Page 436 - Mechanical Engineers' Handbook (Volume 4)

3 Refrigeration Cycles and System Overview 425

Figure 3 Ideal cascade refrigeration cycle. 3

rejects heat to the evaporator of the high-temperature cycle, which condenses the refrigerant
of the low-temperature cycle. Refrigerants are selected for each cycle with pressure–
temperature characteristics that are well suited for application at either the higher or lower
portion of the cycle. For extremely low temperatures, more than two refrigerants may be
cascaded to produce evaporator temperatures at cryogenic conditions (below 150 C). Ex-
pansion tanks, sized to handle the low-temperature refrigerant as a gas at ambient temper-
atures, are used during standby to hold pressure at levels suitable for economical equipment
design.
Compound cycles using reciprocating compressors or any cycle using a multistage cen-
trifugal compressor allows the use of economizers or intercoolers between compression
stages. Economizers reduce the discharge gas temperature from the preceding stage by mix-
ing relatively cool gas with discharge gas before entering the subsequent stage. Either flash-
type economizers, which cool the refrigerant by reducing its pressure to the intermediate
level, or surface-type economizers, which subcool refrigerant at the condensing pressure,
may be used to provide the cooler gas for mixing. This keeps the final discharge gas tem-