Page 226 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

cost contributions are not dependent on any of the estimating factors provided in Table 8.2. Therefore, the
manufacturing cost is generally insensitive to the estimating factors provided in Table 8.2. The use of the
midrange values is acceptable for this situation.

8.2 Cost of Operating Labor

The technique used to estimate operating labor requirements is based on data obtained from five chemical
companies and correlated by Alkayat and Gerrard [4]. According to this method, the operating labor
requirement for chemical processing plants is given by Equation 8.3:

(8.3)

where N is the number of operators per shift, P is the number of processing steps involving the handling
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of particulate solids—for example, transportation and distribution, particulate size control, and
particulate removal. N is the number of nonparticulate processing steps and includes compression,
np
heating and cooling, mixing, and reaction. In general, for the processes considered in this text, the value of
P is zero, and the value of N is given by
np

(8.4)

compressors
towers
reactors
heaters
exchangers

Equation 8.3 was derived for processes with, at most, two solid handling steps. For processes with a
greater number of solid handling operations, this equation should not be used.

The value of N in Equation 8.3 is the number of operators required to run the process unit per shift. A
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single operator works on the average 49 weeks a year (3 weeks’ time off for vacation and sick leave),
five 8-hour shifts a week. This amounts to (49 weeks/year × 5 shifts/week) 245 shifts per operator per
year. A chemical plant normally operates 24 hours/day. This requires (365 days/year × 3 shifts/day) 1095
operating shifts per year. The number of operators needed to provide this number of shifts is [(1095
shifts/yr)/(245 shifts/operator/yr)] or approximately 4.5 operators. Four and one-half operators are hired

for each operator needed in the plant at any time. This provides the needed operating labor but does not
include any support or supervisory staff.