What is the working principle of an expansion valve?

1. Function of thermal expansion valve:

The thermal expansion valve is installed at the inlet of the evaporator, often referred to as the expansion valve, and has two main functions:

1) Throttling effect: High temperature and high pressure liquid refrigerant passes through the throttle hole of the expansion valve to become a low-temperature and low-pressure mist like hydraulic refrigerant, creating conditions for refrigerant evaporation;

2) Control the flow rate of refrigerant: The liquid refrigerant entering the evaporator evaporates into a gaseous state after passing through the evaporator, absorbing heat and reducing the temperature inside the car. The expansion valve controls the flow rate of refrigerant to ensure that the outlet of the evaporator is completely gaseous refrigerant. If the flow rate is too high, the outlet may contain liquid refrigerant, which may enter the compressor and cause a liquid hammer; If the refrigerant flow rate is too small and the evaporation is completed in advance, resulting in insufficient refrigeration;

2. Types of thermal expansion valves:

The thermal expansion valve is divided into internal balance type and external balance type according to different balance methods; The externally balanced thermal expansion valve is divided into two structural types: F-type and H-type.

1) Internal balance expansion valve structure and working principle:

Structural diagram of an internal balanced F-type thermal expansion valve. The temperature sensing bag is filled with refrigerant and placed on the outlet pipeline of the evaporator. The temperature sensing bag is connected to the upper part of the diaphragm through a capillary tube to sense the temperature of the refrigerant at the outlet of the evaporator. The pressure at the inlet of the evaporator is felt below the diaphragm. If the load of the air conditioning increases and the hydraulic refrigerant evaporates in advance in the evaporator, the temperature of the refrigerant at the outlet of the evaporator will increase, the pressure on the diaphragm will increase, and the valve rod will be pushed to increase the opening of the expansion valve. The flow of refrigerant entering the evaporator will increase, resulting in an increase in cooling capacity; If the air conditioning load decreases, the temperature of the refrigerant at the outlet of the evaporator decreases, and the valve opening decreases according to the same principle of action, thereby controlling the flow of refrigerant.

2) Structure and working principle of external balanced expansion valve:

The principle of the external balance expansion valve is basically the same as that of the internal balance expansion valve, with the difference being that the pressure at the evaporator inlet is felt below the membrane of the internal balance expansion valve; Under the diaphragm of the externally balanced expansion valve, the outlet pressure of the evaporator is felt.

3) H-type expansion valve

The H-type thermal expansion valve has four interfaces that are connected to the refrigeration system, two of which are the same as ordinary thermal expansion valves. One is connected to the liquid storage dryer and the other is connected to the inlet of the evaporator; There are two other interfaces, one connecting the evaporator outlet and the other connecting the compressor inlet. The temperature sensor is directly located in the refrigerant airflow at the outlet of the evaporator. The expansion valve has improved adjustment sensitivity, compact structure, and reliable vibration resistance due to the cancellation of the temperature sensing bulb, capillary tube, and external balance pipe in the F-type thermal expansion valve.