Refrigeration & Equipment Engineering

Unit 4 - Refrigerants

4.4.Defrosting
Evaporators for cold storage and for low temperature applications will necessarily have to be at a temperature lower than O^oC (the freezing percentage of water) and therefore the moisture from the air will not only condense but also freezes (or frost as it is called) on the evaporator surface. Therefore for such an applications frost formation on the evaporator coil is invetible. Also it is obvious that lower the evaporator temperature greater will be the frost formation.

Effects of frost formation on Refrigeration System,

• This reduces the evaporator temperature.
• The frost on the evaporator coil acts as an insulation and retards the heat transfer.
• With decreased heat transfer, the evaporator temperature will go down, causing thicker frost and decreases in evaporator capacity.
• If the frost is allowed to accumulate more, even liquid flood back to the compressor can occur due to the substantial reduction of the evaporator capacity.

For the reasons stated above, the frost formation on evaporator coil of low temperature installation should be melted out periodically. This process of removing the frost by melting is called defrosting.

Defrosting can be accomplished by any one of the following methods,

• Spraying warm water over the frost.
• Using electrical heater embedded in the evaporator (electrical defrosting)
• Circulating hot discharge gas from the compressor (hot gas defrosting)
• Time switches are used (timer) for accomplishing automatic defrosting.