Page 161 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10D 5/30/2001 4:31 PM Page 124

124 Applied Process Design for Chemical and Petrochemical Plants

(e) Assuming a tube length, l: (p) Compare, and if the available area is equal to or greater
than the required area, the selected unit will perform
F 1
No. passes P a (10-99) satisfactorily. If the required area is greater than the
1n¿21l2
available area, select a new unit with more tubes, longer
If this value is not reasonable, reassume the tube tubes, larger tubes, or some combination. Repeat from
length, and/or the size of tubes. Try to keep the step 8, unless the minimum water velocity can safely be
number of passes fewer than 8 except in special changed, then repeat from step 7.
cases, as construction is expensive. (q) Check the effect of winter operating conditions on
(f) From Table 10-9 pick an exchanger shell diameter the importance of (1) maintaining constant yearly
that closely contains the required number of tubes outlet condensate temperature; (2) subcooled con-
at the required number of passes. densate as a result of excess surface area due to
(g) From the actual tube count selected, establish the lower inlet cooling water; and (3) maintaining a
actual number of tubes/pass. They may be a few minimum water velocity in tubes.
tubes more or less than initially figured.
Calculate the flow area/pass (number of tubes/pass) Example 10-10. Total Condenser
(cross-section flow area/tube), ft /pass.
2
2
(h)Calculate velocity in tubes cfs/(ft /pass), ft/sec. Ammonia vapors from a stripping operation are to be
(i) For the film coefficient, tube side, read h i from Fig- condensed. Select the condenser pressure, which sets the
ure 10-50A or 10-50B at the mean water tempera- top of stripper pressure, and design a condenser. Water at
ture and calculated velocity of (h). 90°F is to be used.
Correct to the outside of tube: Flow: 1,440 lb/hr ammonia, at dewpoint.

tube I.D. 1. The condenser operating pressure should be so selected
h io 1h i 21tube dia. film correction, F w 2 a b,
tube O.D. as to give a reasonable temperature difference between
2
Btu>hr 1ft 21°F2 (10-100) the condensing temperature and the water temperature.
The quantity of water required should not be penalized
(j) For the film coefficient, shell side, calculate G o by requiring a small temperature rise in the water.
from Equation 10-73B or 10-76, lb/hr (lin. ft) By referring to a Mollier diagram for ammonia, the
Do not use full tube length as effective, reduce “l” by condensing temperature at 220 psig is 106.6°F. This is
the estimated tubesheet thickness at each end; usually about the lowest operating pressure possible to keep a
1 / 2 in. per tubesheet for low pressure (to 150 psi) and T of greater than 10°F between water and ammonia.
1
3 in. for higher pressure (to about 600 psi) is satisfac- 2. Heat load:
tory.
From Figure 10-67A, read h o , Btu/hr (ft ) (°F). Q 114402 1470.5 Btu>lb latent heat2 680,000 Btu>hr
2
(k) Select fouling factors from tube side and shell side,
from Table 10-12 or 10-13 or your own experience. 3. Minimum water tube velocity: set at 5 ft/sec.
(l) Calculate the overall coefficient: 4. Water required for 10°F temperature rise:

1 Q Wc p T
2
U , Btu>1hr21ft 21°F2 (10-101)
1 1 680,000
r o r io W 68,000 lb>hr
1121102
h o h io
68,000
Usually the tube wall resistance can be neglected, but if gpm 136
18.3321602
you doubt its effect, add to the resistances.
(m) Calculate log mean temperature difference by ft >sec 136 0.303
3
using Figure 10-33 or Equation 10-13. 17.4821602
(n) Area required:
5. Water flow area:
2
A Q>U 1Dt2, ft net
Total tube cross-section flow area 0.303/5 ft/sec 0.0606 ft 2
(o) Area available in assumed unit:
6. Number of tubes, using 1-in., 12 BWG tube:
A = (ft surface/ft tube)(number of tubes total)
2
2
2
(net tube length) (10-102) Tube flow area 0.479 in. /144 0.00333 ft /tube