Page 370 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

commonly included in the compressor range.
a
2. Theoretical reversible adiabatic power = mz RT [({P /P } – 1)]/a where T is inlet
1
1
1
2
1
temperature, R = Gas Constant, z = compressibility, m = molar flow rate, a = (k–1)/k and k =
1
C /C .
p
v
3
Values of R: = 8.314 J/mol K = 1.987 Btu/lbmol R = 0.7302 atm ft /lbmol R
a
3. Outlet temperature for reversible adiabatic process T = T (P /P ) .
1
2
1
2
4. Exit temperatures should not exceed 167–204°C (350–400°F); for diatomic gases (C /C = 1.4).
p v
This corresponds to a compression ratio of about 4.
1/n
5. Compression ratio should be about the same in each stage of a multistage unit, ratio = (P /P ) ,
n
1
with n stages.
6. Efficiencies of reciprocating compressors: 65% at compression ratios of 1.5, 75% at 2.0, and
80–85% at 3–6.
3
7. Efficiencies of large centrifugal compressors, 2.83–47.2 m /s (6000–100,000 acfm) at suction,
are 76–78%.
8. For vacuum pumps use the following:

9. A three-stage ejector needs 100 kg steam/kg air to maintain a pressure of 1 Torr.

10. In-leakage of air to evacuated equipment depends on the absolute pressure, Torr, and the
2/3
3
3
volume of the equipment, V in m (ft ) according to W = kV kg/h (lb/hr) with k = 0.98 (0.2)
when P > 90 Torr, k = 0.39 (0.08) between 3 and 20 Torr, and k = 0.12 (0.025) at less than 1
Torr.

(Adapted from S. M. Walas, Chemical Process Equipment: Selection and Design, Stoneham, MA:
Butterworth, 1988. Copyright © 1988 by Butterworth Publishers, adapted by permission of Butterworth
Publishers, Stoneham, MA. All rights reserved)

Table 11.11 Heuristics for Heat Exchangers

1. For conservative estimate set F = 0.9 for shell-and-tube exchangers with no phase changes, q =
UAFΔT . When ΔT at exchanger ends differ greatly then check F, and reconfigure if F is less
lm
than 0.85.
2. Standard tubes are 1.9 cm (3/4 in) OD, on a 2.54 cm (1 in) triangle spacing, 4.9 m (16 ft) long.
2
2
A shell 30 cm (1 ft) dia., accommodates 9.3 m (100 ft )
2
2
60 cm (2 ft) dia., accommodates 37.2 m (400 ft )
2
2
90 cm (3 ft) dia., accommodates 102 m (1100 ft )
3. Tube side is for corrosive, fouling, scaling, and high-pressure fluids.
4. Shell side is for viscous and condensing fluids.
5. Pressure drops are 0.1 bar (1.5 psi) for boiling and 0.2–0.62 bar (3–9 psi) for other services.
6. Minimum temperature approach is 10°C (20°F) for fluids and 5°C (10°F) for refrigerants.
7. Cooling water inlet is 30°C (90°F), maximum outlet 45°C (115°F).