Page 435 - Mechanical Engineers' Handbook (Volume 4)

424 Refrigeration

of 9 1, so that the simple cycle is used for evaporator temperatures from 2 to 50 C. Below
these temperatures, the application limits of a single-stage compressor are reached. Beyond
that limit, there is a risk of excessively high temperatures at the end of compression, which
may produce lubricant breakdown, high bearing loads, excessive oil foaming at startup, and
inefficient operation because of reduced volumetric efficiency in the compressor.
Centrifugal compressors with multiple stages can generate a pressure ratio up to 18 1,
but their high discharge temperatures limit the efficiency of the simple cycle at these high
pressure ratios. As a result, they operate with evaporator temperatures in the same range as
reciprocating compressors.
The compound cycle (Fig. 2) can achieve temperatures of approximately 100 Cby
using two or three compressors in series and a common refrigerant. This keeps the individual
compressors within their application limits. A refrigerant gas cooler (also called a flash
intercooler) is normally used between compressors to keep the final discharge temperature
from the compressor at a satisfactory level. A common practice is to operate the gas cooler
at about the geometric mean of the evaporating and condensing pressures. This provides
nearly identical pressure ratios for the two compressors. Besides producing very low tem-
peratures, the compound cycle can also be used in applications where multiple evaporators
are needed to produce different temperatures.
Below 100 C, most refrigerants with suitable evaporator pressures have excessively
high condensing pressures. For some refrigerants, the specific volume of refrigerant at low
temperatures may be so great as to require compressors and other equipment of uneconomical
size. With other refrigerants, the specific volume of refrigerant may be satisfactory at low
temperature, but the specific volume may become too small at the condensing condition. In
some circumstances, although none of the above limitations is encountered and a single
refrigerant is practical, the compound cycle is not used because of oil-return problems or
difficulties of operation.
To satisfy these conditions, the cascade cycle is used (Fig. 3). This consists of two or
more separate refrigerants, each in its own closed cycle. The cascade condenser–evaporator

Figure 2 Ideal compound refrigeration cycle. 3