Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit
Contents
1. S TRANE Application Guide Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit August 2012 SYS APG001 EN For more information on dedicated outdoor air systems DOAS refer to the following e Dehumidification in HVAC Systems Trane application manual SYS APM004 EN e Dedicated Outdoor Air Equipment Trane Engineers Newsletter Live program DVD APP CMC043 EN e Water Source and Ground Source Heat Pump Systems Trane application manual SYS APMO10 EN e Dedicated Outdoor Air Systems ASHRAE webcast 2012 www ashrae org TRANE Preface As a leading HVAC manufacturer we deem it our responsibility to serve the building industry by regularly disseminating information that promotes the effective application of building comfort systems For that reason we regularly publish educational materials such as this one to share information gathered from laboratory research testing programs and practical experience This guide discusses HVAC systems that use a dedicated outdoor air unit to treat all of the outdoor air brought into the building for ventilation and zone mounted terminal units to treat the indoor air Treating the outdoor air separately from recirculated return air makes it easy to verify sufficient ventilation airflow and enables enforcement of a maximum humidity limit in occupied zones Trane in proposing these system design and application concepts assumes no respon2. TRANE Defining the Dehumidification Challenge Building professionals expend much time and effort to design HVAC systems that handle both ventilation and dehumidification High occupancy spaces such as classrooms pose a particular challenge especially when the system of choice delivers a constant volume mixture of outdoor and recirculated return air Why The answer lies in the fact that the sensible and latent cooling loads on the HVAC equipment do not peak at the same time When it s hot outside the sensible cooling load often far exceeds the latent cooling load Figure 1 By contrast when it s cooler but humid outside the latent cooling load can approach or even exceed the sensible cooling load Constant volume mixed air HVAC equipment traditionally is selected with sufficient cooling capacity to handle the design load at the peak outdoor dry bulb condition and controlled by a thermostat that matches the sensible cooling capacity of the coil with the sensible cooling load in the space Therefore as the sensible cooling load in the space decreases the cooling capacity both sensible and latent provided by the HVAC equipment also decreases In most climates the combination of less latent cooling capacity and a lower SHR sensible heat ratio in the space elevates the indoor humidity level at part load conditions An off the shelf packaged unitary air conditioner may further aggravate this situation Such equipment
3. 50 7 45 40 45 40 ah 35 40 30 O Oo 20 20 OA CA 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 dry bulb temperature F Ventilation only mode If the outdoor air dew point is lower than or equal to the Dehumidification Enable Setpoint and the outdoor air dry bulb temperature is lower than the Cooling Enable Setpoint but warmer than the Heating Enable Setpoint the unit operates in the Ventilation Only Mode In this mode the fan continues to operate but both the compressors and heater are turned off SYS APG001 EN Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 15 TRANE 16 References American Society of Heating Refrigeration and Air Conditioning Engineers Inc ASHRAE 2012 Dedicated Outdoor Air Systems ASHRAE webcast Atlanta GA ASHRAE www ashrae org Trane Murphy J and B Bakkum 2011 Water Source and Ground Source Heat Pump Systems SYS APM010 EN La Crosse WI Trane Murphy J and B Bradley 2002 Dehumidification in HVAC Systems SYS APM004 EN La Crosse WI Trane Murphy J R Moffitt P Solberg and J Harshaw 2011 Dedicated Outdoor Air Equipment APP CMC043 EN DVD Trane Engineers Newsletter Live program La Crosse WI Trane Stanke D and B Bradley 2001 Dedicated Ventilation Systems Engineers Newsletter volume 30 number 3 La Crosse WI Trane Stanke D and B Bradley 2000 Dehumidify with Constant Volume Systems
4. 21 C and 24 C For our example assume that the air is reheated to 71 F 21 7 C If the system design is based on cold conditioned air rather than neutral temperature air then the dry bulb temperature from the dedicated outdoor air unit depends on the supply air dew point In our example assuming that saturated air leaves the cooling coil then the leaving air dry bulb temperature is 52 F 11 1 C Note For simplicity our example does not include the effect of fan heat A draw through fan arrangement will increase the dry bulb temperature of the conditioned outdoor air The slightly warmer air offsets less of the sensible load in the space which will affect the selection criteria for the local HVAC terminals Figure 9 Plotting the humidity ratio to determine the equivalent dew point temperature 86 En 80 nn 60 80 80 A Dr total oe 75 0 peak WB 5 75 o capacity Qr lt a 91 F DB 79 FWB 129 E 2 42 4 Btu lb 2 708 74 F DB 60 RH gon 3 75 2 grains lb a 65 3 space target 808 6 OY a eee Wes gt E y sp o a 2 55 8 AW de z lt 50 gt 45 7 45 40 40 Kells ee 60 576 grains b gave weni 40 ii 40 21 4 Btu lb 56 30 20 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 dry bulb temperature F 12 Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN SYS APG001 EN S TRANE Control of the Dedicated OA Unit The most common approach to controlling the dedicat
