Turbine impuse and reaction

Introduction:

          Steam and gas turbines are power generating machines in which the pressure energy of the fluid is converted into mechanical energy. This conversion is due to the dynamic action of fluid flowing over the blades. These blades are mounted on the periphery of a rotating wheel in the radial direction. Today the steam turbine stands as one of the most important prime movers for power generation. It converts thermal energy into mechanical work by expanding high pressure and high temperature steam. The thermal efficiency of steam turbine is fairly high compared to steam engine. The uniform speed of steam turbine at wide loads makes it suitable for coupling it with generators, centrifugal pumps, centrifugal gas compressors, etc.

 Classification of Steam Turbines:

            Based on the action of steam on blades, steam turbines are classified into impulse turbines and reaction turbines (or impulse reaction turbines).

Impulse Steam Turbine: Impulse or impetus means sudden tendency of action without reflexes. A single-stage impulse turbine consists of a set of nozzles and moving blades as shown in figure 6.1. High pressure steam at boiler pressure enters the nozzle and expands to low condenser pressure in the nozzle. Thus, the pressure energy is converted into kinetic energy increasing the velocity of steam. The high velocity steam is then directed on a series of blades where the kinetic energy is absorbed and converted into an impulse force by changing the direction of flow of steam which gives rise to a change in momentum and therefore to a force. This causes the motion of blades. The velocity of steam decreases as it flows over the blades but the pressure remains constant, i.e. the pressure at the outlet side of the blade is equal to that at the inlet side. Such a turbine is termed as impulse turbine. Examples: De-Laval, Curtis, Moore, Zoelly, Rateau etc.

Fig. 6.1 Impulse turbine

 Impulse Reaction Steam Turbine: In the impulse reaction turbine, power is generated by the combination of impulse action and reaction by expanding the steam in both fixed blades (act as nozzles) and moving blades as shown in figure 6.2. Here the pressure of the steam drops partially in fixed blades and partially in moving blades. Steam enters the fixed row of blades, undergoes a small drop in pressure and increases in velocity. Then steam enters the moving row of blades, undergoes a change in direction and momentum (impulse action), and a small drop in pressure too (reaction), giving rise to increase in kinetic energy. Hence, such a turbine is termed as impulse reaction turbine. Examples: Parson, Ljungstrom etc.

Difference between Impulse and Reaction Turbines:

            The differences between impulse and reaction turbines are as follows:

Impulse Turbine	Reaction Turbine
ü  Complete expansion of the steam take place in the nozzle, hence steam is ejected with very high kinetic energy.	ü  Partial expansion of the steam takes place in the fixed blade (acts as nozzle) and further expansion takes place in the rotor blades.
ü  Blades are symmetrical in shape.	ü  Blades are non-symmetrical in shape, i.e. aerofoil section.
ü  Pressure remains constant between the ends of the moving blade. Hence relative velocity remains constant i.e.,	ü  Pressure drops from inlet to outlet of the moving blade. Hence relative velocity increases from inlet to outlet i.e.,
ü  Steam velocity at the inlet of machine is very high, hence needs compounding.	ü  Steam velocity at the inlet of machine is moderate or low, hence doesn’t need compounding.
ü  Blade efficiency is comparatively low.	ü  Blade efficiency is high.
ü  Few number of stages required for given pressure drop or power output, hence machine is compact.	ü  More number of stages required for given pressure drop or power output, hence machine is bulky.
ü  Used for small power generation.	ü  Used for medium and large power generation.
ü  Suitable, where the efficiency is not a matter of fact.	ü  Suitable, where the efficiency is a matter of fact.