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Trane TRG-TRC014-EN User's Manual

1. Proper air diffusion is an important comfort consideration especially in VAV systems Because VAV systems require the diffuser to provide proper space air mixing over a wide range of airflows diffusers that are specifically intended for use in VAV applications should be used to prevent cold air dumping at low airflow rates 33 S TRANE period two Components of a VAV System Linear Slot Diffuser linear slot supply air diffuser supply duct H p panne y 5 F 128 C sir from space 5ft 1 5m Linear slot diffusers are generally preferred for VAV air distribution They use a principle known as the Coanda effect to distribute air to the conditioned space The Coanda effect occurs when air is discharged at a relatively high velocity along the surface of the ceiling This creates an area of low pressure that causes the supply air to hug the ceiling As it travels along the ceiling air from the space is drawn into and mixed with the supply air stream When the air settles to the occupied levels of the space it has reached an average temperature The design of linear slot diffusers allows them to effectively distribute air over a wide range of airflows making them the preferred diffuser for VAV systems They also offer acoustical benefits over other types of diffusers Proper selection and placement of linear slot diffusers generates air movement throughout the space eliminating areas of stagn
2. 4 Constant airflow to the space 4 Acoustics a Fan energy consumption Series fan powered units are generally considered the premium VAV system because while the central system operates as a variable volume system the spaces receive constant supply airflow Increased air motion in the space improves comfort at all load conditions and the constant airflow simplifies diffuser selection 20 TRG TRCO14 EN S TRANE period two Components of a VAV System Most designers also believe that series units offer improved acoustical conditions in the space The constant sound of the series unit with the fan operating whenever the space is occupied is generally preferred to the on off sound generated by the cycling fan in the parallel unit However because the fan runs continuously whenever the space is occupied a series fan powered unit consumes more energy than a parallel fan powered unit The development of high efficiency motors has lessoned the energy consumption difference The fan in a series fan powered unit also costs more than an equivalent parallel fan powered unit because it generally requires a larger terminal fan Fan powered terminal units without a heating coil are typically used to provide tempering for those zones that require year round cooling and have relatively high minimum airflow settings such as the densely occupied interior zones of a building i e a conference room A fan powered terminal unit with a heating
3. 67 S TRANE 68 period five Application Considerations VAV Systems period five Application Considerations When designing a VAV system for a building there are several considerations to address m System level ventilation m Freeze protection for coils E Part load space humidity control Building pressure control This is by no means the entire list of considerations when designing a VAV system but is representative of some of the key issues TRG TRC014 EN TRG TRCO14 EN S TRANE period five Application Considerations System Level Ventilation VAV VAV 2 VAV S PENEI 1000 cfm 1000cfm 1000cfm _ 3000 cfm Primary air PA 0 47 mes 0 47 mis 0 47 mils 1 41 ms gt 200 cfm 200 cfm 300 cfm _ 700 cfm OA required 0 09 m s 0 09 mils 0 15 mils 7 0 33 m s ventilation fraction 0 20 0 20 0 30 System Level Ventilation During operation a typical VAV system delivers a mixture of outdoor air and recirculated air to the multiple spaces it serves The volume of supply air that enters a space is controlled by a temperature sensor in that space In contrast the ventilation requirement for a space typically remains constant as the supply airflow varies with thermal load The challenge is to maintain the proper amount of ventilation air to each individual space while varying the supply air to that space in response to its load ASHRAE Standard 62 1999 Ventilation for Accepta
4. Air Conditioning Clinic VAV Systems One of the Systems Series gt TRG TRC014 EN VAV Systems One of the Systems Series A publication of The Trane Company S TRANE Preface VAV 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 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 concept of variable air volume or VAV systems 2001 American Standard Inc All rights reserved TRG TRCO014 EN TRG TRCO14 EN S TRANE Contents period one period two period three period four period five period six What Is Variable Air Volume 1 Why VAV occccccccccsscsssccseeeceeeeeeeeeseeeeseeeeeteettteestennnees 7 Components of a VAV System 12 System Configurations 00 00 0 40 Perimeter SPACES siensiassdeedadscnsscecn
5. S TRANE period one What Is Variable Air Volume the air volume reduction creates an opportunity to reduce the fan energy required to move this air The amount of energy saved depends on the method used to modulate the capacity of the fan to be discussed in Period Four Second the reduced airflow across the cooling coil causes the refrigeration system to throttle back in order to stabilize the primary air temperature In turn this results in a reduction in refrigeration energy compared to full load Why VAV Comfort 4 Dedicated terminal units 4 Dedicated thermostats Another reason for VAV is to cost effectively provide improved comfort A VAV system is capable of controlling space temperature in many spaces with dissimilar cooling and heating requirements while using only one central air handling unit AHU This is accomplished by providing one VAV terminal unit and thermostat for each independently controlled space When the sun is beating against the west side of the building in the late afternoon a VAV system can provide an increased amount of cool supply air to keep the spaces on the west exposure comfortable while throttling back the airflow to the spaces on the east exposure so as not to overcool them TRG TRCO014 EN TRG TRCO14 EN S TRANE period one What Is Variable Air Volume System Comparison constant volume constant volume single zone terminal reheat VAV e Constant fan e Constant fan e Fa
6. TRG TRCO14 EN 51 S TRANE 52 period four Fan Modulation VAV Systems period four Fan Modulation To accommodate the variable volume requirements imposed by a VAV system the system supply fan must be selected and controlled in such a manner that it is capable of modulating over the required airflow range without entering an unstable area of operation First certain VAV system basics should be reviewed including E The fan performance curve E The system resistance curve m The fan modulation curve Fan Performance Curve 672 5 Pal static pressure 24 000 cfm 11 3 ms airflow Fan Performance Curve The fan performance curve of a centrifugal fan is a plot of the airflow capacity at a given speed rpm versus the static pressure it generates While a centrifugal fan produces both static and velocity pressures it is traditional to TRG TRC014 EN TRG TRCO14 EN amp manwe period four Fan Modulation plot performance on the basis of the static pressure component only Performance curves published for some packaged air handlers will include the effects of the dampers filters and coils that are in the unit In this case the static pressure capacity of the fan must be able to overcome the rest of the static pressure losses in the system supply duct terminal units diffusers and grilles and so forth To deliver 24 000 cfm 11 3 m s of air this example fan running at 1 000 rpm generates 2 7 i
7. space thermostat set points The morning warm up cool down mode typically occurs as a transition from the unoccupied mode to the occupied mode It establishes the occupied comfort conditions for the building as rapidly as possible because they were allowed to drift from occupied set point during the unoccupied mode usually to save energy In this mode the building does not initially require ventilation because it is not occupied but it may eventually be provided for a preoccupancy purge diluting the contaminants that accumulated during the unoccupied mode During the morning warm up cool down mode the system is controlled as follows m The AHU fan operates continuously to provide primary air to the spaces for cooling or heating If separate perimeter heat is installed and heat is demanded the perimeter heat source operates and the AHU fan remains off m The supply fan is controlled to maintain the static pressure set point for the system E The outdoor air damper is closed unless ventilation is needed for preoccupancy purge E The terminal units may be fully open allowing wild uncontrolled warm up or cool down or they may modulate to achieve the occupied temperature set points for a controlled warm up or cool down Morning warm up or cool down mode is ended when the perimeter zone thermostats or a single representative thermostat reaches its occupied set point Then the system switches to the occupied mode
8. cooling only terminal units that modulate in response to the changing space cooling load They also have a minimum airflow setting to serve ventilation requirements Interior Spaces Reheat Required VAV reheat terminal unit Some types of interior spaces such as conference rooms require some amount of tempering to avoid overcooling the space at part loads This is common for spaces in which the occupancy varies from full occupancy to two or three people TRG TRC014 EN TRG TRCO14 EN S TRANE period three System Configurations Typically either VAV reheat or fan powered terminal units are used to provide the tempering needed to balance the reduced cooling load Small Buildings Changeover Bypass VAV System Many smaller buildings cannot afford commercial or applied equipment Often constant volume light commercial equipment like a small packaged rooftop unit will be used Nevertheless it may still be desirable to accommodate many independently controlled spaces economically For these applications a special VAV system called a changeover bypass VAV system is available Changeover Bypass VAV System constant volume supply fan VAV to spaces Like a typical VAV system a changeover bypass VAV system contains an airflow modulation device for each individually controlled space This device modulates supply airflow in response to the space load Instead of modulating the central supply fan however this sy
9. duct system static pressure at the inlet of the terminal unit pressure independent VAV control scheme that directly controls the actual volume of primary air that flows to the space The position of the modulation device is not directly controlled and is basically a by product of regulating the airflow through the unit Because the airflow delivered to the space is directly controlled it is independent of inlet static pressure primary air Conditioned air delivered by a central supply fan to a terminal unit series fan powered terminal unit A fan powered VAV terminal unit consisting of a primary airflow modulation device and a small integral TRG TRC014 EN S TRANE Glossary constant volume fan packaged so that the airflow paths are in series This provides a constant volume of supply air to the space when operating static regain duct design method Method of designing an air duct system that strives to maintain a fairly consistent static pressure throughout the entire duct Recommended for sizing the supply ducts upstream of the terminal units in a VAV system The design of a static regain duct system often requires the use of a computer program supply air Conditioned air delivered to a space supply duct static pressure control Method of VAV system static pressure control that mounts the static pressure sensor somewhere in the supply duct system allowing the supply fan to decrease speed and lower the static pressure in the
