Page 73 - Chemical process engineering design and economics

58 Chapter 2

propeller agitator for 3 hp in January 1990 is $2800, and the correlation range is 1
to 7 hp. Bringing the equipment cost up-to-date will be discuss later.
Equation 2.6 will be linear when plotted on log-log coordinates. The slope
of the line is the capacity exponent, n. In most cases, the equipment size, cost, and
capacity exponents in Table 2.9 were taken from Peters and Timmerhaus's log-log
plots [4]. If the log-log plot was not linear, it was approximated by a straight line
to maintain the simple relationship given by Equation 2.6. If you cannot find a
capacity exponent for a piece-of-equipment, Lang [14] suggested using six tenths.
This is called the six tenths rule. Drew and Ginder [33], however, found that six
tenths is appropriate for pilot-scale equipment and seven tenths for large equip-
ment. Because most exponents are less than one, doubling the equipment capacity
will not double the equipment cost, which is an example of the economy of scale.

Correcting Equipment Cost for Design, Material of Construction, Tempera-
ture, and Pressure

Sometimes, the cost literature contains equipment cost at base conditions, CB i in
Equation 2.7. The base conditions are a low temperature and pressure, carbon steel
construction, and a specific design. If you need the actual cost of equipment, CA i,
at other conditions, multiply the base cost by correction factors. Thus,

f
= rfpf M fDCBi (2.7)
C Ai
f
f
f
where T corrects for temperature, P for pressure, M for material of construction,
and f D for a specific design. Table 2.10 contains values of f T, f P, and f M for some
equipment. For the case where the equipment is only available in one design, f =
D
1. The factors in Equation 2.7 depend on the type of equipment, and thus using the
same correction factors for all equipment is an approximation. Also, if the equip-
ment operates at extreme conditions of temperature, pressure, or with a corrosive
fluid, the correction factors in Table 2.10 will be too low.
For shell-and-tube heat exchangers, the correction factors are defined differ-
ently. The shell material may be different than the tube material. If the process
fluid is corrosive, for example, then the tube material could be stainless steel.
Also, it is good practice to place the high-pressure fluid on the tube side to reduce
the cost of metal. Table 2.11 contains material factors obtained from Guthrie [13]
for combinations of shell-and-tube materials. Also, use the pressure and design
correction factors given in Table 2.11 instead of Table 2.10. Because Guthrie [13]
does not give any temperature correction factors use the factors given in Table
2.10, which will increase the heat-exchanger cost. To underestimate is worse than
to overestimate, up to a point. Using Table 2.11, then, for heat exchangers the cost
equation is

C Ai = f T (f P + f D )f M C B i (2.8)

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