Refrigerant letdown valve

Refrigerant motor-driven compressor Refrigerant condenser Refrigerant letdown valve Evaporator Refrigerant receiver... 

The refrigerant liquid partially flashes to a vapor as it flows through the letdown valve. The flashing represents the conversion of the sensible heat of the refrigerant to latent heat of vaporization. In Fig. 22.1, the refrigerant is chilled from 100 to 40°F. Approximately 25 percent of the liquid flashes to a vapor to provide this autorefrigeration. 

The partially vaporized refrigerant flows into the evaporator. In Fig. 22.1, the evaporator shown is similar to a kettle-type reboiler (see Chap. 5). The process fluid flows through the tube side of the kettle evaporator. The refrigerant liquid level is maintained by the letdown valve. The refrigerant vapor flows from the top of the kettle, to the compressor suction. 

Now let s assume that there is a refrigerant receiver between the condenser and letdown valve. This traps any uncondensed vapors. The accumulation of these vapors raises the pressure in the receiver. This puts backpressure on the condenser. The higher condenser pressure promotes more complete condensation of the refrigerant vapors. 

If, in the heat pumps, the energy of compression is not recovered but is wasted in letdown valves (as the pressure of the working fluid is reduced to the low pressure of the evaporator (Joule-Thomson cycle), the liquefaction efficiency will be low (35-60%). This range of efficiencies is a function of the liquefier size and refrigerant used. If the letdown valves are replaced by turbo expanders (Brayton cycle), which recover some of the compression energy during pressure letdown, and if helium or neon refrigerants are used, the efficiency can theoretically reach 80-90%. 

Letdown valve Used to reduce the pressure of a liquid refrigerant. 

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