Why Purge System Is Required For Refrigerant and How Does It Operate in Chiller Compressor

Fig: Refrigerant Purging System Diagram

Why Purge System Is Required For Refrigerant and How Does It Operate in Chiller Compressor

Purge operation system for refrigerant in chiller compressor is carry-out to remove air and moisture; 

Low-pressure chillers typically include a purge system to remove air and moisture that may leak in, while minimizing the emission of refrigerant.

The purge consists of a small refrigeration system, a pump-out system, controls, and a filter drier. 

The purge’s refrigeration system includes a small compressor, a finned-tube air-cooled condensing coil, an expansion valve, and an evaporator coil inside of the purge tank. 

The tank with the evaporator coil separates condensable refrigerant from non-condensable air. 

Because the purge evaporator operates at a lower temperature and pressure than the chiller condenser, a mixture of refrigerant vapour and the air is drawn from the chiller condenser, just above the level of the liquid refrigerant. (This is where air typically concentrates in a low-pressure chiller.) 

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The mixture enters the purge tank, and the refrigerant condenses on the cold evaporator tubes and returns to the chiller condenser as a liquid. 

The air does not condense but instead accumulates in the top of the tank. Eventually, enough air accumulates to cover a large portion of the coil. 

The air insulates the coil, reducing the amount of heat transferred and the temperature of the refrigerant leaving the purge evaporator coil. 

This temperature is called the purge suction temperature. The drop in purge suction temperature signals the need for a pump-out sequence.

When the purge suction temperature drops below the setpoint, a controller turns on the pump-out compressor and opens the isolation valves. 

Since the air contains a very small amount of refrigerant, it is pumped from the purge tank into a filtration canister. 

This canister adsorbs nearly the entire remaining refrigerant, and the air is then piped to the chiller vent line. 

When the purge suction temperature rises again, the controls close the valves and turn off the pump-out compressor.

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A filter drier is located in the refrigerant drain line, between the purge tank and the chiller condenser. 

The filter drier removes moisture, acid, and dirt from the liquid refrigerant before it returns to the condenser.

The purge controls can also be used to track and record how often pump-out occurs. 

Leaks can be detected early by comparing pump-out activity over the last 24 hours to the 30-day average. 

So, you might get why purge system is required for refrigerant and how does it operate in the chiller.