Expansion Device Orifice Plates of Centrifugal Water Chiller

Expansion Device Orifice Plates of Centrifugal Water Chiller
Expansion device Orifice Plates of Centrifugal Water Chiller

An expansion device is used to maintain the pressure difference between the high-pressure (condenser) and low-pressure (evaporator) sides of the refrigeration system, as established by the compressor. This pressure difference allows the evaporator temperature to be low enough for the refrigerant to absorb heat from the water being cooled, and the condenser temperature to be high enough for the refrigerant to reject heat to water at normally available temperatures. High-pressure liquid refrigerant flows through the expansion device, causing a pressure drop that reduces the refrigerant pressure to that of the evaporator. This pressure reduction causes a small portion of the liquid to boil off, or “flash,” cooling the remaining refrigerant to the evaporator temperature.

The expansion device is also used as a liquid refrigerant metering system, balancing the refrigerant flow rate with the evaporator load condition. In our example centrifugal chiller, the expansion device used is a set of 2 orifice plates. At full load, a large amount of refrigerant is moving through the chiller. The column of liquid refrigerant in the liquid line pressurizes the liquid at its base. During passage through the orifice plates, the liquid refrigerant undergoes a pressure drop equal to the head (H1) before some of it flashes to vapour.

As the load decreases, less refrigerant moves through the chiller and the level of the liquid column drops. Now, as the liquid refrigerant passes through the orifice plates, it only undergoes a pressure drop equal to the lower head (H2) before some of it flashes to vapour. This causes additional flashing at the orifice plate which, in turn, feeds less liquid to the evaporator.

 Other types of expansion devices found in centrifugal chillers include: float valves, expansion valves (thermostatic or electronic), and variable orifices.

Reference: Trane Chiller


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