Page 204 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

(A.3)

This equation is clearly different from Equation 7.10 for process vessels. Moreover, the value predicted
by this equation (using the appropriate constants) gives values of F much smaller than those for vessels
P
at the same pressure. This difference arises from the fact that for other equipment, the internals of the
equipment make up the major portion of the cost. Therefore, the cost of a thicker outer shell is a much
smaller fraction of the equipment cost than for a process vessel, which is strongly dependent on the
weight of the metal. Example 7.11 considers the effect of pressure on a shell-and-tube heat exchanger.

Example 7.11

a. Repeat Example 7.10 except consider the case when the operating pressures in both the shell-
and the tube-side are 100 barg.
b. Explain why the pressure factor for the heat exchanger is much smaller than for any of the
process vessels shown in Figure 7.6.

Solution

a. From Example 7.10, (2001) = $25,000, F = 1
P
From Table A.2, for 5<P<140 barg, C = 0.03881, C = –0.11272, C = 0.08183
1
2
3
Using Equation A.3 and substituting for P = 100 barg and the above constants,
2
log F = 0.03881 – 0.11272log (100) + 0.08183[log (100)] = 0.1407
10
10 P
10
F = 10 0.1407 = 1.383
P
From Equation A.4:

b. Compared with Figure 7.6, this pressure factor (1.383) is much less than any of the vessels at P
= 100 barg. Why?

The answer lies in the fact that much of the cost of a shell-and-tube heat exchanger is associated with the
cost of the tubes that constitute the heat exchange surface area. Tubing is sold in standard sizes based on
the BWG (Birmingham wire gauge) standard. Tubes for heat exchangers are typically between 19.1 and
31.8 mm (3/4 and 1-1/4 inch) in diameter and between 2.1 and 0.9 mm (0.083 and 0.035 inch) thick,
corresponding to BWGs of 14 to 20, respectively. Using Equation 7.9, the maximum operating pressure of
a 25.4 mm (1 inch) carbon steel tube can be estimated (assume that CA is zero), the results are as follows.