Page 79 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 57

Types
Steam turbines are available in two types: axial-flow turbines and radial-flow turbines.
Axial-flow steam turbines are those in which high-pressure steam is introduced into the
turbine inlet at one end of the turbine and steam flows along the turbine’s axis of
rotation driving finned (bladed) wheels, or stages, that spin much like a windmill spins
under the influence of the wind. Axial-flow steam turbines are further delineated into
several basic types, including
• Noncondensing (backpressure) turbines
• Condensing turbines
• Automatic extraction turbines
• Nonautomatic extraction turbines
• Induction (mixed-pressure) turbines
• Induction-extraction turbines
Axial-flow turbines are also defined by the type of stages and blades. The blades can
either be impulse or reaction. Impulse blades are fixed to the turbine wheel and undergo
rotation from the force of the steam hitting the turbine blades, while reaction blades also
undergo rotation due to the nozzle effect as the steam leaves the blades.
Radial-flow steam turbines are dramatically different from their axial-flow counter-
parts. In a radial-flow steam turbine, high-pressure steam enters the turbine in the
center of the turbine impeller and decompresses radially, perpendicular to the turbine’s
axis of rotation. This drop in steam pressure (and energy) provides the motive force that
causes the rotation of the turbine and, thus, the rotation of the shaft driving any mechanical
device or generator. Multistage radial-inflow steam turbines are factory prepackaged
equipment that include two or more impellers connected through reduction gearing
with steam piping installed between stages to transport steam from one stage to the
next. Condensate, if any, is removed between stages, since turbines (of all types) operating
at high rpm can be severely damaged if subjected to trace water droplets.
A noncondensing backpressure steam turbine’s exhaust is under pressure, and is,
therefore, called a backpressure turbine. The backpressure can be at any pressure required
by the low-pressure secondary steam system, so long as that pressure is lower than the
turbine inlet pressure. The greater the pressure difference the more potential for genera-
tion of power. Backpressure steam turbines provide an energy efficient method to reduce
steam pressures compared to using pressure reducing valves which lose much of the
steam energy. Most of the energy difference between the steam entering and leaving a
backpressure turbine is converted to shaft horsepower so the process is quite efficient. For
example, if a steam boiler can produce 200-psig steam and only 60 psig is needed for
distribution, a backpressure steam turbine can be used to generate power operating on
the energy difference between 200- and 60-psig steam. The power produced is a function
of the steam pressure difference across the backpressure turbine and the steam flow.
Steam flow is related to the thermal loads served and usually varies.
Another application may serve part of steam plant needs where different pressures
are needed. As an example, a hospital may need 150-psi steam for sterilizers and 15-psi
steam for domestic water and space heating, absorption cooling, and other process.
When the steam is produced by waste heat recovery from a prime mover like a com-
bustion turbine the application is similar. Steam is generated at pressures higher than