5. Engineers Newsletter volume 29 number 4 La Crosse WI Trane Stanke D B Bradway A Hallstrom and N Bailey 1998 Managing Building Moisture SYS AM 15 La Crosse WI Trane Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN TRANE Trane optimizes the performance of homes and buildings around the world A business of Ingersoll Rand the leader in creating and sustaining safe comfortable and energy efficient environments Trane offers a broad portfolio of advanced controls and HVAC systems comprehensive building services and parts For more information visit www Trane com Trane has a policy of continuous product and product data improvement and reserves the right to change design and specifications without notice 2012 Trane All rights reserved SYS APGOO1 EN 3 August 2012 We are committed to using environmentally CED ingersou Rand Supersedes SYS APG001 EN January 2003 conscious print practices that reduce waste
6. HVAC unit The DOAS in Figure 3 also uses a dedicated outdoor air unit to handle the ventilation load Ductwork carries the conditioned outdoor air CA to each local HVAC terminal or air handler typically blower coils horizontal fan coils or water source heat pumps discharging it near or directly into the inlet The conditioned outdoor air then mixes with recirculated return air RA and passes through the cooling coil of the local terminal or air handler which delivers the mixed supply air SA to the space Figure 3 dedicated OA gt outdoor air unit local HVAC unit Ol M IQ M RA SA Advantages Disadvantages e Helps ensure the required amount of outdoor air reaches each local unit because the OA is ducted directly to each intake e Avoids the cost and space needed to install additional ductwork and separate diffusers e Easier to ensure that outdoor air is adequately dispersed throughout the zone because outdoor air is distributed by the local fan e Measurement and balancing is more difficult than if the OA was delivered directly to the zone via separate diffusers e May require a field fabricated plenum or section of duct to connect the outdoor air duct and mix it with recirculated air prior to entering the local HVAC unit e Fans inside the local units must operate continuously to provide ventilation during scheduled occupancy rather than cycling off e If the
7. dedicated OA system operates during unoccupied periods for after hours humidity control or preoccupancy purge for example the fans inside the local units typically must operate also SYS APG001 EN Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 3 Ss TRANE Dedicated OA System Configurations Table 1 Comparison of different dedicated OA system configurations continued Conditioned OA delivered to the supply side of each local HVAC unit The DOAS in Figure 4 delivers the conditioned outdoor air CA directly to the supply side of each local HVAC terminal where it mixes with supply air from the local HVAC terminal before being delivered to the occupied space The local equipment conditions only recirculated air RA Figure 4 dedicated outdoor air unit local HVAC y unit SA RA RA SA Advantages Disadvantages Helps ensure the required amount of outdoor air reaches each unit because the OA is ducted directly to the supply side of each unit Avoids the cost and space needed to install additional ductwork and separate diffusers Affords the opportunity to downsize local units reducing installed cost and energy use if the conditioned outdoor air is delivered at a cold temperature rather than reheated to neutral Easier to ensure that outdoor air is adequately dispersed throughout the zone because outdoor air is distributed by the local fan e Measurement and balancing is more d
8. is designed to operate with a supply airflow to cooling capacity ratio of 350 to 400 cfm ton In hot humid climates offsetting the ventilation load for high occupancy spaces may require that the unit delivers no more than 200 to 250 cfm ton in order to achieve the dew point needed for adequate dehumidification Selecting a packaged unitary air conditioner with enough cooling capacity tonnage to meet the high ventilation load results in excess supply airflow that is more supply airflow than is necessary to meet the sensible cooling load To avoid overcooling the space the air conditioner must deliver the supply air at a warmer dry bulb temperature Unfortunately this reduces the dehumidification capacity of the coil and raises the humidity level in the space especially at part load The right combination of cooling capacity and supply airflow large compressors small fan simply may not exist in packaged air conditioners with prematched refrigeration and air handling components SYS APG001 EN Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 1 TRANE Dedicated OA System Configurations One way to overcome the challenges imposed by a constant volume mixed air system is to design it as a dedicated outdoor air system DOAS The design approach outlined in this guide permits each component of the HVAC system to do what it does best Zone level heating and cooling equipment provides occupants with air circulation and therm
