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Steam
turbines are made in a variety of sizes ranging from
small 1 hp (0.75 kW) units (rare) used as mechanical
drives for pumps, compressors and other shaft driven
equipment, to 2,000,000 hp (1,500,000 kW) turbines
used to generate electricity. There are several
classifications for modern steam turbines.
Steam Supply and Exhaust Conditions
These types include condensing, noncondensing,
reheat, extraction and induction.
Noncondensing or backpressure turbines are most
widely used for process steam applications. The
exhaust pressure is controlled by a regulating valve
to suit the needs of the process steam pressure.
These are commonly found at refineries, district
heating units, pulp and paper plants, and
desalination facilities where large amounts of low
pressure process steam is available.
Condensing turbines are most commonly found in
electrical power plants. These turbines exhaust
steam in a partially condensed state, typically of a
quality near 90%, at a pressure well below
atmospheric to a condenser.
Reheat turbines are also used almost exclusively in
electrical power plants. In a reheat turbine, steam
flow exits from a high pressure section of the
turbine and is returned to the boiler where
additional superheat is added. The steam then goes
back into an intermediate pressure section of the
turbine and continues its expansion.
Extracting type turbines are common in all
applications. In an extracting type turbine, steam
is released from various stages of the turbine, and
used for industrial process needs or sent to boiler
feed water heaters to improve overall cycle
efficiency. Extraction flows may be controlled with
a valve, or left uncontrolled. Induction turbines
introduce low pressure steam at an intermediate
stage to produce additional power.
Casing or Shaft Arrangements
These arrangements include single casing, tandem
compound and cross compound turbines. Single casing
units are the most basic style where a single casing
and shaft are coupled to a generator. Tandem
compound are used where two or more casings are
directly coupled together to drive a single
generator. A cross compound turbine arrangement
features two or more shafts not in line driving two
or more generators that often operate at different
speeds. A cross compound |
