Page 195 - Plant design and economics for chemical engineers

COST ESTIMATION 169
80,000 1 I I I I Ill,, I I 11
60,000
50,000
40,000

10,000
8.000
6,000 I I III1 ia,. lb90
100 200 500 1,000 2,000 5,000
Outside heat-transfer area, sq ff
\
FIGURE 6-5 \
Application of “six-tenth-factor” rule to costs for shell-and-tube heat exchangers.

Estimating Equipment Costs by Scaling
It is often necessary to estimate the cost of a piece of equipment when no cost
data are available for the particular size of operational capacity involved. Good
results can be obtained by using the logarithmic relationship known as the
six-tenths-factor rule, if the new piece of equipment is similar to one of another
capacity for which cost data are available. According to this rule, if the cost of a
given unit at one capacity is known, the cost of a similar unit with X times the
capacity of the first is approximately (X)“.6 times the cost of the initial unit.

capac. equip. a O6
Cost of equip. a = cost of equip. b (1)
capac. equip. b
The preceding equation indicates that a log-log plot of capacity versus
equipment cost for a given type of equipment should be a straight line with a
slope equal to 0.6. Figure 6-5 presents a plot of this sort for shell-and-tube heat
exchangers. However, the application of the 0.6 rule of thumb for most pur-
chased equipment is an oversimplification of a valuable cost concept since the
actual values of the cost capacity factor vary from less than 0.2 to greater than
1.0 as shown in Table 5. Because of this, the 0.6 factor should only be used in
the absence of other information. In general, the cost-capacity concept should
not be used beyond a tenfold range of capacity, and care must be taken to make
certain the two pieces of equipment are similar with regard to type of construc-
tion, materials of construction, temperature and pressure operating range, and
other pertinent variables.