The impacts of selecting the wrong expansion valve

When it comes to selecting a thermal expansion valve, the basic parameters that need to be determined include evaporation temperature, condensation temperature, subcooling degree, pressure drop of the liquid separator, refrigerant, refrigeration capacity, interface size, whether MOP function is required, and the range of use for evaporation temperature. We need to clarify several concepts.

Firstly, the nominal capacity of the thermal expansion valve. Taking TG valve as an example, there will be a nominal cooling capacity of 32Kw on the top cover of the valve body. Here, 32Kw is the actual cooling capacity of R410A refrigerant corresponding to the valve measured by the thermal expansion valve under ASERCOM standard operating conditions, with a static superheat of 4 degrees and an open superheat of 4 degrees. The capacity of the valve is approximately proportional to the magnitude of the superheat, with a linear adjustable range of 30% to 120%. In addition, the static superheat of the valve can be adjusted within the range of 0 to 8 degrees, which has a lot of room for selection. At the same time, the testing standards are relatively close to the actual operating conditions of the water-cooled air conditioning unit, and the capabilities under other operating conditions will definitely be very different. The larger the high and low pressure difference, the greater the capacity of the valve. Based on the above reasons, we need to know that the selection of a 10HP thermal expansion valve for the air conditioning unit we usually refer to is not accurate.

Secondly, evaporation temperature and condensation temperature. The low and high pressure used in valve selection correspond to the outlet and inlet pressure of the corresponding valve, while the evaporation temperature and condensation temperature commonly referred to in air conditioning units usually refer to the refrigerant pressure at the outlet of the two heat exchangers. In the selection process, we need to consider the pressure drop generated by the liquid pipe from the condenser outlet to the inlet of the thermal expansion valve and all accessories installed in the pipeline, such as the accumulator, sight glass, solenoid valve, drying filter, etc. The low pressure side needs to consider the pressure drop generated by the evaporator itself, and if the equipment has a liquid separation head and capillary tube, it should also be taken into account.

Thirdly, undercooling. The greater the degree of undercooling, the greater the capacity of the thermal expansion valve. If the size of the designed undercooling of the system cannot be determined, it can be estimated based on experience. The undercooling of an air-cooled system is generally around 2 degrees, and that of a water-cooled system is generally around 4 degrees. Systems with an economizer need to consider the size of the economizer matching.

Shanghai KUB Refrigeration Equipment Co., Ltd.

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