way Valve Control A three way control valve is one method used to regulate the flow of chilled water through a cooling coil As the space cooling load decreases the modulating valve directs less water through the coil decreasing its capacity The excess water bypasses the coil and mixes downstream with the water that flows through the coil As a result the temperature of the water returning from the system decreases as the space cooling load decreases Systems that use three way valves have the following characteristics m The temperature of the water returning from the system varies as the cooling load varies m The water flow through each load terminal water through the coil plus water bypassing the coil is relatively constant at all load conditions m The pump energy is constant at all loads because the use of three way valves results in constant water flow throughout the system m Water flow balance is very critical to proper operation because the flow is constant TRG TRCO16 EN 29 S TRANE period two Chilled Water System Design Two Way Valve Control A two way modulating valve is similar to a three way valve in that the water flow through the coil is modulated proportionately to the load The primary difference is that the two way valve does not bypass any unused water it simply throttles the amount of water passing through the coil
13. approximately 100 F 37 8 C At these same outdoor conditions the refrigerant condensing temperature in an air cooled condenser is approximately 125 F 51 7 C A lower condensing temperature and therefore a lower condensing pressure means that the compressor needs to do less work and consumes less energy This efficiency advantage may lessen at part load conditions because the dry bulb temperature tends to drop faster than the wet bulb temperature see Figure 12 As a result the air cooled chiller may benefit from greater condenser relief Additionally the efficiency advantage of a water cooled chiller is much less when the additional cooling tower and condenser pump energy costs are considered Performing a comprehensive energy analysis is the best method of estimating the operating cost difference between air cooled and water cooled systems S TRANE 10 period one Types of Water Chillers air cooled or water cooled Comparison air cooled water cooled a Lower maintenance 4 Greater energy efficiency a Packaged system a Longer equipment life 4 Better low ambient operation Another advantage of an air cooled chiller is its delivery as a packaged system Reduced design time simplified installation higher reliability and single source responsibility are all factors that make the factory packaging of the condenser compressor and evaporator a major benefit A water cooled chiller has the additional requireme
14. by the downstream chiller This strategy may be desirable in systems that benefit from preferentially loading the upstream chiller Examples include m Using a heat recovery chiller in the upstream position Because the chiller is at full capacity whenever the system load exceeds 50 percent the amount of heat available for recovery is maximized m Using an absorption chiller in the upstream position An absorption chiller works more efficiently and has a higher cooling capacity with higher leaving chilled water temperatures The absorption chiller in the upstream position provides a warmer leaving chilled water temperature at design conditions 48 F 8 9 C in this example This arrangement preferentially loads the gas burning absorption chiller allowing the system to maximize the use of a lower cost fuel during periods of high electrical energy cost Alternatively equal loading of the two chillers in series can be accomplished using a chiller plant control system to monitor system load and balance chiller loading The set point for the downstream chiller is set equal to the desired system supply water temperature and the set point for the upstream chiller is then dynamically reset to maintain equal loading on both chillers The control system must be stable enough to prevent control hunting or chiller cycling during periods of changing load 39 S TRANE period two Chilled Water System Design chillers piped in series Stag
15. by varying the speed of the motor that rotates the pump impeller water cooled condenser A type of condenser in which water flows through the tubes and absorbs heat from the refrigerant that fills the surrounding shell 115 TRANE ea Literature Order Number TRG TRC016 EN File Number E AV FND TRG TRC016 0201 EN Supersedes CWS CLC 1 CWS CLC 2 CWS CLC 3 and CWS CLC 4 The Trane Company AmAmericarn Standard Company Stocking Location La Crosse www trane com For more information contact your local distict office or Since The Trane Company has a policy of continuous product and product data improvement it reserves the right e mail us at comfort trane com to change design and specifications without notice
16. chilled water system in which the production loop provides less flow than is required by the distribution loop To make up for this deficit water travels from the return side of the distribution loop through the bypass pipe and mixes with the water supplied by the production loop direct fired A type of absorption chiller that uses the combustion of a fossil fuel such as natural gas or oil directly to provide heat to the high temperature generator double effect A type of absorption chiller that uses two generators a high temperature generator and a low temperature generator evaporator A component of the refrigeration cycle where cool liquid refrigerant absorbs heat from air water or some other fluid causing the refrigerant to boil excess flow A condition in a primary secondary chilled water system in which the production loop is providing more flow than is required by the distribution loop This excess water travels from the supply side of the production loop through the bypass pipe and mixes with the water returning from the distribution loop expansion device A component of the refrigeration cycle used to reduce the pressure and temperature of the refrigerant to the evaporator conditions 113 S TRANE 114 Glossary expansion tank A component of a closed piping system that accommodates the expansion and contraction of the water as temperature and therefore density changes fouling Minerals in the water that
17. chiller pairs in a primary secondary system depends on the direction and quantity of water flow in the bypass pipe m Whenever there is deficit flow through the bypass pipe for a specified period of time typically 15 to 30 minutes in a comfort cooling system another pump and chiller pair started m Whenever there is excess flow through the bypass pipe that is greater than the flow being produced by the next pump and chiller pair to be turned off that pump and chiller are turned off To prevent short cycling as the result of a slight increase in load the chiller plant control system will typically allow excess flow of from 110 to 115 percent of the flow produced by the next production pump to be turned off E If neither of the above conditions exist no action is taken 56 TRG TRCO16 EN S TRANE period two Chilled Water System Design Types of Fluid Flow Meters 4 Pressure based Pitot tube Venturi Orifice plate Differential pressure 4 Turbine and impeller a Vortex 4 Magnetic a Ultrasonic The direction and quantity of flow in the bypass pipe may be determined either directly by using a flow meter or indirectly by sensing temperatures Direct flow measurement can be accomplished using a variety of flow meter technologies These include pressure based flow meters pitot tubes venturi meters orifice plates and differential pressure sensors turbine and impeller meters vortex meters magnetic flow meters an
18. chillers piped in parallel Single Pump Parallel Configuration Parallel piping is one common configuration of multiple chiller systems Figure 41 shows a system that uses a single pump to deliver chilled water both to chillers and to the system load terminals This configuration can be used in systems that use constant flow methods of terminal control three way valves or face and bypass dampers or in systems that use variable flow methods of terminal control two way valves Varying the flow through the load terminals using two way valves in this type of system results in variable water flow through the chiller evaporators Chilled water systems that are specifically designed to vary evaporator water flow will be discussed in Period Three This section will focus on systems that use constant flow methods of terminal control Water is pumped through both chillers continuously regardless of whether only one chiller or both chillers are operating This example system is at 50 percent load with one chiller operating and the second chiller off Return water from the system at 54 F 12 2 C continues to flow through the non operating chiller and mixes with the chilled 42 F 5 6 C water produced by the operating chiller The resulting mixed water temperature leaving the plant is 48 F 8 9 C This rise in supply water temperature may result in problems with building comfort or humidity control A chiller plant controller may be used to re
19. compressor A type of compressor that uses centrifugal force generated by a rotating impeller to compress the refrigerant vapor TRG TRC016 EN TRG TRCO16 EN S TRANE Glossary chilled water system Uses water as the cooling media The refrigerant inside the evaporator absorbs heat from the water and this water is pumped to coils in order to absorb heat from the air used for space conditioning coefficient of performance COP A dimensionless ratio used to express the efficiency of a refrigeration machine A higher COP designates a higher efficiency For an electric chiller it is defined as evaporator cooling capacity divided by the electrical energy input For an absorption water chiller it is defined as evaporator cooling capacity divided by the heat energy required by the generator excluding the electrical energy needed to operate the pumps purge and controls compressor A mechanical device used in the vapor compression refrigeration cycle to increase the pressure and temperature of the refrigerant vapor condenser A component of the refrigeration cycle in which refrigerant vapor is converted to liquid as it rejects heat to air water or some other fluid cooling tower A device used to reject the heat from a water cooled condenser by spraying the condensing water over the fill while drawing outdoor air upward through the fill decoupled system See primary secondary system deficit flow A condition in a primary secondary
20. form scaling on the internal surfaces of the heat exchanger tubes generator A component of the absorption refrigeration cycle in which refrigerant vapor boils and is separated from the absorbent solution as it absorbs heat from the primary heat source heat recovery The process of capturing the heat that is normally rejected from the chiller condenser and using it for space heating domestic water heating or another process seems unnecessary helical rotary screw compressor A type of compressor that uses two mated rotors to trap the refrigerant vapor and compress it by gradually shrinking the volume of the refrigerant hybrid system A chilled water system that can use more than one type of fuel indirect fired A type of absorption chiller that uses steam or a hot fluid such as water from an external source to provide heat to the generator Integrated Part Load Value IPLV An equation that predicts chiller efficiency at the ARI standard rating conditions using weighted average load curves that represent a broad range of geographic locations building types and operating hour scenarios both with and without an air side economizer Nonstandard Part Load Value NPLV An equation that predicts chiller efficiency at nonstandard rating conditions using weighted average load curves that represent a broad range of geographic locations building types and operating hour scenarios both with and without an air side economizer primary s
21. may be in the same location as in the primary secondary system or it may be a three way valve on a few of the cooling coils Critical VPF System Requirements 4 Chillers must handle variable evaporator flow 4 System must include a bypass 4 System level controls must limit the rate of flow change 4 Adequate time to design and commission controls 4 Operator must understand the system Although VPF systems have been successfully installed and operated they are more complex both to design and to operate when compared to a primary secondary system The sequencing of chillers and pumps requires a thorough understanding of system dynamics because flow rates will vary through every operating chiller The control system needs to avoid cycling restarting the chiller too soon and maintain the rate of flow variation through the chiller evaporators within the allowable limits This becomes very complicated as the number of chillers increases Another important consideration when investigating VPF systems is the fact that they take more time and planning to design and commission properly than other systems The system design engineer must thoroughly define the control sequence early in the design process and clearly communicate it to the controls provider Also the system operators must understand how the VPF system works therefore training is mandatory The success of a system design is directly related to the ability of the operator to ca
22. move the water through the entire system The primary benefit of this system is the elimination of the separate distribution pump s and the associated electrical and piping connections There is also a small reduction in operating cost because there is seldom excess water flowing through the bypass pipe VPF systems however require chillers that can operate properly when the water flow through the evaporator varies Many of today s chillers can tolerate variable water flow through the evaporator within limits These limits include minimum and maximum flow rates and a limitation on how quickly the flow can vary Exceeding these operating limits may cause control instability or even catastrophic failure The VPF system therefore requires a method of monitoring the flow rate through each chiller and a control system to ensure that the flow through the evaporator stays within the limits for the specific chiller Do not attempt to use a VPF system with chillers that have older analog electric or pneumatic controls that cannot handle variable evaporator flow 64 TRG TRC016 EN S TRANE period three System Variations Notice also that the VPF system must continue to include a bypass Although a control valve prevents flow in the bypass for most system operating conditions the modulating valve and bypass are required to ensure that the water flow through the system remains above the minimum flow limit of the operating chillers This bypass
23. renewed interest Today the utility industry in many regions is going through some degree of deregulation One of the first United States locations to experience this was San Diego California Due to a number of factors the price of electricity TRG TRC016 EN 59 S TRANE 60 period three System Variations doubled and even tripled during periods of high demand in the first summer of deregulation Utility deregulation will occur differently in various locations but the possibility of high electricity costs during peak periods has building owners and operators looking for ways to use different fuels during those periods Another reason for using different fuel types is that it provides the building owner with leverage to negotiate for reduced utility prices from competing utilities If a building uses both electricity and natural gas for cooling and can switch between the two the owner can often negotiate better rates for both cooling and heating Fuel Choice Options thermal storage indirectly coupled gas engine chillers control interface A chilled water system that uses more than one type of fuel is referred to as a hybrid system The most obvious option for using an alternate fuel is an absorption chiller This type of chiller can be powered by natural gas fuel oil or even waste heat in the form of steam or hot water Another option is to use natural gas to operate an engine and generator set that pro
24. that a chiller needs to be turned on or off the next issue is to determine the sequence in which to turn chillers on and off It is assumed that the first chiller in the sequence will always be turned on whenever cooling is required When the system consists of identical chillers the choice of which chiller is turned on or off next has little impact on system efficiency Some design engineers and operators prefer to equalize the run time and the number of starts for all chillers in the system This is typically done by rotating the sequence of chillers on a periodic basis often every few days or weeks This method generally keeps the run time equalized reasonably well and the operator knows exactly when to expect the rotation to occur An alternative approach is to total the actual run hours on each chiller in an attempt to rotate the chillers when a significant imbalance in the run time or the number of starts occurs Rotation that is based on actual run time has the disadvantage of the operator not knowing when rotation will occur In some installations operating personnel prefer to manually initiate rotation On the other hand some design engineers and operators believe that equalizing run times will result in all of the chillers needing to be overhauled or replaced at the same time They tend to operate the most efficient chiller first followed by the next most efficient and so on With this approach all chillers are turned on at least o
25. the ambient temperature inside the building Soft loading either delays turning on additional chillers or varies the chilled water set point allowing the operating chillers to gradually catch up to the building pull down load This results in a very smooth pull down prevents overshooting the set point and operates only the equipment required to satisfy the actual system load TRG TRCO16 EN S TRANE period four Chiller Plant Control constant volume pumping system Chilled Water Set Point Control Constant flow chilled water systems frequently require individual chiller set point control Its purpose is to help maintain the system supply water temperature by compensating for the bypass of return water through non operating chillers The chiller plant control system adjusts the individual set points for the operating chiller to overcool the water before it mixes with the higher temperature water that bypasses through the non operating chiller The result is that the chilled water supplied to the system is as close as possible to the desired temperature There are limits to the amount of overcooling Depending on the design of the chilled water system one of two situations may exist Either the chiller may not have been selected to produce cold enough water or the temperature required may be below the freezing point of the water being cooled In either case the control system must be intelligent enough to limit overcool
