Page 186 - Chemical Process Equipment - Selection and Design

158 FLUID TRANSPORT EQUIPMENT

L .-
2; lo
2 E 76
2.0 74
0 .-
LE 72
70
00 000000 00 000000
00 000000 00 000000
mwrT)(DQ)g rT)d oooooo'
E: 0 0 00 0 0 0- 0 0- o-op, 0-0
- N n*rT)afDcog
Suction Volume CFM
TABLE OF CORRECTION FACTORS DUE TO COMPRESSION RATIO
Compression Inlet Volume in CFM 60,000
Ratio 1500 2000 3000 4000 5000 7500 15,000 30,000 &over
1.35 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.75 .983 .990 .990 .994 .996 .996 .996 .996 1.0
2.25 .976 .977 .986 .985 .991 .995 .995 .995 1.0
2.80 - .969 .980 .980 .989 .992 .993 .994 1.0
5.00 - -
.965
10.00 - - - .970 .972 .985 .991 .996 .997
.944*
.985*
.989*
.995
15.00 - - - - .955* .960* .971* .986* .993
.935*
.957*
Notes: Asterisk indicates figures applying only to high molecular weight hydrocarbons. Polytropic Efficiency
Factors apply on one compressor body with six or less impellers.

Efficiencies
Compression Ratio (Engine-driven')
1.1 50-60
1.2 60-70
1.3 65-80
1.5 70-85
2.0 75-88
2.5 80-89
3.0 82-90
4.0 85-90
'Multiply by 0.95 for motor-driven
V PRESSURE RATIO Po/ Ps compressors.

Figure 7.27. Efficiencies of centrifugal and reciprocating compressors. (a) Polytropic efficiencies of centrifugal compressors as a function of
suction volume and compression ratio (Clurk Brothers Co.). (b) Relation between isentropic and polytropic efficiencies, Eqs. (7.22) (7.23).
(c) Isentropic efficiencies of reciprocating compressors (De Lava1 Handbook, McGruw-Hill, New York, 1970). Multiply by 0.95 for motor
drive. Gas engines require 7000-8000 Btu/HP.

nonideal for any other reason, the isentropic condition becomes

EXAMPLE 7.6
Polytropic and Isentropic Efficiencies
Take qp-= Oy7.5, k = 1.4, aid PJP, = 3. From Eq. (7.39), n = 1.6154
and qs = o,7095, With Figure 7.27(b), = 3~.2857 = 1,3687, After the final isentropic temperature T, has been found by trial,
qs = 0.945qp = 0.709. The agreement is close. the isentropic enthalpy change is obtained from
(AH), = J rT2r CL dT + AH; - AH&. (7.48)
T1