Application Guide Dedicated Outdoor Air Systems Trane DX Outdoor Air Unit August 2012 SYS-APG001-EN
Preface For more information on dedicated outdoor air systems (DOAS), refer to the following: • Dehumidification in HVAC Systems, Trane application manual (SYS-APM004-EN) • “Dedicated Outdoor Air Equipment,” Trane Engineers Newsletter Live program (DVD; APP-CMC043-EN) • Water-Source and Ground-Source Heat Pump Systems, Trane application manual (SYS-APM010-EN) • “Dedicated Outdoor Air Systems” ASHRAE webcast, 2012 (www.ashrae.
Table of Contents Defining the Dehumidification Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Dedicated OA System Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Comparison of Different Dedicated OA System Configurations . . . . . . . . . . 3 Cold or Neutral Air? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Designing a Dedicated OA System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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. Figure 1.
Dedicated OA System Configurations One way to overcome the challenges imposed by a constant-volume, mixedair 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 thermal comfort by modulating the cooling-coil capacity to match the sensible-cooling load in the space.
Dedicated OA System Configurations Table 1. 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).
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 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.
Dedicated OA System Configurations Compared to a neutral-air 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.
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.
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.
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: The examples throughout this guide are based on “0.
Designing a Dedicated OA System Table 2. Design weather conditions for cooling/dehumidifying in Jacksonville, Fla. (a) Design condition Enthalpy Peak dry bulb, mean-coincident wet bulb 96°F (35.7°C) DB, 76°F (24.5°C) WB 39.3 Btu/lb (91.4 kJ/kg) Peak dew point, mean-coincident dry bulb 76°F (24.6°C) DP, 84°F (28.8°C) DB 41.5 Btu/lb (96.5 kJ/kg) Peak wet bulb, mean-coincident dry bulb 79°F (26.1°C) WB, 91°F (32.8°C) DB 42.4 Btu/lb (98.
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 “100percent outdoor air ventilation system.” Accordingly, per Section 6.2.4 of ASHRAE 62.
Designing a Dedicated OA System Classroom 102 because it requires the driest air (lowest humidity ratio, Wca ). 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 outdoor air.
Control of the Dedicated OA Unit The most common approach to controlling the dedicated 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.
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.
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.
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.
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