Expansion Device :
The expansion device in refrigeration system serves two function. One is the thermodynamic function of the expanding the liquid refrigerant from the condenser pressure to evaporator pressure. The other is the control function which may involve the supply of the liquid to evaporator at the rate at which it is evaporated.
These are two types of expansion devices :
1.) Variable Restriction type
2.) Constant Restriction type
- In variable type the area of flow keeps on changing depending on the type of control, viz. Automatic expansion valve and thermostatic expansion valve.
- The constant restriction type devices is the capillary tube.
Types of Expansion Valve used in refrigeration system :
Capillary Tube :
- The capillary tube is used as expansion valve (device) in small capacity hermetic sealed refrigeration units such as in domestic refrigerator, water cooler, room air-conditioner and freezer. A fine mesh screen is provided at the inlet of the tube in order to protect it from containts. A small filter drier is used on some system to provide additional freeze-up applications.
- Due to frictional resistance offered by a small diameter tube, the pressure drops. Since the frictional resistance is directly proportional to the length and inversely proportional to the diameter.
Hand Operated Expansion Valve :
- The use of hand operated valve is limited to system operating under nearly constant loads for long period of time. Such in ice making plants and cold storages.
- It is not suitable for installation where the load varies and the compressor runs intermittently to maintain a constant temperature.
Automatic (Or Constant pressure) Expansion Valve :
- It maintain constant pressure regardless of the load on the evaporator.
- It is used with dry expansion evaporators where the load is relatively constant.
Thermostatic Expansion Valve :
It is commonly used in commercial and industrial refrigeration system. This is also called constant super-heat valve because it maintain a constant super- heat of the vapour refrigerator coil, by controlling the flow of liquid refrigerant through the evaporator.
Low Side Float Valve :
- The low side float valve is located in the low pressure side (between evaporator and compressor suction line) of the refrigeration system.
- It maintain a constant level of the liquid refrigerant in the evaporator under all loading condition regardless of the evaporator and float chamber by opening and closing a needle valve.
- The major advantage of the low side float valve is that it maintains a constant liquid level in the evaporator under all loading condition regardless of the evaporator pressure and temperature.
High Side Float Valve :
- The high side float valve is located on the high pressure side (between condenser and evaporator) of the refrigeration system.
- It controls the flow of liquid refrigerant to the evaporator according to the load and maintains a constant liquid level in the evaporator and float chamber by opening or closing of the needle valve.