1. Introduction to Thermal Expansion Valve
Thermostatic expansion valve is an important control component in the refrigeration system. It is mainly used to control the rate at which the refrigerant changes from a high-pressure liquid to a low-pressure gas, thereby regulating the flow of refrigerant into the evaporator. This process is essential to maintaining the best cooling effect on the evaporator coil and the overall energy efficiency of the system.
The working principle of the thermal expansion valve is based on several key factors:
Superheat control: The thermal expansion valve controls the flow of refrigerant by monitoring the superheat of the refrigerant vapor at the outlet of the evaporator (that is, the part with a temperature higher than the saturation temperature corresponding to its pressure). Superheat control ensures that the refrigerant in the evaporator is completely evaporated, avoiding liquid refrigerant from entering the compressor and preventing "liquid hammer".
Pressure regulation: The thermal expansion valve also promotes evaporation by reducing the pressure of the refrigerant, which is one of the key steps in the refrigeration cycle. The reduction in pressure causes the refrigerant to boil and absorb heat from the surrounding environment, thereby achieving a cooling effect.
Flow control: The thermal expansion valve dynamically adjusts the flow of refrigerant according to the heat load demand of the evaporator. Under low load conditions, it allows less refrigerant to pass; under high load conditions, it allows more refrigerant to pass.
The structure of a thermal expansion valve usually includes a diaphragm, a temperature bulb (a sensitive element that senses superheat), a spring (provides closing force), and an adjustable control device. The refrigerant or oil in the temperature bulb will expand or contract due to the temperature change at the outlet of the evaporator, thereby changing the position of the diaphragm and controlling the degree of opening of the valve.
2. What is the MOP of a thermal expansion valve
The MOP function of a thermal expansion valve refers to the maximum operating pressure. This function is designed to protect the compressor in the refrigeration system from exceeding the safe range of suction pressure. When the refrigeration system is running, the MOP function can prevent the evaporator pressure from rising abnormally, thereby protecting the compressor from damage caused by excessive suction pressure. In other words, MOP protects the compressor and other components from overpressure by limiting the maximum working pressure of the refrigeration system, ensuring that the refrigeration system can operate under safe and stable conditions
In a thermal expansion valve without an MOP function, most of the medium filled in the temperature bulb exists in liquid form. When the temperature bulb is heated or cooled by the compressor suction line, the internal medium will evaporate into gas or condense back into liquid accordingly, resulting in pressure changes, which will affect the opening of the expansion valve and adjust the flow rate of the refrigerant.
3. Expansion valve installation
The expansion valve must be installed in the liquid pipeline before the evaporator, and its temperature bulb should be fixed on the suction pipeline as close to the evaporator as possible. If there is an external pressure balance, the balance pipe must be connected to the suction pipeline immediately after the temperature bulb.
Since the oil at the bottom of the pipeline may generate false signals, the temperature bulb can never be located at the bottom of the suction pipe, and the temperature bulb cannot be installed after the heat exchanger, because in this position it will send an incorrect signal to the expansion valve. The temperature bulb must also not be installed near a large mass component, because this will also cause an incorrect signal to be sent to the expansion valve.
As mentioned above, the temperature bulb must be installed in the horizontal part of the suction pipeline immediately after passing the evaporator. The temperature bulb must not be installed on the collecting pipe or riser after the oil storage tank.
4. Expansion valve adjustment
The factory setting of the expansion valve can meet the requirements of most applications, and the factory static superheat is generally set to 4℃. Turning the adjustment lever clockwise (close down) increases the static superheat of the valve, and turning the adjustment lever counterclockwise (open up) decreases the static superheat of the valve. Generally speaking, the static superheat changes by 1~2℃ for each rotation of the adjustment lever.
If the superheat in the evaporator is too high, the cause may be insufficient liquid refrigerant supply. Reduce the superheat by gradually turning the setting lever of the expansion valve counterclockwise until fluctuations are observed.
