Ice will build on the evaporator and therefore decrease in efficiency when boiling temperature is below 0C (as it will be for the majority of refrigeration). There are three ways of ice can be removed:
natural defrost when temperature is above 4C - when the refrigeration system, turn off the fans continue to operate to allow the air to defrost fin block;
Electric defrost - defrosting heaters embedded in fin block, periodically switches to defrost fin block when the compressor is switched off;
hot or cold steam defrost - hot discharge of vapors or warm saturated vapour from the top of the receiver is distributed through the evaporator, to melt the ice.
In all cases there is a drain pan and line to allow condensation to drain away. If the air temperature is below 0C they are heated and insulated.
Energy efficient defrosting depends on the following factors
initiation defrost only when it becomes necessary, determined through the loss of productivity
;
using the most efficient method of applying the necessary amount of heat;
making sure that the thawing of the heat spreads evenly throughout fin block;
stop defrost cycle as soon as fin block completely clear of ice;
reduction of volumes of thawing of heat absorbed by the process fluid or product;
minimization of frost build on ensuring the evaporating temperature is, as high as possible and low humidity of the evaporator.
Defrost on demand (where sensors are used, so defrost is initiated only when necessary and stopped only when fin block of ice) is always more effective than timed defrost, as it adjusts to different levels of ice, which will usually occur.