Expansion valves serve two purposes:
- Controlling the amount of refrigerant entering the evaporator: As much of the evaporator surface as possible should be covered with liquid refrigerant without liquid being carried over to the compressor. If the capacity of the evaporator increases, the expansion valve should allow a larger flow of refrigerant, and vice versa. A smaller refrigerant mass flow results in a higher level of superheating, because less surface area is required for evaporation.
- Maintaining the pressure difference between the condenser (high pressure) and the evaporator (low pressure): The pressure difference created by the work of the compressor is maintained by the expansion device.
Expansion valves do not directly control the evaporation temperature. Instead, they regulate the superheating by adjusting the mass flow of refrigerant into the evaporator, and maintain the pressure difference between the highpressure and low-pressure sides. The evaporation temperature depends on the capacity of the compressor and the characteristics and efficiency of the evaporator. The function of an expansion valve is shown in Figure 4.1.
There are seven main types of expansion devices:
- Thermal expansion valves (TEVs)
- Manual valves
- Capillary tubes
- Automatic valves
- Electronic expansion valves
- Low-pressure float valves
- High-pressure float valves