Controlling Clean Room Temperature

THE CHALLENGE:
The clean room does not care whether outdoor conditions are cold or warm, damp or dry, clean or dirty, windy or calm. Seasons change, weather fronts roll through, exhaust hoods cycle on and off but production under constant environmental conditions must go on. In order to control a class 1,000 clean room on a 24/7 year round basis at a constant dry bulb temperature of 70 degrees and maintain the relative humidity at 55 degrees is a psychrometric challenge of major proportions. Direct expansion refrigeration and indirect gas fired heating will be used as the basis for design.

THE SOLUTION:
High efficiency filtration is used with indirect fired outdoor air heating that feeds treated outdoor air to a modulation direct expansion cooling system. A smaller quantity of treated outdoor air is blended with a much larger recirculating air quantity (320 air changes per hour). A cooling load varying from zero to one hundred percent with an air quantity as low as 125 CFM per ton must be employed. Modulated humidification and reheat are necessary.

A DDC control system must cause the components to respond such that temperature and humidity circles are drawn on the chart recorder. The combination represents true "environmental control" and should not be confused with "air conditioning". Heating and humidifying must not be allowed to cycle on and off but rather must modulate. In order for direct expansion type cooling to handle widely varying loads and still not cycle off, lowside and highside capacities must remain equal at all times. Carefully selected refrigeration devices are essential. Such sophisticated techniques are required for industrial processes but seldom for the maintenance of human comfort.

Engineering Bulletin -Volume 2, Issue #2
by: Robert Carpenter, Application Engineer
6-18-02

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