Refrigeration Cycle the chiller trane

Refrigeration Cycle the chiller trane

Refrigeration Cycle trane

Refrigeration Cycle

A pressure–enthalpy (p-h) chart illustrates the refrigeration cycle of the centrifugal water chiller

Refrigeration Cycle

First, let’s review the components of a 2-stage centrifugal chiller in the context of the refrigeration cycle.
Refrigerant vapor leaves the evaporator and flows to the compressor, where it is compressed to a higher pressure and temperature. High-pressure refrigerant vapor then travels to the condenser where it rejects heat to water, and then leaves as a saturated liquid. The pressure drop created by the first expansion device causes part of the liquid refrigerant to evaporate and the resulting mixture of liquid and vapor enters the economizer. Here, the vapor is separated from the mixture and routed directly to the inlet of the second-stage impeller. The remaining saturated liquid refrigerant enters the second expansion device.

low-pressure

low-pressure

In chillers designed to use a low-pressure refrigerant, the evaporator and the piping leading to the suction side of the compressor operate at pressures that are lower than atmospheric pressure. Therefore, if small leaks exist in either of these sections, air will leak into the chiller instead of refrigerant leaking out

  

Purge System

Purge System

Air inside a chiller reduces the surface area available for heat transfer. It also increases the refrigerant pressure in the condenser, which increases the pressure difference required across the compressor and causes more power to be consumed. In extreme cases, air can cause the compressor to surge, limiting the chiller’s ability to produce cold water. Finally, infiltration of moist air can cause corrosion and other harmful chemical reactions inside the chiller.

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

Purge operation

Purge operation

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 noncondensable air. Because the purge evaporator operates at a lower temperature and pressure than the chiller condenser, a mixture of refrigerant vapor and 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.) The mixture enters the purge tank, and the refrigerant condenses on the cold evaporator tubes and returns to the chiller condenser as 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 set point, 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 all of the 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.

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. 


 
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