ASHRAE Journal - August 2011 - 76

iaq applications

Doas Misconceptions
By stanley a. Mumma, ph.D., p.E., Fellow/Life Member ASHRAE

Proper Control Key to System Economics

edicated outdoor air systems (DOAS) have many favorable indoor environmental quality (IEQ) attributes such as good ventilation per-

formance and decoupled space humidity control. But, two design/operation misconceptions exist that may be barriers to its adoption. Total energy recovery (TER) control and supply air (SA) thermal conditions waste energy, destroying a DOAS’s economics, and preventing its wider application.

DOAS Defined
A DOAS is generally designed to provide at least the minimum 100% outdoor air (OA) to each individual occupied space (as specified by ASHRAE Standard 62.1-2010). In addition, the OA is cooled and dehumidified, generally mechanically with the aid of TER, to accommodate the entire space latent loads. In all but very high occupancy density rooms, space dry-bulb temperature (DBT) control is achieved with parallel terminal equipment selected to meet the design sensible loads not met by the DOAS supply air.

Enhanced IEQ with a DOAS
DOAS can help provide excellent IEQ. Some of the best enhancements to IEQ are discussed briefly here. Ensured ventilation. A DOAS, by virtue of its 100% OA design (i.e., no recirculated air), can ensure proper space ventilation even in multi-space applications. On the contrary, as a general rule multizone recirculating systems (all-air systems) are challenged in this area, and virtually always require overventilation of some spaces to meet the ventilation requirements of critical spaces. It is recom76 ASHRAE Journal

mended that a DOAS be equipped with demand controlled ventilation (DCV) when serving highly variable occupancy situations, which reduces summer cooling energy use and essentially eliminates the use of terminal reheat while still providing the ensured ventilation. Decoupled humidity control from sensible control. A DOAS, by virtue of its ability to provide latent load control independent of temperature control, can provide excellent energy efficient humidity control—a key part of acceptable IEQ. Minimize the spread of odors, infections, microbes, and other contaminants between conditioned spaces. Since a DOAS uses no recirculated air, contaminant transfer between conditioned spaces is minimized, which is an especially important advantage in school applications and other public places of assembly. On the contrary, all-air systems, often supplying 80% recirculated air, can unfortunately often be efficient contaminant transport systems, even when fitted with particle filters. Enhanced isolation from unexpected outdoor events. A DOAS with parallel terminal space heating/cooling equipment has a relatively low air change rate per
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hour due to low SA flow rates when compared to all-air systems. As a result they have been correctly identified as slow to dilute contaminants that get into a space. However, if unexpected outdoor events are a concern, and the agent of concern is known, a much higher efficiency DOAS filter can be used compared to that for an all-air system, greatly reducing the comparable exposure. This is achieved with a DOAS at lower first and operating cost compared to all-air systems. More on this topic is at http://doas-radiant.psu.edu/ Mumma_Filter_Paper_LO_09_032_Summer_Mtg.pdf. Building pressurization. Pressurization is an important design requirement for enhanced IEQ. With a properly designed DOAS, this requirement can be met cost effectively even when the building is unoccupied. The feature also makes it possible to provide reserve latent capacity very cost effectively. More on this topic is at http://doas-radiant. psu.edu/DOAS_Pressurization_Paper. pdf. Most terminal equipment choices provide excellent thermal comfort. A wide range of sensible terminal equipment is available to complement a DOAS (fan coil units, ceiling radiant cooling panels, chilled beams, and various types of unitary equipment). Because a DOAS removes both the space and OA latent loads, the terminal equipment need only provide sensible cooling. This avoids standing water in the terminal equipment condensate pans that could otherwise serve as septic amplifiers.

total Energy Recovery Requirement
A key component in a DOAS is total energy recovery (TER—frequently an enthalpy wheel [EW]), a requirement for
August 2011

ASHRAE Journal - August 2011

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