“AC versus EC”

It seems that most of society thinks of “Environmental Control” as “Air Conditioning”. That is all that one hears around the bridge table or sees in advertising. What a misnomer!

“Air Conditioning” is really a refrigerating process used to keep humans cool enough so that the temperature level around them is satisfactory. We design systems for perhaps 75° F and 50% RH and then control the temperature with a thermostat. This entirely disregards the 50% RH requirement.

We can get away with this because the human body can tolerate rather wide swings in vapor pressure without complaint. Vapor pressure is the pressure at which water molecules bounce off the surfaces in which they are contained. Envision a child’s balloon that has been blown up using human breath. It contains both dry air and water vapor. Each are creating different pressures on the inner surface of the balloon and causing it to change shape. The water molecules bounce off the surface and are either absorbed or adsorbed into any material that is hygroscopic.

Air Conditioning systems normally must deliver rather large quantities of air to the occupied space (perhaps six space changes per hour) versus the lower quantity of new outdoor air required for human occupancy (perhaps 15 CFM per person). This means that only small percentages of outdoor air (10/20 percent) are required. The constant change in seasons and movement of fronts generally is what changes the levels of absolute humidity and dry bulb temperatures. The lower the percentage of outdoor air blended into a system, the less influence it has on the space psychrometrics.

In many procedures, the material being processed is hygroscopic enough to change color, shape, dimensions, odor and/or density. Many manufacturing techniques fail when changes occur resulting in costly rejections. In these instances “Air Conditioning” techniques become inadequate and true “Environmental Control” becomes necessary.

True EC is designed to draw circles of both temperature and absolute humidity for 24 hours a day, year round, on the chart recorder without the saw tooth control that results from component cycling. True modulating control is mandatory.

When large quantities of outdoor air are required, the cooling, heating, dehumidifying and humidifying devices must respond, without cycling, to loads that vary from zero to one hundred percent. This will happen as rapidly as overnight, during which time the refrigeration system must not be allowed to cycle on and off with load variations.

All of this EC requires components that normally do not appear in AC systems. Such items are:
Evaporator temperature modulation
Compressor unloading without shut off
Hot gas bypass
Double suction risers
SCR electric power modulation
Desiccant dehumidification
Humidifier modulation
Multiple compressors
Redundancy
Optimization
Electronic control
Modulating humidistats
Specific air distribution
Special filtration
Corrosion resistance

Let it be said that such systems should be called “Environmental Control” rather than “Air Conditioning”. The attitudes that systems should cost so much per square foot or per ton are obsolete. Such “rules of thumb” must be abandoned.

Engineering Bulletin -Volume 2, Issue #3
by: Kenneth W. Wicks, M.E.
ASHRAE Fellow
8-08-02

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