The principle explanation of the thermal expansion valve of the chiller
Thermal expansion valves are widely used in industrial chillers. It can not only control the liquid supply of the evaporator, but also throttle the saturated liquid refrigerant. According to the different structure of thermal expansion valve, it is divided into two types: internal balance type and external balance type. Considering that the refrigerant flowing through the evaporator produces a certain pressure loss, in order to reduce the opening superheat and increase the utilization rate of the heat transfer area of the evaporator, generally from the expansion valve outlet to the evaporator outlet, the evaporation temperature corresponding to the pressure drop of the refrigerant If the temperature drops more than 2 ~ 3 ℃, the external balance type thermal expansion valve should be selected.
The working principle of external balance thermal expansion valve is based on the balance of force. When the water chiller is in operation, the upper part of the elastic metal diaphragm is affected by the pressure P3 of the working medium in the temperature sensing package, and the lower part is affected by the outlet pressure P1 of the evaporator and the spring force P2. Under the action of three forces, the diaphragm swells up or down, so that the valve hole is closed or opened to adjust the liquid supply of the evaporator. When the amount of liquid entering the evaporator is less than the heat load of the evaporator, the superheat of the vapor at the outlet of the evaporator increases, and the pressure above the diaphragm is greater than the pressure below, thus forcing the diaphragm to swell downward through the ejector rod Compress the spring and lift the valve needle to open the valve hole, the amount of liquid supply will increase. Conversely, when the amount of liquid supplied is greater than the heat load of the evaporator, the superheat of the steam at the outlet is reduced, the pressure in the temperature-sensing system is reduced, and when the force above the diaphragm is less than the force below, the diaphragm is swollen upward When the spring is extended, the ejector rod moves upward and closes the valve hole, and the liquid supply to the evaporator decreases accordingly. The superheat of the thermal expansion valve is composed of the opening superheat and the effective superheat. The opening superheat is related to the preload of the spring, and the effective superheat is related to the strength of the spring and the stroke of the valve needle.