I have a situation that I cannot explain so I need the assistance of someone with a good understanding of thermodynamics. I have 36 AC to DC power supplies located in an equipment rack. These power supplies are failing after some time. The failures appear to be moisture related. We put moisture indicating labels in the power supply and their color changed indicating the presence of moisture. We replaced the metal cover on the power supply with clear plexiglass and we also saw condensation form inside the enclosure after a number of hours. The environmental conditions were 30C and 45%RH. The dew point should be far below room temperature under these conditions. This was installed at a location close to sea level. There are large cooling fans on the cabinet door pulling air out of the cabinet. So, the pressure inside the cabinet is negative. Could this large amount of airflow pulling through the unit cause the moisture in the air to condense? Could the negative pressure cause the dew point to drop? Can someone explain this? I have been designing power electronics for decades and have never seen this before.
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Too much cooling air?
Too much cooling air?
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After reading the question I understood the following points:
The AC to DC power supplies are failing consistently
The moisture indicator shows the presence of moisture
The environmental conditions are 30C and RH=45%
The pressure inside the cabinet is negative (less than atmospheric)
Some points yet to be known:
What is the value of negative pressure?
What is the dew point temperature at that negative pressure?
What is the value of moisture the indicator is showing inside the equipment rack?
Up to what moisture level content the power supplies can withstand?
Some facts:
1. The dew point temperature at 30C and RH=45% is 17C.
2. With decreasing pressure the saturation temperature also decreases.
Some measurements need to be done before concluding, but low pressure can be the reason for failure as you said.