Pumpdown System Control

Pumpdown System Control

Refrigerant vapor will always migrate to the coldest part of the system, and if the compressor crankcase can bacome colder than other parts of the system,  refrigerant in the condenser, receiver,  and evaporator will vaporize,  travel througn the system, and condense in the compressor crankcase.

Because of the difference in vapor pressure of oil and refrigerant, refrigerant vapor is attracted to refrigeration oil,  and even though no pressure or temperature difference exists to cause a flow,  refrigerant vapor will migrate through the system and condense in the oil until the oil is saturated. During off cycles extending several hours or more, it is possible for liquid refegerant to almost completely fill the compressor crankcase due to the oil attraction. For example in a system using R-12 refrigerant which is allowed to equalize at an ambient temperature of 70° ., the oil-refrigerant mixture in the crankcase will end up about 70% refrigerant before equilibrium is reached.

The most positive and dependable means of keeping refrigerant out of the compressor crankcase is the use of a pumpdown cycle. By closing a liquid line solenoid valve, the refrigerant can be pumped into the condenser and receiver, and the compressor operation controlled by means of a low pressure control. The refrigerant can thus be isolated during periods when the compressor is not in operation, and migration of refrigerant to the compressor crankcase is prevented.

Pumpdown control can be used on all thermostatic expansion valve systems with the addition of a liquid line solenoid valve, provided adequate receiver capacity is available. Slight refrigerant leakage may occur through the solenoid valve, causing the suction pressure to rise gradually, and a recycling type control is recommended to repead the pumpdown cycle as required. The accasional short cycle usually is not objectionable.

A pumpdown cycle is highly recommended whenever it can be used. If a not-recycling pumpdown circuit is required, then consideration should be given to the pumpdown for more dependable compressor protection.