In reaction turbines, the steam expands through the fixed nozzle , where the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades (nozzles), changes its direction and undergo further expansion. The change in its direction and the steam acceleration applies a force. The resulting impulse drives the blades forward, causing the rotor to turn. Ther is no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. In this type of turbine the pressure drops take place in a number of stages, because the pressure drop in a single stage is limited.
The main feature of this type of turbine is that in contrast to the impulse turbine, the pressure drop per stage is lower, so the blades become smaller and the number of stages increases. On the other hand, reaction turbines are usually more efficient, i.e. they have higher “isentropic turbine efficiency”. The reaction turbine was invented by Sir Charles Parsons and is known as the Parsons turbine.
In the case of steam turbines, such as would be used for electricity generation, a reaction turbine would require approximately double the number of blade rows as an impulse turbine, for the same degree of thermal energy conversion. Whilst this makes the reaction turbine much longer and heavier, the overall efficiency of a reaction turbine is slightly higher than the equivalent impulse turbine for the same thermal energy conversion.
Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its periphery. The rotor blades are usually designed like an impulse blade at the rot and like a reaction blade at the tip.