The purpose of the expansion device is to regulate the pressure change between the liquid leaving the condenser and the evaporator. There are a number of devices that can be used, but the two most commonly used in industrial refrigeration are:

1. Thermostatic Expansion Valve

The thermostatic expansion (TX) valve is generally used on simpler systems using one of the Freon refrigerants. This piece of equipment has a needle valve that is controlled by a thermal bulb that is attached to the outlet of the coil. As this thermal bulb senses the evaporated refrigerant gas increasing, it opens the needle valve a little more to allow a higher flow rate. The problem with using this type of valve on ammonia systems is instability, as it has a tendency to overfeed.

2. Hand Expansion Valve

The hand expansion valve is commonly used on both flooded and pumped liquid overfeed systems in series with a solenoid valve. When adequate liquid is in the accumulator, the liquid feed solenoid shuts off the flow until such time that more is needed to maintain an adequate pool of liquid. In a world of automation it may sound strange to use the term "hand". The hand expansion is a needle valve and should be manually adjusted for teh highest flow rate required and no more. When adjusted properly the solenoid valve should be on 75% of the time. This will reduce the load fluctuations caused by excessive flash gas if too much liquid is allowed to flow.