10. experienced at reduced airflows may overpressurize the VAV terminals resulting in greater than desired space airflow and noise problems Additionally if the fan enters an unstable area of operation at this high static pressure fan vibration called surge and severe air pulsation within the duct system may result This may lead to objectionable noise levels and potentially a mechanical fan failure 55 amp ranw 56 period four Fan Modulation Fan Modulation Curve static pressure airflow Fan Modulation Curve The fan modulation curve illustrates the static pressure requirement of the VAV system over the range of airflows This requirement consists of a fixed component and a variable component The VAV system requires a minimum amount of static pressure to properly operate the terminal unit modulation devices and diffusers This static pressure is the fixed component The second component of this static pressure curve is variable This is the amount of static pressure required to overcome the system pressure losses due to the ducts fittings coils filters and so forth at various airflows These losses vary with changes to the system airflow This curve describes the fan modulation needed to balance the static pressure that is required to offset these variable system losses demand with the static pressure produced by the fan supply TRG TRCO014 EN TRG TRCO14 EN period four Fan Modulation
11. levels of the space it has reached an average temperature constant volume system Type of air conditioning system that varies the temperature of a constant volume of air supplied to meet the changing load conditions of the space cooling only terminal unit Simplest type of single duct VAV terminal unit It has the capability of varying the airflow but has no method for space heating diffuser Device connected to the end of the supply duct system used to distribute the supply air effectively to the conditioned space direct digital control DDC Method of terminal unit control using an electric motor to operate the airflow modulation device activator It uses a microprocessor that enables digital communication between the unit controller and a central building automation system discharge dampers Method of supply fan modulation that matches the static pressure supplied by the fan with the static pressure required by the system This is accomplished by using a modulating damper a little bit downstream of the fan to create a static pressure drop in the system dual duct terminal unit VAV terminal unit consisting of two primary airflow modulation devices one for cool primary air and one for warm primary air These units can be controlled to provide either a constant volume or a variable volume of supply air to the space electronic control Method of terminal unit control using an electric motor to operate the airflow modulation device actu
12. lights are on In the winter when it is cold outside these spaces require heating to offset the skin heat loss through the exterior walls and windows Most interior spaces require year round cooling due to the absence of skin heat losses and the fairly consistent amount of heat generated by people lights 40 TRG TRC014 EN TRG TRCO14 EN S TRANE period three System Configurations and equipment Interior spaces on the top floor of a building often need to be treated as a perimeter space due to the heat gain loss from the roof Each space is typically served by an individual VAV terminal unit allowing independent cooling and heating control Perimeter Spaces overhead heating perimeter heating Perimeter Spaces Before a VAV terminal unit can be selected to serve a perimeter space the designer must determine the heating load for that space This will determine whether the heating load can be satisfied by supplying warm air through overhead diffusers or if the heating load must be handled by a separate perimeter heating system e g baseboard wall fin The guideline for heating a perimeter space is based on the skin heat loss per unit length of perimeter wall 41 S TRANE 42 period three System Configurations Baseboard Perimeter Heating heat loss gt 450 Btu hr ft 430 Wim cooling only VAV terminal unit baseboard heating system If the heat loss of the perimeter space excee
13. not allowed to shut off completely because of the space requirement for ventilation air Series Fan Powered terminal primary B mixing fan air A series fan powered terminal unit consists of an airflow modulation device and a small constant volume fan packaged so that their airflow paths are in series The terminal unit fan operates continuously whenever the space is occupied The fan draws air from either the primary air stream or the plenum based on the thermostat in the space This results in a constant volume of supply air delivered to the space at all times 19 S TRANE period two Components of a VAV System Series Fan Powered 100 maximum primary airflow Q o is 2 N 2 z plenum air minimum T primary 3s airflow primary air 0 design design heating load space load cooling load In the cooling mode the primary airflow is reduced as the cooling load in the space decreases The total supply airflow to the space remains constant a combination of cool primary air and warm plenum air If the space cooling load drops below the minimum primary airflow setting for the unit or if space heating is required a heating coil can be used to further warm the supply air to the space The maximum airflow for the airflow modulation device typically equals the terminal fan airflow This means that at the design cooling load primary airflow equals supply airflow Parallel Versus Series Fan Powered
14. of a building It is seldom used because of the high first cost incurred to provide two duct systems A dual duct terminal unit can be controlled to provide either a variable volume or a constant volume of supply air to the space TRG TRC014 EN TRG TRCO14 EN S TRANE period two Components of a VAV System Variable Air Volume to the Space 100 maximum heating tempering 4 primary i airflow warmi primary alr minimum primary airflow airflow to space cool primary air 0 design design heating load space load cooling load As before this graph indicates how the air supplied to the space by the terminal unit varies as the space loads change The vertical axis indicates the total airflow cool primary air plus warm primary air supplied to the space The horizontal axis indicates the space load To deliver a variable volume of supply air to the space in a two fan dual duct system in the cooling mode the dual duct unit is controlled in the same manner as the cooling only unit The cool primary airflow is reduced as the cooling load in the space decreases When the space cooling load drops below the minimum cool primary airflow setting for the unit overcooling the space the second modulating device begins to open This allows the warm primary air to mix with the cool primary air and provide warmer supply air to the space As the cooling load decreases further and the space requires heating more warm primar
15. profiles in the spaces E The need for multiple independently controlled spaces m The existence of a common return air path Variable Thermal Load Profiles east facing space 100 west facing space 3 75 o S amp 50 S 5 25 Q 0 6 12 6 a m noon p m A key feature of a VAV system is its ability to realize both fan and refrigeration energy savings at part load conditions A building with construction and utilization characteristics that cause the thermal load profiles of the spaces to vary throughout the day and year is an excellent application for a VAV system The fact that the west facing space in this example has a very different load profile than the east facing space allows the VAV system to take advantage of this energy savings at all hours of the day TRG TRC014 EN S TRANE period one What Is Variable Air Volume Independent Space Control 4 Dedicated terminal units 4 Dedicated thermostats Another feature of the VAV system is the ability to efficiently satisfy the comfort requirements of many different spaces within the building As we saw from the comparison of the VAV and terminal reheat systems the VAV system is the most efficient at performing this task Common Return Air Path Because the VAV system uses one central cooling coil and fan to serve many spaces the need for a common path to allow the air to return from these spaces is significant A return air plenum is commonly us
16. system delivers the primary air at a constant temperature and varies the airflow to maintain the required space temperature at all load conditions VAV Full Load Supply 40 000 Btu hr es Airflow 1 085 x 75 F 55 F Supply 11724W___ L 0 87 itis Airflow 1 210 x 23 9 C 12 8 C Similar to the constant volume system at design conditions the VAV system will balance the same 40 000 Btu hr 11 724 W space sensible load with the same 1 840 cfm 0 87 m s of 55 F 12 8 C supply air 6 TRG TRCO14 EN S TRANE period one What Is Variable Air Volume VAV Part Load Supply _ 20 000 Btu hr L gon cfm Airflow 1 085 x 75 F 55 F Supply 5862 W_____ Z 0 43 m s Airflow 1 210 x 23 9 C 12 8 C However at part load the VAV system produces a balance by modulating the volume of constant 55 F 12 8 C supply air In this example the air volume required to balance the lesser 20 000 Btu hr 5 862 W space sensible load is 20 000 Btu hr Supply Airflow 1 085 x 75 F 55 F 920 cfm _ 5 862 W 7 7 Supply Airflow 1210x23 9 C 128 C 0 43m7S That is 920 cfm 0 43 m s of 55 F 12 8 C supply air Why VAV Energy Savings a4 Reduced fan energy a Reduced refrigeration energy Why VAV One reason to use a VAV system is the potential for part load energy savings The part load energy savings inherent with the VAV system are twofold First TRG TRCO14 EN 7
17. the system static pressure back down to the set point Fan Modulation Methods 4 Discharge dampers 4 Inlet vanes 4 Fan speed control 4 Variable pitch blade control Fan Modulation Methods The four methods commonly used to modulate fan capacity are discharge dampers inlet vanes fan speed control and variable pitch blade control We will examine these now Discharge Dampers discharge dampers Discharge dampers match the static pressure supplied by the fan with the static pressure required by the system They accomplish this by adding static pressure loss to the system a little bit downstream of the fan TRG TRC014 EN TRG TRCO14 EN period four Fan Modulation Discharge Dampers actual system resistance curve design system resistance curve discharge damper SP loss static pressure As the VAV terminal units modulate shut the system resistance curve shifts upwards The fan begins to ride up the constant speed performance curve from the design operating point A trying to reach the balance point with this new system resistance curve As a result the fan delivers a lower airflow at a higher static pressure The system static pressure controller senses this higher pressure and sends a signal to the discharge dampers instructing them to begin closing This has no effect on the performance curve for the fan but it does add a new static pressure loss to the system This m
18. 14 EN TRG TRCO14 EN S TRANE period three System Configurations Occupied Mode 4 Main supply fan operates continuously 4 Constant primary air set point 4 Main supply fan controlled to maintain the system static pressure set point a Outdoor air damper delivers proper amount of ventilation air 4 Terminal units maintain respective occupied space thermostat set points In a typical commercial building the occupied mode occurs during daytime operation The building must be ventilated and the comfort cooling or heating temperature set points must be maintained in all occupied zones The occupied mode is characterized as follows m The main supply fan operates continuously E The primary air temperature is controlled to a constant set point m The supply fan is controlled to maintain the static pressure set point of the system m The outdoor air damper is controlled to deliver the proper amount of ventilation air E Allterminal units are controlled to maintain their respective occupied space temperature set points 49 S TRANE period three System Configurations Unoccupied Mode 4 Main supply fan operates only as needed to maintain thermostat set points 4 Main supply fan controlled to maintain the system static pressure set point 4 Outdoor air damper is closed 4 Terminal units maintain respective unoccupied space thermostat set points In a typical commercial facility the unoccupied mode oc
