Troubleshooting Humidity Problems in a Manufacturing Environment

A Michigan medical device manufacturer was performing certain processes in a clean facility designed to maintain a temperature of 70+ / -2ºF and 40+ / -2% RH. The owner observed, over a period of time, the mechanical system failure to maintain the desired humidity level. This consultant was called in to examine the mechanical system to determine why there were extended periods of "out of spec" performance of this system. It was observed that a "standard" packaged air conditioning unit, similar to that typically used in ordinary comfort cooling applications, had been installed along with a humidifier and electric reheat package. The humidification equipment was observed to be functioning adequately in response to the system controls. The major problem with humidity control was observed during periods of high ambient humidity and temperature. The use of a "standard" air conditioning unit was suggested by the consultant as a possible cause of the problem. Further review of the design documents showed that the installed unit did not have the required controls to maintain the desired level of dehumidication. The unit would cool adequately however the humidity controller would continually call for operation of the equipment to remove moisture from the airstream, even after room temperature had been satisfied. The evaporator coil of the unit would develop a coating of ice that reduced airflow, created an abnormally low evaporator refrigerant pressure and would ultimately cause the equipment to shut down on a safety control before satisfying the dehumidification requirement.

The solution to the problem was to reconfigure the refrigerant piping of the air conditoning unit. An evaporator pressure regulator was added to insure that suction pressure would not drop to a point where freezing would occur. This regulator was set to maintain a leaving dew point consistent with the 40% RH requirement. The result was that the air conditioning unit could now run for an extended period, removing moisture from the airstream, with no ice formation and no premature shutdown. The alternative solution involved replacing the air conditioner with a "process" unit more suited to this dehumidification application, but such a solution would have resulted in unacceptable downtime as well as being more costly to implement.

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