An expansion device is used to maintain the pressure differential between the high side (condenser) and low pressure side (evaporator) of the refrigeration system as established by the compressor. This pressure differential allows the evaporation temperature to be low enough for the refrigerant to absorb heat from the water to be cooled, and the condensation temperature to be sufficiently high to allow it to reject to water at normal temperatures. High pressure liquid refrigerant flows through the expansion device, causing a pressure drop that reduces the refrigerant pressure to the evaporator pressure. This pressure reduction causes a small portion of liquid (or flash) to vaporize, cooling the remaining refrigerant to the evaporator temperature. The expansion device is also used as a measurement / calibration system, balancing / matching refrigerant flow with the evaporator charge condition. In our case, the expansion device used is a set of two orifice plates. At full charge, a large amount of refrigerant is circulating through the chiller. The liquid column in the liquid line exerts pressure on the liquid at the base of the column. During passage through the orifice plate, the coolant undergoes an equal pressure drop before part of it vaporizes.
As the charge decreases, less refrigerant circulates through the chiller and the liquid column level decreases. Now, as the liquid refrigerant passes through the orifice plate, it only undergoes pressure drop equal to the smallest column of liquid before part of it vaporizes. This causes additional vaporization in the orifice plate which in turn supplies the evaporator with less liquid.
Other types of expansion devices found in centrifuges include: Float valves, expansion valves (thermostatic or electronic) and variable orifices.
