- It is a variable-restriction type expansion valve
- It maintains constant pressure in the evaporator, the so-called constant pressure expansion valve.
- A screen or strainer is usually installed at the liquid inlet of the valve in order to prevent the entrance of foreign materials, which may cause the stoppage of the valve.
- Its main actuating fore is the evaporating pressure.
A schematic diagram of an automatic expansion valve
The valve consists of the following:
- Needle and seat
- Pressure bellows or metallic diaphragm
- Spring with an adjusting screw
The opening and closing of the valve depending upon the following:
- Spring pressure – acting on the top of the diaphragm.
- Atmospheric pressure – acting on the top of the diaphragm.
- Evaporator pressure – acting below the diaphragm.
- The operation of the automatic expansion valve is based on maintaining a constant pressure in the evaporator.
- When the compressor is running, the valve maintains an evaporator pressure in equilibrium with spring pressure and atmospheric pressure.
- As the name implies, the operation of the valve is automatic and once the tension of the spring is adjusted for the desired evaporator pressure, the valve will operate automatically so that the desired evaporator pressure is maintained regardless of evaporator loading.
- When the evaporator pressure falls down, the diaphragm moves downwards to open the valve. This allows more liquid refrigerant to enter the evaporator. It increases the evaporator pressure till the desired pressure is reached.
- When the evaporator pressure raises, the diaphragm moves upwards, which reduces the opening of the valve, and decreases the flow of liquid refrigerant to the evaporator. It lowers the pressure till the desired pressure is reached.
- When the compressor stops, the liquid refrigerant continues to flow into the evaporator. It increases the pressure in the evaporator. It causes the diaphragm to move upwards and the valve is closed. It remains closed until the compressor starts again.
- It is not suitable for varying-load requirements.
- Poor efficiency under heavy load conditions
The automatic expansion valve is best applied only to small equipment having relatively constant loads such as:
- Domestic refrigerators and freezers.
- Small retail ice-cream cabinets.
- Milk coolers.