26. to acquaint a nontechnical audience with various fundamental aspects of heating ventilating and air conditioning We have taken special care to make the clinic as uncommercial and straightforward as possible Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text This particular clinic introduces the reader to chilled water systems 2001 American Standard Inc All rights reserved TRG TRCO16 EN TRG TRCO16 EN Contents period one Types of Water Chillers 0000000000 en 1 Vapor Compression Water Chillers cccccececeees 3 Air Cooled or Water Cooled Condensing 00 6 Packaged or Split Components 0ccccceseeeeeeees 11 Absorption Water Chillers cccccccsseccceeeseeneeeees 15 Equipment Rating Standards cccccccceeeeeeeeeees 18 period two Chilled Water System Design 26 Load Terminal Control cccccccccesseeceeeeeeeeeeeees 28 Parallel Configuration ccccceccceeeeeeceeeeseeeeeeeeees 35 Series Configuration a ssossreresrrss creisis 38 Primary Secondary System Operation ce 53 period three System Variations 59 Alternate Fuel Choice oo ccccccceecsssseeeeeeeeeeeeeeees 59 LOW FIOW SYSTEMS oicceecccccccceccceeeeeeeessaaeeeeeeeeeeeeeees 61 Variable Primary Flow Systems cccccceeeeeeeeeeeees 64 Preferential Loading cccc
27. to as a decoupled system This configuration is unique because it dedicates separate pumps to the production and distribution loops A bypass pipe that connects the supply and return pipes is the key component in decoupling the system The chillers in the production loop receive a constant flow of water while the coils in the distribution loop controlled by two way modulating valves receive a variable flow of water 41 S TRANE period two Chilled Water System Design Primary Secondary System Rules a The bypass pipe should be free of restrictions Sized for minimal pressure drop Avoid random mixing of supply and return water streams No check valve The bypass pipe is common to both production and distribution loops The purpose of the bypass pipe is to hydraulically decouple the production primary and distribution secondary pumps Because water can flow freely between the supply and return pipes for both loops a change in flow in one loop does not affect the flow in the other loop The actual extent of hydraulic decoupling depends on the pressure drop due to the bypass pipe Total decoupling is accomplished only if the bypass pipe is free from restrictions and large enough to produce no pressure loss at all flow rates Because zero pressure loss is not practical some insignificant pump coupling will exist Bypass pipes are typically sized so that the water velocity in the pipe will be 10 to 15 ft s 3
28. 3 kWh at night By using either ice or chilled water to store cooling capacity at night when the cost of electricity is low and then using that stored energy to help cool the building during the day when the cost of electricity is high total electric costs can be reduced substantially Although thermal storage does not use a different fuel it is certainly an option for avoiding high electricity costs during peak periods Chiller Efficiency Improvements 09 8 0 a COP amp o8 70 amp R g 0 7 60 E 06 50 3 5 kWiton 8 05 40 3 1970 1980 1920 2000 year Low Flow Systems Building owners are becoming more conscious about how improved efficiency reduces system operating costs and overall environmental impact Typically the largest piece of equipment in the chilled water system is the water chiller However it is also the most efficient piece of equipment in the system Figure 68 shows the dramatic improvements in chiller efficiency at standard ARI conditions since 1970 High efficiency compressors and motors economizers on multiple stage centrifugal compressors more heat transfer tubes and tubes with special geometry to enhance heat transfer in both the evaporator and condenser have all contributed to these efficiency improvements Manufacturers continue to strive to improve chiller efficiency by redesigning chiller components TRG TRCO16 EN 61 62 TRANE period three System Variations Greater Focus on System Effic
29. E period four Chiller Plant Control Operator Interface There is an amazing amount of information available within a chilled water system Often the problem is not a lack of information but how to interpret that information Therefore a clear and concise interface between the control system and the system operator is extremely important Information that should be communicated to the operator includes m Chiller water system temperatures Chiller status on or off Information specified by ASHRAE Guideline 3 Any pending control actions chiller about to turn on or off Status of system time delays E Status of ancillary equipment pumps cooling towers and so forth In addition the chiller plant control system should notify the operator of problems that are occurring or are about to occur in the system These warning or diagnostic messages may point to a single piece of equipment malfunctioning or be indicative of system changes that may cause problems Diagnostics that occur at the chiller control panel should be communicated to the chiller plant control system TRG TRCO16 EN 101 amp ranw 102 period four Chiller Plant Control chiller operating log ASHRAE Guideline 3 4 Chilled water inlet and outlet 4 Compressor refrigerant suction temperatures and pressures and discharge temperatures 4 Chilled water flow A Oil pressures temperature and a Evaporator refrigerant levels temperature and pressures Ref
30. HVAC system design ASHRAE IESNA Standard 90 1 1999 Energy Standard for Buildings Except Low Rise Residential Buildings went into effect in October 1999 ASHRAE is the American Society of Heating Refrigerating and Air Conditioning Engineers and IESNA is the Illuminating Engineering Society of North America This standard addresses all aspects of buildings except low rise residential buildings It contains specific requirements for both water chillers and chilled water systems standard 90 1 1999 efficiency requirements Electric Vapor Compression Chillers chiller type capacity minimum efficiency air cooled all capacities 2 8 COP 3 05 IPLV water cooled reciprocating all capacities 42 OOP 5 05 IPLV helical rotary scroll lt 150 tons 528 KW 4 45 COP 5 2 IPLV 150 to 300 tons 528 to 1 056 kW 4 9 COP 56 IPLV gt 300 tons 1 056 KM 5 5 COP 6 15 IPLV centrifugal lt 150tons 528 KW 5 0 COP 525 IPLV 150 to 300 tons 528 to 1 056 kW 5 55 COP 5 9 IPLV gt 300 tons 1 056 kW 6 1 COP 64 IPLV as of October 29 2001 Standard 90 1 contains minimum full and part load efficiency requirements for packaged water chillers The table in Figure 27 is an excerpt from Table 6 2 1C of Addendum J to the standard It includes the minimum efficiency requirements for electric vapor compression chillers operating at standard ARI conditions The standard also contains tables of minimum efficiency requirements for these ch
31. Some systems however will use one three way valve at the load terminal furthest from the distribution pump to ensure that cold water is immediately availability to all terminals in the system TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design Varying Distribution Flow pressure difference variable speed control riding the pump curve The distribution pump is typically equipped with a variable speed drive that is controlled to maintain a certain pressure difference between the supply and return water piping In response to a reduced cooling load the two way valve modulates closed restricting the flow of water through the coil This causes an increase in system differential pressure which can be measured and used to signal a reduction in the speed of the distribution pump An alternative is to allow the pump to ride its pump curve As the two way valves modulate closed the increase in system pressure causes the pump to ride up its performance curve A to B resulting in a reduction to 50 percent of design flow in this example This method however generally results in less energy savings than a pump with a variable speed drive Also proper pump selection is important and part load operating conditions must be considered In variable flow systems ASHRAE IESNA Standard 90 1 1999 Section 6 3 4 1 requires the use of a modulation device such as a variable speed drive on pump mo
32. art Load Value NPLV Weighted average load curves Based on an average single chiller installation Non standard operating conditions The IPLV predicts chiller efficiency at the ARI standard rating conditions using weighted average load curves that represent a broad range of geographic locations building types and operating hour scenarios both with and without an airside economizer The NPLV uses the same methods to predict chiller efficiency at non standard rating conditions Although these weighted average load curves place greater emphasis on the part load operation of an average single chiller installation they will not by definition represent any particular installation Additionally ARI notes that more than 80 percent of all chillers are installed in multiple chiller systems Chillers in these systems exhibit different unloading characteristics than the IPLV weighted formula indicates Appendix D of Standard 550 590 1998 explains this further The IPLV equations and procedure are intended to provide a single number part load performance number for water chilling products The equation was derived to provide a representation of the average part load efficiency for a single chiller only However it is best to use a comprehensive analysis that reflects the actual weather data building load characteristics operational hours economizer capabilities and energy drawn by auxiliaries such as pumps and cooling towers when
33. ast majority of chilled water systems employ pumping schemes that maintain a constant flow rate of water through each chiller evaporator Even in the most carefully designed chilled water systems however the flow through the chillers will still vary slightly due to system TRG TRCO16 EN S TRANE period two Chilled Water System Design effects System effects include pump system curve interaction dynamic head variations and variation in distribution system flow There are benefits to maintaining a constant water flow rate through the chiller evaporator Constant flow provides more stable and simple chiller and system operation However there is potential for energy savings by varying the water flow in the distribution system Applying these two seemingly conflicting principles to chilled water systems requires careful planning and a thorough understanding of hydraulic system operation Due to advances in technology however many of today s chillers can operate with variable evaporator water flow Chilled water systems that are specifically designed to vary evaporator water flow are discussed in Period Three This period focuses on systems that employ constant water flow through the chiller and either constant or variable water flow through the rest of the distribution system Single Chiller System air cooled chiller purp three way valve Another factor that influences chilled water system design is the number of c
34. ater supplied to the distribution loop is 44 3 F 6 8 C Control of the water temperature supplied by the distribution loop is compromised due to this mixing When this deficit flow condition exists starting an additional chiller and pump increases the supply water flow from the production loop It also changes the supply and demand relationship in order to restore the temperature of the chilled water supplied to the distribution loop TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design Excess Flow 2 000 gomat 42 F 126 Lis at 5 6 C 2 000 gpm at 54 6 F 126 L s at 12 6 C 1 800 gomat 42 F 114 U s at 5 6 C WI 1 800 gomat 56 F 114 Us at 13 3 C When the flow of chilled water from the production loop exceeds the demand of the distribution loop the direction of flow in the bypass pipe reverses Chilled water flows from the supply side of the production loop through the bypass pipe and mixes with warm water returning from the distribution loop This is called excess flow In this example the pumps operating in the production loop are supplying 2 000 gpm 126 L s of water while the distribution pump is pumping 1 800 gpm 114 L s to meet the demand of the cooling coils The result is that 200 gpm 13 L s of supply water flows through the bypass pipe to be mixed with the water returning from the production loop The temperature of the water returning to the chillers decreases to
35. cceeeeeeeeeeeeeeeeeeeetneeeeees 66 Heat Recovery cecccccceeeeeeeceeeeeeeeeeeeeeeeeseaaaeeeeeees 68 Asymmetric Design ccceeeeeeeeeeeeeeeeeeeeeeseaeeeeees 72 Free Cooling se icida deasa AAE ENEA 75 Application Outside the Operating Range ofthe Chiller ee ee iaaa 78 period four Chiller Plant Control 0000 00000000e 79 Chiller Sequencing cccceceecccceeeeeceeeeseeeeeeeeeeneeeees 82 Failure Recovery and Contingency Planning 90 Systemi TUNNO osecctssutustercins arsenate di EEE 92 SyStEM OptiMIZation cece cceeeeeeeeeeeeaeees 96 Operator Interface oo ccceecccccceeeeeeeeeeeeeeeeeeees 100 period five Review asierea 103 QU Z 5 ca ex cacacansstarctcdatacceptstctatahasaictatatazetenatenaenets 108 PRINS WU NS iieo E rns ener 110 Glossary eee nc ee ne 112 ii S TRANE TRG TRC016 EN S TRANE period one Types of Water Chillers Chilled Water Systems period one Types of Water Chillers Water chillers are used in a variety of air conditioning and process cooling applications They cool water that is subsequently transported by pumps and pipes The water passes through the tubes of coils to cool air in an air conditioning system or it can provide cooling for a manufacturing or industrial process Systems that employ water chillers are commonly called chilled water systems When designing a chilled water system one of the first issues that must be addressed is to determine w
36. d ultrasonic transit time meters The accuracy ease of installation required maintenance and cost of these meter technologies vary widely The accuracy and reliability of the flow meter will directly impact the efficiency and reliability of the chilled water system High quality flow meters are critical to proper system operation When using a flow meter it is important to understand the range of flows and velocities that the specific device can accurately measure The accuracy of some flow meters is dependent on the velocity of the flow and the development of a smooth flow profile in the stream being measured To obtain accurate measurements several diameters of straight pipe may be required both upstream and downstream of the meter Finally in order to give accurate results many types of flow meters require periodic calibration This is often overlooked in the maintenance of chilled water systems TRG TRC016 EN 57 S TRANE 58 period two Chilled Water System Design Temperature Based Calculations supply tee controller The advent of microprocessor based controls has led to another method for determining flow in the bypass pipe Temperature sensors are placed in the supply and return pipes of the production and distribution loops and in the bypass pipe With these temperatures a chiller plant control system programmed with fluid mixing equations can determine the quantity of excess or deficit flow that exists at a
37. de range of loads This is especially true of centrifugal and helical rotary chillers This fact allows constant flow chilled water systems similar to the system shown in Figure 91 to use the system supply and return water temperatures to determine system load By sensing a rise in the temperature of the water leaving the chiller plant the control system can determine when the operating chillers can no longer maintain the desired temperature Often the supply water temperature is allowed to drift a predetermined amount before an additional chiller is turned on to ensure that there is enough load to keep an additional chiller operating Deciding when it is appropriate to turn a chiller off is more complex The control system may monitor the system AT that is return water temperature minus supply water temperature This information along with the capacities of the operating chillers allows the control system to determine when a chiller can be turned off To help stabilize system operation the control system should use logic to prevent load transients from causing unwarranted chiller cycling In constant flow systems that are suffering from low AT syndrome airside systems that return water to the plant at lower temperatures than desired some of the load terminals may starve for flow before the capacity of the operating chiller is exceeded To preserve system efficiency this situation is best dealt with by solving the airside proble
38. dition to turning chillers on and off there are other functions of the chiller plant control system that help prevent system flow instability from disrupting chiller operation Flow instability can often be caused by normal valve and pump operation The first is time delays Excessive cycling can be detrimental to the life of a motor For this reason turning a large motor such as those used in large chillers on and off should be minimized Chilled water systems typically have a large thermal mass water in the system and benefit from the diversity and slow rate of change of the system cooling load Fast reactions therefore are typically not required In fact a response that is too fast will often cause system instability waste energy and cause unnecessary wear on mechanical equipment To achieve stable and accurate control many chiller plant control systems provide time delays that can be adjusted by the operator to help minimize chiller cycling The first time delay is the load confirmation timer Its purpose is to delay turning on an additional chiller for a period of time following the initial indication that an additional chiller is required This confirms that the indicated load is not a transient condition that would cause the chiller to be turned on and then quickly turned off The second time delay which works in conjunction with the first is a staging interval timer Its purpose is to allow the system time to respond after a ch
39. doing nothing more than shutting it down when a safety setting is violated Improved control accuracy allows chillers to be applied in systems and applications that were previously avoided When problems occur diagnostic messages aid troubleshooting Modern chiller controls also interface with a chiller plant control system for integrated system operation TRG TRCO016 EN TRG TRCO16 EN amp rame period four Chiller Plant Control What Is Important 4 When to turn a chiller on or off 4 Which chiller to turn on or off 4 How to recover from an equipment failure 4 How to optimize system efficiency a How to communicate with the operator There are primarily five issues to address in a chiller plant control system 1 2 When should a chiller be turned on or off After we know that a chiller must be turned on or off which one should it be If we attempt to turn on a chiller pump or cooling tower and there is a malfunction what do we do next How can we minimize the energy cost of operating the system How can the chiller plant control system effectively communicate with the operator 81 amp ranw period four Chiller Plant Control Chiller Sequencing 4 Turning on an additional chiller 4 Turning off a chiller 4 Which chiller to turn on or off pl an ery z Chiller Sequencing Chiller sequencing refers to making decisions about when to turn chillers on and off and in what ord