19. Fan Control Loop EEE static pressure sensor controller To achieve this balance a simple control loop is used First the static pressure is sensed from a particular location in the system Second a controller compares this static pressure reading and the set point of the system Finally the fan capacity is controlled to deliver the required airflow at a static pressure that maintains this set point at the location of the system sensor VAV Fan Modulation actual system R resistance curve design system resistance curve Q Q Q Q g 2 3 D n 2 a m s D airflow We will use an exaggerated example to illustrate this system operation Assume that the load on the system decreases causing all or part of the VAV terminal units to modulate closed This causes the system resistance curve to shift upwards In response the fan begins to ride up the constant speed performance curve from the design operating point A trying to reach the balance point with this new system resistance curve As a result the fan delivers a lower airflow at a higher static pressure 57 amp ranw 58 period four Fan Modulation The system static pressure controller senses this higher pressure and sends a signal to the supply fan to reduce capacity Modulating the fan capacity results in a new fan performance curve and the system balances at an operating point B along the fan modulation curve bringing
20. airflow that must be introduced at the system level at the central air handler to satisfy the ventilation requirements The BAS then resets the outdoor airflow set point and the outdoor airflow measuring device controls the damper accordingly This method called ventilation reset ensures that all spaces are properly ventilated at all load conditions while minimizing wasteful overventilation TRG TRC014 EN S TRANE period five Application Considerations Freeze Protection for Coils warm air OA oef 12 2 C cold air cooling coil Freeze Protection for Coils As mentioned proper VAV system ventilation often leads to a high percentage of outdoor air at part load conditions Streams of air at vastly different temperatures are difficult to mix and may cause the mixed air stream to stratify resulting in a risk of freezing the water inside the cooling coil Air blending devices mixing baffles and high velocity mixing dampers can provide effective mixing but usually require more space in the air handler and more fan power As an alternative preheating the outdoor air before it enters the mixing box decreases this temperature difference improves mixing and offers freeze protection for cooling coils Measures to Prevent Coil Freeze Up 4 Drain coils during cold weather 4 Keep water in motion a Add glycol to the water 4 Use face and bypass dampers to limit amount of cold airflow through coil a Preheat th
21. alance the low space cooling load When space heating is required the heating coil further warms the primary air to satisfy the space heating load TRG TRC014 EN TRG TRCO14 EN S TRANE period two Components of a VAV System During the heating mode the primary airflow must often be greater than the minimum setting of the unit for the cooling mode When warm buoyant air is supplied from the ceiling a certain velocity is required to effectively deliver it down to the occupied portion of the space Increased airflow may also be needed to meet the minimum requirement for proper operation of the heating coil VAV reheat is more efficient than the constant volume reheat system discussed earlier because heat is provided at reduced airflow and only when required Fan Powered Terminal Units warm parallel plenum air cool primary air series Another method that a single duct VAV terminal unit can employ to heat a space is to mix warm plenum air with cool primary air Because this method uses a small fan to draw warm air from the plenum and mix it with the primary air they are called fan powered terminal units They come in two configurations parallel and series 17 S TRANE period two Components of a VAV System Parallel Fan Powered plenum dD air terminal mixing fan primary air air A parallel fan powered terminal unit consists of a primary airflow modulation device and a small i
22. an outlet method previously discussed However using this method typically allows for fan energy savings compared to the fan outlet method TRG TRC014 EN 65 amp ranw period four Fan Modulation Optimized Static Pressure Control sensor located damper at fan outlet static positions pressure supply SS sensor fan communicating BAS pm EN jg ee speed or inlet vane position The optimized static pressure control method combines the location related benefits of fan outlet control with operating cost savings that exceed those of supply duct static pressure control A single static pressure sensor is located at the fan outlet and the controller dynamically adjusts the static pressure set point based on the position of the modulating devices in the VAV terminal units The DDC VAV controllers know the position of their individual air modulation devices and because they are pressure independent they will modulate to maintain the required airflow The building automation system BAS continually polls the VAV terminal units looking for the most open VAV damper The controller resets the static pressure set point so that at least one VAV terminal the one requiring the highest inlet pressure is nearly wide open The result is that the supply fan generates only enough static pressure to get the required flow through this critical terminal unit This method allows the sensor to be factory installed a
23. anging the equation and using the constant supply airflow of 1 840 cfm 0 87 m s the supply air temperature required to balance a lesser 20 000 Btu hr 5 862 W space sensible load is Sensible Heat Gain S ly DB S DB gt T T ppiy pace Constant x Supply Airflow 20 000 Btu hr Supply DB 75 F 7 ose x 1 840 cfm 65 F Supply DB 23 9 C eee 1 210 x 0 87m3 S Therefore as the space sensible load drops from 40 000 Btu hr to 20 000 Btu hr 11 724 W to 5 862 W this system modulates the temperature of the constant 1 840 cfm 0 87 m s supply air from 55 F to 65 F 12 8 C to 18 3 C Removing less energy from the air takes less energy from the central system Therefore with this particular system refrigeration energy savings are realized at part load conditions although control of space humidity levels suffers due to the warmer supply air condition TRG TRC0O14 EN 3 TRANE period one What Is Variable Air Volume Constant Volume Multiple Spaces single thermostat It s stuff However because this type of system can respond to the demands of only one thermostat it can serve only those building spaces with similar cooling requirements If a building has many spaces with diverse cooling needs each must be served by its own system Constant Constant Volume Multiple Spaces Spaces 80 F 26 7 C 65 F 18 3 C 55 F air mixing 12 8 C 55 F 65 F 12 8 C 18 3 C reh
24. ant and stratified air For applications with special air coverage requirements such as the blanketing of an outside wall or window area one way discharge diffusers are commonly used Such coverage is particularly useful for overcoming the downdraft problems that can occur when large volumes of heated air are distributed through ceiling diffusers 34 TRG TRCO14 EN TRG TRCO14 EN S TRANE period two Components of a VAV System Ceiling Diffuser Compatibility hanger wire i ee a ceiling tile EE Da cross T structural T Because variable air volume systems are installed in and distribute air along the surfaces of ceilings it is essential that the diffusers be compatible with the ceiling design The T bar ceiling is possibly the most popular design It is constructed of a grid of inverted T shaped members suspended by wires from the floor or roof structure above Cross members added to the load bearing T bars generate a symmetrical ceiling pattern Finally ceiling tiles are laid in place and supported by this grid 35 S TRANE period two Components of a VAV System Ceiling Diffuser Compatibility cross T ceiling air slot flange tile mounting feet air slot structural T The linear slot diffuser is placed on the structural T bars with the mounting feet resting on the flanges The cross member fits in a slot in the diffuser air opening aligning the unit with th
25. antnndensndisadenenes 41 Interior SPACES 2 cceceicibidecceceacncsededssensessseeeenennannncs 46 Changeover Bypass VAV System c0ccccceeeee eens 47 System Level Control Modes ccccccceeeseeeeeeeeees 48 Fan Modulation 00000000000000ccce cece 52 Fan Performance Curve ccccececcceeeeeeeeeeeeee 52 System Resistance Curve cccccceeeecceeee eens 53 Fan Modulation Curve ccccceecccceeseeeeeeeeeees 56 Fan Modulation Methods c ccccccceeeeeeees 58 System Static Pressure Control a se 64 Application Considerations 0 0 00 68 PMV ironmim i aki 76 QuiZ ssc vtasinnnecaceccat nina stenanstnnwndeceadiaciesnin dt enainnentaaidices 81 Answers Grr te docateccncnnatnanctaditinnnensdesd nneibadatgsiebabiioe 84 MOSSE esisisenducaceatinpsnecacsnvesbohacaceacnniedaanadsinniacee 85 iii S TRANE TRG TRC014 EN S TRANE period one What Is Variable Air Volume VAV Systems period one What Is Variable Air Volume A variable air volume VAV air conditioning system varies the volume of constant temperature air that is supplied to meet the changing load conditions of the space Constant Volume Variable Temperature System cooling coil space For the purpose of comparison we will look at a traditional constant volume CV variable air temperature system This system delivers a constant volume of air to the space and to maintain the
26. are several methods used to control building pressure in a VAV system The method that is commonly considered the best suited for VAV applications is direct control of a central relief fan 74 TRG TRC014 EN S TRANE period five Application Considerations A sensitive static pressure controller directly compares the variations between the space static pressure and the outdoor reference static pressure The controller modulates the capacity of a central relief fan to maintain a fixed static pressure difference usually a slightly positive building pressure The space static pressure sensor is typically located in the lobby or near the main entrance TRG TRC014 EN 75 S TRANE 76 period six Review VAV Systems period six Review Now we will review some of the main concepts from this clinic on variable air volume VAV systems Review Period One central air handler thermostat A variable air volume VAV air conditioning system varies the volume of constant temperature primary air to meet changing load conditions in the space VAV systems can provide both fan and refrigeration energy savings at part load while efficiently serving many independently controlled spaces with dissimilar cooling and heating load requirements Variable thermal load profiles in the spaces the need for multiple independently controlled spaces and the existence of a common return air path are three building characteristics that fa
27. ates how the air supplied to the space by the terminal unit varies as the space loads change The vertical axis indicates the total airflow supplied to the space The horizontal axis indicates the space load The supply airflow to the space is reduced as the cooling load in the space decreases Responding to the space thermostat the primary airflow is modulated between maximum and minimum settings The maximum setting is TRG TRC014 EN TRG TRCO14 EN S TRANE period two Components of a VAV System determined by the design cooling load of the space and the minimum setting is normally determined by the space ventilation requirement or minimum airflow for proper diffuser selection Most cooling only units are applied to spaces that have no need for heat These units would operate in the region on the right hand portion of this chart modulating between design and minimum primary airflow When cooling only units are applied to spaces that do have heating requirements the heat is provided by a remote source such as finned radiation along the wall In these spaces when the cooling load drops below the minimum airflow setting for the unit overcooling the space the remote heat source activates When space heating is required the remote heat source satisfies the space heating load Most terminal unit controllers provide an output signal to control this remote source of heat Space Heating with a VAV System terminal mixing fan plenum ai
