Conditioning outside air in a humid climate: proper dehumidification and control of outside air is a tough task in stickier climates. Deploying a DOAS is often a good first step toward effective ventilation in these situations. Consider the benefits, the LEED advantages, and how to clear potential hurdles like effective reheat to achieve neutral temperature air

Engineered Systems, Oct, 2005 by John E. Bergman

NEUTRAL TEMPERATURE AIR

A DOAS should not be confused with an air conditioner. There usually is an air conditioner for the space, which is used to offset the sensible-only load and is controlled by a drybulb thermostat.

A DOAS is an outside air conditioner that conditions the outside air only to offset the latent load and is controlled by a separate humidistat or dewpoint control. Thus, it is ideal that the DOAS provide the necessary dry air but at a drybulb temperature close to the design drybulb temperature, or neutral air. This requires reheat, which, if not done correctly, is socially unacceptable, an energy hog, and counter to most codes.

Two acceptable methods of reheat are refrigerant reheat and the use of a precool-reheat heat exchanger. Refrigerant reheat utilizes the heat rejected by the compressor performing the dehumidification duty and, thus, requires no outside energy source. The precool/reheat heat exchanger is a heat exchanger in the DOAS that transfers the sensible heat in the outside air from the entering side of the cooling/dehumidification coil to the leaving side. It can add 15 [degrees] to 20 [degrees] to the coil leaving air temperature and thus deliver neutral temperature without an additional energy source.

ACCURATE CONTROL

The problem with controlling a conventional system with one air handler performing both the sensible and latent cooling functions is that, while it may perform adequately at full load conditions with approximately a 0.70 sensible heat ratio, it cannot cope with part load conditions when the sensible heat ratio may become as low as 0.30. On a cool, damp morning, the supply air temperature will be modulated above the required dewpoint temperature and the building will be humidified.

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Accurate control of the building is attained by controlling the drybulb temperature and the humidity separately--the "divide and conquer" approach. The "sensible" air handler is controlled solely by drybulb control and its supply air temperature is modulated as necessary to maintain design drybulb temperature.

The DOAS is controlled solely by humidity controls and its supply air dewpoint temperature is modulated as necessary to maintain design humidity conditions. With the latent and sensible loads controlled independently, a mixing will occur in the space between the ventilation air and the recirculated air that will always fall on the sensible heat ratio line. The load will be absorbed in the correct proportions, and the space design point will be attained under all sensible ratios.

Separate latent treatment has an additional benefit in that all the condensate removal is at the DOAS and none is at the sensible air handler or fancoil. This eliminates potential maintenance and IAQ problems.

EFFECTIVE AND VERIFIABLE VENTILATION

Simply introducing the correct amount of ventilation air to the building is not enough. It must be distributed properly so that the correct amount is introduced to each space. Since the DOAS usually is producing neutral temperature air, it may be separately ducted to a dedicated diffuser in each space. This allows easy verification of ventilation. This ductwork can be low pressure and uninsulated. This overcomes one of the vexing problems of VAV systems, which require complicated minimum outside air controls, high VAV box minimum position settings, and reheat in order to provide effective ventilation.