DOAS Brings Fresh Air Into Urban Buildings.
Today, mechanical HVAC systems handle the job of ventilation, and ANSI/ASHRAE Standard 62.1 developed the ventilation requirements for acceptable indoor air quality. Depending on the type of occupancy, ASHRAE Standard 62.1-2001 called for a minimum of 15 to 20 cfm (7 to 9 L/s) of outdoor air (OA) per person. Standard 62.1-2010 and later standards offer three alternative procedures for determining minimum outdoor airflow rates.
Today's architects get credit for bringing more outdoor air into interior spaces. Under the LEED v4 rating system, for example, supplying 30% more outdoor air to each breathing zone than required by Standard 62.12013 is worth one additional point.
Technology Advances Along With Standards
As standards developed over the last decade, so has technology. In 2001, six ASHRAE technical papers introduced a new HVAC concept, "dedicated outdoor air system (DOAS)," as a new paradigm that ensures delivery of adequate outdoor air.1 DOAS can be defined as a dedicated system that uses separate equipment to condition all OA used for ventilation, whether DOAS introduces the OA directly into zones or through conventional HVAC units. Because it functions as an independent system, DOAS makes it easy to supply the correct amount of outdoor air and verify compliance with Standard 62.1.
Over the last decade, DOAS has evolved to meet the ventilation challenges of urban buildings. New, higher-density commercial buildings demand higher indoor air quality (IAQ) and advanced zoning. Older buildings must be extensively updated to accommodate proper ventilation technology. In both cases, forced ventilation is needed to maintain acceptable air quality, because occupants usually cannot open windows for energy, noise and security reasons.
In contrast, DOAS is now the go-to solution, because it can handle today's ventilation, energy and size/space challenges.
Advantages of a Dedicated OA Design
Enhancing ventilation is the critical function of DOAS. Several studies have shown a correlation between proper ventilation and improved productivity.2,3 Also, architects know the importance of moisture management to prevent or reduce the risk of mold growth. DOAS decreases indoor CO2 concentrations, plus it removes excess humidity and condensation, which reduces the risk of "sick building syndrome." DOAS does not recirculate air, which provides benefits to indoor air quality. Instead, DOAS introduces outdoor air to dilute contaminants rather than spread a concentrated volume throughout the building--thereby implementing the old HVAC proverb: "Dilution is the solution to pollution." Contaminants in outdoor air can be prevented from entering the DOAS by proper design and filtration of fresh air intakes.
Overall system efficiency is also improved by decoupling the sensible and latent loads. It may seem illogical that energy costs would be reduced by adding a dedicated system, but by handling the latent load, DOAS allows cooling components to shut off when the dry-bulb temperature is met. Limiting the need for reheat (as required by ASHRAE/IES Standard 90.1) also saves energy. Energy recovery technology helps reduce heating and cooling loads by transferring energy between the exhaust airstream and outdoor airstreams. Finally, compared to a typical mixed-air HVAC system, DOAS optimizes outdoor air intake, minimizing the total OA heating and cooling loads.
Component sizes and the overall system footprint can be reduced with DOAS. For example, when a DOAS enthalpy exchanger and cooling coil handle the latent load, then terminal units, chillers, air handling units, electrical requirements and more can be downsized. Downsizing will lower the initial cost of the project, as well as cut operating costs down the road.
To realize these advantages, DOAS must be properly designed and configured for the application. DOAS can be integrated into almost any type of HVAC system.
In the most common configuration, DOAS delivers conditioned outdoor air directly to each zone through its own duct system and space diffusers (Figure 1). In this configuration, the DOAS can be sized to handle the outdoor air load and the building's dehumidification load, allowing the sensible load to be handled by either local HVAC units or the central cooling system.
In a second type of configuration, DOAS supplies the intake of local HVAC units (Figure 2). Here, the conditioned air mixes with return air that is delivered to the space through the local unit. The local unit may be installed in a ceiling plenum, closet or roof. This configuration is often used with rooftop units, variable refrigerant flow terminals, fan coils, and active chilled beams.
In a third configuration, DOAS delivers outdoor air to the supply side of local HVAC units (Figure 3). In this configuration, the conditioned air mixes with supply-side air. Only then is the supply air delivered to the zone through diffusers used in common by both the DOAS and local units. The local units only condition the recirculated air. This configuration can use cold outdoor air without reheating to meet the space's temperature setpoint. Consequently, the local HVAC equipment can be downsized, because it is doing less cooling.
The benefits of DOAS technology in an urban environment go beyond simply supplying outdoor air into buildings to comply with standards. DOAS actually solves the size, energy and performance problems of using only traditional HVAC equipment to meet OA requirements. Thanks to industry efforts in the preceding decade, DOAS excels at optimizing ventilation, dehumidification and zoning to improve occupant com fort with a low energy impact.
(1.) Mumma, S.A. 2014. "Tribute to William (Bill) J. Coad, 1931 to 2014." www.doas.psu.edu/Tribute_to_BiILCoad.pdl
(2.) Kumar, S., W.J. Fisk. 2002. "The role of emerging energy-efficient technology in promoting workplace productivity and health: final report." Lawrence Berkeley National Laboratory. http:// escholarship.org/uc/item/0sw558qr.
(3.) Fisk, W.J. 2002. "How IEQ affects health, productivity." ASHRAE Journal (5).
John Tuleya is a product manager, Applied DX product, at Johnson Controls, York, Pa.
Caption: FIGURE 1 DOAS used to supply outdoor air directly to each zone.
Caption: FIGURE 2 DOAS ducted to intakes of local HVAC units.
Caption: FIGURE 3 DOAS directly ducted to supply side of local HVAC units.
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|Title Annotation:||COLUMN: HVAC APPLICATIONS|
|Date:||Mar 1, 2017|
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