40. duces electricity and then use that electricity to run a standard electric chiller This indirect coupling of the gas engine to the chiller allows the flexibility of operating the chiller using the gas engine during times of high electricity costs and operating the chiller on utility line electricity during times of low electricity costs A second benefit of indirect coupling is that the engine can be sized to provide enough power for the chiller the pumps and in a water cooled system the cooling tower If the engine is also to be used for emergency backup the pumps and cooling tower would not need a second generator to provide them with power An alternative to this approach is to directly couple the engine and chiller A significant drawback of this approach is that the building owner does not have the flexibility to switch between natural gas and electricity the chiller must always operate on natural gas Also only the chiller is connected to the engine If emergency backup is necessary a second generator is required to operate the pumps and cooling tower TRG TRCO16 EN S TRANE period three System Variations A third method of using an alternate fuel is actually to use the same fuel electricity but to use it at a different time The highest electricity costs occur at the time of highest demand For example a real time pricing rate for electricity may be 0 50 kWh at times of peak demand during the day but only 0 0
41. duction loop to the load of the distribution loop The operation of a primary secondary system focuses on the direction and amount of flow in the bypass line At the supply tee which connects the supply and bypass pipes a supply and demand relationship exists The total water flow from all operating production chiller pumps is the supply flow The demand flow is the total water flow required to meet the loads on the cooling coils Whenever the supply and demand flows are unequal water will either flow into or out of the bypass pipe at the supply tee 53 S TRANE 54 period two Chilled Water System Design Deficit Flow 1 000 gpm at 42 F 63 Lis at 5 6 C 1 000 gpm at 56 F 63 Us at 133 C 1 200 gomat 44 3 F 76Ls at 68 C NK 1 200 gomat 56 F 76 Us at 13 3 C If production supply is inadequate to meet the load demand a deficit of supply water exists To make up for this deficit the distribution pump will pull water from the return pipe of the distribution loop through the bypass pipe This is called deficit flow In this example the pumps operating in the production loop are supplying 1 000 gpm 63 L s of water while the distribution pump is pumping 1 200 gpm 76 L s to meet the demand of the cooling coils The result is that 200 gom 13 L s of system return water flows through the bypass pipe to be mixed with the supply water from the production loop The temperature of the mixed w
42. e applied to practically any type of water chiller It can be accomplished either by operating at higher condensing temperatures and recovering heat from the water leaving the standard condenser or by using a separate condenser as shown in Figure 76 for a centrifugal chiller In smaller chillers heat recovery is sometimes accomplished using a device called a desuperheater A desuperheater is a device that is connected to the refrigeration circuit between the compressor and condenser to recover heat from the hot refrigerant vapor TRG TRC016 EN S TRANE period three System Variations Heat Recovery Chiller Options heat recovery auxiliary dual condenser condenser heat pump e Second full e Second smaller e No extra size condenser size condenser condenser Large heating Preheating loads Large base heating loads Woderate loads or continuous High hot water hot water operation temperatures temperatures High hot water Controlled Uncontrolled temperatures Degrades e Improves chiller Controlled chiller efficiency efficiency Good heating efficiency For water cooled centrifugal chillers there are generally three methods of implementing heat recovery The dual condenser or double bundle heat recovery chiller contains a second full size condenser that is connected to a separate hot water loop It is capable of more heat rejection and higher leaving hot water temperatures than an auxiliary condenser The am
43. e flow rate through the chiller is 610 gpm 38 5 L s When the second same size pump and chiller are turned on the flow rate through the system increases to 870 gpm 54 9 L s but the flow through each chiller drops to 435 gpm 27 4 L s This is an instantaneous reduction of 175 gpm 11 L s or 30 percent through the first chiller The temperature of the water leaving the chiller and the temperature of the refrigerant in the evaporator drop as a result of this drastic flow reduction New advanced chiller controls may allow the refrigerant temperature to drop below the fluid s freezing point for a brief period of time while the compressor unloads The evaporator low temperature safety may however turn off the chiller if the controls and compressor cannot react quickly enough The unload before start function partially unloads the operating chillers raising the refrigerant temperature in the evaporator before the flow reduction occurs The chillers are allowed to reload as soon as the additional chiller is turned on TRG TRCO16 EN 93 amp ranw 94 period four Chiller Plant Control Soft Loading soft loading one chiller supply water temperature a Q Q 44 C two chillers 0 30 60 operating time minutes Another control function that is desirable is called soft loading It is typically used when the system has been off for an extended period of time and the chilled water temperature is the same as
44. e increasing the system AT With the lower water temperature and increased AT the coil requires less water flow to handle the same load Cooling tower energy can be reduced by increasing the condenser water temperature This allows the tower fans to cycle or slow down Condenser water pumping energy can be reduced by increasing the AT through the condenser side of the system thereby pumping less water This is achieved by reducing the water flow through the condenser Obviously looking at only a single component presents a conflicting picture for energy reduction and a change in one component has an impact on other components To truly optimize the chiller plant all components must be analyzed together 96 TRG TRCO16 EN amp ranw period four Chiller Plant Control Chilled Water Reset 4 Pros 4 Cons Reduces chiller energy Increases pump energy Can work in constant in variable flow systems flow systems Can cause loss of space humidity control Complicates chiller sequencing control As previously stated as the chilled water temperature set point is reset upwards the chiller will use less energy In constant flow systems this chilled water reset strategy is fairly simple to implement and can be controlled based on the drop in return water temperature In a variable flow system however as the chilled water temperature increases the pumping energy also increases While the COP of the chiller is approximat
45. e mode cooling only 0 57 kW ton not centrifugal chiller 6 2 COP applicable heat recovery 0 60 kW ton 0 69 kW ton centrifugal chiller 5 9 COP 5 1 COP cooling mode conditions evaporator AT 44 F to 54 F 6 7 C to 12 2 C condenser AT 85 F to 95 F 29 4 C to 35 0 C heat recovery mode conditions evaporator AT 44 F to 54 F 6 7 C to 12 2 C condenser AT 85 F to 105 F 29 4 C to 40 6 C There is usually an efficiency penalty associated with the use of heat recovery with a chiller The cost of this efficiency penalty however is typically much less than the energy saved by recovering the free heat The energy consumption of a heat recovery chiller will be higher than that of a cooling only chiller because of the higher pressure differential at which the compressor must operate In this example the energy consumption of a centrifugal chiller operating in heat recovery mode producing 105 F 40 6 C condenser water is 0 69 kW ton 5 1 COP The efficiency of the same chiller operating in the cooling only mode no heat being recovered and producing 95 F 35 C condenser water is 0 60 kW ton 5 9 COP A comparable cooling only chiller of the same capacity and operating at the same cooling only conditions consumes 0 57 kW ton 6 2 COP In this example the heat recovery chiller uses four percent more energy in the cooling only mode than the chiller designed and optimized for cooling only operati
46. ecause absorption chillers use water as the refrigerant however they are exempt from this standard For many chilled water systems in which the chillers are located indoors the standard requires the refrigeration equipment to be installed in a mechanical equipment room The requirements for this mechanical equipment room include refrigerant monitors and alarms mechanical ventilation pressure relief piping and so forth TRG TRC016 EN 25 S TRANE period two Chilled Water System Design Chilled Water Systems period two Chilled Water System Design Proper design of a chilled water system can greatly impact the first cost operating costs and flexibility of the HVAC system The purpose of this period is to discuss the design of reliable chilled water systems Chilled Water System Components cooling tower lt i 1 errs a chiller The conventional chilled water system consists of combinations of the following primary components m Water chillers Load terminals chilled water cooling coils in comfort cooling applications Cooling towers in water cooled systems Chilled and condenser water pumps Chilled and condenser water distribution systems that include piping an expansion tank control valves check valves strainers and so forth 26 TRG TRCO16 EN S TRANE period two Chilled Water System Design Chilled Water System This period focuses on the chilled water side of t
47. econdary decoupled system A configuration of a multiple chiller system that uses separate production and distribution pumps to hydraulically decouple the production capacity of the chillers from the load of the distribution system reciprocating compressor A type of compressor that uses a piston that travels up and down inside a cylinder to compress the refrigerant vapor refrigerant migration A method of free cooling that allows the chiller to be used as a heat exchanger without operation of the compressor It is possible when the condensing temperature of the refrigerant is low enough for refrigerant to migrate from the evaporator to the condenser scroll compressor A type of compressor that uses two opposing scrolls to trap the refrigerant vapor and compress it by gradually shrinking the volume of the refrigerant single effect A type of absorption chiller that uses a single generator TRG TRC016 EN TRG TRCO16 EN S TRANE Glossary swing chiller A smaller capacity chiller used in a multiple chiller system It is alternated on and off between the larger chillers operation to serve as a smaller incremental step of loading variable primary flow VPF system A type of chilled water system that is designed to vary the flow of water throughout the entire system through the evaporator of each operating chiller as well as through the cooling coils variable speed drive A device used to vary the capacity of a centrifugal pump
48. ect fired There are two fundamental differences between the absorption refrigeration cycle and the vapor compression refrigeration cycle The first is that the compressor is replaced by an absorber pump and generator The second is that in addition to the refrigerant the absorption refrigeration cycle uses a secondary fluid called the absorbent The condenser expansion device and evaporator sections however are similar Warm high pressure liquid refrigerant D passes through the expansion device and enters the evaporator in the form of a cool low pressure mixture of liquid and vapor A Heat is transferred from the relatively warm system water to the refrigerant causing the liquid refrigerant to boil Using an analogy of the vapor compression cycle the absorber acts like the suction side of the compressor it draws in the refrigerant vapor B to mix with the absorbent The pump acts like the compression process itself it pushes the mixture of refrigerant and absorbent up to the high pressure side of the system The generator acts like the discharge of the compressor it delivers the refrigerant vapor C to the rest of the system The refrigerant vapor C leaving the generator enters the condenser where heat is transferred to cooling tower water at a lower temperature causing the refrigerant vapor to condense into a liquid This high pressure liquid refrigerant D then flows to the expansion device which creates a pressure d
49. ed by the heat recovery condenser Controlling heat recovery capacity based on the temperature of the hot water leaving the heat recovery condenser can cause operational problems for a centrifugal chiller This is explained best by using a map of centrifugal compressor operation see Figure 79 Control based on the temperature of the water leaving the heat recovery condenser causes the condenser to evaporator pressure differential to remain relatively high at all loads line A to B High pressure differentials at low cooling loads increases the risk of a centrifugal compressor operating in its unstable region commonly known as surge The preferred method is to control heat recovery capacity based on the temperature of the hot water entering the heat recovery condenser This allows the condenser to evaporator pressure differential to decrease as the chiller unloads line A to C thereby keeping the centrifugal chiller from surging and resulting in more stable operation If high leaving hot water temperatures are required at low cooling load conditions another method to prevent surge is to use hot gas bypass on the centrifugal chiller For other types of chillers that are not prone to surge operating at these high pressure differentials at low cooling loads may cause the chiller to consume more energy than it recovers in the form of heat TRG TRCO16 EN 71 S TRANE period three System Variations Asymmetric Design 4 Different chil
50. een the cooling tower water and the chilled water inside a centrifugal chiller through the use of refrigerant migration When the temperature of the water from the cooling tower is colder than the desired chilled water temperature the compressor is turned off and automatic shutoff valves inside the chiller refrigerant circuit are opened This allows refrigerant to circulate between the evaporator and condenser without the need to operate the compressor Because refrigerant vapor migrates to the area with the lowest temperature refrigerant boils in the evaporator and the vapor migrates to the cooler condenser After the refrigerant condenses into a liquid it flows by gravity back into the evaporator There are no additional fouling concerns because the cooling tower water flows through the chiller condenser and is separated from the chilled water loop Although not as effective as a plate and frame heat exchanger it is possible for refrigerant migration in a centrifugal chiller to satisfy many cooling load requirements up to 40 percent of the chiller s design capacity without operating the compressor This can increase further if the system can accommodate warmer chilled water temperatures at part load conditions 77 S TRANE 78 period three System Variations Application Outside Range of Chiller fro 240 gpm at 45 F gp 15 Us at 7 2 C 240 gomat 56 7 F _ 15 Us at 13 7 C 80 gomat 45 F 5 Us at 7 2
51. el optimization Remember that you cannot control your way out of a poor system design Although the control system can attempt to minimize the impact of a poor design it cannot eliminate the cause of the poor design Second even a properly installed system with good components requires system level control to make those components work together effectively Third even if the components are working together the system not the individual components needs to be optimized Remember the meter is on the building Finally a very important issue that is related to chiller plant control is the issue of interfacing with the person who is operating the system Simplicity is important and it gives the system a much better chance of working without intervention by the operator TRG TRCO16 EN period five Review S TRANE An American Standard Company For more information refer to the following references m Multiple Chiller System Design and Control Applications Engineering Manual Trane literature order number SYS APM001 EN m Refrigeration Cycle Air Conditioning Clinic Trane literature order number TRG TRC003 EN m Refrigeration Compressors Air Conditioning Clinic Trane literature order number TRG TRC004 EN m Centrifugal Water Chillers Air Conditioning Clinic Trane literature order number TRG TRC010 EN E Absorption Water Chillers Air Conditioning Clinic Trane literature order number TRG TRC011 EN
52. ely 6 5 the COP of the pump is about 0 65 Often the increase in pump energy will be more than the amount of chiller energy saved especially because the chiller will often operate at part load conditions Another potential problem with resetting the chilled water temperature upward is that space humidity control can be compromised if the water gets too warm Finally the chiller plant control system must account for the changing supply water temperature ASHRAE IESNA Standard 90 1 1999 Section 6 3 4 3 requires the use of chilled water temperature reset in systems larger than 25 tons 88 kW It does however exclude variable flow systems and systems where space humidity control will be compromised In Period Three the concept of designing for reduced chilled water temperature and flow rates was briefly discussed Some engineers feel that designing the system for low flow rates and a lower supply water temperature thus minimizing pump energy use might be a better answer than attempting to reset the temperature upward TRG TRCO16 EN 97 amp ranw period four Chiller Plant Control Condenser Water Temperature codling tower chiller 85 F 70 F 55 F opima aac 2110 1289 control condenser water temperature set point kWh 8 8 8 S annual energy consumption 8 8 Lowering the temperature of the condenser water can also reduce the energy consumption of the chiller Depending on the system load and outdoor condi
53. ems Group 3600 Pammel Creek Road La Crosse WI 54601 7599 www trane com An American Standard Company Response Card We offer a variety of HVAC related educational materials and technical references as well as software tools that simplify system design analysis and equipment selection To receive information about any of these items just complete this postage paid card and drop it in the mail Education materials J Air Conditioning Clinic series J Engineered Systems Clinic series LJ Trane Air Conditioning Manual LJ Trane Systems Manual Software tools LJ Equipment Selection LJ System design amp analysis Periodicals Lj Engineers Newsletter Other LJ Thank you for your interest About me Name Title Business type Phone fax E mail address Company Address TRANE The Trane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse WI 54601 7599 www trane com An American Standard Company Chilled Water Systems One of the Systems Series A publication of The Trane Company S TRANE Preface Chilled Water Systems A Trane Air Conditioning Clinic The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research testing programs and field experience The Trane Air Conditioning Clinic series is one means of knowledge sharing It is intended
