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THERMOSTATIC EXPANSION VALVE.- The thermostatic expansion valve (fig. 3-16) is mounted close to the evaporator and meters the flow of refrigerant into the evaporator, depending upon system demand. Efficient evaporator operation is dependent upon the precise metering of liquid refrigerant into the heat exchanger for evaporation. As was previuosly stated, if heat loads on the evaporator were constant, a fixed opening size could be calculated and used to regulate the refrigerant supply. However, since the system encounters varying heat loads, a variable opening device is needed to prevent starvation or flooding of the evaporator, which would seriously affect the evaporator and system efficiency. This variable opening effect is accomplished by the thermostatic expansion valve, which senses evaporator
condi- tions and meters refrigerant to satisfy them. By sensing the temperature and the pressure of the gas leaving the evaporator, the expansion valve prevents the evaporator from being flooded, and thereby returning liquid refrigerant to the compressor. The valve consists of a housing containing an inlet port, an equalizer port, and 25 outlet ports. The flow of refrigerant to the outlet ports is controlled by positioning a metering valve pin. Valve pin positioning is controlled by the pressure created by the remote sensing bulb in the power section, the superheat spring setting, and the evaporator discharge pressure as supplied by the external equalizer port. The remote sensing bulb is a closed system and is filled with refrigerant. The bulb itself is placed in a well, attached to the evaporator. The pressure within the bulb corresponds to the pressure of the refrigerant leaving the evaporator. This force is felt on top of the diaphragm in the power head section of the valve, and any increase in pressure will cause the valve to move towards an open position. The bottom side of the diaphragm has the forces of the superheat spring and the external equalizer port pressure acting in a direction to close the valve pin. The valve position at any instant is determined by the resultant of these three forces. If the temperature of the gas leaving the evaporator increases above the desired superheat value, it will be sensed by the remote bulb. The
Figure 3-16.- Evaporator and thermostatic expansion valve. pressure generated in the bulb is transmitted to the diaphragm in the power section of the valve, causing the valve pinto open. A decrease in the temperature of the gas leaving the evaporator will cause the pressure in the remote bulb to decrease, and the valve pin will move toward the closed position. The superheat spring is designed to control the amount of superheat in the gas leaving the evaporator. A vapor is superheated when its temperature is higher than that necessary to change it from a liquid to a gas at a certain pressure. This ensures that the Freon returning to the compressor is in the gaseous state. The superheat spring is adjustable and is factory set to provide approximately 9 of superheat in this particular vapor cycle system. Superheat setting is calculated in relation to evaporator size and heat loads applied; therefore, it should never be tampered with in the field as serious inefficiencies will result. The equalizer port is provided to compensate for the effect the inherent evaporator pressure drop has on the superheat setting. The equalizer senses evaporator discharge pressure and reflects it back to the power head diaphragm, adjusting the expansion valve pin position to hold the desired superheat value. The purpose of the multioutlet configuration of the valve is to ensure an even distribution of the refrigerant in the evaporator.
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