THE DEFROST CYCLE

The refrigeration cycle results in frost formation on the surface of the evaporator. This frost will eventually build up to the point where it will restrict the air flow causing a loss of refrigeration capacity. To prevent this, a timeclock (I), usually set to repeat every 6 or 8 hours, initiates a defrost cycle which melts the frost.

The clock de-energizes (closes) the liquid line solenoid valve which causes the compressor to pumpdown and shut off from the low pressure control. The clock also energizes the drain pan heater (J) in the evaporator and timer relay which after a two minute delay energizes (opens) the 3-way valve and hot gas solenoid valve (K) which then builds up pressure in the evaporator causing the low pressure control to close and start the compressor.

The hot discharge gas from the compressor flows through the 3-way valve, Hot gas valve and check valve forcing all the liquid left in the liquid line into the evaporator. If pressure builds up too high the 3-way valve pressure control (L) will de-energize the solenoid valve and allow pressure to relieve through the condenser. Pressures within the evaporator will remain steady and once all the frost has melted the pressure will rise until the defrost termination pressure control (M) energizes the timeclock's internal solenoid terminating the defrost cycle. The 3-way valve, hot gas valve solenoids are then de-energized. The liquid line solenoid valve opens and the compressor continues to run. The evaporator fans do not start up until the pressure in the evaporator is low enough to close the Fan delay control (N). By delaying the fans this allows any moisture left on the coil to drain away or freeze. As soon as the evaporator fans are energized the system will then resume back to the refrigeration cycle.

The cycle continues until the room temperature is satisfied.

This de-energizes the liquid line solenoid, initiating a pumpdown cycle that reduces the suction pressure to the cut-out setting on the low pressure control which de-energizes the compressor.

To view the Refrigeration Cycle, Click on the button below