54. entering the cooling coil This can reduce or totally eliminate the requirement for mechanical cooling for much of the year in many climates In water cooled systems there are also several types of waterside economiczers The most direct method but typically the least desirable is to use a Strainer cycle In this system the condenser and chilled water systems are connected When the outdoor wet bulb temperature is low enough cold water from the cooling tower is routed directly into the chilled water loop Although the strainer cycle is the most efficient waterside economizer option it greatly increases the risk of fouling in the chilled water system and cooling coils with the same type of contamination that is common in open cooling tower systems A strainer or filter can be used to minimize this contamination but the potential for fouling prevents widespread use of the strainer cycle system TRG TRCO16 EN 75 S TRANE period three System Variations waterside economizer Plate and Frame Heat Exchanger distribution plate and frame heat exchanger A second type of waterside economizer is the plate and frame heat exchanger In this case water from the cooling tower is kept separate from the chilled water loop by a plate and frame heat exchanger This is a popular configuration because it can achieve a high heat transfer efficiency without the potential for cross contamination With the addition of a second condenser wate
55. er Typically turning chillers on and off is performed with the goal of matching the capacity of the chiller plant to the system cooling load In order to do this successfully the design of the chilled water system must provide the control system with variables that are good indicators of system load The hydraulic design and size of the chilled water system will determine the possible method s for effectively monitoring system load Typical methods for load monitoring include E In series or parallel piped systems the supply and return water temperatures and sometimes chiller current draw are monitored E Ina primary secondary system the system supply and chiller return water temperatures and or the direction and quantity of flow in the bypass pipe are typically measured E Ina variable primary flow system the system supply water temperature and the system flow rate may be monitored m Direct measurement of the system load in tons kW or amperes has also been used in some systems Other methods are also possible It is imperative that the chilled water system be designed with the control variables in mind otherwise the result may be a system that is impossible to efficiently control 82 TRG TRCO16 EN S TRANE period four Chiller Plant Control load indicators Temperature chiller plant controller Today s chiller controls can very accurately control the chiller s leaving water temperatures over a wi
56. erior reliability reduced sound levels and relatively low cost have contributed to the popularity of the centrifugal chiller Centrifugal compressors are generally available in prefabricated chillers from 100 to 3 000 tons 350 to 10 500 kW and up to 8 500 tons 30 000 kW as built up machines 4 TRG TRC016 EN TRG TRCO16 EN S TRANE period one Types of Water Chillers These various types of compressors are discussed in detail in the Refrigeration Compressors Air Conditioning Clinic Variable Speed Drives The capacity of a centrifugal chiller can be modulated using inlet guide vanes IGV or a combination of IGV and a variable speed drive adjustable frequency drive AFD Variable speed drives are widely used with fans and pumps and as a result of the advancement of microprocessor based controls for chillers they are now being applied to centrifugal water chillers Using an AFD with a centrifugal chiller will degrade the chiller s full load efficiency This can cause an increase in electricity demand or real time pricing charges At the time of peak cooling such charges can be ten or more times the non peak charges Alternatively an AFD can offer energy savings by reducing motor speed at low load conditions when cooler condenser water is available An AFD also controls the inrush current at start up Certain system characteristics favor the application of an adjustable frequency drive including m A substantial
57. ethod of load terminal control should be used in the distribution loop of a primary secondary chilled water system 8 Deficit flow in the bypass pipe of a primary secondary system is an indication to turn an additional chiller on or off TRG TRCO16 EN TRG TRCO16 EN S TRANE Quiz Questions for Period 3 9 Why does a variable primary flow VPF system require a bypass in the system 10 Identify one scenario where preferential loading is beneficial to a chilled water system 11 What is the benefit of using a smaller capacity swing chiller in a multiple chiller system Questions for Period 4 12 Making decisions about when to turn chillers on and off is commonly referred to as chiller 13 Lowering the temperature of the water leaving the cooling tower increases or decreases the energy consumption of the chiller and increases or decreases the energy consumption of the cooling tower fans 14 An increase in the condenser approach temperature that is the temperature difference between the water and the refrigerant inside the condenser may be a sign of what 109 S TRANE Answers 1 Centrifugal helical rotary reciprocating and scroll 2 Air cooled chiller advantages include lower maintenance costs a pre packaged system for easier design and installation and better low ambient operation Water cooled chiller advantages include greater energy efficiency at least at design conditions and longer e
58. evaporator in the form of a cool low pressure mixture of liquid and vapor A Heat is transferred from the relatively warm air or water to the refrigerant causing the liquid refrigerant to boil The resulting vapor B is then drawn from the evaporator by the compressor which increases the pressure and temperature of the refrigerant vapor The hot high pressure refrigerant vapor C leaving the compressor enters the condenser where heat is transferred to ambient air or water at a lower temperature Inside the condenser the refrigerant vapor condenses into a liquid This liquid refrigerant D then flows to the expansion device which creates a pressure drop that reduces the pressure of the refrigerant to that of the evaporator At this low pressure a small portion of the refrigerant boils or flashes cooling the remaining liquid refrigerant to the desired evaporator temperature The cool mixture of liquid and vapor refrigerant A travels to the evaporator to repeat the cycle The vapor compression refrigeration cycle is reviewed in detail in the Refrigeration Cycle Air Conditioning Clinic S TRANE period one Types of Water Chillers Compressor Types helical rotary The type of compressor used generally has the greatest impact on the efficiency and reliability of a vapor compression water chiller The improvement of compressor designs and the development of new compressor technologies have led to more efficient and reliab
59. ften desirable to preferentially load that chiller sequencing it as a base chiller first on and last off Other chillers can then be turned on when the heat recovery chiller cannot handle the cooling load alone A variation on this idea is an absorption chiller fueled by waste heat It is preferentially loaded to handle as much of the cooling load as possible before turning on other chillers The absorption chiller would be sequenced as a base chiller to make use of the free energy operating this chiller 88 TRG TRCO16 EN S TRANE period four Chiller Plant Control Variable Primary Flow Systems __ optional bypass with three way valve The variable primary flow system introduced in Period Three is designed to operate with variable flow through the chiller evaporators Sequencing chillers in this type of system cannot be based solely on temperature because in a properly operating system the supply and return water temperatures will be nearly constant Determining when to turn chillers on or off is not a simple task For control stability and chiller reliability the flow rates through the chillers and the rate of flow change must be kept within allowable ranges Therefore control of a variable primary flow system must E Include a method to determine system load Many systems measure flow rates and temperatures E Ensure that flow rates through the chillers are within the allowable minimum and maximum limits
60. g from the load terminals will be at least as high as it is at design load conditions and may actually rise at part load conditions This warm return water is especially advantageous in systems using heat recovery free cooling or preferential loading of chillers These options will be discussed in Period Three TRG TRCO16 EN 51 S TRANE period two Chilled Water System Design Primary Secondary System Rules 4 All chillers should be selected for the same leaving chilled water temperature and AT For simplicity of system control all of the chillers in a primary secondary system should be selected to operate with the same leaving water temperature and with the same temperature difference AT This allows all operating chillers to be loaded to equal percentages Control of supply water temperature is fairly simple The set points of the individual chillers are all equal to the desired system supply water temperature Because water flows only through operating chillers there is no water mixing in the production loop and the production loop supplies the water temperature corresponding to the individual chiller set points 52 TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design System Operation Primary Secondary System Operation We have seen that the production and distribution loops of the primary secondary system act independently The next consideration is to match the capacity of the pro
61. gered Set Points An alternative method of controlling chillers in series involves staggering the set points of the two chillers This results in the downstream chiller operating first and being preferentially loaded Any portion of the load that the downstream chiller cannot meet is handled by the upstream chiller The example in Figure 46 shows the system operating at about 80 percent of design cooling load As we mentioned earlier with three way valves at the coils the temperature of the water returning to the chillers decreases at part load At 80 percent load the return water temperature is 52 F 11 1 C instead of the 54 F 12 2 C at 100 percent load The upstream chiller is partially loaded cooling the water to the 48 F 8 9 C set point while the downstream chiller remains fully loaded cooling the water the rest of the way to 42 F 5 6 C 40 TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design Primary Secondary Configuration Primary Secondary Decoupled Configuration If the water flow through the chillers production could be hydraulically isolated from the water flow through the coils distribution many of the problems encountered in parallel and series configurations could be eliminated Figure 47 shows a configuration that separates or decouples the production capacity from the distribution load This scheme is known as a primary secondary system also referred
62. gestions for good design practice for any given system they simply define a common rating point to aid comparisons Trends toward improved humidity control and system level energy efficiency have led many design engineers to reduce the flow rates on both the chilled and condenser water sides of the system This results in smaller motors in the pumps and cooling tower fans as well as smaller pipes and control valves in the distribution system The right column of this table shows one possible set of conditions for a low flow system For comparison 1 5 gpm ton 0 027 L s kW through the evaporator results in a 16 F 8 9 C AT and 2 0 gpm ton 0 036 L s kW through the condenser results in a 15 F 8 3 C AT chiller pumps cooling tower fans low flow Low Flow Systems 750 000 600 000 300 000 150 000 0 base case Figure 71 shows the combined annual energy consumption of the chiller chilled and condenser water pumps and cooling tower fans for these two system designs In fact a growing number of design engineers and utilities have published papers or manuals that recommend that system flow rates be reduced A number of them have found that using lower flow rates can reduce both installed and operating costs annual energy consumption kWh 8 S there are times you can have your cake and eat it too In most cases larger AT s and the associated lower flow rates will not only save installation cost but will usua
63. gs In a distributed pumping system a dedicated distribution pump is located out in the system at each building instead of all the pumps being housed in the chiller plant This configuration offers the potential for additional pump energy savings because each pump only needs to pump the water required for the building it serves 49 S TRANE period two Chilled Water System Design Tertiary Pumping In very large systems a primary secondary tertiary pumping configuration is sometimes used The primary pumps circulate water through the chillers The secondary distribution pumps circulate the water around the distribution loop The individual load terminals are decoupled from the distribution loop and each load terminal has a dedicated tertiary pump A load terminal in this case may be an individual cooling coil or an entire building In systems that use tertiary pumping the load terminal must be controlled so that only the quantity of water required is drawn from the distribution loop Water must not be allowed to flow into the return piping until it has experienced the proper temperature rise The two way valve modulates to maintain the design return water temperature A constant volume tertiary pump circulates water through the load terminal Some tertiary pumping systems use a small bleed line to ensure that water will be immediately available when the tertiary pump is started and to provide an accurate control signal when
64. he system that is the water that flows through the chiller evaporator and out through the load terminals Specifically we will review methods of load terminal control and various multiple chiller system configurations These topics apply to systems using both air cooled and water cooled chillers Fundamentally the function of the chilled water system is to transport the cooling fluid from the chillers to the load terminals and back to the chillers Assuming that the distribution system is adequately sized we will concentrate on the hydraulic interaction between the load terminals and the chillers TRG TRC016 EN 27 S TRANE 28 period two Chilled Water System Design Load Terminal Control Options 4 Three way modulating valve 4 Two way modulating valve 4 Face and bypass dampers Load Terminal Control The purpose of load terminal control is to modulate the flow of air or water through the coil to maintain building space comfort This is accomplished by measuring the temperature of the supply air or space The temperature then converted to an electronic signal that modulates the capacity of the cooling coil to match the changing load in the space Three methods of load terminal control are commonly used in chilled water systems E Three way modulating valve control m Two way modulating valve control E Face and bypass damper control Each of these methods has a different effect on the operation of the system
65. herefore typically require dedicated boilers Typical COPs for these chillers are 0 9 to 1 2 The direct fired absorption chiller includes an integral burner rather than relying on an external heat source Common fuels used to fire the burner are natural gas fuel oil or liquid petroleum Additionally combination burners are available that can switch from one fuel to another Typical COPs for direct fired double effect chillers are 0 9 to 1 1 based on the higher heating value of the fuel Higher energy efficiency and elimination of the boiler are largely responsible for the increasing interest in direct fired absorption chillers These types of absorption chillers have the added capability to produce hot water for heating Thus these chiller heaters can be configured to produce both chilled water and hot water simultaneously In certain applications this flexibility eliminates or significantly down sizes the boilers TRG TRCO16 EN 17 S TRANE period one Types of Water Chillers Equipment Rating Standards 4 Air Conditioning amp Refrigeration Institute ARI STANDARD for Standard 550 590 1998 WATER centrifugal and helical rotary Seer water chillers ie Standard 560 1992 cree absorption water chillers AR Stantara 550990 Equipment Rating Standards The Air Conditioning amp Refrigeration Institute ARI establishes rating standards for packaged HVAC equipment ARI also certifies and labels eq
66. hich type of water chiller to use This period discusses the primary differences in chiller types absorption water chiller centrifugal water chiller The refrigeration cycle is a key differentiating characteristic between chiller types The vapor compression and absorption refrigeration cycles are the two most common cycles used in commercial air conditioning TRG TRC016 EN 1 S TRANE period one Types of Water Chillers Water chillers using the vapor compression refrigeration cycle vary by the type of compressor used Reciprocating scroll helical rotary and centrifugal compressors are common types of compressors used in vapor compression water chillers Absorption water chillers make use of the absorption refrigeration cycle Driving Sources compressor driven heat driven Vapor compression water chillers use a compressor to move refrigerant around the system The most common energy source to drive the compressor is an electric motor Absorption water chillers use heat to drive the refrigeration cycle They do not have a mechanical compressor involved in the refrigeration cycle Steam hot water or the burning of oil or natural gas are the most common energy sources for these types of chillers TRG TRC016 EN TRG TRCO16 EN S TRANE period one Types of Water Chillers Vapor Compression Cycle Vapor Compression Water Chillers In the vapor compression refrigeration cycle refrigerant enters the
