COWARD ENVIRONMENTAL SYSTEMS, INC.
500 Birchfield Drive,Suite 501, Mt. Laurel, NJ 08054
856-727-4400 -fax 856-727-4344
e-mail: cesi@cowardenvironmental.com

One Hundred Percent Outside Air

We hear it every day. "I have a room that I want to maintain at 75 degrees F and 50% RH year round with 100% outside air. What size equipment should I use?" Or "We must maintain our air conditioned room at a positive pressure at all times. What does it take?"

Our response must be such that the client is made aware of the very specific requirements of his request. If one stops to think about the reasons for control of the environment: heating, cooling, dehumidifying, humidifying, contamination control, pressurization and air distribution, many factors become apparent. The paramount realization must be that the task becomes one of control of "mass transfer" in addition to the more common task of control of "heat transfer." It is this fact that differentiates "Air Conditioning" from "Environmental Control".

This statement is valid because of the behavior of nature and its influence on "Outdoor Air". The passage of weather fronts (high and low barometric pressure fronts) means that the outdoor air not only changes in temperature, but also in absolute moisture content with its resultant change in vapor pressure and total heat content. The change in vapor pressure means that the moisture content of exposed hydroscopic materials will vary and the generated static electric charge will change. The change in heat content means a similar change in equipment load.

The typical "Air Conditioning" system is designed to control room dry bulb temperature only and is therefore insensitive to "mass transfer" or total heat load. The loads reflected on equipment handling 100% outdoor air varies much more than if just dry bulb temperature control is considered.

To the refrigeration section of the system, this means the use of components that allow compressors to run continuously in variable unloaded condition. The fact that compressors must not be allowed to short cycle means that they must remain "off" for perhaps a minimum of three minutes. In a system that delivers 100% outside air, it means that during those three minutes the occupied space is out of controlled conditions.

The solution to this problem normally requires the use of a modulated refrigerant evaporator temperature control in lieu of the typical cycling of solenoid valves. During the weather periods requiring heating, when the entering air temperatures vary from below zero to perhaps 65 degrees, both preheat and reheat are normally required. This heating can be satisfactorily accomplished with either gas, electric or steam devices so long as those devices include the ability to modulate as opposed to cycling on and off. Certainly care must be taken to properly remove whatever contaminants are in the outside air before it passes through the heat and mass exchangers.

Attention to the need to add moisture to the cold, dry air should be of concern. It must be realized that 100% outdoor air systems become severe dehumidifiers during the cold weather season and just as severe humidifiers during the warm weather season. Systems that contain only heating can allow the room conditions to drop to as low as 5% relative humidity with the resultant static electricity and health problems.

The use of outdoor air for room pressurization presents unique but readily solvable problems even when the areas of exfiltration vary (door openings, conveyor belt openings, cycling hood exhausts, etc.). We must be able to prevent the intrusion of heavier or contaminated gases while still maintaining proper psychometric conditions.

The satisfying of exhaust hood make-up air requirements frequently requires certain "mass transfer" to prevent excessive system loading. It is wise to consider the use of "tail water" from the air conditioning system to accomplish this "mass transfer" load without the use of additional chilled water. High percentages of outside air demand increased attention to the total conditioning scheme to assure proper "environmental" control.

By:

Kenneth W. Wicks

ASHRAE Fellow

7-99