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Driving out the dampness

How can you get rid of moisture inside cabinets and enclosures?

November 19, 2010

Dear Control Engineering: I was reading the article about cabinet dryers. How does that technology work? The article doesn’t explain it.

Compressed air cabinet dryers.In this particular case, the approach dries out the interior of the enclosure by purging it with very dry air. If you install one, you have to run a compressed air line to the enclosure and install one of the filter setups. The filter is the interesting part. It’s a two stage process: The first part is a normal coalescing filter that removes oil and water droplets in the line. Those are pretty common with air systems. The second filter (the longer vertical section) is a membrane module that consists of a bundle of hollow membrane fibers which are permeable to water vapor. As the air passes through the center of these fibers, water vapor permeates through the walls of the fiber and dry air exits from the other end of the fiber. A small portion of the dry air is redirected along the shell side of the membrane fiber to carry away the moisture which surrounds the fiber and the water vapor is vented to atmosphere. The combination of a coalescing filter and a membrane filter provides air with a dew point of -7 °F (-22 °C) even if the air coming into the system is saturated. That’s really dry air.

In operation, you add some sort of port and bleed this dry air into the cabinet. Any moisture that gets inside will evaporate and be carried out of the enclosure as the dry air leaks out. Since the interior is slightly pressurized, it also reduces the likelihood that dust will get inside, and it makes it harder for water to get in as well. This type of purging technique is also used in explosive environments as a safety measure.

As the article says, no electricity is required, however this approach does consume compressed air. The amount will depend on the size of the cabinet and severity of water leaking. Both types of filters empty accumulated moisture automatically so they require very little maintenance.

Peter Welander, pwelander(at)