Thermal expansion valve: the core automatic control component of the refrigeration system

As an indispensable automatic control component of the refrigeration system, the thermal expansion valve can be divided into two types: external balance type and internal balance type according to different balancing methods. It is usually installed between the liquid storage cylinder and the evaporator. It uses the temperature change sensed by the temperature sensing bag as a signal to adjust the opening of the valve, thereby changing the flow of refrigerant. After the medium-temperature and high-pressure refrigerant passes through its throttling effect, it will turn into low-temperature and low-pressure wet steam, and then absorb heat in the evaporator to achieve the cooling effect.

1. The function of thermal expansion valve
Throttle and reduce pressure
The refrigerant condensed by the condenser is in a high-temperature and high-pressure state. The thermal expansion valve can throttle and reduce the pressure, turning it into a low-temperature and low-pressure vapor-liquid mixture that is easy to evaporate. Then it enters the evaporator to evaporate and absorb external heat.

Adjust flow
The thermal expansion valve can automatically adjust the refrigerant flow into the evaporator based on the temperature signal obtained by the temperature sensing package to meet the changing needs of the refrigeration load.

Maintain a certain degree of superheat and prevent liquid shock and abnormal overheating
By adjusting the flow rate, the thermal expansion valve can maintain a certain degree of superheat in the evaporator, ensure that the total volume of the evaporator is effectively utilized, and prevent liquid refrigerant from entering the compressor and causing liquid hammer. At the same time, it can also control the superheat within a certain range to prevent abnormal overheating.

2. The composition and working principle of the thermal expansion valve
Thermal expansion valve is mainly composed of induction mechanism, actuator, adjustment mechanism and valve body. When the temperature of the temperature sensing package decreases, P1 (the pressure generated by the filling medium in the temperature sensing package, acting on the top of the diaphragm) < P2 (evaporation pressure, acting on the bottom of the diaphragm) + F (spring force, acting on the bottom of the diaphragm). At this time, the valve opening decreases, and the refrigerant flow rate decreases proportionally; when the temperature of the temperature sensing package increases, P1 > P2 + F, the valve opening increases, and the refrigerant flow rate increases proportionally.

3. Superheat degree of thermal expansion valve
Usually, the thermal expansion valve has been matched and debugged before leaving the factory, and can output maximum energy while operating stably. To ensure that the thermal expansion valve is at the best matching debugging point, it must be ensured that it has appropriate superheat. The sum of the static assembly superheat and the effective superheat of the thermal expansion valve is what we usually call the working superheat, and its value is generally 5 - 8°C.

Only by ensuring that the superheat is within this value range can the expansion valve output the maximum cooling capacity while avoiding wet stroke. Under normal circumstances, the superheat is required to be controlled between 5 - 8°C. If it is found that the superheat is not within this range, adjustments need to be made.

For refrigeration equipment that has just been put into operation, the thermal expansion valve does not need to be adjusted. However, after several years of continuous use, due to valve needle wear, impurities in the system, partial blockage of the valve hole, and weakened spring elasticity, the opening of the expansion valve will be affected, causing it to deviate from the normal operating point, manifesting as a small or excessive opening of the expansion valve.

If the valve opening is too small, there will not be enough refrigerant in the evaporator to evaporate, resulting in insufficient cooling capacity and reduced cooling effect. If the expansion valve is opened too large, that is, the liquid supply to the evaporator exceeds the evaporator load, part of the refrigerant will not have time to evaporate in the evaporator, and will enter the compressor together with the gaseous refrigerant, causing a wet stroke, which may damage the compressor.

Shanghai KUB Refrigeration Equipment Co., Ltd.

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