A process for producing refrigeration or liquefaction at cryogenic temperatures usually involves ambient compression of a process fluid with heat rejection to a coolant. During the compression process, the enthalpy and entropy of the fluid are decreased. At the cryogenic temperature where heat is absorbed, the enthalpy and entropy are increased. The reduction in temperature of the process fluid is usually accomplished by heat exchange with a colder fluid and then followed by an expansion. This expansion may take place using either a throttling device (isenthalpic expansion) with only a reduction in temperature or a work-producing device (isentropic expansion) in which both temperature and enthalpy are decreased. Because of liquid withdrawal, a liquefaction system experiences an unbalanced flow in the heat exchanger while a refrigeration system with no liquid withdrawal system usually operates with a balanced flow in the heat exchanger, except where a portion of the flow is diverted through the work-producing expander.
Principles The performance of a real refrigerator is measured by the coefficient of performance, COP, defined as
Q heat removed at low temperature . .
W net work input
Another means of comparing the performance of a practical refrigerator is by the use of the figure of merit, FOM, defined as
where COP is the coefficient of performance of the actual refrigerator system and COR is the coefficient of performance for the thermody-namically ideal system. For a liquefier, the FOM is generally specified as
where Wt is the work of compression for the ideal cycle, W is the work of compression for the actual cycle, and irij is the mass rate liquefied in the ideal or actual cycle.
The methods of refrigeration and/or liquefaction generally used include (1) vaporization of a liquid, (2) application of the JouleThomson effect in a gas, producing engine. Normal commercial refrigeration generally is accomplished in a vapor-compression process. Temperatures to about 200 K can be obtained by cascading vapor-compression processes in which refrigeration is accomplished by liquid evaporation. Below this temperature, isenthalpic or isentropic expansions are generally used either singly or in combination. With few exceptions, refrigerators using these methods also absorb heat by vaporization of the liquid.
If refrigeration is to be accomplished at a temperature range where no suitable liquid exists to absorb heat by evaporation, then a cold gas must be available to absorb the heat. This is generally accomplished by using a work-producing expansion engine.
Expansion Types of Refrigerators A thermodynamic process
2T P2" Heat< exchanger*
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