9. this guide are based on 0 4 percent data from the ASHRAE climatic data tables This percentage indicates that the temperature is likely to equal or exceed the design value for 35 hours each year Some design engineers choose to use more extreme conditions others base their designs on the 1 percent or 2 percent values which represent more hours Ss TRANE Designing a Dedicated OA System In most climates the peak latent ventilation load occurs at a lower dry bulb temperature and higher dew point than the outdoor air condition that produces the peak sensible ventilation load The ASHRAE Handbook Fundamentals is a popular source for climatic data representing the outdoor design conditions for many locations To aid the design of cooling and dehumidifying systems the handbook includes Peak dry bulb and mean coincident wet bulb temperatures sensible design condition Peak dew point and mean coincident dry bulb temperatures latent design condition Peak wet bulb and mean coincident dry bulb temperatures enthalpy design condition Table 2 p 10 lists the 0 4 percent cooling design data for Jacksonville Fla Plotting these values on the psychrometric chart Figure 8 p 10 illustrates that the highest outdoor enthalpy exists at the peak wet bulb condition In this case the enthalpy of the outdoor air is 8 percent higher than it is at the peak dry bulb sensible cooling design condition Note Us
10. 1 000 grams kg and 60 min hr equals 0 69 3 0 A different air condition or elevation will result in a different value Step 5 Classroom 101 QL 5 250 Btu hr 0 69 x 450 cfm x 75 2 gr Ib W Weg 58 3 gr lb QL 1 5 kW 3 0x0 21 m3 sx 10 8 g kg W a Y C Weg 8 34 g kg Ss TRANE Designing a Dedicated OA System Step 4 Determine the total airflow that the dedicated outdoor air unit must deliver If a centralized piece of equipment brings in outdoor air and then delivers only outdoor air not mixed with any recirculated air to one or more ventilation zones ASHRAE Standard 62 1 classifies this as a 100 percent outdoor air ventilation system Accordingly per Section 6 2 4 of ASHRAE 62 1 the system level intake airflow Vot delivered by the dedicated OA unit should be the sum of the calculated zone outdoor airflows Voz Vot 2 Voz Given the zone outdoor airflow requirements Voz listed in Table 3 the dedicated OA unit in this example must deliver a total outdoor airflow of 1815 cfm 0 86 m3 s Table 3 Design criteria for a DOAS serving four classrooms in Jacksonville Fla example Space characteristics Classroom 101 Classroom 102 Classroom 103 Classroom 104 sensible load 29 750 Btu hr 26 775 Btu hr 26 927 Btu hr 28 262 Btu hr 8 7 kW 7 8 kW 7 9 kW 8 3 kW Latent load 5 250 Btu hr 5 465 Btu hr 5 697 Btu hr 5 250 Btu hr 1 5 kW 1 6 kW 1 7 kW 1 5 kW Sensi
11. 110 9 dry bulb temperature F Sensible cooling mode If the outdoor air dew point is lower than or equal to the Dehumidification Enable Setpoint and the outdoor air dry bulb temperature is higher than the Cooling Enable Setpoint the unit operates in the Sensible Cooling Mode Figure 12 In this mode compressor capacity is staged modulated to cool the outdoor air OA to the desired leaving air dry bulb temperature CA Figure 12 Sensible cooling mode 85 180 160 80 80 140 _ 75 Z _75 lt 3 2 ge 120 2 amp 70 5 70 8 A 2 US 100 3 Se 65 e 65 Rs 3 3 So 80 2 60 5 5 E 55 S 55 8 50 OS Bo 3 lt 45 O o E Boe 40 CA OA 40 40 35 30 20 30 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 dry bulb temperature F 14 Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN Ss TRANE Control of the Dedicated OA Unit Sensible heating mode If the outdoor air dew point is lower than or equal to the Dehumidification Enable Setpoint and the outdoor air dry bulb temperature is lower than the Heating Enable Setpoint the unit operates in the Sensible Heating Mode Figure 13 In this mode heater capacity is staged modulated to warm the outdoor air OA to the desired leaving air dry bulb temperature CA Figure 13 Sensible heating mode 85 180 160 80 80 140 gt 75 i 75 lt 3 2 ge 120 2 2 7 E 70 3 70 8 Sos 100 z oo 80 2 00 3 5 E 55 g 55 0 60 o a 50 lt
