Trane DX Application Manual
Outdoor air unit
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Summary of Contents for Trane DX
- Page 1 Application Guide Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit SYS-APG001A-EN October 2019...
- Page 2 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.
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Page 3: Table Of Contents
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Page 4: Defining The Dehumidification Challenge
74°F dry bulb and 50% relative humidity delivers no more than 200 to 250 cfm/ton in order to achieve the dew point needed for adequate dehumidification. SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit... -
Page 5: Dedicated Oa System Configurations
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-APG001A-EN... -
Page 6: Comparison Of Different Dedicated Oa System Configurations
• 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 SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit... - Page 7 • 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 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN...
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Page 8: Cold Or Neutral Air
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. - Page 9 In classrooms or offices, however, sensible cooling loads in the zones are relatively high during daytime hours. In fact, for some climates, Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN...
- Page 10 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. SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit...
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Page 11: Designing A Dedicated Oa System
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-APG001A-EN... - Page 12 In this example, combining the 75°F (23.9°C) setpoint for the space with a maximum relative humidity of 50 percent corresponds to a humidity ratio of 64.9 grains/lb (9.3 g/kg) or a dew point of approximately 55°F (13°C). SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit...
- Page 13 Jacksonville, Fla. Table 3 (p. 11) lists the latent load for each space; in this case, the latent loads presumably remain constant whenever the building is occupied. Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN...
- Page 14 , must be 48.0 grains/lb (6.92 g/kg). (∴ W = 6.92g/kg) Table 3 shows the results of this calculation for all four classrooms. Although the highest latent load exists in Classroom 103, the “critical space” is SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit...
- Page 15 The slightly warmer air offsets less of the sensible load in the space, which will affect the selection criteria for the local HVAC terminals. Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN...
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Page 16: Control Of The Dedicated Oa Unit
Heating Enable Setpoint ≤ Outdoor Air Dry-Bulb Temperature ≤ Cooling Enable Setpoint Outdoor Air Dew Point ≤ Dehumidification Enable Setpoint Sensible heating Outdoor Air Dry-Bulb Temperature < Heating Enable Setpoint SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit... -
Page 17: Dehumidification Mode
(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 10. Dehumidification mode sensible cooling dry-bulb temperature, °F... -
Page 18: Sensible Heating Mode
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-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit... -
Page 19: References
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. - Page 20 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. © 2019 Trane All rights reserved...