67. hillers used Single chillers are sometimes used in small systems less than 100 tons 35 kW while larger or critical systems typically use multiple chillers Many single chiller systems resemble the one shown in Figure 39 This system uses a single pump to move water through the chiller and load terminals The load terminals are controlled using three way modulating valves The pump delivers a constant flow of water throughout the entire system and flow balance is relatively easy TRG TRCO16 EN 33 S TRANE period two Chilled Water System Design Multiple Chiller Systems a4 Redundancy 4 Part load efficiency Multiple chiller systems are more common than single chiller systems for the same reason that most commercial airplanes have more than one engine redundancy provides reliability Additionally because cooling loads typically vary widely multiple chiller systems can often operate with less than the full number of chillers During these part load periods the system saves the energy required to operate the additional chillers pumps and in water cooled systems cooling tower fans There are several configurations used to connect multiple chillers in these systems Some of these configurations work well others do not Next we will look at the most commonly used system configurations including their advantages and drawbacks 34 TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design
68. iency Realize however that the chiller is only one component of the chilled water system Although chiller efficiency is important overall system efficiency is more important because the building owner pays to operate the entire system not just the chiller Said another way The meter is on the building With this in mind many system design engineers are looking for ways to optimize the efficiency of the entire system not just the chiller Trend Toward Lower Flow Rates electric driven chiller yesterday today evaporator 2 4 gpm ton 1 5 gpm ton flow rate 0 043 L s kW 0 027 L s kW leaving 44 F 41 F chilled water 6 7 C 5 C temperature condenser 3 0 gpm ton 2 0 gpm ton flow rate 0 054 L s kW 0 036 L s kW entering 85 F 85 F condenser water 29 4 C 29 4 C temperature One approach to increase overall system efficiency has been to reduce pump and cooling tower energy by reducing the amount of water being pumped through the system In the past the conditions shown in the center column of the table in Figure 70 were often used when designing a water cooled chilled water system These flow rates result in a 10 F 5 6 C temperature difference AT through both the evaporator and the condenser In fact they are the standard conditions at which electric vapor compression chillers are rated by TRG TRC016 EN S TRANE period three System Variations ARI They are not however sug
69. iller has been turned on This prevents more chillers from turning on than are actually required particularly during periods of pull down or rapid load variation The third time delay is a minimum cycle timer This timer should have the highest priority It requires a fixed period of time between turning an individual chiller on and turning it back off This ensures that the chiller is not cycled too frequently It is important to understand that these timers are lower priority than the safeties built into the individual chiller controls At all times the individual TRG TRCO16 EN amp anwe period four Chiller Plant Control chiller safeties must be capable of shutting the chiller down to avoid equipment damage Unload Before Start 2 purrps head pressure system flow rate 610gpm 870gom 385Us 54 9Us The next control function is to partially unload the operating chillers before an additional chiller and pump are turned on Depending on the system configuration there can be very rapid variations in water flow through the chiller evaporator when a pump is turned on or off or when a control valve is opened or closed Partially unloading the chiller prior to such variations allows the chiller to continue to operate without interruption This can be explained by looking at a flow diagram for a chilled water system with multiple pumps see Figure 36 This diagram shows that with one pump and chiller operating th
70. illers operating at nonstandard conditions The test procedure for these chillers is ARI Standard 550 590 1999 Notice that these requirements go into effect on October 29 2001 23 S TRANE 24 period one Types of Water Chillers Note Addendum J updates the minimum IPLV efficiency requirements in the standard to match the methods included in the most current version of the ARI rating standard At the time this booklet was printed Addendum J had not been formally adopted as part of the standard However its adoption was deemed sure enough to include in this publication The standard requires that both full and part load conditions be met For example the efficiency of a water cooled centrifugal chiller with a capacity greater than 300 tons 1 056 kW must be 6 1 COP or better at ARI standard conditions This is equivalent to 0 576 kW ton The part load IPLV efficiency must also be 6 4 based on the efficiency units of COP or better This is equivalent to an IPLV of 0 549 kW ton Coefficient of performance COP is a unitless expression of efficiency defined as useful energy out divided by energy input A higher COP designates a higher efficiency 3 516 kW 2e ton COP standard 90 1 1999 efficiency requirements Water Cooled Absorption Chillers chiller type capacity minimum efficiency single effect all capacities 0 7 COP double effect indirect fired all capacities 1 0 COP 1 05 IPLV direct fired all capac
71. ing in order to prevent damage to the chiller Additionally the control system must know when to turn another chiller on to meet the system chilled water temperature set point Turning an additional chiller on may be required to meet the system demand for flow even though the operating chiller may not be fully loaded TRG TRC016 EN 95 amp ranw period four Chiller Plant Control System Optimization 4 Chiller Decrease condenser water temperature Increase chilled water temperature 4 Chilled water pump variable flow system Increase chilled water AT 4 Cooling tower Increase condenser water temperature 4 Condenser water pump variable flow system Increase condenser water AT System Optimization The chiller plant control system can also be used for system optimization For the purposes of this discussion we will define optimization as minimizing the energy used by the chiller plant including chillers chilled water pumps condenser water pumps and cooling tower while still maintaining comfort or satisfying process loads The first step is to examine the energy use of the major components of the chiller plant to see what can be done to minimize each component individually The chiller energy usage can be reduced by lowering the condenser water temperature or by raising the chilled water temperature In a variable flow system chilled water pumping energy can be reduced by lowering the chilled water temperature whil
72. ions may allow the system design engineer to provide added value to the building owner and operator in the areas of improved reliability greater flexibility reduced installed costs and lower operating costs Topics included TRG TRCO16 EN Hybrid chilled water systems using chillers that operate on different fuels Low flow systems that use lower chilled water temperatures and lower flow rates Variable primary flow systems that are designed to vary the water flow through the chiller evaporator Configurations that allow a chiller to be preferentially loaded specifically in the case of a high efficiency heat recovery or alternate fuel chiller Heat recovery chillers that are capable of providing heat to another part of the system Asymmetric system designs using chillers of different capacities or efficiencies such as the swing chiller concept Several free cooling options that reduce cooling energy costs Applications in which the required conditions are outside of the normal operating range of the chiller 105 amp ranw 106 period five Review Review Period Four 4 Chiller sequencing a Failure recovery i ant 4 Contingency planning melee a System tuning ae a System optimization ap Res a Operator interface Zz a 1 ii Period Four discussed the issues related to chiller plant control including chiller sequencing failure recovery contingency planning system tuning and system lev
73. iping The possibility of system contamination and leaks increases when field installed piping and brazing are required Additionally longer design time is generally required for the proper selection sizing and installation of this split system TRG TRC016 EN S TRANE period one Types of Water Chillers Remote Air Cooled Condenser Another popular configuration is to use an outdoor air cooled condenser connected to a packaged compressor and evaporator unit also called a condenserless chiller that is located in the indoor equipment room Again the components are connected with field installed refrigerant piping The primary advantage of this configuration is that the compressors are located indoors which makes maintenance easier during inclement weather and virtually eliminates the concern of refrigerant migrating to the compressors during cold weather TRG TRC016 EN 13 S TRANE period one Types of Water Chillers Indoor Air Cooled Condenser The final configuration includes a packaged compressor and evaporator unit that is located in an indoor equipment room and connected to an indoor air cooled condenser The air used for condensing is ducted from outdoors through the condenser coil and rejected either outdoors or inside the building as a means for heat recovery Indoor condensers typically use a centrifugal fan to overcome the duct static pressure losses rather than the propeller fans used in conventional o
74. is reached the flow may again be increased Either the refrigerant pressure in the condenser or the condenser evaporator refrigerant pressure differential can be monitored and used to control the temperature or flow rate of the condenser water to prevent this pressure differential from dropping below the limit TRG TRCO16 EN 99 amp ranw 100 period four Chiller Plant Control Operator Training and Support oes ko gi Operator Interface System level communication and control is very important Today the amount of communication between the components chillers cooling towers pumps control valves and so forth has increased immensely allowing many chilled water systems to be fully automated In some facilities however the largest energy user in the HVAC system the chiller plant has not progressed beyond manual control In some cases it was reduced to manual control shortly after the building was commissioned Why does this occur Chillers are large with very expensive pieces of equipment which if damaged by incorrect operation can cost the owner a substantial amount of money to repair or replace Operators are therefore very sensitive to chiller plant operation If the operator does not understand how the system is designed and controlled it is likely that the system will be put into a manual control mode Therefore initial and ongoing operator training and support is critical TRG TRC016 EN S TRAN
75. ities 1 0 COP 1 0 IPLV as of October 29 2001 The table in Figure 28 includes the minimum efficiency requirements for absorption water chillers For an absorption chiller COP is defined as evaporator cooling capacity divided by the heat energy required by the generator excluding the electrical energy needed to operate the pumps purge and controls Again these minimum efficiency requirements take effect on October 29 2001 The test procedure for these chillers is ARI Standard 560 1992 Note that the efficiency requirement for single effect absorption chillers is higher than many manufacturers have offered in the past Section 6 3 4 of Standard 90 1 includes additional requirements for the design and operation of chilled water systems These requirements will be mentioned in later periods TRG TRCO16 EN S TRANE period one Types of Water Chillers ASHRAE Standard 15 1994 a Safety standard for refrigerating systems 4 Mechanical equipment room Satety Code tor Retrigerstion Refrigerant monitors Alarms 2 e Mechanical ventilation i Pressure relief piping Another standard that is related to chilled water systems ASHRAE Standard 15 1994 Safety Code for Mechanical Refrigeration is intended to specify requirements for safe design construction installation and operation of refrigerating systems This standard covers mechanical refrigeration systems of all sizes that use all types of refrigerants B
76. ity is the first distinguishing characteristic Air cooled chillers are typically available in packaged chillers ranging from 7 5 to 500 tons 25 to 1 580 kW Packaged water cooled chillers are typically available from 10 to 3 000 tons 35 to 10 500 kW TRG TRC016 EN S TRANE period one Types of Water Chillers air cooled or water cooled Maintenance 4 Water treatment a Condenser tube brushing 4 Tower maintenance 4 Freeze protection a Makeup water cooling tower A major advantage of using an air cooled chiller is the elimination of the cooling tower This eliminates the concerns and maintenance requirements associated with water treatment chiller condenser tube cleaning tower mechanical maintenance freeze protection and the availability and quality of makeup water This reduced maintenance requirement is particularly attractive to building owners because it can substantially reduce operating costs Systems that use an open cooling tower must have a water treatment program Lack of tower water treatment results in contaminants such as bacteria and algae Fouled or corroded tubes can reduce chiller efficiency and lead to premature equipment failure TRG TRCO16 EN 7 TRANE period one Types of Water Chillers air cooled or water cooled Low Ambient Operation Air cooled chillers are often selected for use in systems that require year round cooling requirements that cannot be met with an airside economizer
77. le water chillers The reciprocating compressor was the workhorse of the small chiller market for many years It was typically available in capacities up to 100 tons 350 kW Multiple compressors were often installed in a single chiller to provide chiller capacities of up to 200 tons 700 kW Scroll compressors have emerged as a popular alternative to reciprocating compressors and are generally available in hermetic configurations in capacities up to 15 tons 53 kW for use in water chillers As with reciprocating compressors multiple scroll compressors are often used in a single chiller to meet larger capacities In general scroll compressors are 10 to 15 percent more efficient than reciprocating compressors and have proven to be very reliable primarily because they have approximately 60 percent fewer moving parts than reciprocating compressors Reciprocating and scroll compressors are typically used in smaller water chillers those less than 200 tons 700 kW Helical rotary or screw compressors have been used for many years in air compression and low temperature refrigeration applications They are now widely used in medium sized water chillers 50 to 500 tons 175 to 1 750 kW Like the scroll compressor helical rotary compressors have a reliability advantage due to fewer moving parts as well as better efficiency than reciprocating compressors Centrifugal compressors have long been used in larger water chillers High efficiency sup
78. ler capacities 10 000 4 Different chiller S 8 000 efficiencies gt bo i 6 000 chiller 4 000 E lt lead gs 20 chiller 50 50 60 40 chiller split Asymmetric Design Many system design engineers seem to default to using chillers of equal capacity in a multiple chiller system There are benefits to using chillers of varying capacities to more favorably match the system loads Remember that when a chiller is started so is the associated ancillary equipment pumps and cooling tower In general the smaller the chiller the smaller the ancillary equipment Operating the least number of chillers and the smallest chiller possible to meet the system load minimizes system energy consumption Figure 80 examines the use of a 60 40 split that is one chiller the lead chiller is sized for 40 percent of the total system capacity and the second chiller the lag chiller for 60 percent Notice that the number of hours of chiller and ancillary equipment operation is reduced by 15 percent by changing from two chillers of equal capacity to one chiller at 40 percent capacity and the second chiller at 60 percent This is because up to 60 percent load only one chiller is operating Below 40 percent load only the lead chiller is operating between 40 and 60 percent load only the lag chiller is operating In the same system with chillers of equal capacity both chillers and their ancillary equipment are operating when the load i
79. lly save energy over the course of the year This is especially true if a portion of the first cost savings is reinvested in more efficient chillers With the same cost chillers at worst the annual operating cost with the lower flows will be about equal to standard flows but still at a lower first cost Source Kelly David W and Chan Tumin Optimizing Chilled Water Plants Heating Piping Air Conditioning January 1999 TRG TRC016 EN 63 S TRANE period three System Variations Variable Primary Flow Systems AN variableflow dy pups S MES check valves ___ optional bypass with three way valve Variable Primary Flow Systems One of the reasons that many chilled water systems are installed using the primary secondary configuration is that in the past chillers could not respond well to varying water flow through the evaporator Therefore the production loop was designed for a constant flow through the chillers and the distribution loop was designed for variable flow to take advantage of the pump energy savings The system was hydraulically decoupled to meet these two goals Alternatively in a variable primary flow VPF system the flow of water varies throughout the entire system through the evaporator of each operating chiller as well as through the cooling coils The VPF system differs from the primary secondary system in that it no longer hydraulically decouples the two loops The variable flow pumps
80. low 30 TRG TRCO16 EN S TRANE period two Chilled Water System Design Face and Bypass Damper Control The final method of modulating the coil capacity to match the cooling load is through the use of face and bypass dampers A linked set of dampers varies the amount of air flowing through the coil by diverting the excess air around the coil As the cooling load decreases the face damper closes reducing the airflow through the coil and reducing its capacity At the same time the linked bypass damper opens allowing more air to bypass around the coil A unique characteristic of this method of load terminal control is that the coil is allowed to run wild meaning that the water flow through the coil is constant Similar to the three way valve systems that use face and bypass dampers have the following characteristics m The temperature of the water returning from the system varies as the cooling load varies m The water flow through each load terminal and therefore pump energy are constant at all load conditions An advantage of face and bypass control with a wild cooling coil is that it can better dehumidifiy of the conditioned air when compared to varying the water flow through the coil As the airflow through the coil decreases at part load conditions assuming that the temperature of the water entering the coil is constant the temperature of the air leaving the coil also decreases That is the air is cooled f
