Electric Power Plants

The Turbine Nozzle

The turbine nozzle performs two functions:

1.It transforms a portion of the energy of the fluid, acquired in the heat exchanger and evidenced by a high pres-sure and temperature, into kinetic energy.

2. a) In the impulse turbine it directs the high-velocity fluid jet against blades which are free to move in order to convert the kinetic energy into shaft work; b) In the reaction turbine the nozzles, which are free to move, discharge high-velocity fluid. The reactive force of the fluid against the nozzle produces motion, and work is done.

For the first function to be performed efficiently, the nozzle walls must be smooth, streamlined, and so proportioned as to satisfy the changing conditions of the -steam or gas flowing through the nozzle.

For the second function the nozzle should discharge the fluid at the correct angle with the direct ion of blade motion to allow a maximum conversion of kinetic energy into work.

The main consideration in nozzle design is to provide a nozzle of proper wall contour. The contour of the walls depends upon the conditions of the fluid required by the turbine and upon certain properties of the fluid which are influenced by these established conditions. For nozzle design the engineer has at his disposal four fundamental tools or relations. They are: 1)the first law of thermodynamics; 2)the equation of continuity of flow; 3)the characteristic equation of continuity of flow; 4)the equation of the process.

The two main types of power plants traditionally have been the fossil-fuel steam-electric plant and the hydroelectric plant. Other types, including internal-combustion-engine plants and nuclear plants also have been built. The selection of a particular type of generating plant and its location involves consideration of a number of factors such as plant, fuel, and transmission line costs; availability of cooling water; and environmental considerations.

For several reasons, the relative importance of the various types of power plants has been shifting. Good sites for new hydroelectric plants have become scarce in many countries. Distribution networks have been extended so that less expensive power from large steam-electric stations has been replacing power from smaller diesel-generator units. Nuclear-electric power plants have been built instead of fossil-fuel steam-electric plants because the cost of coal and oil has been increasing.

In the United States in 1970, fossil-fuel steam-electric plants ac-counted for 76% of the power generated, hydroelectric plants for 16%, and nuclear plants for 2%.

In 2000 45% of the electric power in the United States is generated from fossil-fuel steam-electric plants, 45% from nuclear plants, and10% from hydroelectric plants.

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