12. 5 70 75 80 85 90 95 100 105 110 dry bulb temperature F In some cases the local HVAC terminals may also help to dehumidify the space when the sensible cooling load is high yielding an indoor humidity that is drier than the maximum upper limit As a rule of thumb size the dedicated outdoor air unit so that it offsets both the ventilation load and the space latent loads at the peak outdoor enthalpy condition Selecting the Dedicated OA Unit The following steps establish the required airflow dew point and dry bulb temperature for the conditioned air Step 1 Determine the entering air condition Three factors dictate the capacity required from the dedicated outdoor air unit airflow the enthalpy of the entering outdoor air and the enthalpy of the conditioned air leaving the cooling coil If the outdoor airflow is constant then the basis of design is the condition resulting in the greatest difference in enthalpy across the cooling coil Indoor latent loads fluctuate with occupancy and processes as well as with ambient conditions and wind through infiltration These variables can make it difficult to discover when the greatest enthalpy difference occurs However if the latent loads within the space are relatively constant and infiltration is minimal assume that the greatest enthalpy difference occurs at the highest outdoor air enthalpy Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN The examples throughout
13. E ae je is 2 80 amp AL peak WB A S du 7 6 24 Bulb 40 5 N peakDP a 3 a amp 30 amp Z BEER f 2 2 amp 7 15 Bul 7 05 E X 2 70 j rn Te L 3 70 9 S E TPA ve E o nn 100 8 25 y 5 4 A peak DB gt UE El Re Ve pfisss Btullb gt Mee g 9 L 2 20 60 space if EN E 7 2 55 8 a gt lt Bso g Bas 40 30 20 0 30 35 40 45 50 55 60 65 70 75 80 85 90 95 dry bulb temperature F 100 105 110 Step 3 Determine the latent loads in the space The dedicated outdoor air unit will offset the local latent loads in the space it serves as well as the total ventilation load Common sources of latent load include respiration from people processes such as cooking and the infiltration of humid outdoor air through cracks and other openings in the building structure For this example the dedicated outdoor air handler serves four classrooms of a school in Jacksonville Fla Table 3 p 11 lists the latent load for each space in this case the latent loads presumedly remain constant whenever the building is occupied 10 Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN In the Q equations at right 0 69 and 3 0 are derived from the properties of air they are not constants At the standard air condition which is 69 F 21 C dry air at sea level the product of density the latent heat of water vapor and a conversion factor for units 7 000 grains lb
14. ads However when a chilled water or DX cooling coil is used for dehumidification a by product of that process is that the dry bulb temperature of the air leaving the coil is colder than the zone Figure 6 If the dehumidified outdoor air DH is reheated to neutral CA most of the sensible cooling performed by the dedicated OA unit is wasted If the dedicated OA system delivers air directly to each zone see Figure 2 p 3 or to the supply side of each local HVAC unit see Figure 4 p 4 the dehumidified outdoor air DH can be delivered cold rather than reheated to neutral The low dry bulb temperature of the conditioned OA offsets part of the sensible cooling load in the zone reducing the energy used by the local unit At design conditions this means that the local unit can be sized for less airflow and less cooling capacity than in a neutral air system Figure 6 Sensible cooling is a by product of cold coil dehumidification 80 sensible cooling 60 40 oo O 2 1 75 m m z 2 5 20 amp 1 E i E i 8 coil curves 3 E 00 5 1 3 Q I 1 3 I E 80 g 5 155 y 3 9 50 e Y pS eS SE 5 DH CA EN a 40 ON 40 ae 35 60 30 40 20 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 dry bulb temperature F SYS APG001 EN Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 5 Ss TRANE Dedicated OA System Configurations Compared to a neutral air
15. al comfort by modulating the cooling coil capacity to match the sensible cooling load in the space Any local latent cooling occurs coincidentally the latent cooling load does not affect the selection of zone level HVAC equipment Meanwhile a central dedicated outdoor air unit sufficiently dehumidifies and tempers the outdoor air to meet both the latent cooling load and the ventilation requirements for all spaces served by the system Dividing the building s cooling load in this fashion can make it easier to effectively ventilate and dehumidify occupied spaces Key concepts to remember when undertaking such a design include the following Always provide conditioned air that is drier than the air in the space This practice minimizes the cooling capacity required from the local HVAC terminals and adequately controls the indoor humidity without additional zone level dehumidification enhancements Deliver cold conditioned air whenever possible and use recovered energy to reheat during mild weather Providing cold conditioned air from the DOAS minimizes the cooling loads at the local HVAC terminals During mild weather spring and fall modulate the amount of recovered energy used by the DOAS for reheat only warm the conditioned air enough to minimize inefficient reheat at the local HVAC terminals Neutral temperature conditioned air which has a dry bulb temperature approximating that of the air in the space increase
