OVERVIEW

In a steam turbine, high-enthalpy (high pressure and temperature) steam is expanded in the nozzles (stationary blades) where the kinetic energy is increased at the expense of pressure energy (increase in velocity due to decrease in pressure). The kinetic energy (high velocity) is converted into mechanical energy (rotation of a shaft increase of torque or speed) by impulse and reaction principles. The impulse principle consists of changing the momentum ( m V) of the flow, which is directed to the moving blades by the stationary blades. The jet s impulse force pushes the moving blades forward. The reaction principle consists of a reaction force on the moving blades due to acceleration of the flow as a result of decreasing cross-sectional area.

Figure 3.1 illustrates a turbine with impulse blading. It has one velocity-compounded stage (the velocity is absorbed in stages) and four pressure-compounded stages. The velocity is reduced in two steps through the first two rows of moving blades. In the moving blades, velocity decreases while the pressure remains constant.

Figure 3.1: Turbine with impulse blading. Velocity compounding is accomplished in the first two stages by two rows of moving blades between which is placed a row of stationary blades that reverses the direction of steam flow as it passes from the first to the second row of moving blades. Other ways of accomplishing velocity compounding involve redirecting the steam jets so that they strike the same row of blades several times with progressively decreasing velocity.

Figure...