81. m Typical causes of low AT syndrome include a poorly balanced flow system dirty filters or coils poorly performing air handler controls incorrect coil control valves or undersized air handlers TRG TRCO16 EN 83 amp ranw 84 period four Chiller Plant Control load indicators Flow chiller plant controller In a primary secondary system the direction and quantity of flow in the bypass pipe is an excellent indicator of when to turn a chiller on or off As discussed in Period Two the water flow in the bypass pipe can be measured directly using a flow meter or indirectly by measuring system water temperatures and applying flow mixing equations The rules applied to the bypass flow to determine when to turn a chiller on and off are m When there is a deficit flow a chiller may be added m When there is excess flow greater than that of the next chiller to be turned off plus a 10 to 15 percent safety factor that chiller may be turned off E If neither of the above conditions exists do nothing As an alternative to measuring flow in a primary secondary system with four or less chillers system supply and chiller plant return water temperatures may be used to decide when to turn achiller on or off This is similar to the logic applied to constant flow systems It is simple and has a low installed cost but it is less accurate than flow determination especially as the number of chillers increases Low AT syndro
82. m Helical Rotary Water Chillers Air Conditioning Clinic Trane literature order number TRG TRC012 EN m ASHRAE Handbook Refrigeration m ASHRAE Handbook Systems and Equipment Visit the ASHRAE Bookstore at www ashrae org For information on additional educational materials available from Trane contact your local Trane sales office request a copy of the Educational Materials price sheet Trane order number EM ADV_1 or visit our online bookstore at www trane com bookstore TRG TRCO16 EN 107 S TRANE 108 Quiz Questions for Period 1 1 What are the four types of compressors commonly used in vapor compression water chillers 2 List one advantage of an air cooled chiller and one advantage of a water cooled chiller 3 True or False The IPLV equation included in the ARI standards for rating chillers was derived to provide a representation of the average part load efficiency for a system with multiple chillers Questions for Period 2 4 What are the three most common methods of load terminal coil control 5 The system shown in Figure 117 contains two chillers piped in parallel with a single constant volume pump What is the drawback of this system configuration when only one chiller is operating 6 Ina conventional primary secondary chilled water system each production pump delivers a constant or variable flow of water and the distribution pump delivers a constant or variable flow of water 7 What m
83. m full load and part load chiller efficiency requirements as well as requirements for the design and operation of chilled water systems Review Period Two 4 Load terminal control Three way valve Two way valve Face and bypass dampers 4 Parallel configuration 4 Series configuration 4 Primary secondary configuration primary secondary system Period Two examined the methods of load terminal control including using three way modulating valves two way modulating valves and face and bypass dampers We then examined several common system configurations including chillers piped in parallel in series and in a primary secondary arrangement The majority of the time was spent discussing the design and operation of the primary secondary or decoupled chilled water system The primary secondary system eliminates many of the hydraulic problems associated with multiple chiller systems It provides a reliable and energy efficient supply of chilled water and its simplicity and flexibility make it easy to design expand and operate TRG TRCO16 EN period five Review Review Period Three 4 Hybrid systems 4 Low flow systems 4 Variable primary flow systems 4 Preferential loading 4 Heat recovery 4 Asymmetric design 4 Free reduced energy cooling 4 Application of a chiller outside its normal operating range Period Three reviewed several variations in the design of chilled water systems These variat
84. me can also affect the operation of primary secondary systems Unlike constant flow systems the primary secondary system will maintain the required system flow and supply water temperature and therefore maintain occupant comfort However it accomplishes this by turning on additional chillers before all operating chillers are fully loaded This may reduce overall system efficiency Although some have proposed solutions such as putting a valve in the bypass line lowering the supply water temperature or controlling the system differently these are only band aids that mask the actual problem and often cause other operational difficulties Fixing the root cause of low AT syndrome in the distribution system is the best course of action for proper and efficient system operation TRG TRCO16 EN S TRANE period four Chiller Plant Control load indicators Capacity chiller plant controller Another method of monitoring system cooling load is to measure the system water flow rate and temperatures directly and then calculate the load Although it would appear that direct measurement of the actual system load would be an excellent way to determine when to turn chillers on and off this method has several drawbacks It requires the use of flow meters with high accuracy and high turndown capacities Although flow meters have become more accurate and less expensive they require special installation conditions for reliable accuracy
85. n requiring 50 percent of design water flow the energy consumed by the variable flow distribution pump is 12 5 percent of full load 0 50 0 125 The total installed pump capacity required in a primary secondary system is typically less than in a system not designed for primary secondary pumping This is because the total system head production plus distribution is divided between pumps Each pump is more efficient because it works against a lower head Furthermore the distribution pump is sized to meet the diversified block system load not the sum of peaks coil loads This can represent a 20 to 25 percent reduction in the size of the distribution pump TRG TRC016 EN 45 S TRANE 46 period two Chilled Water System Design Primary Secondary System Rules A 4 Load terminals should use two way modulating control valves Obviously in order to achieve variable flow at the distribution pump the load terminals must be configured to vary the system flow This requires the use of a modulating two way control valve for each coil Typically no three way valves or wild coils need to be used in a primary secondary system Their use decreases the energy savings potential from the variable flow distribution pump In fact ASHRAE IESNA Standard 90 1 1999 Section 6 3 4 1 requires the use of modulating two way control valves and thus variable water flow in most systems Primary secondary systems comply with this requirement
86. nce a month to ensure that they will be able to start when required 86 TRG TRCO16 EN TRG TRCO16 EN S TRANE period four Chiller Plant Control Chiller Rotation When the system consists of chillers with different capacities efficiencies or fuel types the question of which chiller to turn on or off next becomes more complex Although each system requires a complete analysis there are some general principles that apply to most systems In systems with chillers of different capacities such as the swing chiller concept introduced in Period Three the goal is to operate the least number of chillers and the smallest chiller possible This typically minimizes overall system energy consumption by closely matching the capacity of the plant to the system load thus reducing the energy used by ancillary equipment In systems with chillers of different efficiencies it makes sense to operate the most efficient chillers first and the least efficient chillers last If different fuel types are involved the control system may receive data on the costs of natural gas and electricity and calculate the real time cost of operating the electric versus gas driven chillers 87 amp ranw period four Chiller Plant Control Heat Recovery standard electric chillers The system might also benefit from having a heat recovery chiller fully loaded As discussed in Period Three to maximize the amount of heat recovered it is o
87. ndenser tubes By monitoring system and equipment trends the operator has a chance to fix minor issues before they cause operational problems TRG TRCO16 EN TRG TRCO16 EN period five Review Chilled Water Systems period five Review We will now review the main concepts that were covered in this clinic on chilled water systems Review Period One 4 Vapor compression water chillers Air cooled versus water cooled 4 Absorption water chillers 4 Equipment rating standards a ASHRAE IESNA Standard 90 1 1999 In Period One we learned about the different types of vapor compression and absorption water chillers Vapor compression chillers are differentiated primarily by the type of compressor used and whether they use an air cooled or water cooled condenser Absorption chillers are primarily differentiated by whether they are indirectly or directly fired We compared the use of air cooled versus water cooled chillers Air cooled chiller advantages include lower maintenance costs a prepackaged system for easier design and installation and better low ambient operation Water cooled chiller advantages include greater energy efficiency at least at design conditions and longer equipment life 103 amp ranw 104 period five Review ARI Standards 550 590 and 560 are common industry standards used for rating water chiller performance ASHRAE IESNA Standard 90 1 1999 the energy standard contains minimu
88. nt to understand that you cannot control your way out of a bad system design The chiller plant consists of chillers pumps pipes coils cooling towers temperature sensors control valves and many other devices It is similar to an orchestra with many instruments The existence of these pieces does not guarantee that the system will work properly There needs to be an orchestra conductor In the case of a chilled water system that conductor is a chiller plant control system How well the plant works depends on how well the control system gets all the pieces to work together 79 amp ranw 80 period four Chiller Plant Control Chiller Controls a Start stop 4 Chilled water temperature control a Monitor and protect 4 Adapt to unusual conditions The largest change to chillers in the last decade has undoubtedly been in the area of controls In the past chillers were pneumatically controlled and they were protected by turning them off if the flow rates or temperatures changed too quickly Today s microprocessor based controls provide accurate chilled water temperature control as well as monitoring protection and adaptive limit functions These controls monitor chiller operation and prevent the chiller from operating outside its acceptable limits They can also adapt to unusual operating conditions keeping the chiller operating by modulating its components and sending a warning message rather than
89. nts of condenser water piping pump cooling tower and associated controls Water cooled chillers typically last longer than air cooled chillers This difference is due to the fact that the air cooled chiller is installed outdoors whereas the water cooled chiller is installed indoors Also using water as the condensing fluid allows the water cooled chiller to operate at lower pressures than the air cooled chiller In general air cooled chillers last 15 to 20 years while water cooled chillers last 20 to 30 years To summarize the comparison of air cooled and water cooled chillers air cooled chiller advantages include lower maintenance costs a prepackaged system for easier design and installation and better low ambient operation Water cooled chiller advantages include greater energy efficiency at least at design conditions and longer equipment life TRG TRCO16 EN TRG TRCO16 EN S TRANE period one Types of Water Chillers Packaged Air Cooled Chiller co air cooled chiller Packaged or Split Components Water cooled chillers are rarely installed with separable components Air cooled chillers however offer the flexibility of separating the components in different physical locations This flexibility allows the system design engineer to place the components where they best serve the available space acoustic and maintenance requirements of the customer A packaged air cooled chiller has all of the refrigeration com
90. number of part load operating hours m The availability of cooler condenser water m Chilled water reset control Chiller savings using condenser and chilled water temperature reset however should be balanced against the increase in pumping and cooling tower energy This is discussed in Period Four Performing a comprehensive energy analysis is the best method of determining whether an adjustable frequency drive is desirable It is important to use actual utility costs not a combined cost for demand and consumption charges Depending on the application it may make sense to use the additional money that would be needed to purchase an AFD to purchase a more efficient chiller instead This is especially true if demand charges are significant S TRANE period one Types of Water Chillers Condenser Types air cooled water cooled Air Cooled or Water Cooled Condensing The heat exchangers in the water chiller the condenser and evaporator have the second greatest impact on chiller efficiency and cost One of the most distinctive differences in chiller heat exchangers continues to be the type of condenser selected air cooled versus water cooled Air Cooled or Water Cooled water cooled air cooled Otons 500tons 1 000tons 1 500tons 2 000tons 2 500tons 3 000 tons 0 kW 1 759kW B5174 5276kM 034kM 8793kW 10 551 KY chiller capacity When comparing air cooled and water cooled chillers available capac
91. ny time Because a small change in temperature may indicate a relatively large change in load or flow in the bypass pipe it is important to use accurate calibrated sensors to ensure acceptable system operation The primary advantage of this method is that it does not depend on flow velocity in the pipe Also it is reliable and cost effective because the temperature sensors are relatively low cost devices Integrating the chiller controls with a chiller plant control system however is imperative for efficient system operation The control of chilled water systems will be discussed further in Period Four TRG TRCO16 EN S TRANE period three System Variations Chilled Water Systems period three System Variations Period Two discussed several standard configurations of chilled water systems In addition there are many variations available to reduce installed costs enhance the efficiency of the system improve reliability or increase operational flexibility These variations are worth examining because improving the reliability or efficiency of the system by even a small percentage can result in a large payback over the life of a building Electric Utility Deregulation 0 30 0 20 0 10 price of electricity kWh San Diego CA 000 Mey June July August September Alternate Fuel Choice Many building owners and operators are investigating the use of fuels other than electricity There are several reasons for this
92. on It is therefore important to perform a life cycle cost analysis to determine when heat recovery is a viable option It should also be noted that the chiller can only recover the amount of heat transferred into the evaporator plus the energy input to the compressor Therefore the more load on the chiller evaporator the more heat can be recovered To maximize the heat available for recovery a heat recovery chiller is often piped in one of the preferential loading configurations described earlier An advantage of the sidestream position is that the heat recovery chiller is not required to maintain a chilled water temperature set point It can be loaded just enough to satisfy the heating load while the more efficient cooling only chillers provide the rest of the cooling The heat removed from the chilled water loop benefits the system by precooling the return water 70 TRG TRCO16 EN S TRANE period three System Variations Control of a Heat Recovery Chiller 120 Ng e i tay g 100 xo ae SOA oe N B A l meta 80 unloading with E c constant leaving hot g me unloading with water temperature ia constant entering hot g 60 water temperature amp 0 100 50 percent load The control of a heat recovery centrifugal chiller although seemingly simple is critical to reliable chiller operation Typically either the temperature or the flow of the water entering the standard condenser is modulated to meet the capacity requir
93. on to operate at any temperature difference In other words it does not need to supply water at the same temperature that the other operating chillers do The chiller in this position precools the system return water reducing the load on the downstream chillers In this example a heat recovery chiller is located in the sidestream position so that it can be preferentially loaded to maximize the amount of heat recovered thus reducing the overall building energy consumption Because a heat recovery chiller is typically less efficient than a standard cooling only chiller the heat recovery chiller only has to provide as much cooling as is required to meet the heat recovery load letting the more efficient cooling only chillers meet the rest of the cooling load One drawback of the sidestream arrangement is that it does not add water flow capability to the system it simply reduces the load on other chillers Therefore the other system pumps must ensure that the system flow requirements are met For this reason the capacity of the sidestream chiller is often smaller than the other chillers in the plant Either of these arrangements allows a chiller to be preferentially loaded Preferential loading is typically most beneficial in the following applications E In a system that has a high efficiency chiller along with several standard efficiency chillers the high efficiency chiller can be preferentially loaded to reduce system energy consumption E
94. ontrol of Condensing Pressure Related to the issue of condenser water temperature control is the control of condensing pressure Every chiller requires a minimum refrigerant pressure difference between the evaporator and the condenser in order to ensure that refrigerant and oil circulate properly inside the chiller This pressure difference varies based on the chiller design and operating conditions The chiller must develop the required pressure difference within a certain amount of time as specified by the manufacturer or the chiller controls will turn it off due to a safety limit During some start up conditions this pressure difference may be difficult to achieve within the time required An example of such a condition is an office building that has been unoccupied during a cool autumn weekend The temperature of the water in the sump of the cooling tower is 40 F 4 4 C Monday is sunny and warm and the building cooling load requires a chiller to be started Because the chiller is operating at part load and the tower sump is relatively large the minimum pressure difference may not be reached before the chiller is turned off on a safety If however the flow of water through the condenser is reduced the minimum pressure difference can be obtained The lower flow rate increases the temperature of the water leaving the condenser which results in a higher refrigerant pressure inside the condenser After the minimum pressure difference