16. ble heat ratio SHR 0 85 0 83 0 83 0 84 Required outdoor airflow 450 cfm 450 cfm 480 cfm 435 cfm q 0 21 m3 s 0 21 m3 s 0 23 m3 s 0 20 m3 s Humidity ratio of 58 3 grains Ib 57 6 grains lb 58 0 grains Ib 57 7 grains lb conditioned air Wca 8 34 g kg 8 24 g kg 8 29 g kg 8 25 g kg Step 5 Determine which zone requires the driest conditioned outdoor air Because the dedicated outdoor air unit will offset the latent loads in each space as well as the total ventilation load the conditioned outdoor air must be dry enough to enforce the maximum humidity limit in the worst case space Use the following equation to calculate the required conditioned air humidity ratio Wea for each space Q 0 69 x Vaa x Wsp Wea QL 3 0 x Vaa X Wsp Wea 1 where Or latent load in the space Btu hr kW Voa conditioned outdoor airflow cfm m s which is supplied to the space by the dedicated outdoor air handler Wea humidity ratio of the conditioned outdoor air grains lb grams kg Wsp maximum limit for the humidity ratio in the space grains lb grams kg For example to assure that the humidity Wsp in Classroom 101 does not exceed the maximum limit of 75 2 grains lb 10 8 g kg the humidity ratio of the conditioned outdoor air Wea must be 58 3 grains lb 8 34 g kg Table 3 shows the results of this calculation for all four classrooms Although the highest latent load exists in Classroom 103 the critical spa
17. ce is SYS APG001 EN Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 11 S TRANE Designing a Dedicated OA System Classroom 102 because it requires the driest air lowest humidity ratio Wea Supplying the conditioned outdoor air at a humidity ratio of 57 6 gr lb 8 24 g kg will offset the latent load in each classroom and assure that the humidity in Classroom 102 does not exceed the maximum limit lower humidities will result in the other classrooms Step 6 Determine the required dew point for the conditioned Step 6 outdoor air With the help of a psychrometric chart Figure 9 we find that a Qy 4 5 x 1 815 cfmx 42 4 21 4 Btu lb humidity ratio of 57 6 grains lb 8 24 g kg is equivalent to a dew point 171 518 Btu hr 14 3 tons temperature of 52 F 11 1 C Qq 1 2x 0 86 m3 s x 98 6 49 8 kJ kg Dehumidifying 1 815 cfm 0 86 m s of outdoor air from the peak wet bulb 50 4 kW condition to a 52 F 11 1 C dew point requires 14 3 tons 50 4 kW of cooling capacity Incidentally lowering the maximum humidity limit to 50 percent would require 16 9 tons 59 4 kW an 18 percent increase in capacity Step 7 Determine the supply air dry bulb temperature for the dedicated outdoor air handler If the system design requires neutral temperature conditioned air then the air leaving the dedicated outdoor air unit must be reheated to the desired dry bulb temperature This is typically between 70 F and 75 F
18. d earlier delivering the conditioned OA at a dry bulb temperature colder than the zone temperature offsets part of the sensible cooling load in the zone As the zone sensible cooling load decreases due to changes in outdoor conditions solar heat gain and or internal loads it is possible that the cold conditioned OA may provide more sensible cooling than the zone requires As a result the temperature in the zone begins to drop At these conditions depending on the type of local HVAC equipment being used it may be desirable to heat or reheat the outdoor air before delivering it directly to the zones For many applications another approach to avoid overcooling is to implement demand controlled ventilation This control strategy reduces the quantity of outdoor air delivered to a zone when there are fewer people in that zone This often avoids overcooling altogether and reduces the energy used to condition and deliver that air In applications where zone sensible cooling loads differ greatly at any given time In hotel guest rooms or dormitories the sensible cooling loads can be drastically different from zone to zone The result is that if the conditioned OA is delivered cold it may be more likely that some zones will experience overcooling For these applications it may be simpler to deliver the conditioned OA at a neutral dry bulb temperature because the benefit of delivering the air cold occurs less frequently In classrooms or o
19. ed OA system is to turn it on when the building is expected to be occupied The same time of day schedule that is used to start and stop the local HVAC terminal equipment is used to start and stop the dedicated OA system The fan in the dedicated OA unit is activated to bring in the required amount of outdoor air for ventilation and cooling dehumidification or heating is modulated to maintain the discharge air at the desired conditions With OA control the operating mode for the dedicated OA unit is based on the current outdoor air conditions Outdoor temperature and humidity sensors are used to calculate the outdoor air dew point and compare it to the desired leaving air conditions This determines whether the unit operates in Dehumidification Mode Sensible Cooling Mode Sensible Heating Mode or if conditions are sufficient for the unit to operate in Ventilation Only mode Figure 10 and Table 4 Figure 10 Dedicated OA unit control modes OA control 85 80 60 80 80 40 gt 75 a K 5 75 a Pr BAHR A 20 amp 2 7 dehumidification mode Z PE E El 00 y 65 e 65 3 Po 3 80 Z 60 dehumidification enable setpoint 55 a Se EER le Ben serpent Dm Pp lain IE 40 m 45 os UN W OER 40 RSE a alt mode 20 T 4 gt j i 0 30 70 75 80 95 100 105 110 dry bulb temperature F Table 4 Dedicated OA unit control modes OA control Control
