CLASSIFICATION OF STEAM TURBINES

On the basis of principles of operation, steam turbines are classified as follows:

1. Impulse turbine
2. Reaction turbine
3. Impulse-reaction turbine

1. Impulse Turbine: This type of turbine is called a simple impulse turbine because the expansion of steam takes place only in one set of nozzles, as shown in Fig. 7.1. The pressure of steam falls from that in the steam nozzle to that existing in the condenser, whereas the steam flows through it. Hence, the pressure in the wheel chamber is practically equal to the condenser pressure.The high-velocity jet coming out from the nozzle impinges on the blades, so that the whole kinetic energy is converted into mechanical energy. In practice, this type of turbine is used for small power ratings. The rotor diameter is kept small and consequently, the rotational speed becomes very high. This is known as De-Laval turbine.Figure 7.1 Simple impulse turbine
2. Reaction Turbine: In a reaction turbine, there are guide blades instead of nozzles which convert the pressure energy into kinetic energy. The steam passing over the moving blades has the difference of pressure at the inlet tip and exit tip; hence, there is a drop of pressure in steam while passing over the moving blades. The fixed blade serves the purpose of nozzles which changes the direction of steam and at the same time, allows it to expand to a higher velocity. The pressure of steam falls as it passes over the moving blades. The diameter of each stage of a reaction turbine must increase after each group of blade rings in order to accommodate the increased volume of steam at lower pressure. Figure 7.2 Impulse-reaction turbine
3. Impulse-Reaction Turbine: If the pressure of steam at outlet from the moving blades of the turbine is less than that at the inlet side of the blades, the drop in pressure suffered by steam during its flow through the moving blades causes a further generation of kinetic energy within the blades and adds to the preparing force which is applied to the turbine shaft; such a turbine is called impulse-reaction turbine.This type of turbine is shown in Fig. 7.2. In this type of turbine, there is application of both principles, namely, impulse and reaction. There are several rows of moving blades fixed to the shaft and equal number of fixed blades attached to the casing. The fixed blades correspond to the nozzles of an impulse turbine. Steam is admitted for the entire circumference and therefore, there is all-round admission. In passing through the first set of fixed blades, the steam undergoes a small pressure drop and the velocity is increased. It enters the first set of moving blades and suffers the change of direction and therefore momentum. This gives an impulse to the blades. However, here, the passage to the blades is so designed that there is also a small drop in pressure in the moving blades, giving rise to an increase in kinetic energy. This drop in pressure gives rise to reactions in the direction opposite to that of added velocity. Thus, the driving force is the vector sum of impulse and reaction turbines. Normally, the turbine is known as reaction turbine. This is also called Parson’s reaction turbine.