Expansion valve is an important part of refrigeration system, which is generally installed between the liquid storage tank and evaporator. The expansion valve makes the liquid refrigerant with medium temperature and high pressure become wet steam with low temperature and low pressure through its throttling, and then the refrigerant absorbs heat in the evaporator to achieve refrigeration effect. The expansion valve controls the flow of the valve through the change of superheat at the end of the evaporator to prevent the insufficient utilization of evaporator area and cylinder knocking.

In short, the expansion valve is composed of valve body, temperature sensing package and balance pipe.

Temperature sensitive bag

The temperature sensing bag is filled with refrigerant which is in the saturated state of gas-liquid equilibrium, and this part of refrigerant is not connected with the refrigerant in the system. It is usually tied to the outlet pipe of the evaporator, which is in close contact with the pipe to feel the superheated steam temperature at the outlet of the evaporator. Because the refrigerant inside is saturated, it transmits the saturated pressure to the valve body according to the temperature.

Balance tube

One end of the balance pipe is connected to the evaporator outlet slightly away from the temperature sensing package, and is directly connected with the valve body through the capillary. The function is to transfer the actual pressure at the evaporator outlet to the valve body. There are two diaphragms in the valve body. The diaphragm moves upward under the action of pressure to reduce the refrigerant flow through the expansion valve and seek balance in the dynamic state.

Quality judgment

The ideal working state of expansion valve should be to change the opening in real time and control the flow with the change of evaporator load. But in fact, due to the hysteresis of heat transfer, the reaction of expansion valve is always slow. If we draw a time flow chart of expansion valve, we will find that it is not a smooth curve, but a broken line. The quality of the expansion valve is reflected in the amplitude of the fold. The larger the amplitude, the slower the response and the worse the quality of the valve.

1 function of thermal expansion valve:

The thermal expansion valve is installed at the inlet of evaporator, which is often called expansion valve

1) throttling function: the liquid refrigerant with high temperature and high pressure flows through the throttle hole of the expansion valve to become the mist like hydraulic refrigerant with low temperature and low pressure, creating conditions for the evaporation of the refrigerant;

2) control the flow of refrigerant: the liquid refrigerant entering the evaporator evaporates from liquid state to gaseous state after passing through the evaporator to absorb heat and reduce the temperature in 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 large, the outlet contains liquid refrigerant, which may enter the compressor to produce liquid hammer; if the flow rate of refrigerant is too small, evaporation is completed in advance, resulting in insufficient refrigeration;

2 types of thermal expansion valve:

The thermal expansion valve is divided into internal balance type and external balance type according to different balance modes, and the external balance type thermal expansion valve is divided into F-type and H-type structures.

1) structure and working principle of internal balanced expansion valve:

Internal balance type F thermal expansion valve structure diagram. The temperature sensing package is filled with refrigerant and placed on the outlet pipe of the evaporator. The temperature sensing package is connected with the upper part of the diaphragm through a capillary tube to feel the refrigerant temperature at the outlet of the evaporator. The pressure at the inlet of the evaporator is felt under the diaphragm. If the air conditioning load increases and the hydraulic refrigerant is evaporated in advance in the evaporator, the refrigerant temperature at the outlet of the evaporator will increase, the pressure on the diaphragm will increase, the valve rod will be pushed to increase the opening of the expansion valve, the refrigerant flow into the evaporator will increase, and the refrigerating capacity will increase; if the air conditioning load decreases, the refrigerant temperature at the outlet of the evaporator will decrease, and the same principle will make the refrigerant flow into the evaporator increase The valve opening is reduced to control the refrigerant flow.

2) Structure and working principle of external balanced expansion valve: the principle of external balanced expansion valve is basically the same as that of internal balanced expansion valve, and the difference is that the inlet pressure of evaporator is felt under the diaphragm of internal balanced expansion valve, while the outlet pressure of evaporator is felt under the diaphragm of external balanced expansion valve.

3) H expansion valve

The H-type thermal expansion valve has four interfaces to connect with the refrigeration system, two of which are the same as the ordinary thermal expansion valve, one is connected with the liquid storage dryer and the other is connected with the evaporator inlet; the other two interfaces are connected with the evaporator outlet and the compressor inlet. The temperature sensing package is directly in the refrigerant flow at the evaporator outlet. The expansion valve has the advantages of high sensitivity, compact structure and reliable anti vibration due to the elimination of temperature sensing package, capillary tube and external balance connecting pipe in F-type thermal expansion valve.