Control principle of the expansion valve

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Working principle of thermal expansion valves: Thermal expansion valves control the flow rate of refrigerant entering the evaporator by sensing the superheat of the gaseous refrigerant at the outlet of the evaporator. Based on the difference in balancing methods, thermal expansion valves are divided into externally balanced and internally balanced types. In dedicated air conditioning systems for computer rooms, external balance thermal expansion valves are generally used. The air-cooled dedicated air conditioners currently in use for server rooms, such as HIROSS, STULZ, ISOVEL, AIREDELE, and Faya, all adopt this structure. The structure of a thermal expansion valve is shown in Figure 1: It consists of a sensing mechanism, an actuating mechanism, an adjusting mechanism, and a valve body. The sensing mechanism is filled with Freon refrigerant, and the temperature sensor is located at the outlet of the evaporator; there is a temperature difference between the temperature at this outlet and the evaporation temperature, which is commonly referred to as superheat. Once the temperature sensor detects the temperature at the evaporator outlet, it brings the entire sensing system to the corresponding saturation pressure Pb. As shown in Figure 1, this pressure is transmitted through the diaphragm to the push rod and then to the valve spool. On the diaphragm at the upper part of the pressure chamber, only Pb is present; below the diaphragm, there is the spring force Pt of the adjustment spring and the evaporation pressure P0. When these three forces are in equilibrium, Pb = Pt + Po. When the heat load on the evaporator increases, the outlet superheat becomes higher, Pb increases, and Pb > Pt + Po. This combined effect causes the stem and valve core to move downward, resulting in the expansion valve opening more widely and the refrigerant flow rate increasing proportionally. Conversely, as the thermal expansion valve opens less, the refrigerant flow decreases proportionally. Therefore, the dedicated air conditioning for server rooms enables self-regulation of the cooling system by means of a thermal expansion valve that controls the superheat.

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Thermal expansion valves are widely used in central air conditioning chiller units. It can both control the liquid supply to the evaporator and throttle the saturated liquid refrigerant. Based on the differences in their structural design, thermal expansion valves are divided into internal balance type and external balance type. Considering that the flow of the refrigerant through the evaporator results in a certain pressure loss, and in order to reduce the superheat at startup and improve the utilization rate of the evaporator’s heat transfer area, when the temperature drop in the evaporator caused by the pressure drop of the refrigerant from the outlet of the self-acting expansion valve to the outlet of the evaporator exceeds 2–3°C, an externally balanced thermal expansion valve should be used. The working principle of an external balance type thermal expansion valve is based on force balance. During operation, the upper part of the elastic metal diaphragm is subjected to the pressure P3 of the working fluid inside the temperature sensing bulb, while the lower part is subjected to the pressure P1 at the evaporator outlet and the spring force P2. Under the action of three forces, the diaphragm bulges upward or downward, thereby closing or opening the valve orifice to regulate the liquid supply to the evaporator. When the amount of liquid entering the evaporator is less than what is required to meet the evaporator’s heat load, the superheat of the vapor exiting the evaporator increases. The pressure above the diaphragm becomes greater than the pressure below it, which forces the diaphragm to bulge downward. This, in turn, compresses the spring through the push rod and pushes the valve needle open, thereby increasing the flow rate of liquid supplied. Conversely, when the supply volume of liquid is greater than what is required by the heat load of the evaporator, the superheat of the vapor at the outlet decreases. The pressure in the temperature-sensing system drops, and when the force acting on the diaphragm from above is less than that from below, the diaphragm bulges upward. This causes the spring to stretch, the push rod to move upward, and the valve orifice to narrow, thereby reducing the amount of liquid supplied to the evaporator. The superheat of a thermal expansion valve consists of opening superheat and effective superheat; the opening superheat is related to the pre-tension of the spring, while the effective superheat is related to the strength of the spring and the stroke of the valve needle. The spring of the expansion valve is designed for standard operating conditions; under these conditions, the unit maintains a high COP value whether it is operating at full load or at variable loads. However, under conditions of high pressure difference, the evaporation pressure decreases, which reduces the amount of liquid required by the evaporator. In reality, however, with the suction superheat remaining unchanged, the decrease in evaporation pressure leads to a corresponding drop in the outlet pressure P1 of the evaporator. This increases the pressure difference across the diaphragm, resulting in an increased opening degree of the main valve and thus an increased supply of liquid ； However, under conditions of low pressure difference, the evaporation pressure rises, resulting in an increased demand for liquid volume by the evaporator. In reality, with the suction superheat remaining unchanged, the increase in evaporation pressure leads to a corresponding rise in the outlet pressure P1 of the evaporator; this reduces the pressure difference across the diaphragm, causing the main valve to open less and thus reducing the amount of liquid supplied ； The same is true under variable load. Therefore, the thermal expansion valve needs further improvement in regulating the liquid supply volume under variable operating conditions.