John Garbini had tried almost everything. As facility support manager for an assembly plant in Reading, Pennsylvania, he was stuck with a nagging problem: When the humidity in the plant’s Class 8 cleanrooms wasn’t too high, it was too low.

He had tried dehumidifiers. He had tried a large industrial desiccant unit. Balanced dampers? Reduced air pressure? Been there, done that.

Angiotech, a pharmaceutical specialty and medical device manufacturer, uses the Reading plant to assemble and package sterile, disposable scalpels and surgical needles with attached sutures. High humidity in the cleanrooms could not only encourage bacterial growth, it could prematurely degrade a type of moisture-activated suture that is designed to slowly dissolve in human flesh after the surgical incision heals. Low humidity, on the other hand, promotes static electricity, which interferes with computerized packaging and labeling equipment and gives employees small shocks.

“Temperature and humidity swings were normal,” Garbini recalls. “Our employees were either too hot or too cold. It was a challenge to maintain the proper temperature while still dehumidifying the space. The air-handling compressors were constantly slamming on and off, but the main problem was that I could never get the humidity low enough to stay within our specs, which are more stringent than the federal cleanroom standards.”

Today, both temperature and humidity are holding steady in Angiotech cleanrooms. The solution came in the form of the APR Control, a device manufactured by Rawal Devices, Inc., of Woburn, Massachusetts. The device smoothly modulates a direct-expansion air conditioning system's capacity, allowing it to dehumidify the space without overcooling it. The device varies refrigerant flow according to suction pressure, which changes with the temperature of air crossing the evaporator coil. Because it continuously monitors the heat content of return air, it maintains the system in a dehumidifying mode more efficiently than thermostats and humidistats and without risk of coil icing, liquid slugging, or excessive compressor cycling.

The device allows an air conditioning system to
dehumidify without overcooling.