An Oxymoron

The concept that the typical “Economizer” application offers free cooling during periods of cooler outdoor ambient air temperature is widespread. In certain psychrometric requirements however, the cooling is anything but “free” and may not even be applicable. An oxymoron is born.

When the quantity of moisture in the air in the environment being considered is critical, cooling and humidifying or dehumidifying may be required. The fact that outdoor air psychrometric conditions are the result of weather fronts continuously passing by means that its moisture content is always changing.

Many industrial and hospital applications require that the vapor pressure to which the process/patients are being subjected is critically important. Low moisture content allows for the creation of static electricity and the drying of human membrane. Nasal passages are unable to add enough moisture to the ingested air to support the body’s need. The vapor pressure resulting from high moisture content can cause hygroscopic materials to change shape, color and workability. It can make breathing difficult for certain persons and cause the formation of molds.

When an economizer delivers increased quantities of dry outdoor air, it lowers the moisture content of the supply air. To maintain desired absolute humidity levels will require rehumidification at the cost of adding 1,000 BTU per pound of water added. This function is certainly not “free”. It becomes even more costly when the colder air must be preheated before it can hold the extra moisture required. Once this has happened, the advantage of cooling from the colder air is lost.

During this consideration, it must be realized that a typical economizer modulating damper blade configuration does not represent thorough mixing of the outdoor and return air. Even parallel blades directed toward the opposing parallel blades do not guaranty mixing before the “rain” occurs when the outdoor air dry bulb temperature is below the dew point temperature of the return air.

One can conclude that the natural desire to get “something for nothing” must be constrained and that the use of the “free cooling” concept should be limited to projects where humidity control is not required.

Engineering Bulletin -Volume 2, Issue #6
by: Kenneth W. Wicks, M.E. – ASHRAE Fellow

06-01-05

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