28. ator The motor can drive the modulation device open or closed equal friction duct design method Method of designing an air duct system that results in an equal static pressure drop per foot of duct Equal friction duct systems can be easily designed by hand fan modulation curve Curve that illustrates the static pressure requirement of the VAV system fan over the range of airflows fan outlet static pressure control Method of VAV system static pressure control that mounts the static pressure sensor near the outlet of the main supply fan and maintains a constant static pressure at the sensor fan performance curve Plot of the airflow capacity of a specific fan at a given speed rpm versus the static pressure it generates 87 S TRANE 88 Glossary fan speed control Method of supply fan modulation that affects the capacity of a fan by varying the speed of rotation commonly accomplished using a variable speed drive on the fan motor fan powered terminal unit Type of single duct VAV terminal unit that can provide heating for a space by mixing warm plenum air with the cool primary air using a small terminal fan inlet vanes Method of supply fan modulation that affects the capacity of a fan by pre swirling the air in the direction of fan wheel rotation before it enters the wheel The inlet vanes lessen the ability of the fan wheel to bite the air reducing its airflow capacity interior space A conditioned space t
29. ble Indoor Air Quality includes a procedure that can be used to calculate the quantity of outside air required at the central air handling unit in order to satisfy the ventilation requirements of the individual spaces At part load conditions VAV systems typically require a constant quantity of ventilation air resulting in an increased percentage of outdoor air at the air handler Fixed position dampers will result in underventilated VAV systems 69 amp ranw 70 period five Application Considerations Ventilation Reset outdoor airflow measurement and control central air handler with controls VAV terminal units with DDC controls gt communicating BAS With all three components DDC controls on the VAV terminal units and air handler a method to measure outdoor airflow at the air handler and a communicating building automation system BAS a VAV system can use the multiple spaces equation Equation 6 1 from Standard 62 to dynamically operate the VAV system Each VAV terminal unit continuously monitors primary airflow The DDC controller also knows the ventilation air requirement With this information the controller can continuously calculate the space ventilation fraction of the space or the ventilation airflow required divided by the primary airflow being delivered The BAS periodically gathers this data from all the VAV terminal units and solves Equation 6 1 This determines the amount of outdoor
30. coil is typically used for spaces that require seasonal cooling and heating such as the exterior zones of a building Units with heating coils are the most common of the fan powered terminal units Single Duct VAV Systems central air handler single duct VAV terminal units The types of terminal units discussed so far are used in single duct VAV systems Single duct VAV systems use a central return air path that allows the air from the spaces to come back to the air handler At the air handler a portion of this return air is recirculated and mixed with outdoor air introduced for space ventilation purposes This mixture of outdoor and recirculated return air or primary air is then conditioned and delivered to the VAV terminal units through the supply duct system The remainder of the return air is exhausted from the building TRG TRCO14 EN 21 S TRANE 22 period two Components of a VAV System Dual Duct warm primary air cool a primary air A supply air b A dual duct terminal unit consists of two airflow modulation devices with controls packaged in a sheet metal enclosure One controls the cool primary air and the other controls the warm primary air Depending on the method of control these two air streams may mix in the dual duct unit before proceeding downstream to the space This VAV terminal unit is intended for zones that require seasonal cooling and heating typically the exterior zones
31. curs at night The building does not require ventilation because it is not occupied and the temperature in the perimeter spaces must be prevented from getting too cold perhaps 60 F 15 6 C or too hot perhaps 85 F 29 4 C Temperature control of the interior spaces is typically ignored because proper control of the perimeter spaces normally adequately limits the interior space temperatures The top floor of the building may require temperature limiting for all spaces During the unoccupied mode the following occurs E The main supply fan cycles on whenever any perimeter space or a centrally located nighttime thermostat demands heating or cooling If separate perimeter heat is installed and heat is demanded it will operate and the main supply fan remains off m The supply fan is controlled to maintain the static pressure set point of the system E The outdoor air damper is closed m All terminal units with demand for central heating or cooling maintain their respective unoccupied temperature set points All other terminal units remain off 50 TRG TRCO14 EN S TRANE period three System Configurations Morning Warm up Cool down Mode 4 Main supply fan operates continuously 4 Main supply fan controlled to maintain the system static pressure set point a Outdoor air damper is closed unless required for preoccupancy purge 4 Terminal units are either fully open or modulated to maintain their respective occupied
32. ds 450 Btu hr per linear foot 430 W per linear meter of outside wall an under the window or baseboard heating system is typically used With this much heat loss supplying a high quantity of warm air from overhead diffusers can cause downdrafts leading to occupant discomfort The cooling requirements of these perimeter spaces are served by a cooling only terminal unit The diffusers are located in the center of the room in order to evenly supply cool air to the space During heating mode the terminal unit provides minimum airflow to the space to meet the ventilation requirement The perimeter heat source is separate but can be controlled by the VAV terminal unit controller Having only one controller for the space ensures proper sequencing of the cooling and heating systems TRG TRC014 EN TRG TRCO14 EN S TRANE period three System Configurations Overhead Supply Perimeter Heating diffusers in center of space lt 250 Btu hr ft 430 W m diffusers blanket perimeter wall a l 250 450 Btu hr ft Sal 240 430 Wim If the heat loss of the perimeter space is less than 450 Btu hr per linear foot 430 W per linear meter of outside wall downdrafts are less problematic and heated air supplied through ceiling diffusers can provide a satisfactory solution Again the rate of heat loss is used to determine the proper diffuser location If the heat loss is between 250 and 450 Btu hr per linear foot 240 and 430 W p
33. e allowed to drift from occupied set point during the unoccupied mode usually to save energy TRG TRC014 EN 89 TRANE ee Literature Order Number TRG TRC014 EN May 2001 File Number E AV FND TRG TRC014 0501 EN The Trane Company Supersedes TRG TRC004 EN May 1999 Worldwide Applied Systems Group y i 3600 Pammel Creek Road Stocking Location La Crosse La Crosse WI 54601 7599 www trane com Since The Trane Company has a policy of continuous product improvement it reserves the right to change An American Standard Company design and specifications without notice
34. e ceiling tile pattern The weight of the linear slot diffuser is partially carried by the T bar structure but should also be augmented by hanger wires to ensure a vertical orientation and provide additional weight support After the diffuser is mounted ceiling tiles are trimmed and laid in place completing the job 36 TRG TRCO14 EN TRG TRCO14 EN S TRANE period two Components of a VAV System Supply Duct System central lt air handler _ diffuser a flexible J i _t duct sheet metal aa supply duct pon p 5 VAV my terminal lt gt lt unit Supply Duct System The supply duct system transports the primary air from the air handler to the VAV terminal units and then on to the space diffusers A successful design achieves the following mE Minimizes the static pressure and associated power requirements of the supply fan Minimizes the installed cost without sacrificing system efficiency Supplies air to each VAV terminal unit without excessive noise Accommodates space limitations without excessive pressure drop Minimizes design time 37 S TRANE 38 period two Components of a VAV System Duct Design a Equal friction method Equal static pressure drop per unit length of duct Design can be performed by hand a Static regain method Relatively constant static pressure throughout system Desirable for VAV system duct design Design often requires a computer pro
35. e outdoor air heating coil heat recovery Even with effective mixing the temperature of the mixed air entering the coil may approach that of the outdoor air during cold weather This may cause water inside the cooling coil to freeze TRG TRCO14 EN 71 amp ranw 72 period five Application Considerations Coils can be protected from freezing in many ways such as m Draining the coils during cold weather E Keeping the water in motion m Adding glycol to the water Using face and bypass dampers to limit the amount of cold airflow through the coil E Preheating the outdoor air with either a conventional heating coil or heat recovered from the exhaust air stream Part Load Space Humidity Control part load space conditions CVsystem 75 F 66 RH 23 9 C VAV system 75 F 58 RH 23 9 C z 1 3 D mixed air c a A B Io i l 55 F 1 1 60 F 175 F 12 8 C 4 15 6 C 1 23 9 c Cee reer reer dry bulb temperature Part Load Space Humidity Control VAV systems do a good job of indirectly controlling space relative humidity RH at full and part load conditions A typical VAV system supplies air at a constant low dew point temperature at all load conditions In this example both the constant volume and variable volume systems deliver the same quantity and temperature A 55 F 12 8 C of air to the space at design load conditions However at part load the VAV system sti
36. eat coil Other constant volume system designs can serve the cooling requirements of more than one space with a central fan and cooling coil However to do so the cool primary air must be either reheated or mixed with warm air to produce the supply temperatures needed to balance the various space cooling loads TRG TRC014 EN TRG TRCO14 EN S TRANE period one What Is Variable Air Volume Terminal Reheat System cooling coil thermostat The terminal reheat system uses a central air handler and cooling coil to deliver cool primary air to all the spaces Each space has its own heating coil to temper the air to satisfy the space load Of course any heat added to meet the part load requirements of a space becomes a cooling load that the refrigeration system must overcome This can result in a nearly constant refrigeration load even when the building is at part load conditions Therefore reheating cooled air to achieve part load supply air temperature control is not very energy efficient and is used only in special constant volume applications or when there is a free source of heat i e heat recovery Primary air PA is the air delivered by a central supply fan to a terminal unit Supply air SA is the air delivered to a space S TRANE period one What Is Variable Air Volume Variable Air Volume VAV System i variable speed drive thermostat In contrast a variable air volume VAV
37. ed If return ductwork is required to connect each conditioned space to the central air handling unit the system becomes more difficult to balance and control TRG TRC014 EN 11 S TRANE period two Components of a VAV System VAV Systems period two Components of a VAV System Next we will examine the individual components that compose a VAV system Components of a VAV System return air path central air handler A simple VAV system includes the following Central air handler with a variable volume supply fan a cooling coil possibly a heating coil controls filters a mixing box and a return or relief fan Supply duct m VAV terminal unit or box with a thermostat and supply diffusers for each independently controlled space Thermostat and unit controller for each terminal unit Return plenum or duct The VAV terminal units modulate the airflow supplied to each space 12 TRG TRC014 EN TRG TRCO14 EN S TRANE period two Components of a VAV System VAV Terminal Units primary air y a Heating coil a Filter 4 Mixing fan airflow modulation device j supply ntroller controlle air VAV Terminal Units A VAV terminal unit is a sheet metal assembly installed upstream of its respective space diffusers The unit consists of an air modulation device control hardware and depending on the system application possibly a heating coil a filter and a small terminal mi
