Page 165 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10D 5/30/2001 4:31 PM Page 128

128 Applied Process Design for Chemical and Petrochemical Plants

Because the value of h o read from Figure 10-67A is still 272
3
ft >sec 0.606
about 2,500, use h o 1500. 17.4821602
Overall coefficient: 0.606
ft>sec 18.6
10.0015521212
1
U
1 1
0.001 0.002 This velocity is too high for satisfactory operation.
1500 1140 Therefore, the only way to get more flow area is more
2
220 Btu>hr 1ft 21°F2 tubes and requires the same size shell as the previously
designed unit. I recommend using the unit with 1-in.
Water flow of 95.2 gpm gives: tubes.
680,000 Btu>hr 21. Nozzles should be sized with or checked against the
Water temperature rise 14.3°F sizes of the incoming or outgoing lines. Often the
112195.2218.3321602
exchanger nozzle must be larger than the pipe in order
to keep velocities low to prevent erosion or high pres-
Log mean temperature difference:
sure drop.
Summer: based upon water 90°F S 104.3°F
LMTD 7.2°F as calculated previously Water Connections
Winter: based upon water 70°F S 84.3°F
LMTD 29.0°F as calculated previously Flow rate 136 gpm
Design for (136) (1.5) 204 gpm
Area required: Maximum allowable velocity 6 ft/sec
Referring to Cameron hydraulic tables:
680,000 Select 4-in. nozzle, vel 5.3 ft/sec
Summer: A 430 ft 2
1220217.22 Select head loss (0.046 ft) (6 in./12) 0.023 ft liquid
680,000
Winter: A 106 ft 2 Condensed Ammonia Liquid Out
12202129.02
Referring to Cameron Miscellaneous Liquid Table in
Area available in selected unit:
Fluid Flow Chapter, Vol. I:
1,440 lb>h.
A 10.19632 1842 115.52 256 ft 2 gpm 4.87
160218.33210.592
To make this unit acceptable for summer operation Design rate 14.872 11.52 7.8 gpm
(the calculated required surface is greater than that
available), assume that the water rate can be increased, Select 3-in. nozzle, head loss less than 0.00035 ft (negligi-
thereby decreasing water T and increasing LMTD. ble). Use large nozzle to ensure free drainage of unit and no
vapor binding in outlet line. Actually a 1-in. connection
106.6 106.6 would safely carry the liquid flow with a head of about 0.08
95 90 ft of liquid. A condenser must be free draining and capable
11.6 16.6 of handling surges.
LMTD = 14°
Ammonia Vapor Inlet
Summer area required (not making any correction for
change in water film coefficient or condensing coeffi- Design rate (1440) (1.5) 2160 lb/hr
cient):
Referring to Figure 10-63, at 220 psia and 17 mol wt, the
7.2 maximum suggested vapor velocity through a nozzle is
A 286a b 147
14
1402 11.22 48 ft>sec max.
680,000
Water rate 136,000 lb>hr For a 3-in. nozzle, Schedule 40,
15°2112
Cross-section area to flow 0.0513 ft 2
136,000 Sp. Vol NH 3 vapor 1.282 ft /lb
3
gpm 272
18.3321602 Total flowing ft /hr (2,160) (1.282) 2,770
3