Defrost refrigeration control is critical in low temp refrigeration equipment.
Most defrost systems utilize a time clock. The clock can be adjusted to begin defrost at a preset time.
However,termination of the defrost can be accomplished in one of three ways.
1. Time Initiated-Time Terminated: Time termination is the most common. After a preset time period, the clock ends the defrost cycle.
2. Time Initiated-Pressure Terminated: As the defrost proceeds, the pressure in the evaporator increases. The cycle ends when a preset pressure limit causes the clock to override any remaining time.
3. Time Initiated-Temperature Terminated: A sensing element installed on the evaporator ends the defrost at a preset limit. Any remaining time is overridden.
What advantage is served using pressure or temperature defrost termination?
The refrigeration equipment is returned to normal operation in a more efficient manner. Wasted time in defrost is eliminated.
What methods of defrost are used?
1. Air Over Defrost: This method of defrost uses the evaporator fans to continually pull air across the evaporator. The air must be warmer than the frost and eventually melts the ice. Obviously this method is not used in low temp equipment!
2. Electric Defrost: By far the most common, it uses a defrost heater mounted on the evaporator to melt the ice. During defrost the evaporator fans stop. This keep the heat from warming the product.
3. Hot Gas Defrost: A hot gas defrost valve mounted on the discharge line directs hot refrigerant gas into the evaporator. This hot gas melts any frost or ice. Once again, the evaporator fans are off.
A few extra words about hot gas defrost!
Hot gas defrost required the compressor to remain running at all times. It also increase the risk of liquid refrigerant damaging the compressor.
As the hot gas melts the evaporator ice, it condenses into liquid. This liquid refrigerant can flood the compressor.
What to do?
In most hot gas defrost systems a suction line accumulator is installed. It catches this liquid and returns it to gas, before it reaches the compressor.
One more piece of information is needed. Do you have an operating condensate drain pan heater? It's a must on low temp equipment.
Remember, without a properly operating defrost refrigeration control, that low temp freezer will not operate correctly.
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