In order to understand the principles of thermostatic expansion valve operation, a review of its major components is necessary. A sensing bulb is connected to the TEV by a length of capillary tubing which transmits bulb pressure to the top of the valve’s diaphragm. The sensing bulb, capillary tubing, and diaphragm assembly is referred to as the thermostatic element. The thermostatic element on all standard Sporlan TEVs is replaceable.
The diaphragm is the actuating member of the valve. Its motion is transmitted to the pin and pin carrier assembly by means of one or two pushrods, allowing the pin to move in and out of the valve port. The superheat spring is located under the pin carrier, and a spring guide sets it in place. On externally adjustable valves, an external valve adjustment permits the spring pressure to be altered.
There are three fundamental pressures acting on the valve’s diaphragm which affect its operation: sensing bulb pressure P1, equalizer pressure P2, and equivalent spring pressure P3 (see Figure 1). The sensing bulb pressure is a function of the temperature of the thermostatic charge, i.e., the substance within the bulb. This pressure acts on the top of the valve diaphragm causing the valve to move to a more open position. The equalizer and spring pressures act together underneath the diaphragm causing the valve to move to a more closed position. During normal valve operation, the sensing bulb pressure must equal the equalizer pressure plus the spring pressure, i.e.: P1 = P2 + P3
Equivalent spring pressure is defined as the spring force divided by the effective area of the diaphragm. The effective area of the diaphragm is simply the portion of the total diaphragm area which is effectively used by the bulb and equalizer pressures to provide their respective opening and closing forces. Equivalent spring pressure is essentially constant once the valve has been adjusted to the desired superheat. As a result, the TEV functions by controlling the difference between bulb and equalizer pressures by the amount of the spring pressure.
Article contributed by Glen Steinkoenig, product manager, Thermostatic Expansion Valves, Sporlan Division of Parker Hannifin
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