Plant Study. A plant study shall be conducted before authorization
to develop definitive designs is provided. The study will evaluate plant
operating parameters with a primary emphasis on economic factors. Major
emphasis will also be given to energy conservation and environmental concerns.
The documented study shall determine:
Plant load and operating characteristics,
Fuels to be burned,
Required dependability of service,
Predicted lifespan of the plant,
Estimated cost (Capital and operating),
Figure 1 provides the predominant costs which affect the plant. It
may be used as a basis for comparing plant alternatives. Table 1 provides a
list of predominant design considerations for design of thermal plants.
Thermal Generating Equipment
The combination of number and size of thermal generating
equipment in a plant determines the plant capacity. A plant may be installed
with a single boiler able to provide the required plant capacity for
applications where loss of the boiler will not adversely affect the operation.
Most applications cannot tolerate the complete loss of a thermal plant. These
applications require that two boilers provide 135 to 150 percent of the
plant's maximum steam demand. The continuous capacity of the plant with a
boiler not operating shall be greater than the essential plant load. A spare
boiler may be needed to provide the necessary backup.
The recommended method for selecting the number of boilers in this
situation is to use three equal capacity boilers which can each carry 50
percent of the maximum load. A variation of this scheme is to use two equal
capacity boilers each capable of providing two thirds of the maximum load and
a smaller boiler capable of carrying one third of the maximum load. The
maximum steam plant demand shall not require operation of more than three
simultaneously fired boilers. A smaller peaking and off-season boiler should
be included if increased efficiency at low loads economically justifies the
installation. Avoid installation of initial main boilers whose capacities are
smaller than those to be added when the plant is expanded. Avoid
unnecessarily large numbers of small boilers.