95. ount of heat rejected is controlled by varying the temperature or flow of water through the standard condenser Chiller efficiency is degraded slightly in order to reach the higher condensing temperatures An auxiliary condenser heat recovery chiller makes use of a second but smaller condenser bundle It is not capable of rejecting as much heat as the dual condenser chiller Leaving hot water temperatures are also lower so it is often used to preheat water upstream of the primary heating equipment or water heater It requires no additional controls and actually improves chiller efficiency because of the extra heat transfer surface for condensing A heat pump chiller is a standard chiller no extra shells are required used and controlled primarily for the heat it can produce in the condenser The evaporator is connected to the chilled water loop typically in the sidestream position discussed earlier but it only removes enough heat from the chilled water loop to handle the heating load served by its condenser This application is useful in a multiple chiller system where there is a base or year round heating or process load or where the quantity of heat required is significantly less than the cooling load The heating efficiency of a heat pump chiller is the highest of any heat producing device TRG TRCO16 EN 69 S TRANE period three System Variations Heat Recovery Chiller Efficiency cooling heat recovery chiller type mod
96. ponents compressor condenser expansion device and evaporator located outdoors A major advantage of this configuration is factory assembly and testing of all chiller components including the wiring refrigerant piping and controls This eliminates field labor and often results in faster installation and improved system reliability Additionally all noise generating components compressors and condenser fans are located outdoors easing indoor noise concerns Finally indoor equipment room space requirements are minimized 11 S TRANE 12 period one Types of Water Chillers Remote Evaporator Barrel refrigerant piping An alternative to the packaged air cooled chiller is to use a packaged condensing unit condenser and compressor located outdoors with a remote evaporator barrel located in the indoor equipment room The two components are connected with field installed refrigerant piping This configuration locates the part of the system that is susceptible to freezing evaporator indoors and the noise generating components compressors and condenser fans outdoors This usually eliminates any requirement to protect the chilled water loop from freezing during cold weather This configuration is particularly popular in schools and other institutional applications primarily due to reduced seasonal maintenance for freeze protection A drawback of splitting the components is the requirement for field installed refrigerant p
97. quipment life 3 False The IPLV equation was derived to provide a representation of the average part load efficiency for a single chiller system only 4 Three way modulating control valve two way modulating control valve and face and bypass dampers 5 When only one chiller is operating warm return water continues to flow through the non operating chiller and mixes with the chilled water leaving the operating chiller The temperature of the mixture of these two streams is higher than the desired supply water temperature possibly resulting in building comfort or space humidity control problems Constant variable Two way modulating control valves Turn on o on Q To ensure that the water flow through the system remains above the minimum flow limit of the operating chiller s 10 Preferential loading is typically most beneficial in the following scenarios E In a system that has a high efficiency chiller along with several standard efficiency chillers the high efficiency chiller can be preferentially loaded to reduce system energy consumption E In a system with a heat recovery chiller preferentially loading the heat recovery chiller maximizes the amount of heat recovered thus reducing the overall system energy consumption E n a system with an alternate fuel chiller such as an absorption chiller preferentially loading the alternate fuel chiller during times of high electricity costs minimizes system energy cost 11 The s
98. r pump and proper piping modifications this heat exchanger can operate simultaneously with the chiller As much heat as possible is rejected through the heat exchanger while the chiller handles any excess cooling load If the plate and frame heat exchanger is piped in the sidestream position it can be used for more hours in the year because it does not need to maintain a leaving chilled water temperature set point It can provide some useful cooling at any time that it can precool the system return water If simultaneous free cooling and mechanical cooling are performed care must be taken to control the evaporator to condenser pressure differential inside the chiller When very cold condenser water flows through the chiller condenser for an extended period of time operational problems may result due to a low pressure differential between the evaporator and the condenser Using a three way modulating bypass valve to mix the warm water leaving the chiller condenser with the cold water entering the condenser or a two way modulating valve and a variable speed condenser water pump can eliminate this problem Consult with the chiller manufacturer to determine the limits for the specific chiller being used This issue is discussed further in Period Four 76 TRG TRCO16 EN TRG TRCO16 EN S TRANE period three System Variations waterside economizer Refrigerant Migration The final method of free cooling is to transfer heat betw
99. rigerant level a Evaporator approach Vibration levels temperature a Condenser water inlet and a Addition of refrigerant or oil outlet temperatures and pressures a Condenser water flow 4 Condenser refrigerant temperature and pressures 4 Condenser approach temperature ASHRAE Guideline 3 Reducing Emission of Halogenated Refrigerants in Refrigeration and Air Conditioning Equipment and Systems includes a list of recommended data points to be logged daily for each chiller Much of this data may be available from the display on the chiller control panel It is also helpful to the operator if this information is available at the chiller plant control system and presented in a clear format In addition to current status historical operating information is valuable for keeping the equipment operating at peak efficiency and for identifying operating trends that signal either impending problems or a drop in system performance For example the condenser approach temperature is the temperature difference between the water leaving the condenser and the refrigerant inside the condenser If there has been a problem with water treatment in the cooling tower fouling may build up inside the tubes in the chiller condenser This will cause the difference between the condenser water and refrigerant temperatures to increase reducing chiller efficiency By noting an increase in this approach temperature the operator can schedule cleaning of the co
100. rop that reduces the pressure of the refrigerant to that of the evaporator repeating the cycle The absorption refrigeration cycle is discussed in more detail in the Absorption Water Chillers Air Conditioning Clinic 15 S TRANE period one Types of Water Chillers Absorption Chillers Offer Choice 4 Avoid high electric demand charges 4 Minimal electricity needed during emergency situations a Waste heat recovery 4 Cogeneration Absorption water chillers generally have a higher first cost than vapor compression chillers The cost difference is due to the additional heat transfer tubes required in the absorber and generator s the solution heat exchangers and the cost of the absorbent This initial cost premium is often justified when electric demand charges or real time electricity prices are a significant portion of the electric utility bill Because electric demand charges are often highest at the same time as peak cooling requirements absorption chillers are often selected as peaking or demand limiting chillers Because the absorption chiller uses only a small amount of electricity backup generator capacity requirements may be significantly lower with absorption chillers than with electrically driven chillers This makes absorption chillers attractive in applications requiring emergency cooling assuming the alternate energy source is available Some facilities such as hospitals or factories may have excess s
101. rry out the design intent TRG TRCO16 EN 65 S TRANE 66 period three System Variations Preferential Chiller Loading distribution Preferential Loading To take full advantage of a high efficiency heat recovery or alternate fuel chiller the system may need a method to preferentially load these chillers The following two system configurations are variations of the primary secondary system In the basic primary secondary system all operating chillers are loaded to equal percentages In this first preferential loading configuration the preferentially loaded chiller is moved to the distribution side of the bypass pipe This chiller is preferentially or most fully loaded when it is turned on because it always receives the warmest system return water In this example an absorption chiller is located on the distribution side of the bypass pipe so that it can be preferentially loaded during periods of high electricity costs TRG TRC016 EN TRG TRCO16 EN S TRANE period three System Variations Sidestream Configuration The second preferential loading configuration shown in Figure 75 ensures that the chiller in the sidestream position receives the warmest entering water temperature and that it can be fully loaded whenever the system load is high enough This arrangement is unique because it not only allows preferential loading but it also permits the chiller or other cooling device in the sidestream positi
102. rt load flow rate all of the coils will receive less water regardless of their actual need Typically however some coils receive full water flow and others receive little or no water In either case heavily loaded coils will usually be starved for flow Examples of spaces with constant heavy loads that may suffer include computer rooms conference rooms photocopy rooms and rooms with high solar loads TRG TRCO16 EN S TRANE period two Chilled Water System Design chillers piped in parallel Dedicated Pumps Figure 43 shows an example of the pump system curve relationship When both pumps are operating the system receives 100 percent of design flow When only one pump is operating the intersection of the pump s performance curve with the system curve results in about 65 percent of design flow This configuration also presents problems to chiller operation The starting or stopping of a pump for one chiller affects the flow through the other chiller Using this same example if one chiller is operating and a second chiller and pump are started the total water flow in the system does not double The system and pump performance curves will rebalance resulting in an increase in system flow of only 35 percent of total flow The new total flow rate however is now divided equally between the two chillers This results in a rapid reduction in water flow through the original operating chiller from 65 percent of total
103. rtial loss of cooling capacity It allows a building operator to act more quickly by having a plan in place and by proactively preparing the facility Such a plan often includes working with suppliers to temporarily lease cooling equipment During initial construction it is easy and cost effective to provide piping stubs which are built into the chilled water system for quick connection and easily accessible electrical connections When equipment leasing is combined with these simple additions to the system a contingency plan can be put into action quickly and the system can produce chilled water again in a short period of time It is important to first identify the minimum or critical cooling capacity required With multiple chillers in a facility it may be acceptable to have less than full capacity in an emergency situation For example the chiller plant may consist of 1 800 tons 6 330 kW but the minimum capacity required in an emergency situation may only be 1 200 tons 4 220 kW Therefore it is also important to identify a contingency plan if Chiller 1 fails if Chiller 2 fails if Chillers 2 and 3 fail and so on TRG TRCO16 EN 91 amp ranw 92 period four Chiller Plant Control System Timers 4 Load confirmation timer Avoids transient conditions a Staging interval timer Allows time for the system to respond to turning a chiller on 4 Minimum cycle timer Prevents excessive cycling System Tuning In ad
104. s This typically requires a reduced number of passes in the evaporator and may impact chiller efficiency This efficiency impact however is often offset by the gain in system efficiency due to thermodynamic staging System pressure drop also increases because the pressure drops through the chillers are additive This can result in increased pump size and energy costs This increase in pumping energy can be substantially reduced by designing the system for a higher system AT and therefore a reduced water flow rate Because of the pressure drop limitations it is difficult to apply more than two chillers in series Systems involving three or more chillers typically use either the primary secondary configuration or parallel sets of two chillers in series 38 TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design chillers piped in series Equal Set Points set point 42 F 5 6 C Temperature control in a series system can be accomplished in several ways depending on the desired operating sequence The first method shown in Figure 45 has both set points adjusted to the desired system supply water temperature Assuming equally sized chillers either chiller can meet the load below 50 percent Above 50 percent load both chillers operate and the upstream chiller is preferentially loaded This means that the upstream chiller is operated at full capacity and any portion of the load that remains is handled
105. s 50 percent or greater 72 TRG TRCO16 EN S TRANE period three System Variations Swing Chiller small capacity swing chiller Another benefit of unequal chiller capacities is that the system load can be more closely matched with the operating chiller and ancillary equipment capacity increasing overall system efficiency Figure 81 shows a system that includes two large chillers of equal capacity along with one smaller capacity chiller The smaller capacity chiller called a swing chiller in this combination presents an opportunity for significant overall system energy savings TRG TRC016 EN 73 S TRANE period three System Variations Swing Chiller OOOO Oo Chiller 100 swing chiller 80 swing chiller 60 chiller 2 40 20 swing chiller A chiller 1 0 chiller sequence percent cooling load The smaller swing chiller is turned on to handle the low cooling loads either during the night or during unoccupied periods of time When the building load exceeds the capacity of the swing chiller it is turned off and a larger chiller is turned on The larger chiller handles the building cooling load alone until it becomes fully loaded Then the swing chiller is turned on again The swing chiller is alternated on and off between the larger chillers operation to serve as a smaller incremental step of loading This sequence more favorably matches the capacity of the chiller plant to the system load
106. set the set point of the operating chiller downward in an attempt to compensate for this condition and more closely maintain the desired supply water temperature Reducing the set point of the operating chiller has its limits however depending on the operating characteristics and evaporator freeze limits of the specific chiller The more chillers in the system the worse the problem becomes For this reason this configuration is seldom used in systems with more than two chillers Additionally ASHRAE IESNA Standard 90 1 1999 Section 6 3 4 2 prohibits this type of system when the pump is larger than 10 hp 7 5 kW The standard requires that in systems that contain more than one chiller piped in parallel system water flow must be reduced when a chiller is not operating 35 S TRANE 36 period two Chilled Water System Design chillers piped in parallel Dedicated Pumps t odil starved for flow If separate dedicated pumps are used with each chiller a pump and chiller pair can be turned on and off together as the cooling load varies This solves the temperature mixing problem that occurred in the previous single pump configuration but it presents a new problem in a system that uses a constant flow method of terminal control Below 50 percent load only one chiller and one pump are operating The total water flow in the system decreases significantly typically 60 to 70 percent of full system flow Ideally at this pa
107. sting methods for all types and sizes of water chillers It covers factory designed prefabricated water chillers both air cooled and water cooled using the vapor compression refrigeration cycle ARI Standard 560 A publication titled Absorption Water Chilling and Water Heating Packages that promotes consistent rating methods for many types and sizes of absorption water chillers in which water is the refrigerant and lithium bromide is the absorbent It covers single effect chillers operating on steam or a hot fluid indirect fired double effect chillers operating on steam or a hot fluid and direct fired double effect chillers operating on natural gas oil or liquid petroleum LP ASHRAE American Society of Heating Refrigerating and Air Conditioning Engineers ASHRAE Guideline 3 A publication titled Reducing Emission of Halogenated Refrigerants in Refrigeration and Air Conditioning Equipment and Systems that includes a recommended list of data points to be logged daily for each water chiller ASHRAE Standard 15 A publication titled Safety Code for Mechanical Refrigeration that specifies safe design construction installation and operation of refrigerating systems ASHRAE IESNA Standard 90 1 A publication titled Energy Standard for Buildings Except Low Rise Residential Buildings that provides minimum requirements for the energy efficient design of buildings except low rise residential buildings including the HVAC system centrifugal
