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Refrigeration Systems 365

(A)

Figure 11-60. Typical closed and open impeller designs for use in a
mechanical expansion turbine. (Used by permission: Bul.
2781005601. ©Atlas Copco Comptec, Inc.)

(B)

gases than valve throttling when the final pressure is the
same. Each gas needs to be examined separately for the spe-
cific operating conditions. Pressure reduction refrigeration
is used extensively in the natural gas recovery
operation for production of low temperatures for vapor
condensation and local power recovery. See Figures
11-59A—C. Figure 11-60 illustrates typical impeller designs
for mechanical expansion turbines.
The design of low-temperature systems, whether mechan-
(C) ical expansion turbine or throttling valve, is a special tech-
nology and cannot be adequately covered in this chapter.
References on the subject include 20, 21, 23, 60.

Nomenclature

BHP = Bhp = brake horsepower.
btu = British thermal unit.
Figure 11-59. Representative expander refrigeration systems for tem- e o = overall compression efficiency, fraction.
perature requirements and/or power recovery. (A) Expander coupled gpm = gallons/minute.
to a generator recovers pressure-loss energy for conversion to elec-
tric power. (B) System for power recovery from reactors in chemical hp = horsepower.
processing plants. (C) Typical waste-heat recovery system using h = enthalpy of a vapor or gas at a specific state of temper-
expansion turbines to generate electrical power. (Used by permis- ature and pressure, Btu/lb.
sion: Bul. 2781005601. ©Atlas Copco Comptec, Inc.) h 2 (corr) = enthalpy corrected for overall efficiency and heat loss,
Btu/lb.
k = ratio of specific heat, c p /c v .
K = equilibrium constant.
represents the comparison of throttling (Joule-Thompson
n = polytropic compression exponent.
Effect) of nitrogen with the use of an expansion turbine
2
P = absolute pressure, lb/in. abs, psia.
starting at the same conditions. Note that the expander as a 2
P = pressure drop, lb/in. , psi.
general rule will produce a lower temperature for most Q = system heat loss, Btu/hr.