Page 276 - Chemical engineering design

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COSTING AND PROJECT EVALUATION
6.6.1. Lang factors
The factorial method of cost estimation is often attributed to Lang (1948). The fixed
capital cost of the project is given as a function of the total purchase equipment cost by
the equation:
Cf D f L Ce 6.6
where Cf D fixed capital cost,
Ce D the total delivered cost of all the major equipment items: storage tanks,
reaction vessels, columns, heat exchangers, etc.,
f L D the “Lang factor”, which depends on the type of process.

f L D 3.1 for predominantly solids processing plant
f L D 4.7 for predominantly fluids processing plant
f L D 3.6 for a mixed fluids-solids processing plant
The values given above should be used as a guide; the factor is best derived from an
organisation’s own cost files.
Equation 6.6 can be used to make a quick estimate of capital cost in the early stages of
project design, when the preliminary flow-sheets have been drawn up and the main items
of equipment roughly sized.

6.6.2. Detailed factorial estimates
To make a more accurate estimate, the cost factors that are compounded into the
“Lang factor” are considered individually. The direct-cost items that are incurred in the
construction of a plant, in addition to the cost of equipment are:
1. Equipment erection, including foundations and minor structural work.
2. Piping, including insulation and painting.
3. Electrical, power and lighting.
4. Instruments, local and control room.
5. Process buildings and structures.
6. Ancillary buildings, offices, laboratory buildings, workshops.
7. Storages, raw materials and finished product.
8. Utilities (Services), provision of plant for steam, water, air, firefighting services (if
not costed separately).
9. Site, and site preparation.
The contribution of each of these items to the total capital cost is calculated by multiplying
the total purchased equipment by an appropriate factor. As with the basic “Lang factor”,
these factors are best derived from historical cost data for similar processes. Typical values
for the factors are given in several references, Happle and Jordan (1975) and Garrett
(1989). Guthrie (1974), splits the costs into the material and labour portions and gives
separate factors for each. In a booklet published by the Institution of Chemical Engineers,
IChemE (1988), the factors are shown as a function of plant size and complexity.
The accuracy and reliability of an estimate can be improved by dividing the process
into sub-units and using factors that depend on the function of the sub-units; see Guthrie
(1969). In Guthrie’s detailed method of cost estimation the installation, piping and