Page 190 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10E 5/30/2001 4:33 PM Page 153

Heat Transfer 153

checks: LMTD: 250 S 135
1,920 1,940 96.7 d 93.6
U t 1930 153.3 41.4
2
LMTD 85°F
1,930
2
U 138 Btu>hr 1ft 21°F2
104 90 Correction factor:
250 135
t g 115°F R 37.1
p v 1.471 psi at 115°F 96.7 93.6
p g 31.0 1.471 29.529 96.7 93.6
P 0.0198
250 93.6
358,540
t w ¿ 92.3 91.1°F F 0.975 (approx., Figure 10-34)
299,000112
Corrected LMTD 85 (0.975) 82.8°F
Clean U 66.4 (Part 9)
New gas rate 970(14) 34.6(18) 14,223 lb/hr
Fouling assumed 0.004
14,223 (overall, 0.0015 tube side, 0.0025 shell side)
2
G 22,800 lb>hr 1ft 2
0.624 1
2
U1dirty2 52.3 Btu>hr 1ft 21°F2
1
This is only 5% of the original G s and still within normal 66.4 0.004
expected accuracy of this method. For improved 938,700
accuracy, the physical properties, R e , h o , and diffusivity Area required this zone 216.0 ft 2
152.32182.82
K G, could be recalculated for the conditions of this
interval. This particular problem does not seem to
b. Cooler-condenser zone, see plot of Figures 10-87
warrant the extra work, although many problems may
and 10-88.
require a recalculation at every interval or two.
Q 1,066,850
Weighted t 26.7
Use h o 65 a q 39,990
Latent heat at 115°F 1028.7 Btu/hr a b
t avg.
Assume condensate temperature, t c , of 94.5°F
p o 0.802 psia at 94.5°F Q 1,066,850
p g 31.0 0.802 30.198 U1clean2 149
A t 1267.92126.72
30.198 29.529 Same fouling applies to this portion of the unit 0.004
p gf 30.03
30.198
2.3 log 1 2
29.529 U1dirty2 93.4 Btu>hr 1ft 21°F2
1
0.004
149
5.41 1.471 0.802
65(115 94.5) 118211,028.72
£ 30.03 § 14.7
Area required for cooler-condenser zone:
14.7
1,066,850
A 427 ft 2
1,140(94.5 91.1) 193.42126.72
1330 2220 3880
3550 3880 Total exchanger area:
Gas cooling zone 216
Close enough check. Cooler-condenser zone 427
Total 643 ft 2
3,550 3,880
U t 3,715 2
2 Available area in assumed unit 645 ft
3,715
2
U 155 Btu>hr 1ft 21°F2 1645 643211002
115 91.1 Percent excess area 0.3%
629
11. Areas required: This excess area is smaller than usual practice; 10%
being a preferred figure. The accuracy of the rela-
a. Gas cooling zone tions used are not claimed to be better than 20%