A company that I worked for was building, or trying to build, a climate-controlled workroom that was supposed to be 60F to 70F, and less than 30 percent relative humidity. The plan was to use two portable air conditioners rated at 12,000 BTUH to cool and dehumidify about 700 cubic feet with 100 percent fresh air.
The working theory of the project supervisor was that the air coming from the air conditioner would be at the same temperature as the thermostat. I tried explaining that most HVAC units put out air at the maximum cooling and dehumidification possible, which then mixes with room air and recirculates until the desired setting is reached. Numerous attempts to properly control the climate of the room failed.
Toward the end of the experiments, I conducted an Internet search into the target dehumidification and temperature level, and found that a refrigeration-type dehumidifier becomes an ice maker. The dew point of 60F and 30 percent is about 30F. So that meant a desiccant-type dehumidifier was necessary.
The next system designer assigned to the job learned from his predecessor’s mistakes, and included an air conditioner, a desiccant dehumidifier, and recirculated all except leakage air. A desiccant dehumidifier uses a desiccant to draw moisture from the air. A separate air circuit uses a heater to heat the air that is then passed over the desiccant to remove the moisture. The desiccant is built into a wheel that rotates, so the desiccant is continuously refreshed.
During testing, the desiccant dehumidifier was cycling off before the humidity setting was reached. We placed an indoor/outdoor thermometer remote sensor inside the exhaust air chamber. The unit was shutting down with an exhaust temperature of 135F. The overheat specification was 150F. The manufacturer sent a replacement overheat sensor. After installing the replacement sensor, we ran the dehumidifier continuously to test the sensor. The unit continued to shut down on overheat, but the exhaust temperature was reaching 158F.
The heating section had two heaters. One was wired to 100 percent capacity, all five segments active. The other was wired with only two segments active. I rewired the element banks to reduce the total heat output, and also tried to balance the two heaters’ output. For this testing, we set the dehumidifier to run continuously to see if the temperature would reach overheat. After several tries, maximum temperature was reaching about 140F.
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