Low Temperature Evaporator Defrost
Low-temperature evaporators require a time clock, controls, and a source of heat to melt their normal accumulation of frost. Also, because frost is produced easily when the coil temperature is below 0°F, the fin spacing must be wide enough to prevent frost bridging between fins for at least 4–6 hours of normal operation.
AC coils may have fin spacing of 15 fins per inch (fpi), whereas medium-temperature units operate at 10 fpi. However, freezer coil fin spacing should be no more than 7 fpi. The wider fin spacing slows the bridging of frost and eventual frost buildup.
The most common defrost system for walk-in and reach-in freezer evaporators is electric resistance strip heaters. They are usually attached to coil fins at the air inlet side of the evaporator. The heaters and all the controls are preassembled at the factory, making it easier for the installers.
Source : Commercial Refrigeration: For Air Conditioning Technicians
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