Page 231 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

The calculation of utility costs can be quite complicated, and the true cost of such streams is often difficult
to estimate in a large facility. For estimating operating costs associated with supplying utilities to
different processes, the approach taken here is to assume that the capital investment required to build a
facility to supply the utility—for example, a cooling tower, a steam boiler, and so forth—has already
been made. This would be the case when a grassroots cost has been used for the fixed capital investment.
The costs associated with supplying a given utility are then obtained by calculating the operating costs to
generate the utility. These are the costs that have been presented in Table 8.3, and the following sections
show how these cost estimates were obtained for the major utilities given in the table.

Cooling Tower Water. In most large chemical, petrochemical, and refinery plants, cooling water is
supplied to process units from a central facility. This facility consists of a cooling tower (or many
towers), water makeup, chemical injection, and the cooling water feed pumps. A typical cooling water
facility is shown in Figure 8.2.

Figure 8.2 Schematic Diagram of Cooling Water Loop

The cooling of the water occurs in the cooling tower where some of the water is evaporated. Adding
makeup water to the circulating cooling water stream makes up this loss. Because essentially pure water
is evaporated, there is a tendency for inorganic material to accumulate in the circulating loop; therefore,
there is a water purge or blowdown from the system. The makeup water stream also accounts for windage
or spray losses from the tower and also the water purge. Chemicals are added to reduce the tendency of

the water to foul heat-exchanger surfaces within the processes. For a detailed discussion and further
information regarding the conditioning of water for cooling towers, the reader is referred to Hile et al. [7]
and Gibson [8]. From Figure 8.2, we can estimate the cost to supply process users with cooling water if
the following are known:
• Total heat load and circulation rate required for process users
• Composition and saturation compositions of inorganic chemicals in the feed water
• Required chemical addition rate
• Desired supply and return temperatures (shown earlier to be 30°C and 40°C, respectively)
• Cost of cooling tower and cooling water pumps
• Costs of supply chemicals, electricity for pumps and cooling tower fans, and makeup water

The estimation of operating costs associated with a typical cooling water system is illustrated in Example