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


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Conscience – “The ability to recognize right and wrong regarding ones behaviour” (Webster)

Unconscionable – “Not restrained by conscience; unscrupulous” (Webster)

If our Engineering profession harbors an inordinate influence over the manner and efficiency with which man converts his natural resources, these definitions become especially important. The application of the philosophy that “the end justifies the means” becomes questionable.

It is obvious that we are converting a finite quantity of irreplaceable energy bearing materials (Oil, Gas, Coal, etc.) to elements that cannot then be used again as an energy source. We can however, prolong the life expectancy of those energy sources by designing systems that convert them as slowly (efficiently) as possible.

A recent example of “unconscionable” took place when a standard pump impeller diameter turned out to be too large for the application. The horsepower used to develop the necessary flow required that the excess “head” be absorbed by installing an orifice through which the false head could be developed. Had this remained, the horsepower wasted over the years of the life of the pump would have been unconscionable.

A simple, inexpensive, machining of the original impeller to reduce its diameter by one half inch resulted in proper performance and a life cycle saving of hundreds of horsepower hours.

A similar phenomenon appears when systems are designed for buildings that contain varying use zones. Frequently large spaces require control of their “occupancy” environments at schedules different from other spaces in the same building. In Sanctuaries, Auditoriums, Gymnasiums, etc. temperature setback schedules differ within the same structure.

The accommodation of such needs is easy to accomplish when the design is still on paper but becomes prohibitively expensive to correct after construction. The energy waste seems to be “unconscionable” when it occurs because of inadequate pre construction consideration.

This means that careful attention must be paid to such functions as piping zone configurations, insulation efficiency, outdoor air infiltration, lighting, life cycle costing, etc.

Perhaps we should consider copying from the safety groups that require the “signing off” of plans by safety experts. We might create “resource use” analysis teams that analyze each new design.

Think about it! !

Engineering Bulletin -Volume 2, Issue #7
by: Kenneth W. Wicks, M.E. – ASHRAE Fellow

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