20. ffices however sensible cooling loads in the zones are relatively high during daytime hours In fact for some climates 6 Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN SYS APG001 EN Ss TRANE Dedicated OA System Configurations classrooms may never reach the point when overcooling occurs during occupied hours especially if demand controlled ventilation is used to reduce outdoor airflow when zone population decreases These applications are typically well suited for delivering the conditioned OA at a cold temperature In applications that require lower than normal dew points If an application has very high indoor latent loads or requires a lower than normal dew point the outdoor air may need to be dehumidified to a very low dew point In this case the corresponding dry bulb temperature of the air leaving the cooling coil may be colder than the HVAC design engineer is willing to discharge directly into an occupied zone below 45 F 7 C for example In this case the dehumidified OA could be reheated to a more traditional supply air temperature 55 F 13 C for example but not reheated all the way to neutral To avoid condensation when conditioned OA is delivered to the ceiling plenum In some applications the dedicated OA system delivers the conditioned outdoor air CA to the ceiling plenum near the intake of each local HVAC terminal see Figure 5 p 4 The outdoor air mixes with recirculated a
21. ide variety of local equipment including water source heat pumps vertical or horizontal fan coils unit ventilators DX direct expansion rooftop units split systems blower coils through the wall air conditioners PTACs variable refrigerant flow VRF terminals passive chilled beams and radiant cooling surfaces Advantages Figure 2 OA dedicated c Y outdoor air unit I pp E f CA y y CA EA Sa A f gt local HVAC unit RA i i Disadvantages Makes it easier to ensure the required amount of outdoor air reaches each zone because separate ventilation diffusers allow easy airflow measurement and balancing Affords opportunity to cycle off the fan inside the local unit reducing fan energy use because outdoor air is not distributed to the zone by the local fan Allows the dedicated OA system to operate during unoccupied periods for after hours humidity control or preoccupancy purge for example without needing to operate the fans inside the local units Affords the opportunity to downsize local heat pumps reducing installed cost and energy use if the conditioned outdoor air is delivered at a cold temperature rather than reheated to neutral e Requires installation of additional ductwork and separate diffusers e May require multiple diffusers to ensure that outdoor air is adequately dispersed throughout the zone Conditioned OA delivered to the intake of each local
22. ifficult than if the OA was delivered directly to the zone via separate diffusers e Fans inside the local units typically must operate continuously to provide ventilation during scheduled occupancy rather than cycling off unless a pressure independent VAV terminal is used to maintain outdoor airflow Conditioned OA delivered to the open ceiling plenum near each local HVAC unit The DOAS in Figure 5 delivers the conditioned outdoor air CA to the ceiling plenum near the intake of each local HVAC terminal The outdoor air mixes with recirculated air RA in the plenum before being drawn in through the intake of the unit The local unit conditions this mixture of outdoor and recirculated air and delivers it to the occupied space through a shared duct system and diffusers Figure 5 dividing wall LA return air inlet to ceiling plenum Source ASHRAE 62 1 2010 User s Manual Figure 5 D OAmerican Society of Heating Refrigerating and Air Conditioning Engineers Inc www ashrae org Advantages Disadvantages e Avoids the cost and space needed to install additional ductwork separate diffusers or field fabricated mixing plenums e More difficult to ensure the required amount of outdoor air reaches each unit since the OA is not ducted directly to each local unit refer to the ASHRAE 62 1 User s Manual for further guidance Conditioned outdoor air may not be able to be delivered at a cold temperature due to c
23. ing the peak dry bulb condition as the basis of design will undersize the dedicated outdoor air unit making it unable to properly dehumidify the outdoor air at certain part load conditions Remember that the primary purpose of the dedicated outdoor air system is to properly control space humidity at all load conditions Step 2 Choose the maximum limit for space humidity The leaving air dew point is determined so that the space humidity level does not exceed some defined upper limit at worst case conditions Some design engineers might choose 50 percent relative humidity RH for the upper limit others might choose to design the system to allow the humidity to rise a little higher e g 60 percent RH at worst case conditions knowing that indoor humidity would then be lower at less extreme conditions Whatever upper limit is chosen by the design team realize that designing the system to maintain a lower indoor humidity level will require larger dedicated OA equipment and increase overall system energy use Note Some types of local cooling equipment such as chilled beams or radiant cooling panels cannot handle any condensation If this type of equipment is used the outdoor air must be dehumidified to a dew point low enough to remove all of the space latent load plus some margin of safety to prevent condensation from forming on the local equipment As an example the upper humidity limit might be 55 F 13 C dew point This allows water a