38. ed VAV terminal unit Similarly fan powered terminal units can be used in perimeter spaces that require seasonal cooling and heating In the heating mode the parallel fan powered unit turns on a small fan as the first stage of heating Doing so allows it to temper the supply air with the heat of the building and lights that is carried by the return air rather than using new energy When activated the small fan also increases airflow to the space improving the mixing of supply air and space air to prevent stagnation Series fan powered terminal units may also be used in this manner They offer the added advantage of supplying constant airflow to the space in both cooling and heating modes Dual Duct VAV heat loss lt 250 Btu hr ft 240 Wim warm dual duct VAV terminal unit Finally dual duct VAV terminal units can also be used to heat and cool perimeter spaces The specific operation of a dual duct system was explained in Period Two 45 S TRANE 46 period three System Configurations Interior Spaces cooling only VAV terminal unit Interior Spaces Because an interior space is surrounded by spaces at the same temperature it does not experience the same heat gain and loss fluctuations as a perimeter space Therefore an interior space typically requires some degree of cooling all year long to overcome the heat generated by people lighting and so forth Most interior spaces are served by
39. em P increase resistance curve 27in HOR Gee mH 6725 Pal i I ce I A i a airflow reduction 18 000 cfm 24 000 cfm 8 5 m s 11 3 m s During operation a VAV system experiences changes in resistance as the VAV terminal units modulate This creates a new system resistance curve In response a constant speed fan rides up the performance curve from the design operating point A to a new balance point B This new operating point is at lower airflow 18 000 cfm 8 5 m s and a higher static pressure 3 15 in H20 784 6 Pal This method of modulating the airflow of the fan is called riding the fan curve TRG TRC014 EN TRG TRCO14 EN period four Fan Modulation Forward Curved Centrifugal Fan input power 3 15 in HOP 784 6 Pal STR snn A 672 5 Pa 18 000 cfm 24 000 cfm 8 5 m s 11 3 ms This riding the fan curve method of modulation can be used with any centrifugal fan However it is most energy efficient when applied to forward curved FC fans The configuration of the input power curves of the FC fan are such that input power requirements drop as the fan operating point moves upward along the constant speed performance curve Riding the fan curve can be used successfully when the airflow modulation range for the system is small On the other hand if the system is required to modulate over a wide range of airflows the increased static pressure
40. enerally available in VAV terminal units They are pneumatic electronic and direct digital control DDC 28 TRG TRCO14 EN TRG TRCO14 EN S TRANE period two Components of a VAV System Pneumatic Control 4 Pneumatic Volume Regulator PVR provides pressure independent control 4 Pneumatic thermostat directly controls terminal unit fan and heat source 4 Minimum and maximum airflow settings adjusted physically on PVR a Compressed air operates modulation device PVR and space thermostat A pneumatically controlled terminal unit has an airflow modulation device operated by a pneumatic actuator consisting of an inflatable bladder and a return spring The return spring positions the damper to a normal state the position it assumes with no pressure applied in the bladder typically fully open If the pneumatic pressure increases in the bladder the return spring compresses and the modulation device begins to close If pneumatic pressure is allowed to bleed from the bladder the force of the return spring expanding begins to open the modulation device A pneumatic volume regulator PVR provides pressure independent control of the VAV terminal unit The PVR is connected to the airflow measurement device and the pneumatic space thermostat The PVR directs the actuator to position the modulation device to deliver the required airflow to the space The pneumatic thermostat signal is used to reset this airflow set point and i
41. er linear meter of outside wall diffusers should discharge directly downward and blanket the perimeter walls with heated air If the heat loss is less than 250 Btu hr per linear foot 240 W per linear meter of outside wall diffusers can be located in the center of the room and still provide adequate blanketing to handle the heat loss 43 S TRANE 44 period three System Configurations VAV Reheat heat loss lt 250 Btu hr ft 240 Wim VAV reheat terminal unit When overhead heating is acceptable VAV reheat fan powered or dual duct terminal units can be used to provide both perimeter cooling and heating In cooling mode the VAV reheat terminal unit modulates in response to the changing space cooling load always maintaining a minimum airflow to serve ventilation requirements For space loads below this minimum airflow the unit maintains this constant minimum airflow and modulates the reheat coil capacity to satisfy the space tempering or heating requirements Reheating previously cooled primary air is not energy efficient However because the VAV system has already reduced the supply airflow it must reheat only a limited amount of air The VAV reheat system is far more energy efficient than a similarly applied constant volume reheat system TRG TRCO014 EN TRG TRCO14 EN S TRANE period three System Configurations Fan Powered VAV heat loss lt 250 Btu hr ft 240 W m parallel fan power
42. fan variable speed drive Fan speed control modulates fan capacity by varying the speed of rotation This is commonly accomplished using a variable speed drive on the fan motor Fan Speed Control actual system resistance curve design system resistance curve static pressure In response to reduced airflow and rising system static pressure adjustable fan speed control produces a system fan interaction that is similar to that produced by inlet vane control By reducing the fan speed the performance curve for the fan shifts downward and the airflow and resulting static pressure decrease until the operating point B balances along the fan modulation curve 61 amp ranw period four Fan Modulation Variable Pitch Blade Control variable pitch blades Finally the capacity of vaneaxial fans can be modulated by adjusting the pitch angle of the actual fan blades This diagram shows a set of fan blades that can swivel to modulate the capacity of the fan This method of fan modulation is typically only found on very large systems Most vaneaxial fans modulate their capacity by varying the speed of the fan Variable Pitch Blade Control actual system i resistance curve design system resistance curve stall region static pressure Note that the fan performance characteristics of this type of fan are slightly different from those of a centrifugal fan Performance is plotted based on airflo
43. gram Designers commonly use two methods to engineer the supply duct system m Equal friction E Static regain Equal friction duct systems are designed for an equal static pressure drop per foot of duct This results in a static pressure that is very high near the fan and which steadily decreases until it is very low near the far terminal units Equal friction duct systems can be easily designed by hand Static regain duct systems strive to maintain a fairly consistent static pressure throughout the entire duct Therefore for VAV systems the static regain method is recommended for sizing the supply ducts upstream of the terminal units This relatively constant static pressure at the terminal unit inlet allows for improved selection and control The design of a static regain duct system often requires the use of a computer program TRG TRCO014 EN TRG TRCO14 EN S TRANE period two Components of a VAV System Duct Design Recommendations 4 Keep as simple and symmetrical as possible a Locate main runs branch runs and terminal units above hallways or unoccupied spaces 4 Minimize use of flexible ductwork upstream of terminal units 4 Use duct lining or a duct silencer in first duct section to attenuate supply fan noise 4 Place balancing dampers upstream of diffusers in all noncritical branches 4 Reducing transitions should be several duct diameters upstream of terminal units Other publications contain more complete de
44. hat is surrounded by other conditioned spaces with no perimeter walls or windows Typically requires some degree of cooling all year long to overcome the heat generated by people lighting and so forth occupied mode Typical daytime operating mode of the system The building must be ventilated and the comfort cooling or heating temperature set points must be maintained in all occupied spaces optimized static pressure control Optimized method of VAV system static pressure control that uses the benefit of DDC control to continuously reset the static pressure set point of the system so that the VAV terminal requiring the highest inlet pressure is nearly wide open parallel fan powered terminal unit A fan powered VAV terminal unit consisting of a primary airflow modulation device and a small integral constant volume fan packaged to provide parallel airflow paths perimeter space A conditioned space with walls and windows that are exposed to the outdoors In most climates these spaces would require seasonal cooling and heating pneumatic control Method of terminal unit control using pressurized air to operate the airflow modulation device actuator The actuator consists of an inflatable bladder and a return spring pressure dependent VAV control scheme that uses the space temperature sensor to directly control the position of the modulating device The actual airflow delivered to the space is a by product of this position and depends on the
45. le pitch vaneaxial fan and fan speed control are similar in performance To obtain a more accurate comparison for specific operating conditions a life cycle cost analysis should be performed 63 amp ranw period four Fan Modulation Controlling System Static Pressure 4 Fan outlet static pressure control 4 Supply duct static pressure control a Optimized static pressure control System Static Pressure Control Each of these fan capacity control methods requires a signal from a controller that monitors static pressure using a sensor located somewhere in the supply duct system This controller compares the sensed pressure to a set point and modulates the fan capacity to maintain a set point at that sensor location There are three common methods for sensing and controlling system static pressure They are fan outlet static pressure control supply duct static pressure control and optimized static pressure control Fan Outlet Static Pressure Control sensor located at fan outlet pressure sensor terminal units For the fan outlet static pressure control method the static pressure sensor is mounted near the outlet of the main supply fan The controller is set to maintain the static pressure required at design flow The appeal of this method is that the sensor can be factory installed and tested resulting in greater reliability and no field installation cost If fire dampers are 64 TRG TRC014 EN a
46. ll supplies 55 F 12 8 C air to the space This results in a lower space relative humidity B verses D than the constant volume system that modulates the coil capacity and supplies warmer C 60 F 15 6 C air at the same part load condition Realize that adding reheat to this constant volume system will improve its ability to control space relative humidity but will also cause it to use more energy This demonstrates the danger of using primary air temperature reset on VAV systems Increasing the primary air temperature on a VAV system will save some cooling energy but will require more fan energy and will result in higher space relative humidity levels ASHRAE Standard 62 1999 recommends that the space relative humidity not exceed 60 If temperature reset is used it should be allowed to be overridden by a space relative humidity sensor TRG TRC014 EN TRG TRCO14 EN S TRANE period five Application Considerations Part Load Supply Air Tempering overcooled space conditions 68 F 74 RH 20 C E 3 mixed air 3 supply air B A 1o i 55 F I L 68 F 12 8 C 4 p 20 C EEEEEEEH PSs TT dry bulb temperature Minimum space ventilation requirements often dictate minimum primary airflow settings on VAV terminal units If the space cooling load requires less airflow than this minimum the space may overcool to B 68 F 20 C in this example instead of the desired condition A unless the s
