Steam Turbine | Working Principle of Impulse and Reaction Steam Turbine


In Impulse Steam Turbine, there are some fixed nozzles and moving blades are present on a disc mounted on a shaft. Moving blades are in symmetrical order. The steam enters the turbine casing with some pressure. After that, it passes through one or more no. of fixed nozzles into the turbine. The relative velocity of steam at the outlet of the moving blades is same as the inlet to the blades. During Expansion, steam's pressure falls. Due to high-pressure drop in the nozzles the velocity of steam increases.

impulse steam turbine

This high-velocity jet of steam flows through fixed nozzles and it strikes the blade with constant pressure. An impulse turbine, steam produced only impulsive force to the blades. Now blades are starting to move in the same direction of the steam flow. Due to change in momentum, turbine's shaft is starting to rotate.

An example of Simple Impulse turbine is a DA-Laval turbine.


Working principle of Impulse Reaction turbine depends on reaction force produced by steam. Here steam flows through the nozzles at the end of the tubes and it is supported on the bearings. The outlet relative velocity of steam is much less than at the inlet to the blades.

reaction steam turbine

In a reaction turbine, nozzles will move on bearing in the opposite direction of the steam flow and the pressure is not constant in this turbine. That's why; a reaction force is always applied on the nozzles and tubes. In this turbine steam produces both impulsive and reactive force. So, the resultant force produces to the rotor is the vector sum of impulsive and reactive force and the reaction force is an unbalanced condition. Generally, this turbine is not used for commercial purpose. Due to this reactive force, it is called reaction turbine.

An example of this turbine is Parson's Turbine.


Impulse Turbine Reaction Turbine
1) In impulse Turbine, only impulsive force strikes to the blades fixed to the rotor 1) In reaction turbine, vector sum of impulsive and reactive force strikes the blades fixed to the rotor.
2) Steam expands completely when it passes through the nozzles and its pressure remains constant. 2) pressure can't expand fully. It partially expands when it pass through the nozzles
and rest on the rotor blades.
3) Blades are symmetrical shape. 3) Blades are asymmetrical shape.
4) Since the velocity of steam is high, speed is high in impulse turbine. 4) But reaction turbine speed is much lower than impulse turbine
because steam velocity is lower in reaction turbine as compared to impulse turbine.
5) For producing same power, the number of stages required are much less. 5) It require more stages to develop same power.
6) The blade efficiency curve is high. 6) The blade efficiency curve is lower than impulse turbine.

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