24. ir RA in the plenum before being drawn in through the intake of the local unit In this configuration the dedicated OA unit should reheat the dehumidified OA to a dry bulb temperature that is above the expected dew point temperature of the air within the ceiling plenum If cold air is dumped into the ceiling plenum it could cool surfaces structural beams electrical conduit ceiling framework At night when the dedicated OA unit is off wind or operating exhaust fans may cause humid outdoor air to leak into the plenum which may lead to condensation on these cold surfaces Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 7 S TRANE Designing a Dedicated OA System In most applications in most climates the dedicated OA unit is sized to dehumidify the outdoor air to remove the moisture or latent load from the entering outdoor air and is often then dehumidified a little further In this case the resulting dew point of the conditioned air is drier than the space dry enough that this quantity of outdoor air also removes most or all of the space latent loads Figure 7 Figure 7 Sizing the dedicated OA unit to offset space latent loads 85 80 60 80 80 40 75 5 75 SS 2 2 OA 20 E 3 a 70 2 00 z gt Q gt 5 65 3 s 60 80 2 space 2p 3 55 dry enough to remove 2 55 3 space latent load 60 a e 50 TEE RM 50 7 45 CA 40 5 245 40 35 40 30 30 20 20 0 30 35 40 45 50 55 60 6
25. mode Outdoor conditions Dehumidification Outdoor Air Dew Point gt Dehumidification Enable Setpoint Outdoor Air Dew Point lt Dehumidification Enable Setpoint Sensible coolly Outdoor Air Dry Bulb Temperature gt Cooling Enable Setpoint Outdoor Air Dew Point lt Dehumidification Enable Setpoint Ventilation only Heating Enable Setpoint lt Outdoor Air Dry Bulb Temperature lt Cooling Enable Setpoint Outdoor Air Dew Point lt Dehumidification Enable Setpoint sensible heating Outdoor Air Dry Bulb Temperature lt Heating Enable Setpoint Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 13 Ss TRANE Control of the Dedicated OA Unit Dehumidification mode If the outdoor air dew point is higher than the Dehumidification Enable Setpoint the unit operates in the Dehumidification Mode Figure 11 In this mode compressor capacity is staged modulated to dehumidify the outdoor air OA to the desired leaving air dew point DH Depending on the application this dehumidified outdoor air may then be reheated using heat recovered from the DX refrigeration circuit i e hot gas reheat to the desired leaving air dry bulb temperature CA Figure 11 Dehumidification mode 85 180 160 80 80 140 _ Z 75 2 2 120 amp OA 2 3 70 8 gt 2 100 3 Z Q ag 2 80 2 60 5 g 2 60 a oe oe 50 2 50 7 N 45 a gt Mi 40 35 40 30 20 20 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105
26. oncerns over condensation within the ceiling plenum rather it must typically be reheated closer to a neutral temperature Fans inside the local units must operate continuously to provide ventilation during scheduled occupancy rather than cycling off If the dedicated OA system operates during unoccupied periods for after hours humidity control or preoccupancy purge for example the fans inside the local units typically must operate also 4 Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN Ss TRANE Dedicated OA System Configurations Cold or Neutral Air Regardless of where the conditioned outdoor air is delivered the dedicated OA unit should dehumidify the outdoor air so that it is drier than the zone This offsets the latent load associated with ventilation and if the dew point temperature of the conditioned outdoor air is lower than the dew point in the zone Figure 6 also offsets some or all of the zone latent loads This approach can adequately limit indoor humidity levels at both full and part load conditions without the need for additional dehumidification enhancements in the local HVAC equipment Many dedicated OA systems are designed to dehumidify the outdoor air and then reheat it to approximately zone temperature neutral Delivering the dehumidified outdoor air at a neutral dry bulb temperature can simplify control because it has no impact on the zone sensible cooling or heating lo
