Title: Pump Down Refrigeration System Lab - Room 170
Course: Canvas Practice Course > Section: Basic Refrigeration Charging Procedures

The student will design and construct an automatic pump-down refrigeration system which operates as follows.   Prior to building the system, the student will prepare and submit an electrical ladder schematic diagram of the system including a defrost timer with electric defrost heat and indicator light.

The automatic pump-down system consists of a normally closed liquid-line solenoid valve installed in the liquid line of a refrigeration system. The solenoid is a normally closed electric shut-off valve that is controlled by a thermostat. The thermostat is located in the refrigerated space.

When the desired box temperature is reached in the space, the thermostat will open and de-energize the liquid-line solenoid valve, closing the valve.

The compressor will continue to run and will pump down the low pressure side of the system up to and including the compressor. This includes part of the liquid line, evaporator, suction line, and crankcase.

This refrigerant will be stored in the condenser and receiver on the high side of the refrigeration system. Most of the refrigerant will be stored in the receiver.

The compressor will be shut off by the action of a low-pressure switch set to open at about 5 psig. This will ensure that no refrigerant will migrate to the compressor during the off cycle.

On a call for cooling, the thermostat will close and energize the liquid-line solenoid valve. This action will send liquid refrigerant to the TXV and into the empty evaporator, thus increasing the evaporator pressure.

Once the cut-in pressure of the low-pressure control is reached, the compressor will start and resume a normal refrigeration cycle.

The cut-in pressure of the low-pressure control should be set at a high enough pressure to ensure that the system will not short-cycle.   A high pressure cut-out will de-energize the condensing unit if the the head pressure becomes excessively high.

The system shall also incorporate a Pump-Down Defrost cycle as described on page 535 of the text book.   The system shall maintain a pump-down condition during the defrost cycle to remove refrigerant from the evaporator during the defrost cycle.