Page 162 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10D 5/30/2001 4:31 PM Page 125

Heat Transfer 125

0.0606 Viscosity 0.085 centipoise
No. tubes
0.00333 From Figure 10-67A or 10-67B, read:
2
h o 2500 Btu/hr(ft ) (°F)
=18.2 per pass to keep the 5 ft/sec water velocity
Although this is satisfactory, because it is so large, use
h o 1500
7. Assumed condenser unit:
In many cases, a “safety” factor of greater than 10—15%
Assume U 200 is not justified.
LMTD 10°
Select the tube-side fouling of water 0.002.
Select the shell-side fouling of ammonia 0.001
680,000 11. Overall coefficient:
A 340 ft 2
120021102
1 1 1
Tube length required: U 1,500 0.001 0.002 864

1
2
Outside area/tube 0.2618 ft /ft 0.00067 .003 0.00116 0.00483
Total tube footage 340/0.2618 1,300 ft U
2
U 207 Btu>hr 1ft 2 1°F2 1neglecting tube wall resistance2
Try 15.25-in. I.D. shell 4 pass, with 78 tubes total, 16-ft
long on 1 / 4 -in. triangular pitch. 12. Log mean temperature difference:
1
Trial area (78) (0.2618) (16-ft long) 326 ft 2
This does not allow for tube area lost inside the two
Condensing
tubesheets of a fixed T. S. unit.
106.6°F. 106.6°F.
8. Flow area: 100°F. Warming 90°F.
6.6 16.6
Number of tube side passes 78/18.2 4.28, use 4 tube passes
Number of tubes/pass 78/4 19.5, use 19
16.6 6.6
Flow area/pass (19) (0.00333) 0.0633 ft 2 LMTD 10.8°F 1Figure 10-332
16.6
In
6.6
0.303 cfs
Velocity in tubes 4.8 fps>pass
2
0.0633 ft >pass
13. Area required:
9. For the film coefficient, tube side, use Figure 10-50A or Q 680,000
A 304 ft 2
10-50B: U t 12072110.82
Mean water temperature 95°F
14. Area available in selected unit:
Vel 4.8
read h i 1,150 Btu/hr (ft ) (°F)
2
A (0.2618) (78) (15.5) 316 ft net
2
correction for 1-in. tube Fw 0.96 at 0.782-in. I.D.
Correction to outside of tube: 680,000 2
U 199 Btu>hr 1ft 21°F2
h io (1,150) (0.96) (0.782/1.0) 864 Btu/hr (ft ) (°F) 13162110.82
2
10. Film coefficient, shell side: 15. Safely factor or percent excess surface:
W 1316 304211002
Condensate loading: G o – % 3.9%
2>3 304
LN t
1440
G o – 5.1 lb>hr 1lin ft2
116 ft 6 in.>1221782 2>3 16. Pressure drop, tube side:
End return loss (from Figure 10-139) at 4.8 ft/sec (0.63)
(allowing 3-in. thickness/tubesheet, thus reducing
(4 passes) 2.52 psi
effective tube length)
For the tube pressure drop, use Figure 10-138:
Thermal conductivity, k a 0.29 Btu/(hr) (ft ) (°F/ft) At water rate 68,000 lb>hr 3,580 lb>hr 1tube21pass2
2
Specific gravity 0.59 19 tubes>pass