DOAS Options

Dedicated Outdoor Air System (DOAS)

As promised in my previous post about DOAS, more information shall be shared in this post. As some sort of review, a simple schematic diagram is given here which will aid us in the foregoing discussions.

Design Options

When designing a dedicated outdoor air system an engineer has to consider two basic options. He can opt for the outdoor air unit to supply air at the same temperature as the inside design temperature or at the temperature required to assure dehumidification of the air.

As a simplified guide the matrix below illustrates how to best design a DOAS:

Neutral air to the space	Cold air to the space
Neutral air to the parallel system	Cold air to the parallel system

Neutral Air

This option requires that the outside air be reheated because the outside air temperature will be reduced below the outside air dewpoint to dry the air. Another challenge is to make use of available sources of heat that should be utilized to avoid having an impact on the cost of the system. As an alternative the design engineer could require that the DOAS include hot gas reheat coils/controls that essentially capture rejected heat from the condenser side of the DOAS unit. The advantage of a "neutral air" concept is that it is easy to mix it with the occupied zone air. The outside air can then be ducted independently from a parallel sensible cooling system. Unfortunately neutral air does not provide any cooling assistance to the parallel system. Although the neutral air DOAS uses energy to lower the ventilation outside air temperature to a point that also provides sensible cooling, the neutral air concept “gives that energy back” by reheating the air to a temperature that does nothing to offset any building sensible cooling load. The parallel system cooling capacity will remain unchanged as will the parallel system air delivery requirements.

Cold Air

A cold air system will not require a reheat load since the outside air temperature will be reduced below the outside air dewpoint to dry the air and then will be distributed in that condition. The result is that the load of the parallel sensible cooling system will be reduced.

But the big challenge is to prevent a draft effect when this cold air is introduced to the warmer zone air. (More on this to be discussed in succeeding posts).

Conclusion

For both the neutral air and cold air system, the economics should be evaluated properly to justify which system is viable to use for certain applications. But in all, the requirements of ASHRAE 62.1 should always be taken as prime consideration.

Next of this series on DOAS is the actual design calculations. Please be sure to subscribe.