Page 220 - Plant design and economics for chemical engineers

192 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

project indirects. As an example of an equipment cost module for heat exchang-
ers, the module would include the basic delivered cost of the piece of equip-
ment with factors similar to Lang factors being presented for supplemental
items needed to get the equipment ready for use such as piping, insulation,
paint,- labor, auxiliaries, indirect costs, and contingencies.
In presenting the basic data for the module factors, the three critical
variables are size or capacity of the equipment, materials of construction, and
operating pressure with temperature often being given as a fourth critical
variable. It is convenient to establish the base cost of all equipment as that
constructed of carbon steel and operated at atmospheric pressure. Factors, such
as are presented in Table 16, are then used to change the estimated costs of the
equipment to account for variation in the preceding critical variables. Once the
equipment cost for the module is determined, various factors are applied to
obtain the final fixed-capital investment estimate for the item completely in-
stalled and ready for operation. Figure 6-6 shows two typical module ap-
proaches with Fig. 6-6~ representing a module that applies to a “normal”
chemical process where the overall Lang factor for application to the f.o.b. cost
of the original equipment is 3.482 and Fig. 6-6b representing a “normal”
module for a piece of mechanical equipment where the Lang factor has been
determined to be 2.456.
The modules referred to in the preceding can be based on combinations of
equipment that involve similar types of operations requiring related types of
auxiliaries. An example would be a distillation operation requiring the distilla-
tion column with the necessary auxiliaries of reboiler, condenser, pumps, holdup
tanks, and structural supports. This type of compartmentalization for estimating
purposes can be considered as resulting in a so-called unit-operations estimate.
Similarly, the functional-unit estimate is based on the grouping of equipment by
function such as distillation or filtration and including the fundamental pieces of
equipment as the initial basis with factors applied to give the final estimate of
the capital investment.
The average-unit-cost method puts special emphasis on the three variables
of size of equipment, materials of construction, and operating pressure as well
as on the type of process involved. In its simplest form, all of these variables and
the types of process can be accounted for by one number so that a given factor
to convert the process equipment cost to total fixed-capital investment can apply
for each “average unit cost.” The latter is defined as the total cost of the
process equipment divided by the number of equipment items in that particular
process. As the “average unit cost” increases, the size of the factor for
converting equipment cost to total fixed-capital investment decreases with a
range of factor values applicable for each “average unit cost” depending on the
particular type of process, operating conditions, and materials of construction.

ESTIMATION OF TOTAL PRODUCT COST

Methods for estimating the total capital investment required for a given plant
are presented in the first part of this chapter. Determination of the necessary