47. mp ranw period four Fan Modulation included in the supply duct this method ensures that the sensor is on the fan side of the damper so that the duct is protected from high pressures Also depending on the layout of the duct system this method may eliminate the need for multiple duct mounted sensors It is not however as energy efficient as the other methods Supply Duct Static Pressure Control sensor located 2 3 down supply duct static pressure sensor terminal units For the supply duct static pressure control method the static pressure sensor is located somewhere in the supply duct system Typically the location of the sensor and consequently the set point of the controller is two thirds of the distance between the supply fan outlet and the critical terminal unit inlet The critical terminal unit is at the end of the supply duct path that represents the largest total pressure drop The sensor is field installed and the controller is set to maintain the pressure corresponding to that location in the duct system at design airflow conditions In larger systems with many terminal units determining the best sensor location for all load conditions can be difficult often determined by trial and error and requiring multiple sensors Field installation and adjustment of one or possibly several duct pressure sensors increases installation cost Without factory testing reliability may also suffer compared to the f
48. n H20 672 5 Pa of static pressure System Resistance Curve system resistance curve 672 5 Pal static pressure 1 05 in H O 261 5 Pa 2 A 2 2 15 000 cfm 24 000 cfm 7 1 m s airflow 11 3 m s System Resistance Curve The system resistance curve is a plot of the pressure drop in terms of static pressure that the system including the supply ductwork duct fittings terminal units diffusers and return grilles coils filters dampers and so forth creates over a range of airflows When the airflow in a system increases the resistance also increases because more friction is experienced When delivering 15 000 cfm 7 1 m s this example system creates 1 05 in H20 261 5 Pal of static pressure resistance 53 amp ranw 54 period four Fan Modulation Operating Point performance system resistance 672 5 Pal static pressure 24 000 cfm airflow 43 mils When the system resistance curve is superimposed on the fan curve the intersection predicts the airflow and static pressure at which the system will balance operating point A Running the fan in this example at 1 000 rpm will satisfy the design requirements of the system by overcoming 2 7 in H20 672 5 Pa of static pressure resistance while moving 24 000 cfm 11 3 m3 s of air Riding the Fan Curve actual system B resistance curve zn HON 2 784 6 Paj static pressure design syst
49. n energy energy energy savings e Refrigeration e Nearly constant e Refrigeration energy refrigeration energy savings energy savings e Delivers comfort Delivers comfort Delivers comfort to only one to many spaces to many spaces thermal zone inefficiently efficiently Reheat energy increases at part load Contrasting the three systems discussed in this example the simple constant volume variable temperature system consumes constant fan energy while providing refrigeration energy savings at part load However it can only deliver comfort to spaces with similar loads The terminal reheat system can serve many spaces with dissimilar load requirements but consumes constant fan energy and nearly constant refrigeration energy at part load It also uses an increasing amount of reheat energy at part load On the other hand the variable air volume system is capable of providing both fan and refrigeration energy savings at part load making it the most energy efficient of these three systems In addition the VAV system can efficiently serve many spaces with dissimilar cooling and heating load requirements S TRANE 10 period one What Is Variable Air Volume VAV Building Characteristics 4 Variable thermal load profiles 4 Multiple independently controlled spaces 4 Common return air path There are some basic building characteristics that favor the application of VAV systems We will discuss three m Variable thermal load
50. nd tested Because the pressure sensor is at the fan outlet it can also serve as the duct high pressure sensor If the terminal units use DDC controls the system level communications are already in place making this the lowest cost highest energy savings strategy 66 TRG TRCO14 EN TRG TRCO14 EN period four Fan Modulation Part Load Comparison control fan static faninput full load method airflow pressure power power full load 24 000 cfm 2 7 in H O 22 hp 100 11 3 mis 672 5 Pa 16 4 kW part load fan outlet 18 000 cfm 2 1 in H O 13 hp 60 8 5 mis 523 1 Pa 9 7 kW supply duct 18 000cfm 1 9 in H O 12 hp 55 8 5 mis 473 3Pa 8 9 kW optimized 18 000 cfm 1 5in H 0 95hp 43 8 5 ms 373 6 Pa 7 1 kW A comparison of these static pressure control methods demonstrates the energy savings potential At this representative part load condition using the optimized static pressure control method allows the supply fan to use only 43 of its full load power versus 55 for the supply duct static pressure control method In addition to the supply fan energy savings because the optimized static pressure control method allows the system to operate as if the static pressure sensor was at each individual terminal unit it ensures that no spaces are starved for air There are also acoustical benefits at part load by operating the supply fan and VAV terminal units at the lowest possible duct static pressure
51. ntegral constant volume fan packaged to provide parallel airflow paths Parallel Fan Powered maximum 100 i primary heating tempering airflow a 2 N 2 z plenum air E minimum w primary z 7 3 airflow imary air primary air 0 design design heating load space load cooling load In the cooling mode the unit is controlled in the same manner as the cooling only unit Primary airflow is reduced as the cooling load in the space decreases When the space cooling load drops below the minimum primary airflow setting for the unit overcooling the space the small fan activates to mix warm plenum air with the cool primary air This increases the total airflow to the space and creates a warmer supply air condition If additional heating is required with the terminal fan on a heating coil can be used to further warm the supply air Operating the fan is a form of energy recovery It tempers the supply air with heat from the building and lights carried by the return air rather than with new energy thus delaying the start of the heating coil 18 TRG TRCO14 EN TRG TRCO14 EN S TRANE period two Components of a VAV System When in cooling mode the supply airflow to the space equals the primary airflow When in tempering and heating modes the space receives a constant supply airflow equal to the minimum primary airflow plus the plenum airflow delivered by the terminal fan Usually the primary airflow is
52. o the space is directly controlled this airflow is independent of the inlet static pressure Coanda effect Baseboard or under the window heating A constant volume supply fan 10 Inlet vanes modulate the capacity of a fan by pre swirling the air in the direction of fan wheel rotation before it enters the wheel By changing the angle of air entry into the fan these modulating inlet vanes lessen the ability of the fan wheel to bite the air 11 Optimized static pressure control 12 ASHRAE Standard 62 1999 13 Draining the coils during cold weather keeping the water in motion adding 86 glycol to the water using face and bypass dampers to limit the amount of cold airflow through the coil or preheating the outdoor air TRG TRCO014 EN TRG TRCO14 EN S TRANE Glossary changeover bypass VAV system A system that allows variable supply air to the spaces but uses a constant volume central supply fan This is accomplished by using a large damper to bypass the excess air This is common in smaller buildings requiring individual space comfort control Coanda effect Concept behind the operation of a linear slot diffuser Air is discharged at a relatively high velocity along the surface of the ceiling creating an area of low pressure that causes the supply air to hug the ceiling As it travels along the ceiling air from the space is drawn into and mixed with the supply airstream When the air settles to the occupied
53. ons for Period 4 10 How do inlet vanes cause the capacity of a fan to change TRG TRCO14 EN 83 S TRANE 84 Quiz 11 What method of static pressure control provides the greatest power savings potential for a VAV system Questions for Period 5 12 What industry standard outlines a method for determining the proper quantity of ventilation air to be introduced at the system level air handler of a multiple space system 13 List two methods used to protect cooling coils from freezing TRG TRC014 EN TRG TRCO14 EN 85 S TRANE Answers 1a 1 382 cfm 0 65 m s 1b 63 F 17 2 C 1c 829 cfm 0 39 m s 2 The reduced airflow across the cooling coil at part load causes the refrigeration system to throttle back in order to stabilize the supply air temperature condition IN turn this results in a reduction in refrigeration energy Variable thermal load profiles in the spaces the need for multiple independently controlled spaces and the existence of a common return air path Series fan powered VAV The air in the ceiling plenum is warmer than the space because it contains heat from the building and lights A pressure independent VAV control scheme directly controls the actual volume of primary air that is flowing to the space The position of the modulation device is not directly controlled and is basically a by product of controlling the actual airflow through the unit Because the airflow delivered t
54. oves the system resistance curve back down to where the system balances at an operating point along the desired fan modulation curve B This brings the system static pressure back down to its set point Other methods are more energy efficient so discharge dampers are rarely used for supply fan modulation 59 amp ranw 60 period four Fan Modulation Inlet Vanes inlet vanes Inlet vanes modulate the capacity of a fan by pre swirling the air in the direction of fan wheel rotation before it enters the wheel By changing the angle of air entry into the fan the modulating inlet vanes lessen the ability of the fan wheel to bite the air This reduces the airflow capacity which in turn reduces the power consumption and the ability to generate static pressure Inlet Vanes actual system resistance curve design system resistance curve inlet vane position static pressure 80 90 100 Ni N N airflow This illustrates how inlet vanes control the fan operating point so that it tracks the desired modulation curve in response to reduced airflow and rising system static pressure By closing the inlet vanes the performance curve for the fan shifts downward The airflow and resulting static pressure fall until the operating point balances along the fan modulation curve B TRG TRC014 EN TRG TRCO14 EN period four Fan Modulation Fan Speed Control supply _
55. ow measuring device on the terminal unit makes this possible The position of the modulation device is not directly controlled and is basically a by product of regulating the airflow through the unit Because the airflow delivered to the space is directly controlled it is independent of inlet static pressure Pressure independent control increases the stability of airflow control and allows minimum and maximum airflow settings to become actual airflows rather than physical positions of the modulation device It is clearly the most popular form of VAV terminal unit control TRG TRCO14 EN 27 S TRANE period two Components of a VAV System Primary Airflow Measurement airflow airflow y i measured j pressure i j difference Accurate measurement of primary airflow is required to enable pressure independent control Most terminal units accomplish this with a multipoint airflow sensor mounted on the inlet This sensor measures a pressure difference between the ports that face the airflow and the ports that face downstream The result is a pressure difference signal that relates to the airflow rate passing through the modulation device Terminal Unit Control Technologies 4 Pneumatic control a Electronic control 4 Direct digital control DDC Now that we understand pressure dependent versus pressure independent control we will look at the technologies available to perform this control Three control technologies are g
