Page 123 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 99
equipment are usually operated by electric motors, although other prime
movers, such as steam engines, internal-combustion engines, and hydraulic
turbines are sometimes employed.
When a design engineer is setting up the specifications for a new plant, a
decision must be made on whether to use purchased power or have the plant set
up its own power unit. It may be possible to obtain steam for processing and
heating as a by-product from the self-generation of electricity, and this factor
may influence the final decision. In some cases , it may be justified by means of
a HAZOP study to provide power to the plant from two independent sources to
permit continued operation of the plant facilities if one of the power sources
fails.
Power can be transmitted in various forms, such as mechanical energy,
electrical energy, heat energy, and pressure energy. The engineer should recog-
nize the different methods for transmitting power and must choose the ones
best suited to the particular process under development.
Steam is generated from whatever fuel is the cheapest, usually at pressures
of 450 psig (3100 kPa) or more, expanded through turbines or other prime
movers to generate the necessary plant power, and the exhaust steam is used in
the process as heat. The quantity of steam used in a process depends upon the
thermal requirements, plus the mechanical power needs, if such a power is
generated in the plant.
Water for industrial purposes can be obtained from one of two general
sources: the plant’s own source or a municipal supply. If the demands for water
are large, it is more economical for the plant to provide its own water source.
Such a supply may be obtained from drilled wells, rivers, lakes, damned streams,
or other impounded supplies. Before a company agrees to go ahead with any
new project, it must ensure itself of a sufficient supply of water for all industrial,
sanitary, and safety demands, both present and future.
The value of an abundance of good water supplies is reflected in the
selling price of plant locations that have such supplies. Any engineering tech-
niques which are required to procure, conserve, and treat water significantly
increase the operational cost for a plant or process. Increased costs of water
processing have made maximum use of the processed water essential. In fact,
the high costs of constructing and operating a waste treatment plant have led to
concentration of industrial wastes with the smallest amount of water, except
where treatment processes require dilution.

STRUCTURAL DESIGN
One of the most important aspects in structural design for the process industries
is a correct foundation design with allowances for heavy equipment and vibrat-
ing machinery used. The purpose of the foundation is to distribute the load so
that excessive or damaging settling will not occur. The type of foundation
depends on the load involved and the material on which the foundation acts. It