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Classification and Correct Adjustment Method of Car Expansion Valve

Classification of car expansion valves

From the balance mode, car expansion valves can be divided into two types: internal balance and external balance.

The balance pressure of the internal balance valve is taken at the entrance of the evaporator, while the external balance valve takes the balance pressure at the exit of the evaporator. Internal balance expansion valves are generally used in household air conditioners. Automotive air conditioners usually have larger cooling capacity, and the pressure loss of refrigerant in the evaporator is also greater. Therefore, only external balance expansion valves are used.

From the structure of superheat adjustment, car expansion valves are divided into internal adjustment and external adjustment types.

  • The internal adjustment type can only be adjusted before the installation of the air conditioning system. Its advantage is that it has a smaller size and a compact structure. It is suitable for mass-produced products.

  • The external adjustment type can be adjusted according to the situation on the car air conditioning system, with great flexibility. However, the increase in parts makes the size and weight larger.

The correct adjustment method of car expansion valve

Before adjusting the thermal expansion valve, it is necessary to confirm that the air conditioning refrigeration anomaly is caused by the thermal expansion valve deviating from the optimal working point, not caused by other reasons such as insufficient fluorine, clogged dry filters, filter screens, fans, belts, etc.

At the same time, it must be ensured that the sampling signal of the temperature sensing package is correct, and the installation position of the temperature sensing package must be correct, and it must not be installed directly below the pipeline to prevent factors such as oil accumulation at the bottom of the pipe from affecting the correct temperature sensing of the package.

Precautions for adjusting the thermal expansion valve

The adjustment of the thermal expansion valve must be carried out under normal operation of the refrigeration unit. Since it is impossible to place a thermometer on the surface of the evaporator, the suction pressure of the compressor can be used as the saturation pressure inside the evaporator. The approximate evaporation temperature can be obtained by checking the table. Measure the temperature of the returning gas pipe with a thermometer, and compare it with the evaporation temperature to check the degree of superheat.

In the adjustment of the car ac expansion valve, if the degree of superheat is too small, the adjustment screw can be rotated clockwise (increase the spring force, reduce the opening degree of the thermal expansion valve) to reduce the flow rate. Conversely, if the degree of superheat is too large, that is, the liquid supply is insufficient, the adjustment screw can be rotated in the opposite direction (counterclockwise) to increase the flow rate.

Since the practical working thermal sensing system of the thermal expansion valve has certain thermal inertia and forms a lag in the signal transmission, the next adjustment can only be carried out after the operation is basically stable. Therefore, the entire adjustment process must be patient and meticulous, and the number of turns of the adjustment screw should not be too much or too fast at a time.

Steps for adjusting the thermal expansion valve

(1) Stop the machine; insert the digital thermometer probe into the insulation layer of the evaporator return air outlet (corresponding to the position of the temperature sensing package). Connect the pressure gauge to the three-way valve of the low-pressure valve of the compressor.

(2) Start the machine; let the compressor run for more than 15 minutes to enter a stable operating state, and make the pressure indication and temperature display reach a stable value.

(3) Read the temperature T1 of the digital thermometer and the temperature T2 corresponding to the pressure measured by the pressure gauge. The degree of superheat is the difference between the two readings T1-T2.

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