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352 Applied Process Design for Chemical and Petrochemical Plants

1
tropic to actual bhp. For a large reciprocating compressor
system, e o 60—70% and 50—65% for small machines. Refer
to the section on compressors for detailed data. e o is not just
the mechanical efficiency of the compressor; it is the prod-
uct of indicated and mechanical compressor efficiencies. 1

Actual Enthalpy. The actual enthalpy of compressed refrig-
erant to account for deviation from isentropic compression
is referenced to Figure 11-48A, point 2.
Correct enthalpy corresponding to isentropic point 2 is: 1

Q
h 2 h 1
h 2 1corr.2 h 1 a b (11-9)
e o W
For large tonnage requirements the heat loss, Q, is often
negligible as far as determining h 2 (corr.) is concerned, but
it must be included as far as total tonnage requirements are
concerned.

Refrigerant Flow Rate. Refer to Figure 11-48A.

200
lb>min>TR (11-10)
h 1 h 3
For subcooled refrigerant, Figure 11-48B,

200
lb>min>TR (11-11)
h 1 h 3
Figure 11-48. Isentropic compression refrigeration cycles.
Note that the temperature at this h 3 is lower than when no
subcooling exists.

200
cfm per TR a b1v 1 2 , at compressor intake (11-12)
h 1 h 3
1
Horsepower. From Figure 11-48A or 11-48B.
Heat Removed by Condenser.
200 h 2 h 1
Theoretical HP>TR a b (11-6) h 2 h 3
42.42 h 1 h 3 200a b, Btu>ton min (11-13)
h 1 h 3
For Economizer System. See Figure 11-48C. Refrigerant flow
1k 12>k
P 2
0.873 k P 1 v 1 ca b 1d in the second (or later) stage is the sum of first-stage weight
P 1
Theoretical HP>TR , (11-7) flow plus the weight flow of the vapor flashed from the econ-
1k 12 1h 1 h 3 2 19
omizer.
Lb flashed gas from economizer/lb vapor from evapora-
for isentropic compression. For polytropic compression, k is
tor
replaced by n,
where, h 3 h 5
h 1c h 3
n k
a b 1efficiency2 a b (11-8)
n 1 k 1 When economizer vapors are mixed with the compressor
vapors, the temperature from this mixing point on is lower
Brake Horsepower. Bhp theoretical hp/e o , where e o is the to the next stage of compression than if the economizer had
overall compression efficiency, the ratio of theoretical isen- not been used. 19