Page 205 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

From the table, it is evident that even the thinnest tube normally used for heat exchangers is capable of
withstanding pressures much greater than atmospheric. Therefore, the most costly portion of a shell-and-
tube heat exchanger (the cost of the tubes) is relatively insensitive to pressure. Hence, it makes sense that
the pressure factors for this type of equipment are much smaller than those for process vessels at the same
pressure.

The purchased cost of the equipment for the heat exchanger in Example 7.11 would be C (2006) =
P
($25,000)(1.383) (500/394) = $43,880. If this equipment cost were multiplied by the bare module factor
for the base case, the cost would become C BM = ($43,880)(3.29) = $144,360. This is 16% greater than

the $124,490 calculated in Example 7.11. The difference between these two costs results from assuming,
in the latter case, that all costs increase in direct proportion to the increase in material cost. This is far
from the truth. Some costs, such as insulation, show small changes with the cost of materials, whereas
other costs, such as installation materials, freight, labor, and so on, are impacted to varying extents. The
method of equipment module costing accounts for these variations in the bare module factor.

Finally, some equipment is unaffected by pressure. Examples are tower trays and packing. This
“equipment” is not subjected to significant differential pressure because it is surrounded by process fluid.
Therefore, in Equation A.3, use C = C = C = 0. Some other equipment also has zero for these constants.
1
2
3
For example, compressor drives are not exposed to the process fluid and so are not significantly affected
by operating pressure. Other equipment, such as compressors, do not have pressure corrections because
such data were not available. Use of these cost correlations for equipment outside the pressure range
shown in Table A.2 should be done with extreme caution.

Materials of Construction (MOCs). The choice of what MOC to use depends on the chemicals that
will contact the walls of the equipment. As a guide, Table 7.9, excerpted from Sandler and Luckiewicz
[14], may be used for preliminary MOC selection. However, the interaction between process streams and
MOCs can be very complex and the compatibility of the MOC with the process stream must be
investigated fully before the final design is completed.

Talbe 7.9 Corrosion Characteristics for Some Materials of Construction