STEAM TURBINE DESIGN
Typical Plants and Cycles
Definition. The cycle of a steam power plant is the group of interconnected
pressures, temperatures, and capacities, and integrated into a practical arrangement to
serve the electrical (and sometimes by-product steam) requirements of a particular
project. Selection of the optimum cycle depends upon plant size, cost of money, fuel
costs, non-fuel operating costs, and maintenance costs.
Smaller Turbines. Turbines under 1,000 kW may be single stage units because
of lower first cost and simplicity. Single stage turbines, either back pressure or
condensing, are not equipped with extraction openings.
Larger Turbines. Turbines for 5,000 kW to 30,000 kW shall be multi-stage,
multi-valve units, either back pressure or condensing types.
a) Back Pressure Turbines. Back pressure turbine units usually exhaust at
pressures between 5 psig (34 kPa gage) and 300 psig (2068 kPa gage) with one or two
controlled or uncontrolled extractions. However, there is a significant price
difference between controlled and uncontrolled extraction turbines, the former being
more expensive. Controlled extraction is normally applied where the bleed steam is
exported to process or district heat users.
b) Condensing Turbines. Condensing units exhaust at pressures between 1
inch of mercury absolute (Hga) and 5 inches Hga, with up to two controlled, or up to
five uncontrolled extractions.
Selection of Cycle Conditions. The function or purpose for which the plant is
intended determines the conditions, types, and sizes of steam generators and turbine
drives and extraction pressures.
Simple Condensing Cycles. Straight condensing cycles or condensing units with
uncontrolled extractions are applicable to plants or situations where security or
isolation from public utility power supply is more important than lowest power cost.
Because of their higher heat rates and operating costs per unit output, it is not likely
that simple condensing cycles will be economically justified for a military power plant
application as compared with that associated with public utility purchased power costs.
A schematic diagram of an uncontrolled extraction-cycle is shown in Figure 11.
Controlled Extraction-Condensing Cycles and Back Pressure Cycles. Back
pressure and controlled extraction-condensing cycles are attractive and applicable to a
cogeneration plant, which is defined as a power plant simultaneously supplying either
controlled extraction-condensing cycle is shown in Figure 12. A schematic diagram of a
back pressure cycle is shown in Figure 13.