Steam turbines can be classified as either reaction, impulse, or a mixture of both; a large steam turbine will almost always have both impulse and reaction stages.

Impulse Turbine

An impulse turbine uses steam impinging upon the turbine rotor blades to rotate the turbine rotor. As the velocity of the steam is directly proportional to the amount of energy transferred to the rotor, the velocity of the steam is increased using nozzles prior to it impinging upon the rotor blades.

Impulse Turbine Design

Reaction Turbine

A reaction turbine uses the reactionary force of the steam changing direction over the blades to rotate the rotor. As steam passes over a reaction turbine blade, a reactionary force is created upon the blade and this causes the turbine rotor to rotate.

Reaction Turbine Design

Despite reaction turbines being classified as ‘reaction’, there is always a small degree of force imparted due to impulse. For this reason, they are also called impulse reaction blades. 

Impulse and Reaction Blades