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Any evaporator surface operating below 32°F collects frost. Unless some
provision is made to remove it, frost builds up and acts as an insulator, thereby
increasing operating cost. In fact, frost has an insulating value up to 50
percent of that of cork! Prevention, defrosting, or periodic removal are common
methods used to solve the frost problem. Defrost cycles are also common in
situations above freezing temperatures where the coil operates below freezing at some
point during the running cycle, and thereby collects frost. One common defrost
method here is to blow the room air (which is above 32°F) over the coil to
melt the frost when the compression cycle is off.
Commercial and domestic equipment use some form of automatic control to
operate the defrost cycle. Common frost prevention and defrosting methods used with
commercial systems are hot gas and electric heater systems. Hot gas defrost
uses the hot discharge (high pressure) gas directly from the compressor piped to
the evaporator with a control valve to begin and end the defrost cycle .
Electric heaters are commonly used to defrost domestic and commercial evaporators.
Although the cost of the electricity to operate these heaters may be
appreciable, defrosting is sure and rapid. Always check the scheduled operating time for
these heaters to be sure they only operate as long as necessary.
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