56. primary air to one of the modulation devices in the dual duct VAV terminal units through the cooling supply duct system Air delivered by the heating air handler contains only recirculated air no outdoor air This air is heated and delivered as warm primary airflow to the other modulation device in the dual duct VAV terminal units through the heating supply duct system The remainder of the return air is exhausted from the building The cooling air modulation device typically has a minimum setting to accommodate the ventilation requirements of the space 25 S TRANE 26 period two Components of a VAV System VAV Terminal Unit Controls VAV Terminal Unit Controls Next we will explore the control of VAV terminal units Before discussing the commonly available control technologies it is important to first understand a fundamental control concept pressure dependent versus pressure independent control Upstream Pressure Affects Airflow 0 70 in H O 175 Pa 1000 cfm Static pressure VAV terminal unit 0 47 mis in supply duct identical damper positions i _ 0 80 in H O 200 Pa 1300 cfm static pressure 0 61 ms in supply duct The modulating device in a VAV terminal unit changes the air resistance of the supply duct path to the space As terminal units modulate the static pressure within the duct system changes The airflow that passes through the terminal unit depends not only on the position of the modula
57. r remote heating heat source coil In addition to controlling this remote source of heat perimeter baseboard radiation in this example single duct VAV terminal units can directly provide heat to a space This can be accomplished by adding a heating coil to each cooling only unit or by mixing the primary air with warm plenum air before it is delivered to the space 15 S TRANE 16 period two Components of a VAV System VAV Reheat S terminal heating coil supply D air VAV reheat terminal units provide supply air tempering or space heating by reheating the cool primary air This is accomplished by adding an electric or hot water heating coil to the discharge of a cooling only unit The VAV reheat terminal unit is typically used for those zones that require seasonal cooling and heating such as exterior zones of a building VAV Reheat 100 heating coil active Siiman a ie airflow heating maximum minimum heating airflow 1 1 i i 1 1 i 1 1 i 1 i minimum cooling airflow airflow to space design design heating load space load cooling load In the cooling mode the unit is controlled in the same manner as the cooling only unit The supply airflow is reduced as the cooling load in the space decreases When the space cooling load drops below the minimum primary airflow setting for the unit overcooling the space the heating coil warms tempers the primary air to b
58. required space temperature at all load conditions varies the temperature of this air In this example the temperature of the air is varied by controlling the capacity of the central cooling coil TRG TRC014 EN 1 S TRANE period one What Is Variable Air Volume Constant Volume Full Load Supply____ SensibleHeatGain ___ Airflow Constant x Space DB Supply DB Supply _ __ 40 000 Btu hr SA Airflow 1 085 x 75 F 55 F Supply 1 oe 0 87 m s Airflow 1 210 x 23 9 C 12 8 C This equation describes the performance of a constant volume variable temperature system at various load conditions Sensible Heat Gain Constant x Space DB Supply DB Supply Airflow cfm With these design conditions space sensible heat gain 40 000 Btu hr 11 724 W space dry bulb DB temperature 75 F 23 9 C supply air dry bulb DB temperature 55 F 12 8 C constant 1 085 1 210 the system balances the load with 1 840 cfm 0 87 m3 s of 55 F 12 8 C supply air 2 40 000 Btu hr Supply Airflow 7085 x 75 F _ 55 F 75 F 55 F 1 840 cfm 11 724 W Supply Airflow 0 87m S 1 210 x 23 9 C 12 8 C 2 TRG TRCO14 EN S TRANE period one What Is Variable Air Volume Constant Volume Part Load Supply _ Space _ __ Sensible Heat Gain _ DB DB Constant x Supply Airflow Supply _ 75 F _ 20 000 Btu hr 65F DB 1 085 x 1 840 cfm 1 210 x 0 87 m3 s Rearr
59. s typically between one and six minutes An electronic pressure transducer enables pressure independent control of the VAV terminal unit The pressure transducer is pneumatically connected to the airflow sensor for airflow measurement and is wired to the electronic controller along with the space temperature sensor The electronic controller positions the modulation device to deliver the required airflow to the space and operates the fan and heat source Minimum and maximum airflow settings are physically adjusted on the electronic controller A 24 volt power supply is required to power the unit actuator and controls 30 TRG TRCO14 EN TRG TRCO14 EN S TRANE period two Components of a VAV System Direct Digital Control DDC building automation system BAS A digitally controlled terminal unit has an airflow modulation device operated with an electric actuator in the same manner as the electronic control option The key difference between electronic control and direct digital control is that the digital controller uses a microprocessor as the intelligence behind the control of the terminal unit This microprocessor enables digital communication between the unit controller and the central building automation system The result is a system that can be monitored from a central point offers control flexibility and enables system optimized control strategies Buildings can be controlled more intelligently because the data s
60. stem supplies constant primary airflow 47 S TRANE 48 period three System Configurations Any unneeded air is diverted to the return air stream allowing individual comfort control of the spaces At part load conditions when more of the primary air bypasses the space the mixture of previously conditioned primary air and recirculated return air cuts energy use at the cooling and heating equipment This explains the use of the term bypass in the name of this system However due to the fan providing a constant airflow no fan energy savings is realized at part load conditions The term changeover refers to how this system handles the cooling and heating requirements of the building The central air handler can provide either cooled or heated primary air to the space terminal units and it makes this decision by periodically polling the spaces Because it can only provide heating or cooling at a given time this system is most appropriately used for smaller buildings that have a minimal number of incidences where heating is required in some spaces and cooling is simultaneously required in others System Level Control Modes 4 Occupied mode 4 Unoccupied mode 4 Morning warm up cool down mode System Level Control Modes When designing a VAV system for a commercial application three system level control modes need to be addressed Occupied mode m Unoccupied mode E Morning warm up cool down mode TRG TRC0
61. system under part load conditions supply duct system Transports the primary air from the central air handler to the VAV terminal units and then on to the space diffusers system resistance curve Plot of the static pressure drop that the system including the supply ductwork duct fittings terminal units diffusers and return grilles coils filters dampers and so forth creates over a range of airflows unoccupied mode Typical nighttime operating mode of the system The building does not require ventilation because it is not occupied and the space temperatures are controlled to unoccupied set points variable pitch blade control Method of fan modulation used on vaneaxial fans The capacity of the fan is modulated by adjusting the pitch angle of the actual fan blades variable air volume system Type of air conditioning system that varies the volume of constant temperature air supplied to meet the changing load conditions of the space VAV reheat terminal unit Type of single duct VAV terminal unit that can provide space heating using a small heating coil VAV terminal unit Sheet metal assembly installed upstream of space diffusers Its purpose is to vary with the purpose of varying the quantity of air delivered to the conditioned space warm up cool down mode Typical operating mode for transition from the unoccupied mode to the occupied mode It establishes the space occupied comfort conditions as rapidly as possible because they wer
62. t directly controls the terminal fan and heat source Minimum and maximum airflow settings are physically adjusted on the PVR Compressed air is required to operate the modulation device and to power the PVR and the space thermostat 29 S TRANE period two Components of a VAV System Electronic Control 4 Electronic pressure transducer provides pressure independent control 4 Electronic controller positions modulation device controls terminal fan and heat source 4 Minimum and maximum airflow settings adjusted physically on electronic controller 4 Electric power supply operates modulation device and electronic controller An electronically controlled terminal unit has an airflow modulation device operated by an electronic actuator that can drive the modulation device open or closed Electrically actuated modulation devices typically do not have a spring to return them to a normal state if power is lost they stop at the position they held when the power loss occurred The actuator motor is operated with three wires common drive open and drive closed To drive the device open the electronic controller applies 24 volts between the common and drive open wire To drive the device closed it applies 24 volts between the common and drive closed wire To stop the actuator no voltage is applied Actuator drive time is determined by the design of the electric motors and gears but i
63. tails related to duct design but here are a few general recommendations Keep the duct layout as simple and symmetrical as possible Place main duct runs and when possible branch runs and terminal units above hallways and other unoccupied areas to ease installation and maintenance and to help attenuate the sound radiated to the occupied spaces Limit the use of flexible ductwork upstream of the terminal unit Apply a duct lining or duct silencer to the first section of the duct system in order to attenuate supply fan noise Add balancing dampers upstream of diffusers in all noncritical branches If needed reducing transitions should be located several duct diameters upstream of terminal units 39 S TRANE period three System Configurations VAV Systems period three System Configurations To permit a complete understanding of the VAV system we must look at it from a system level In this section we will explore a few common space types and how VAV systems address the cooling and heating needs associated with them Perimeter P Versus Interior For simplicity a typical building can be described as having two types of spaces perimeter and interior In many climates perimeter spaces with walls and windows exposed to the outdoors require seasonal cooling and heating Such spaces require cooling in the summer it is warm outside the sun is shining through the windows people are occupying the space and the
64. tion device but also on the static pressure at the inlet With the modulation device in a fixed position a higher static pressure in the upstream duct will push more air through the terminal unit Because the inlet static pressure constantly changes due to the modulation of the terminal units in the system and the resulting variation of TRG TRC014 EN S TRANE period two Components of a VAV System duct static pressure airflow to the space can vary even when the immediate modulation device does not change position Pressure Dependent Control 4 Space sensor controls position of the modulating device 4 Airflow to space depends on SP in upstream duct system Pressure Independent Control 4 Space sensor controls desired airflow a Airflow to space is controlled directly independent of SP in upstream duct system A pressure dependent VAV control scheme uses the space temperature sensor to directly control the position of the modulating device The actual airflow delivered to the space is a by product of this position and depends on the duct system static pressure at the inlet of the terminal unit Although the space temperature sensor will continually correct the position of the modulating device the response can be sluggish and cause unacceptable temperature variations within the space In contrast a pressure independent VAV control scheme directly controls the actual volume of primary air that flows to the space An airfl
