TAKEING THE MYSTERY OUT OF THE TXV
The thermostatic expansion valve has been considered by some to be a mysterious and complex device. As a result, many valves are needlessly replaced when the cause of the system malfunction is not immediately recognized. Actually the thermostatic expansion valve performs only one very simple function. –it keeps the evaporator supplied with enough refrigerants to satisfy all load conditions. It is not a temperature control, suction pressure control, or a humidity control. The refrigeration system can be defined as a closed system in which the process of absorbing and rejecting heat is performed by flowing a refrigerant in a vapor compression cycle. In its simplest form, the refrigeration system consists of five components: the compressor, condenser, evaporator, expansion device, and interconnecting piping. The thermostatic expansion valve, TEV, controls the flow of liquid refrigerant entering the direct expansion, DX, evaporator by maintaining a constant superheat of the refrigerant vapor at the outlet of the evaporator. Superheat is the difference between the refrigerant vapor temperature and its saturation temperature. To measure the superheat is the difference between the actual temperature at the sensing bulb or near the compressor and the saturation temperature corresponding to the suction pressure. The major components of the TXV
• A sensing bulb, connected by a length of capillary tubing, transmits bulb pressure to the top of the valve’s • Diaphragm, all three, cap. Tube, sensing bulb and diaphragm are referred to as, • Thermostatic element, the diaphragm motion is transmitted to the, • Pin and pin carrier assembly by one or two,
• Pushrods, allowing the pin to move in and out of the valve port. • The superheat spring located under the pin carrier, set in by a spring guide. There are THREE fundamental pressures acting on the valve’s diaphragm, • P1 sensing...
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