What is an Expansion Valve?

Figure 1: An expansion valve.

The expansion valve is an important part of the refrigeration system and is generally installed between the liquid storage tank and the evaporator. The expansion valve makes the liquid refrigerant of medium temperature and high pressure pass through its throttling to become wet vapor of low temperature and low pressure.

The refrigerant then absorbs heat in the evaporator to achieve cooling. The expansion valve controls the valve flow through the change of superheat at the end of the evaporator, and prevents insufficient utilization of the evaporator area and cylinder knocking.

1. Composition of the Expansion Valve

Simply put, the expansion valve consists of three parts: the valve body, the sensing bulb, and the balance tube.

Figure 2: Internal balance type expansion valve.

1.1 Sensing Bulb

The sensing bulb is filled with refrigerant in a saturated state of gas-liquid equilibrium, and this part of the refrigerant is not connected with the refrigerant in the system. It is generally tied to the outlet pipe of the evaporator and is in close contact with the pipe to sense the temperature of the superheated steam at the outlet.

1.2 Balance Tube

One end of the balance tube is connected near the outlet of the evaporator, close to the position of the temperature sensing bulb. And the other end is directly connected with the valve body through a capillary tube. The function is to transmit the actual pressure of the evaporator outlet to the valve body.

There are two diaphragms in the valve body. The diaphragm moves upward under the action of pressure, which reduces the refrigerant flow through the expansion valve and seeks balance in the dynamic.

Figure 3: External balance type expansion valve.

2. Expansion Valve Quality Judgment

The ideal working state of the expansion valve should be to change the opening degree in real time and control the flow with the change of the evaporator load. But in fact, due to the hysteresis of heat transfer in the temperature sensed by the bulb, the response of the expansion valve is always a little slow.

If we draw a time flow diagram of the expansion valve, we will find that it is not a smooth curve, but a zigzag line. The quality of the expansion valve is reflected in the amplitude of the twists and turns. The larger the amplitude, the slower the response of the valve and the worse the quality.

3. Types of Thermal Expansion Valves

The thermal expansion valve is divided into internal balance type and external balance type according to different balance methods. External balance thermal expansion valve is divided into F type and H type.

Besides, according to its application range, it can be roughly divided into three types: manual expansion valve, thermal expansion valve, electronic expansion valve, etc.

Figure 4: Electronic expansion valve.

Manual expansion valves are generally used in dry evaporators. Thermal expansion valves are commonly used in Freon refrigeration systems. The advantage of thermal expansion valve is that when the evaporator load changes, it can automatically adjust the flow of refrigerant liquid to control the superheat of the refrigerant at the outlet of the evaporator.

The electronic expansion valve uses the electrical signal generated by the adjusted parameters to control the voltage or current applied to the expansion valve, and then controls the movement of the valve needle to achieve the purpose of adjustment. Compared with the thermal expansion valve, it overcomes the shortcomings of the feedback of the signal, such as large lag, low control precision, and limited adjustment range, and provides conditions for the intelligentization of the refrigeration device.

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