Page 173 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10E 5/30/2001 4:33 PM Page 136

136 Applied Process Design for Chemical and Petrochemical Plants

Water Required Flow area for water (323) (0.233 in. /tube)/144 0.5 ft 2
2
Velocity of water through tubes 2.55 cfs/0.5 5.1 ft/sec
Assume a 7°F rise in sea water temperature: From Figure 10-50A or 10-50B, for water,
2
h i 1,200 Btu/hr (ft ) (°F)
lb water/hr (6,600,000 1,415,000)/(1) (7°) 1,144,000 Referenced to outside surface, tube I.D. 0.532 in.
Water temperature at the dew point: 0.532
2
h io 1,200a b 850 Btu>hr 1ft 21°F2
6,600,000 0.75
t 90 90 5.78 95.8°F
11,144,0002112 Shell-side coefficient: condensing.
2
For log mean temperature differences, see Figure 10-73. Assume / 3 of tube length is used for condensing 10 ft.
Referring to Figure 10-67,
68 16.2
LMTD desuperheating 36.2°F
68
2.3 log Tube loading G o " W/LN t 2/3 (52,400/2)/(10) (646) 2/3
16.2
G o " 26,200/(10)(74) 35.4 lb/lin. ft
LMTD condensing 19 F Propylene properties at 112°F (liquid):
Sp. gr. 0.473
Temperature difference correction for desuperheating f 0.087 centipoise
2
section only; see Figure 10-30 for 1 shell pass, 2 or more tube k f 0.0725 Btu/hr (ft ) (°F/ft)
2
passes. Read, h o 320, use 300 Btu/hr (ft ) (°F)
t 2 t 1 97 95.8
P 0.0173 Overall U for condensing:
T 1 t 1 165 95.8
Assume: water side fouling 0.002
T 1 T 2 165 112 propylene side fouling (oil) 0.0005
R 44.2
t 2 t 1 97 95.8 neglect tube wall resistance
Note that in reading the chart, values are off the scales, 1 1 1
0.0005 0.002
but by approximate interpolation, a value of F 1.0 is not U 300 850
unreasonable. In any case the error in using this value will 1
be small. 0.00333 0.0005 0.002 0.00117 0.0070
U
No correction is necessary for the condensing section. 2
U 142 Btu>hr 1ft 2 1 F2
Assume overall U values to establish initial order-of-mag-
nitude of area required: 16,600,000>22 2
Condensing area A 1,220 ft >unit
114221192
Condensing U 130 Shell-side coefficient: vapor desuperheating or cooling.
Desuperheating U 35 Tube length allowed for this approximately 15 ft 10 ft 5 ft
Refer to Figure 10-81.
6,600,000 1,415,000
Area estimated A Assume a baffle cut of 25% and spacing as shown.
1301192 35136.22
Note that allowance must be made for the entrance nozzle, which
2,670 1,118 3,788 ft 2 often means that baffles cannot be spaced too close to the
tubesheet.
2
Try two parallel units of approximately 1,894 ft each. Tube bundle cross flow area:
a s (D s ) (c') (B)/144 (p)
Select: c' 0.25 in. between tubes
3 / 4 -in. O.D. tubes 12 BWG cupro-nickel 16 ft, 0-in. long a s (29 in.) (0.25) (8 in.)/144 (1-in. pitch)
a s 0.403 ft 2
1,894
No. tubes 645 G s W/a s (52,400/2)/0.403 65,000 lb/hr (ft )
2
10.1962115 ft long tubes2
Vapor properties at 140°F:
c p 0.55 Btu/lb (°F)
Use two passes in tubes; tubes on 1-in. triangular pitch.
k a 0.0128 Btu/hr (ft ) (°F/ft)
2
From the tube count in Table 10-9, a 29-in. I.D. shell will ' 0.0109 centipoise
hold 646 tubes, including an allowance for tie rods. The 0.0109 (2.42) 0.0264 lb/hr (ft)
number of tubes per pass 646/2 323. Vapor density 2.2 lb/ft 3
Tube-side coefficient: (c p /k a ) 1/3 [(0.55) (0.0264)/0.0128] 1/3 1.042
The Reynolds number for Figure 10-54:
8,015,000 Btu>hr total D e 0.73 in. (0.73/12) ft
water flow>unit 573,000 lb>hr
12217°F2 10.73 in.>122165,0002
R e D e G s >
gpm 573,000/(8.33) (60) 1,148 0.0264
ft /sec. 573,000/(62.4) (3,600) 2.55 149,800
3