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Steam Turbines

In a steam turbine cogeneration installation, large gas boilers heat water to produce high-pressure steam. Steam boilers produce steam with temperatures up to 950 degrees under as much as 1300 pounds of pressure. Natural gas usually fuels the boilers, but fuel oil can also be used when necessary, providing uninterrupted service when natural gas is in short supply.

High-pressure steam from the boilers turns the turbines at a speed of 3600 rpm. The rotating turbines drive the AC (Alternating Current) generators to produce electrical power.

Turbine Classification
Steam turbines are classified into two general groups: Impulse Turbines and Reaction Turbines, depending on the method used to cause the steam to be useful.

Turbines may be further classified according to the following:

  • Type and arrangement of staging
  • Direction of steam flow
    • Repetition of steam flow
    • Division of steam flow

A turbine may also be classified by whether it is a condensing unit (exhausts to a condenser at a pressure below atmospheric pressure) or a non-condensing unit (exhausts to another system such as the auxiliary exhaust steam system at a pressure above atmospheric pressure).

Other than the operating and controlling equipment, similarity exists in both the impulse and reaction turbines. These include foundations, casings, nozzles, rotors, bearings, and shaft glands.

The primary function of the nozzles is to convert the thermal energy of steam into kinetic energy. The secondary function of the nozzles is to direct the steam against the blades.

The rotors (forged wheels and shaft) are manufactured from steel alloys. The primary purpose of a turbine rotor is to carry the moving blades that convert the steam's kinetic energy to rotating mechanical energy.