In order to reduce the initial cost of a facilities' HVAC systems, many hours have been spent attempting to design a single HVAC unit to satisfy ventilation requirements as well as mechanical cooling.  High occupancy spaces provide a particular challenge for constant volume, mixed air HVAC units.  However, treating the high latent cooling-load of the outside air with a dedicated outside air unit, and treating separately the high sensible cooling-load of indoor air can provide significant benefits (verification of ventilation flow, better humidity control, etc...).

Splitting the cooling-load with Dedicated Outside Air Systems (DOAS) may be the solution? 

This strategy provides zone-level cooling and heating equipment to provide space comfort by modulating its cooling-capacity to match the sensible-load of the space, therefore, reducing wasted energy associated with excessive supply air-flow.  A separate, centralized, dedicated outside air unit conditions the outside air by satisfying the latent-load and ventilation requirements for all of the spaces. 

There are two ways that the outside air is introduced into a facility, 1) direct from the DOAS to the space, 2) direct from the DOAS to the terminal units or AHU's serving the space.  DOAS that feed outside air directly to the space allow for ease when verifying ventilation flows in critical zones, flexibility in selecting equipment, and prevents forcing ventilation loads on the local units.

However, extra distribution equipment will increase initial cost, proper mixture of air is subject to proper diffuser placement, and there may be a higher cost associated with the fans due to increased total air-flow.  DOAS that feed the return of the zone-level units tend to mix ventilation and recirculation air more thoroughly resulting in more uniform comfort, and slightly lower total air-flow or fan energy.  This type of DOAS is more difficult to balance, and the local terminal unit must run to satisfy ventilation requirements.  Therefore the controls of the local units and the DOAS must be coordinated.

What can be done to optimize the system?  

 If it is feasible to direct most of the facility exhaust back to the AHU of the DOAS, then an energy recovery system could be employed to increase efficiency.  Utilizing a "total-energy flywheel" to precondition the outside air will reduce the cooling and heating loads.  Raising the dew point of the incoming outside air, while limiting the humidity in critical zones will prevent wasted energy associated with over-dehumidification.  Also, resetting and raising the supply dry-bulb temperature of the DOAS will save energy by avoiding re-heating at the terminal units and maximizing the heat-recovery system.

Why is dehumidification difficult with constant volume, mixed air HVAC systems? 

A standard packaged unitary air conditioner is usually designed to operate with a supply-air-flow-to-cooling-capacity ratio of 350 to 400 CFM per ton, while hot humid regions may require no more than 200 to 250 CFM per ton to reach off-coil temperatures below the saturation curve (thus condensing and dehumidifying).  These units are designed to satisfy dry-bulb temperatures and the sensible capacity of the coil is matched to the sensible load of the space.  However, as the temperature approaches set-point, the sensible load decreases and so does the unit's sensible and latent cooling-capacity.  Sensible and latent cooling-loads often do not peak simultaneously.  When the outside air humidity is low and its temperature is high, the sensible cooling-load can exceed the latent cooling-load.  However, when the outside air humidity is high and the temperature is mild or low (spring showers!), then the latent cooling-load can exceed the sensible cooling-load.  This creates a situation where the dry-bulb temperatures in the space may be satisfied, but the relative humidity in the space may be high, which can cause discomfort to your occupants.  This can be corrected by installing an intelligent control system that monitors both temperature and humidity, assuming that your HVAC system has the ability to reheat the supply air.  It is important to note that this simultaneous cooling and heating is effective for dehumidification, however, this process can greatly increase your energy consumption.  Therefore, we strongly suggest that engineers should take extra time to consider this phenomenon when designing your HVAC system so that the cost of dehumidification is minimized.