27. s the cooling capacity required from the local HVAC terminals and requires more reheat at the dedicated outdoor air unit Deliver the conditioned outdoor air directly to each occupied space whenever possible This helps ensure that the required amount of outdoor airflow reaches each occupied space allows the conditioned OA to be delivered at a cold temperature rather than reheated to neutral simplifies the application of demand controlled ventilation when desired and allows the fans in the local HVAC equipment to cycle off without affecting ventilation performance Dedicated outdoor air systems can be designed to deliver conditioned outdoor air either directly to each occupied space or to the individual HVAC terminals or air handlers serving those spaces Evaluate the advantages and disadvantages of each configuration when designing a DOAS application Table 1 summarizes the advantages and drawbacks of each configuration Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN Table 1 Ss TRANE Dedicated OA System Configurations Comparison of different dedicated OA system configurations Conditioned OA delivered directly to each space The DOAS in Figure 2 consists of a dedicated outdoor air unit which delivers conditioned outdoor air CA to each occupied space via separate ductwork and diffusers The local HVAC equipment conditions only recirculated air RA This configuration accommodates a w
28. sibility for the performance or desirability of any resulting system design Design of the HVAC system is the prerogative and responsibility of the engineering professional Trademarks Trane and the Trane logo are trademarks of Trane in the United States and other countries All trademarks referenced in this document are the trademarks of their respective owners 2012 Trane All rights reserved Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS APG001 EN S TRANE Table of Contents Defining the Dehumidification Challenge Dedicated OA System Configurations Comparison of Different Dedicated OA System Configurations Cold or Neutral Air neen Designing a Dedicated OA System Selecting the Dedicated OA Unit Control of the Dedicated OA Unit Dehumidification mode venen Sensible cooling mode 000 eee en Sensible heating mode en en Ventilation only mode eee References eeen Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit ii is Es 5 SYS APG001 EN Figure 1 Cooling loads at different outdoor conditions 40 30 20 cooling coil load MBh sensible latent sensible latent peak dry bulb peak dew point outdoor condition Based on an example classroom which is located in Jacksonville Fla and has a target space condition of 74 F dry bulb and 50 relative humidity
29. system a dedicated OA system that delivers cold air directly to each zone or to the supply side of each local HVAC unit Requires less overall cooling capacity The required capacity of the dedicated OA unit is the same for both configurations but the required cooling capacity of each local unit is less in a cold air system than in a neutral air system Requires less overall cooling energy for much of the year By taking advantage of the sensible cooling already done by the dedicated OA unit the cold air system requires less cooling energy at each local unit The neutral air system throws away this sensible cooling benefit by reheating the air to approximately zone temperature Requires less overall fan airflow and therefore less fan energy The airflow delivered by the dedicated OA unit is the same for both configurations but for those zones that require seasonal cooling and heating the supply airflow delivered by the local HVAC unit is less in a cold air system than in a neutral air system For zones that require year round cooling the local HVAC equipment may not be able to be downsized as much since it may need to be sized based on the warmest temperature expected to be delivered by the dedicated OA unit While the conditioned outdoor air should be delivered cold whenever possible there are situations when the dedicated OA unit should reheat the dehumidified outdoor air To avoid overcooling at part load conditions As explaine
30. t about 58 F 14 C to be sent to the chilled beams or radiant panels without condensation In this example combining the 74 F 23 3 C setpoint for the space with a maximum relative humidity of 60 percent corresponds to a humidity ratio of 75 2 grains lb 10 8 g kg or a dew point of approximately 59 F 15 C SYS APG001 EN Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit 9 Ss TRANE Designing a Dedicated OA System Managing Building Moisture Trane applications engineering manual SYS AM 15 helps designers identify and quantify moisture sources It also presents moisture management techniques for the building envelope occupied spaces and mechanical equipment rooms Table 2 Design weather conditions for cooling dehumidifying in Jacksonville Fla a Design condition Enthalpy BIER 96 F 35 7 C DB 39 3 Btu lb Peak dry bulb mean coincident wet bulb 76 F 24 5 C WB 91 4 kJ kg BE 76 F 24 6 C DP 41 5 Btu lb Peak dew point mean coincident dry bulb 84 F 28 8 C DB 96 5 kJ kg 79 F 26 1 C WB 42 4 Btu lb Peak wet bulb mean coincident dry bulb 91 F 32 8 C DB 98 6 kJ kg a Source 2001 ASHRAE Handbook Fundamentals Chapter 27 Table 1B 0 4 condition Figure 8 Comparison of outdoor air enthalpies at 0 4 cooling design conditions for Jacksonville Fla 55 50 42 e 4 85 ES Je 80 E a Ve A ii o f 2 PS HK AN A RG 40 AMA lp z 60 F
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