Page 163 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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126 Applied Process Design for Chemical and Petrochemical Plants

chart reads p t 6.2 psi>100 ft This is a significant reduction in area and is primarily
due to the increased t during winter operation. Actu-
6.2
Total exchanger p t a b14 passes2116 ft tubes2 ally, the unit will subcool the condensate with the excess
100
surface during the winter. Note that this result is based
3.87 psi on maintaining the same water quantity through the
tubes. If a lower velocity is acceptable for the water
For usual purposes the effect of water temperature is conditions, then a higher temperature rise can be taken,
not great, and Figure 10-138 can be used. The which reduces the liquid subcooling. Very few waters are
preceding value checks Stoever 109 with p t 3.85 psi. acceptable for cooling without excessive scaling when
the velocity falls below 1 ft/sec.
Total tube side p t 2.52 3.87 6.39 psi 6.4 psi 19. Although the previously discussed unit will perform as
Allow: 8 psi required, it may be larger than necessary. The water
velocity of 4.8 ft/sec is not as high as would be pre-
Total shell side p s (refer to the pressure drop section ferred.
of this chapter) can be neglected for pressure units 20. Redesign, using / 4 -in. bimetal tubes.
3
unless an unusual condition or design exists. To check, Try for a minimum tube velocity of 6.5 ft/sec:
follow procedure for unbaffled shell pressure drop.
17. For the specification sheet, see Figure 10-72. gpm 11362 110>52 272 gpm
3
ft >sec. 0.606
18. Winter operation—In order not to allow the water 2
Water flow area 0.606>6.5 0.0934 ft
velocity in the tubes to fall below 3 ft/sec in the winter,
you may have to compromise with the selected unit as 3
For / 4 -in. tubes with an I.D. equivalent to about a 12
based on 90°F water. If the average four-month winter
BWG tube:
temperature drops to 70°F, the quantity of water
required will be reduced as will the velocity through the 2
Tube flow area 0.223>144 0.00155 ft >tube
tubes. The low velocity is the point of concern. Check
Number of tubes 0.0934>0.00155 30.1 tubes>pass
to determine the prevailing conditions.
Assumed unit for trial conditions:
LMTD:
106.6 106.6 680,000
A 226 ft 1using 15° t for estimating2
2
80.6 070 12002115°2
26.6 036.6
LMTD = 31.5°F Outside area/tube 0.1963 ft 2
Number of feet tube 226/0.1963 1,550 ft
This assumes the same quantity of water in order to Using 16-ft tubes:
keep the velocity the same. Number required 1,150/16 72
3
15
Revised tube side film coefficient: For a 4-pass, fixed tubesheet unit, / 4 -in. tubes on / 16 -in.
triangular pitch, shell I.D. 12-in., number of tubes 84:
No. tubes/pass 84/4 21
At 75°F, h i 1,025
Flow area/pass (21) (0.00155) 0.0326 ft 2
3
ft /sec for 6.5 ft/sec (6.5) (0.0326) 0.212
Corrected:
gpm 0.212 (7.48) (60) 95.2
lb/hr (95.2) (8.33) (60) 47,700
2
h io 11,0252 10.962 1.782>1 .02 768 Btu>hr 1ft 2 1°F2
For the film coefficient, tube side, use a mean water temperature
of 85°F instead of 95°F. This will lean a little more to the winter
Assume the shell-side film coefficient unchanged, operation and be safe for summer.
because the previous selection was on conservative side. h i 1,400
3
correction for / 4 -in. tube 1.15
1 0.532
U 201 Btu>hr 1ft 21°F2 h io 11,400211.152a b 1,140 Btu>hr 1ft 21°F2
2
2
1 1 0.75
0.001 0.002
1,500 660
Film coefficient, shell side:
Revised area required:
680,000 1,440
A 107 ft 2 G o – 4.84 lb>hr 1lin ft2
12012131.52 15.51842 2>3