Page 171 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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134 Applied Process Design for Chemical and Petrochemical Plants

By assuming no entrainment in each condenser tube, the
liquid rate out the tube must equal the vapor rate entering
the tubes (assuming no noncondensables), so L/G 1, and
at steady state, F 2 1.
Using the Dowtherm figures cited previously at 20-in. Hg.
vacuum,
20.8 dynes>cm

g 0.0877 lb>ft 3
0.5
a b 15.4
g
0.5
V f F 1 F 2 a b
g
V f 112 112 115.42 15.4 ft>sec
This is the velocity of the vapors in the tube, which will
result in flooding at this low pressure.
For the condition of 20 psig pressure:
13.05 dynes>cm
Figure 10-79. Effect of entrance tube taper on flooding velocity. (Used
g 0.5587 lb>ft 3 by permission: Diehl, J. E., and C.R. Kop Company, Chemical Engi-
neering Progress Symposium, Heat Transfer, V. 65, No. 92, ©1968.
0.5 13.05 0.5 American Institute of Chemical Engineers. All rights reserved.)
a b a b 4.83
0.5587
g
F 1 F 2 1

Because:
0.5 Example 10-11. Desuperheating and Condensing
a b 6 10 Propylene in Shell
g
0.5 1.15
V f 0.71cF 1 F 2 a b d See Figure 10-80.
g
A refrigeration system requires that 52,400 lb/hr of
V f 4.35 ft>sec propylene refrigerant vapor from the compressors be desu-
perheated and then condensed.
By comparing with solving for the same conditions using
a 1-in. tube in the English correlation at 20-in. Hg vacuum, Propylene inlet: 265 psia and 165°F
42
V f 14.02 ft/sec, compared to 15.4 ft/sec from the preced- Propylene dew point: 265 psia and 112°F
ing calculation. Cooling water in: 90°F
42
English’s flooding correlation incorporates an entrain-
ment load of E/G from 0.01—0.05 lb liquid per lb of vapor. Assume that this load can be handled best in two units
The effect of the tapered inlet tube (as earlier discussed) operating in parallel. In this scenario, if one condenser
42
as now determined by English is only significant at the develops trouble, the entire refrigeration system, and con-
60° and 75° tapers, both producing about the same sequently the plant process, is not shut down.
increase in vapor capacity. Diehl’s correlation is shown in
Figure 10-79. Heat Duty

where Heat content of propylene vapor at 165°F 512 Btu/lb
E superficial liquid entrainment rate, lb/hr/ft 2 Heat content of propylene vapor at 112°F 485 Btu/lb
F 1 , F 2 correlation factors defined by equations
Sensible heat duty 52,400 (512 485) 1,415,000 Btu/hr
V f superficial flooding velocity of the vapor, ft/sec
V f7 superficial flooding velocity of the vapor when inlet Latent heat of vaporization at 112°F 126 Btu/lb
tube taper is 70°, ft/sec Latent heat duty 52,400 (126) 6,600,000 Btu/hr