108. system flow to 50 percent This rapid decrease in flow often results in a loss of temperature control and may cause the chiller to shut off on a safety In order to overcome this problem the chiller plant control system should anticipate the starting of additional pumps and unload operating chillers prior to the start of an additional chiller Again this configuration is sometimes acceptable for two chiller systems but is not often used in larger systems because the part load system flow problems are further multiplied TRG TRC016 EN 37 S TRANE period two Chilled Water System Design Chillers Piped in Series Series Configuration Another way to connect multiple chillers is to configure the chiller evaporators in series Series chilled water systems typically use three way valves at the coils to ensure constant system flow With two chillers in series both the temperature mixing and the flow problems associated with the parallel configurations shown previously disappear All of the chilled water passes through both chillers and there is full system water flow at all loads However the flow rate through each individual chiller is equal to the entire system flow rate When compared to chillers piped in parallel at the same system AT this is twice as much water flowing through each chiller This means that the chiller tube pass arrangement must accommodate double the water quantity within acceptable velocity and pressure drop limit
109. systems are being designed with lower chilled water temperatures and lower flow rates The water flow rate required through the system is decreased by allowing a larger temperature difference through the chiller TRG TRCO16 EN S TRANE period one Types of Water Chillers Flow Rates and Temperatures Qun 500 x flow rate x AT Qw 4 184 x flow rate x AT equation for water only The temperature difference AT through the chiller and the water flow rate are related For a given load as the flow rate is reduced the AT increases and vice versa Q 500 x flow rate x AT Q 4 184 x flow rate x AT where m O load Btu hr W E flow rate water flow rate through the chiller gom L s m AT temperature difference leaving minus entering through the chiller OF C Realize that 500 4 184 is not a constant It is the product of density specific heat and a conversion factor for time The properties of water at conditions typically found in an HVAC system result in this value Other fluids such as mixtures of water and antifreeze will cause this factor to change Density of water 8 33 Ib gal 1 0 kg L Specific heat of water 1 0 Btu Ib F 4 184 J kg K 8 33 Ib gal x 1 0 Btu Ib F x 60 min hr 500 1 0 kg L x 4 184 J kg K 4 184 TRG TRCO16 EN 21 S TRANE 22 period one Types of Water Chillers Flow Rates and Temperatures 3J J7 95 F 44 F 100 F 41 F 35 C ge yy
110. t capacities and flow rates because the proper pump needs to be turned on to match the chiller flow rate The drawback of manifolding production pumps is that the chiller flows become hydraulically coupled again If an isolation valve is opened before a pump is started flow through the operating chillers will drop suddenly causing potential control instability If a pump is started before a valve is open the operating chillers will see a momentary flow increase causing control instability or water hammer 44 TRG TRCO16 EN S TRANE period two Chilled Water System Design Distribution Loop The distribution pump circulates water from the supply tee through the load terminals and back to the return tee Although the same water is pumped twice by different pumps there is no duplication of pumping energy The production pumps overcome only the pressure drop through the production loop and the distribution pumps overcome the pressure drop through the distribution loop The distribution pump s should be capable of varying the flow through the distribution loop Typically this is accomplished by using a pump with a variable speed drive to modulate the flow of water through the pump In a properly designed and operating system distribution pump energy consumption will decrease significantly at part load The pump power reduction approaches the theoretical cubic relationship to flow That is when the load is 50 percent of desig
111. team or hot water as a result of normal operations Other processes such as a gas turbine generate waste steam or some other waste gas that can be burned In such applications this otherwise wasted energy can be used to fuel an absorption chiller Finally cogeneration systems often use absorption chillers as a part of their total energy approach to supplying electricity in addition to comfort cooling and heating 16 TRG TRCO016 EN S TRANE period one Types of Water Chillers Absorption Chiller Types There are three basic types of absorption chillers They are typically available in capacities ranging from 100 to 1 600 tons 350 to 5 600 kW Indirect fired single effect absorption chillers operate on low pressure steam approximately 15 psig 205 kPa or medium temperature liquids approximately 270 F 132 C and have a coefficient of performance COP of 0 6 to 0 8 In many applications waste heat from process loads cogeneration plants or excess boiler capacity provides the steam to drive a single effect chiller In these applications absorption chillers become conservation devices and are typically base loaded This means that they run as the lead chiller to make use of the free energy that might otherwise be wasted Indirect fired double effect absorption chillers require medium pressure steam approximately 115 psig 894 kPa or high temperature liquids approximately 370 F 188 C to operate and t
112. the simplest and most reliable failure recovery sequence is to simply turn on the next chiller in the sequence and not try to turn several chillers on and off in an attempt to re optimize the system During an equipment failure it is especially important to notify the operator of the status as well as to help the operator understand where the problem is and what might be the cause The control system must also allow the operator to easily analyze the situation and to intervene if the failure condition will exist for an extended period of time A system that provides this information will ensure that the system itself will be maintained and operated in proper condition 90 TRG TRCO16 EN S TRANE period four Chiller Plant Control Contingency Planning electrical connections In addition to failure recovery it is wise for the system design engineer to work with the building owner to develop a contingency plan for chilled water in the case of an emergency shutdown or an extended breakdown Many organizations have contingency plans for critical areas of their business Some deal with natural disasters and others with the loss of power in critical areas However few have taken the time to think about what a loss of cooling would mean to their facility This is often especially critical for process cooling applications Cooling contingency planning is intended to minimize the losses a facility may incur as a result of a total or pa
113. the two way valve is closed It also keeps the distribution pump from dead heading or trying to pump when all of the two way valves are closed If a bleed line is used it should be of a much smaller diameter than the rest of the piping 50 TRG TRCO16 EN S TRANE period two Chilled Water System Design Distribution Loop Characteristics 4 Reduced pump energy use 4 Distribution loop sized for system diversity 4 Higher return water temperatures Let s summarize When designed and operated correctly the distribution loop of the primary secondary system has the following characteristics E Variable water flow Only the amount of water that is actually used at the load terminals is pumped throughout the distribution loop Under most operating conditions this flow rate is less than the design flow rate resulting in reduced pumping energy E Load diversity Not all of the load terminals peak at the same time Therefore the quantity of water that flows at any given time is less than the constant water flow required in a system using three way valves This allows for reduced distribution pump and pipe sizes mE Higher return water temperature at all loads Properly operating two way valves do not allow unused chilled water to bypass the load terminals Water is only allowed to enter the return pipe after it has accomplished vague useful cooling If the system is operating properly the temperature of the water returnin
114. tions cooling towers typically have the ability to supply colder condenser water than at design conditions This however increases the energy consumption of the cooling tower fans The key to maximizing energy savings is knowing the relationship of cooling tower energy consumption to chiller energy consumption At design conditions a chiller typically uses five to ten times more energy than a cooling tower This would suggest that it might be beneficial to use more cooling tower energy to save chiller energy However there is a point of diminishing return where the chiller energy savings is less than the additional energy used by the cooling tower Figure 106 shows the combined annual energy consumption of a chiller and cooling tower in a system that is controlled to various condenser water temperature set points The third column shows a system that attempts to supply 55 F 12 8 C water from the cooling tower at all times Of course at design conditions the cooling tower may not be able to supply this temperature but it will supply the water at the coldest temperature possible The fourth column shows a system that uses a control system to dynamically determine the optimal condenser water temperature that minimizes the combined energy use of the chiller plus cooling tower It is obvious that this method of optimal control minimizes overall system energy consumption 98 TRG TRCO16 EN S TRANE period four Chiller Plant Control C
115. to 4 5 m s based on the water flowing through the bypass pipe at the design flow rate of the largest chiller in the system Additionally to further minimize pressure drop the bypass pipe is usually relatively short in length To prevent random mixing of the supply and return water streams however the minimum length of the bypass pipe is typically 5 to 10 pipe diameters When designing the bypass pipe an important issue to keep in mind is that the bypass pipe must be kept free of unnecessary restrictions For example a check valve must not be installed in the pipe Restrictions cause hydraulic coupling that can result in unacceptable chiller flow variations or unstable and potentially harmful system pressure variations due to the resulting series pumping effects If a manual isolation valve is required for service it should be large enough to ensure that it does not add significant pressure drop to the bypass pipe 42 TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design Production Loop The production pumps circulate water only from the return tee through a chiller to the supply tee and through the bypass pipe This represents a relatively small pressure loss and therefore relatively low pump energy In addition each pump only operates when its respective chiller is operating A primary secondary system provides a high degree of flexibility in the production loop Not only are the individ
116. tors larger than 50 hp 37 kW that have a pump head greater than 100 ft H20 300 kPa 47 S TRANE period two Chilled Water System Design Multiple Distribution Pumps supply to loads Another advantage of the primary secondary system is that the production loop is not affected by the distribution pumping arrangement For example multiple distribution pumps can be used to vary the flow within the distribution loop Providing variable flow through the application of multiple pumps or through variable speed drives on one or more pumps is more energy efficient than simply riding the pump curve It also provides greater system redundancy 48 TRG TRCO16 EN TRG TRCO16 EN S TRANE period two Chilled Water System Design Multiple Distribution Pumps A variation of the multiple pump configuration is to use separate pumps to deliver water to specific dedicated loads An example is a chilled water system serving a college campus Separate distribution pumps supply water to the east A west B and central C portions of the campus A primary advantage of this configuration is flexibility Expanding the system can be achieved by simply adding another distribution pump to the existing plant and connecting it to the piping that runs to the new building A variation of this multiple dedicated pump configuration is often called distributed pumping It is sometimes used in very large systems that serve multiple buildin
117. ual chiller loops decoupled from the distribution loop they are also decoupled from each another In this configuration the production loop consists of independent pairs of chillers and pumps Each pump is turned on and off with its respective chiller Supply water temperature is maintained by the controls supplied with the chiller Because the bypass pipe prevents flow interaction between chillers there is little worry of flow disturbances In addition the chillers can be of any type size or age or even from different manufacturers Because each chiller has a dedicated pump the chillers can have different evaporator pressure drops 43 S TRANE period two Chilled Water System Design Manifolded Production Pumps Alternatively the production loop can be configured with manifolded pumps and automatic two position isolation valves at each chiller When turning on a chiller a pump is turned on and the isolation valve is opened This manifolded pump configuration provides greater redundancy because the system can change the pump and chiller combinations This redundancy is at the cost of system complexity and somewhat limits the flexibility of selecting chillers of different capacities and types If the production pumps are manifolded the chillers must be selected with the same evaporator water pressure drop or else some method of flow balancing must be employed Pump sizing also becomes an issue if chillers are of differen
118. uipment through programs that involve random testing of a manufacturer s equipment to verify published performance These equipment rating standards have been developed to aid engineers in comparing similar equipment from different manufacturers Chiller full load efficiency is described in terms of kW ton and coefficient of performance COP Additionally two efficiency values developed by ARI that are receiving increased attention are the Integrated Part Load Value IPLV and Non Standard Part Load Value NPLV ARI s part load efficiency rating system establishes a single number to estimate both the full and part load performance of a stand alone chiller As part of ARI Standard 550 590 1998 Water Chilling Packages Using the Vapor Compression Refrigeration Cycle and ARI Standard 560 1992 Absorption Water Chilling Heating Packages chiller manufacturers may now certify their chiller part load performance using the IPLV and NPLV methods This gives the engineering community an easy and certified method to evaluate individual chillers Understanding the scope and application limits of IPLV and NPLV is however crucial to their validity as system performance indicators 18 TRG TRCO16 EN S TRANE period one Types of Water Chillers Part Load Efficiency Rating a Integrated Part Load Value IPLV Weighted average load curves Based on an average single chiller installation Standard operating conditions 4 Non Standard P
119. urther and more moisture is removed TRG TRC016 EN 31 32 TRANE period two Chilled Water System Design Load Terminal Control Options 4 Three way modulating valve Constant water flow Variable system return water temperature 4 Two way modulating valve Variable water flow pump energy savings Constant system return water temperature 4 Face and bypass dampers Constant water flow Variable system return water temperature Enhanced dehumidification capability with wild coils Properly designed operated and maintained any of these three methods can result in good space comfort control However they have different effects on the chilled water system The use of three way valves or face and bypass dampers results in variable return water temperature and relatively constant chilled water flow through the entire system The use of two way valves results in constant return water temperature and variable water flow through the entire system Before choosing one of these control methods it is necessary to determine the effect that it will have on the other parts of the chilled water system Chiller Evaporator Flow a Constant flow is most common a Variable flow is en possible Can reduce energy T lt a consumption ie Use only with 7 evaporator advanced chiller and system controls In the past the water flow rate through the chiller evaporator was to remain as constant as possible The v
120. utdoor air cooled condensers Again the components are connected with field installed refrigerant piping This configuration is typically used where an outdoor condenser is architecturally undesirable where the system is located on a middle floor of a multistory building or where vandalism to exterior equipment is a problem A disadvantage of this configuration is that it typically increases condenser fan energy within compared to a conventional outdoor air cooled condenser Similarly a packaged cooling tower in a water cooled system can also be located indoors This configuration also requires outdoor air to be ducted to and from the cooling tower and again typically requires the use of a centrifugal fan Centrifugal fans use about twice as much energy as a propeller fan but can overcome the static pressure losses due to the ductwork Alternatively the tower sump can be located indoors making freeze protection easier 14 TRG TRC016 EN TRG TRCO16 EN S TRANE period one Types of Water Chillers Absorption Refrigeration Cycle Absorption Water Chillers So far we have discussed water chillers that use the vapor compression refrigeration cycle Absorption water chillers are a proven alternative to vapor compression chillers The absorption refrigeration cycle uses heat energy as the primary driving force The heat may be supplied either in the form of steam or hot water indirect fired or by burning oil or natural gas dir
121. wing chiller is alternated on and off between the larger chillers operation in order to serve as a smaller incremental step of loading This sequence more favorably matches the capacity of the chiller plant to the system load and operates the fewest and smallest pieces of ancillary equipment pumps and cooling towers at any system load 110 TRG TRCO16 EN Answers 12 Sequencing 13 Decreases increases 14 Fouling inside the tubes of the condenser possibly indicating a problem with water treatment in the cooling tower TRG TRCO16 EN 111 S TRANE 112 Glossary absorbent A substance used to absorb refrigerant and transport it from the low pressure to the high pressure side of the absorption refrigeration cycle In absorption water chillers the absorbent is commonly lithium bromide absorber A component of the absorption refrigeration cycle in which refrigerant vapor is absorbed by the absorbent solution and rejects heat to cooling water air cooled condenser A type of condenser in which refrigerant flows through the tubes and rejects heat to air that is drawn across the tubes airside economizer A method of free cooling that involves using cooler outdoor air for cooling instead of recirculating warmer indoor air ARI Air Conditioning and Refrigeration Institute ARI Standard 550 590 A publication titled Standard for Water Chilling Packages Using the Vapor Compression Cycle that promotes consistent rating and te

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