65. uch as airflow damper position fan and heat status is available to perform complex system control strategies and diagnostics from a central monitoring station 31 S TRANE period two Components of a VAV System Direct Digital Control DDC 4 Electronic pressure transducer provides pressure independent control 4 Digital controller positions modulation device controls terminal fan and heat source 4 Minimum and maximum airflow settings adjusted through communication link 4 Electric power supply operates modulation device and electronic controller Similar to electronic control an electronic pressure transducer enables pressure independent control of the VAV terminal unit The digital controller positions the modulation device and operates the fan and the heat source Minimum and maximum airflow settings are adjusted through a communication link to the digital controller A 24 volt power supply is required to power the unit actuator and controls 32 TRG TRCO14 EN TRG TRCO14 EN S TRANE period two Components of a VAV System Diffusers flexible T7 KK j lr A RE sheet metal PA l Y oo di VAV terminal unit diffuser supply duc primary air Diffusers Each VAV terminal unit is commonly connected to a downstream sheet metal duct that is then connected to the remotely located diffusers by flexible ducts Diffusers distribute the supply air effectively to the conditioned space
66. upply air is tempered Overcooling the space causes comfort problems and causes the relative humidity to increase to 74 here To avoid this a method of tempering the supply air is needed This can be done using a terminal heating coil or by mixing in warm air from the plenum or from a warm primary air stream of a dual duct system Realize that if mechanical equipment is used to provide this supply air tempering it must be available not only to handle the space heating requirements but also to handle the tempering requirements during these part load cooling conditions 73 TRANE period five Application Considerations Building Pressure Control Building Pressure Control Maintaining a slightly positive pressure inside buildings especially in humid climates increases comfort and helps avoid indoor air quality IAQ problems related to microbial growth and outdoor contaminant sources Whenever variable quantities of outdoor air are brought into a system building pressurization becomes a concern Resetting the quantity of outdoor air in response to part load conditions or using an outdoor air economizer may lead to undesirable changes in building pressure Varying the quantity of outdoor air requires the system to exhaust a similar quantity of air to avoid overpressurizing or underpressurizing the building Direct Pressurization Control a a outdoor representative static pressure space static pressure There
67. ure in the system There are three common methods for sensing and controlling system static pressure fan outlet static pressure control supply duct static pressure control and optimized static pressure control TRG TRC014 EN period six Review Review Period Five 4 System level ventilation 4 Freeze protection for coils a Part load space humidity control 4 Building pressure control Finally when applying a VAV system to a building there are several considerations that must be addressed They include providing the correct system level ventilation adequately protecting the coils from freezing controlling space humidity over a wide range of loads and controlling building pressure TRANE An American Standard Company For more information refer to the following references E Trane Air Conditioning Manual E Trane product catalogs for VariTrane and VariTrac VAV products Trane literature order numbers VAV PRC002 EN and VAV PRC003 EN m A Guide to Understanding ASHRAE Standard 62 Trane literature order number ISS APG001 EN m ASHRAE Handbook Systems and Equipment TRG TRCO14 EN 79 S TRANE 80 period six Review 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 ADV1 or visit our online bookstore at ww
68. vices in the dual duct VAV terminal units While this single fan configuration requires only one air handler it is very complicated to control efficiently In this example the economizer is controlling the mixed air temperature to 55 F 12 8 C thus saving the energy to operate the cooling coil However the heating coil must warm the air from 55 F 12 8 C to the 105 F 40 6 C primary air temperature If the economizer was not activated the mixed air temperature would be 75 F 23 9 C requiring less heating energy but now the cooling coil must operate Optimizing the energy use of a single fan dual duct system requires a very complicated control system and does not operate as efficiently as a two fan dual duct system Two Fan Dual Duct VAV System cooling air handler dual duct VAV terminal heating units air handler Dual duct systems can be very energy efficient when the mixing of cool and heated air is avoided and two supply fans are used one for heating the other for cooling This arrangement eliminates the inefficient or wasteful reheating of mixed air in a single fan dual duct system and allows an airside economizer to provide cool primary air whenever possible At the cooling air handler a portion of the return air is recirculated and mixed with outdoor air introduced for space ventilation purposes This mixture of outdoor and recirculated return air is then cooled and delivered as cool
69. vor the application of VAV systems TRG TRC014 EN period six Review Review Period Two fan powered VAV terminal unit single duct cooling only VAV terminal unit eta A A simple VAV system is comprised of the following components Central air handler E Supply duct E VAV terminal unit with thermostats and supply diffusers for each independently controlled space Thermostat and unit controller for each terminal unit Return plenum or duct There are various types of VAV terminal units each one unique in its response to changing space loads TRG TRCO14 EN 77 S TRANE 78 period six Review Review Period Three interior spaces perimeter spaces A simple building has two types of spaces perimeter and interior These two space types require different types of VAV terminal units to satisfy their requirements It is important to realize that many VAV systems include a combination of different types of terminal units based on individual space requirements Review Period Four actual system N resistance curve design system resistance curve performance curve Q Q Q static pressure airflow The VAV supply fan capacity is modulated in response to changing building loads Four methods of modulating fan capacity include discharge dampers inlet vanes fan speed control and variable pitch blade control Fan capacity is also controlled based on the static press
70. w at various blade angles versus total pressure static pressure plus velocity pressure The broken line defines the airflow and total pressure range that produces a stall condition for each blade angle Operation in the stall region is characterized by separation of the air at the fan blade surface resulting in instability 62 TRG TRC014 EN TRG TRCO14 EN amp ranw period four Fan Modulation In response to reduced airflow and rising system static pressure the variable pitch vaneaxial fan responds similarly to a centrifugal fan with either inlet vane or variable speed capacity control By changing the angle of the fan blades the performance curve for the fan shifts downward and the airflow and resulting pressure decrease until the operating point B balances along the fan modulation curve Fan Modulation Comparisons 100 BI fan with 90 discharge 80 dampers 5 70 AFfanwith amp 6o inlet vanes S a FC fan with 5 FC fan with inlet vanes 40 discharge se dampers fan speed 30 control 20 variable pitch vaneaxial 10 20 30 40 50 60 70 80 90 100 design airflow These curves describe the performance characteristics of each fan modulation method in terms of the fan power required versus the percent of design airflow Realize that these are generalized curves based on an arbitrary set of test conditions On a generalized basis the forward curved FC centrifugal fan with inlet vanes the variab
71. w trane com bookstore TRG TRC014 EN S TRANE Quiz Questions for Period 1 1 Given a space sensible load of 30 000 Btu hr 8 793 W a space dry bulb temperature of 78 F 25 6 C and a primary air dry bulb temperature of 58 F 14 4 C a What supply airflow is required to balance this load Assume the space sensible load is reduced to 18 000 Btu hr 5 276 W b Fora constant volume system what primary air temperature is required to balance this new load c For a VAV system what supply airflow is required to balance this new load 2 Because the VAV system supplies constant temperature air how is refrigeration energy savings realized at part load 3 List three building characteristics that favor the application of VAV systems in the building Questions for Period 2 4 Which type of single duct VAV terminal unit allows for constant supply airflow to the space 5 Explain why fan powered VAV terminal units can use air from the ceiling plenum to provide the first stage of heating 6 Explain the concept of pressure independent control 7 What is the principle employed by the linear slot diffuser to distribute air within the space Questions for Period 3 8 When the heat loss exceeds 450 Btu hr per linear foot 430 W m per linear meter of exterior wall what type of perimeter heating is generally recommended 9 Is the supply fan in a changeover bypass system constant volume or variable volume Questi
72. xing fan Modulating the airflow to each individual space is accomplished using a temperature controlled mechanical device that varies the airflow resistance in the supply duct to that space The rotating blade damper changes airflow resistance by rotating the damper into the air stream restricting the size of the air passage to the space It is very cost effective and flexible Typically either a pneumatic or electric controller can be used to adjust the damper An understanding of the common VAV terminal unit types is important to understanding VAV systems 13 S TRANE 14 period two Components of a VAV System Single Duct Cooling Only primary vas air B d D supply air The simplest of all VAV terminal units is the single duct cooling only terminal unit It consists of an airflow modulation device with controls packaged in a sheet metal enclosure The unit can only modulate the primary airflow to the space The primary air is supplied by a single central air handler This VAV terminal unit is typically used for those zones that require year round cooling like the interior zones of a building It is the most common and basic type of single duct VAV terminal unit Single Duct Cooling Only maximum 100 remote source of heat gehen R airflow heating 4 tempering I 8 i f I a N 1 2 3 E minimum 5 primary BS airflow 0 design design heating load space load cooling load This graph indic
73. y air is mixed with the minimum amount of cool primary air 23 S TRANE period two Components of a VAV System Constant Air Volume to the Space maximum primary airflow 100 minimum primary airflow airflow to space cool primary air 0 design design heating load space load cooling load To deliver a constant volume of supply air to the space in a two fan dual duct system as the cooling load in the space decreases the amount of cool primary air is reduced and the amount of warm primary air is increased maintaining a constant total supply airflow to the space Because of this constant airflow to the space no fan energy savings is realized at part load conditions Single Fan Dual Duct VAV System cooling coil central air handler OA 40 F 4 4 C heating coil dual duct VAV terminal units ih a 23 9 C Dual duct VAV systems also have a central return air path that allows air from the spaces to come back to the air handler Dual duct systems may however have either one or two central air handlers At the single air handler a portion of the return air is recirculated and mixed with outdoor air This mixture of outdoor and recirculated return air is then diverted through either the cooling coil or the heating coil and delivered down 24 TRG TRCO14 EN TRG TRCO14 EN S TRANE period two Components of a VAV System the respective duct system to the modulation de

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