Thermal testing has proven that natural convection cooling is not adequate for today’s smaller, high power density enclosures. Heat dissipation by forced convection (fan cooling) is the most frequently used method of cooling. Forced air-cooling systems can provide heat transfer rates that are ten times greater than those achievable with natural convection and radiation, but even this is not adequate to cool faster electronic components when they are located in hostile plant environments.

It is necessary to lower the internal enclosure temperature to below the room temperature to reduce hot spot (junction) temperatures and prevent control failure on higher density controls. Research by control manufacturers has shown that for each 18ºF (10ºC) increase in temperature, online production shut-downs will occur twice as often — increasing the failure rate of electronics by 40%. Most manufacturers of electronic components specify 104ºF (40ºC) and 90% humidity for proper operation.

The never-ending pressure to reduce the cost and size of electronics while increasing speed and complexity has created a significant design dilemma. Forced-air fan cooling usually is selected by designers because fans are relatively inexpensive and easy to install. Unfortunately, the factory air pulled into the enclosure by the fans usually contains just enough nearly invisible oil aerosols to coat surfaces of sensitive and expensive electronic boards in control enclosures. This light surface coating of oil attracts and holds dust. The dust eventually forms an insulating blanket over the board, promoting heat buildup and eventually failure.

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