Refrigerating and air-conditioning systems are comprised of four important components: the compressor, the condenser, the throttling or expansion device and the evaporator. The refrigerant keeps on flowing through these components in a continuous cycle. The high pressure and medium temperature condensed refrigerant leaves the condenser and enters the throttling or expansion valve.
In the throttling valve the pressure of the refrigerant reduces suddenly and excessively. With this the temperature of the refrigerant also reduces drastically. This low pressure and low temperature liquid refrigerant then enters the evaporator and absorbs heat from the substance or the space to be cooled.
The throttling valve is fitted between the condenser and the evaporator. The throttling or expansion device is in the form of a small orifice. When refrigerant passes through this small orifice its pressure reduces suddenly due to the friction. The rate of the flow of refrigerant through the throttling device depends on the size and opening of the orifice. It also depends on the difference in pressure on the evaporator and the condenser sides.
There are different types of throttling devices, but in refrigerating and air-conditioning systems, the two most commonly used types are: capillary tube and thermostatic expansion valve. These have been described below:
1) Capillary tube: Instead of the orifice, the capillary is small diameter tubing that offers the restricted flow of the refrigerant. Its internal diameter ranges from 0.020 to 0.090 inches depending upon the capacity of the refrigerating or air-conditioning system. The pressure drop attained through the capillary depends upon its diameter and length. Capillary tubing made of copper is most commonly used.
Capillary tubing is used for small refrigerating and air-conditioning systems like household refrigerators, water coolers, deep freezers, window air-conditioners, split air-conditioners, small packaged air-conditioners etc. For systems in which capillary tubing is fitted, technicians have to be very careful of refrigerant charging as the overcharging can lead excessive high discharge pressures from the compressor, which leads to over loading of the compressor and the chances of refrigerant leakages from the system are also increased.
2) Thermostatic expansion valves: The thermostatic expansion valve is not controlled by the temperature. It works automatically maintaining proper flow of the refrigerant depending upon the heat load in the evaporator. Apart from reducing the pressure of the refrigerant, the thermostatic expansion valve also keeps the evaporator active. These days thermostatic expansion valves used with solenoid valves are more common.
Thermostatic expansion valves are used extensively in medium and large sized refrigerating and air-conditioning systems. They can be used for large water chilling plants, brine chilling plants, large packaged air-conditioners, central air-conditioning plants etc.