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Part 4
The Pump Down System
It is possible to have a system automatically pump down every time a run cycle ends. With Open type compressors this is mandatory so that the off cycle pressure is not applied to the shaft seal. (When operating, the suction pressure is relatively low but during the off cycle the pressures equalize and the low side pressure increases.) Pump downs are also used to empty out the low side at the end of each run cycle. This eliminates the possibility of refrigerant from the evaporator slugging the compressor at start up. It also eliminates the possibility of refrigerant migrating to the compressor sump during the off cycle and washing out the oil at start up. The components of a pump down system are a LPC, a T-Stat and a solenoid valve which is installed in the liquid line. When installed in that location it is called LLSV for Liquid Line Solenoid Valve.


Note that the suction gauge decreases in pressure which is to be expected. However, do you know why the high side pressure did not increase when the system is pumped down? The amount of refrigerant in the high side increased but the pressure does not because of what you have learned about the PT relationship of refrigerants. When there are saturated conditions the pressure will correspond to the pressure that the PT Chart predicts regardless of the quantity of liquid refrigerant.

The schematic below depicts a simple pump down system. Press the "Off Cycle" Button and note that the evap fans are running constantly. The T-Stat is the operating control but does not directly control the compressor. Press "T-Stat On" and see that it allows a circuit to the LLSV. When the LLSV is energized it allows refrigerant flow from the high side to the low side. When the low side pressure builds to the cut in point of the LPC, the Compressor and condenser fans come on. (Press LPC on) The system operates until the T-Stat is satisfied. Press "T-Stat Off" and note that the LLSV is de-energized but the compressor is still operating. It will run until the cut out point of the LPC is reached and that has been set at 1 PSIG. In other words, the low side of the system will pump down to just a slight positive pressure at the end of every run cycle. Press "Off Cycle" to simulate the end of the pump down. The next time the T-Stat calls the sequence repeats.