What is Steam Turbine Degree of Reaction (Reaction Ratio)?

One of steam turbine components are the turbine blades that act as nozzles. Those nozzles, either on the stator or rotor side, convert heat of the steam into kinetic energy. The nozzled shape of turbine blade on the rotor side also serves to convert the steam kinetic energy into mechanical energy as rotation of the rotor.

The convertion of heat energy to kinetic energy is always followed by enthalpy drop at isentropic condition. This enthalpy drop may occur on the side of the stator or rotor blade depending on the turbine design.

Reaction Ratio (also known as the

Simply put, the Reaction Ratio becomes a number indicating the type of a turbine whether it is an impulse turbine, reaction turbine or mixture.

Reaction Ratio (R) = \dfrac {\Delta h_{rotor}}{\Delta h_{stage}}

Where Δhrotor is the amount of decreased enthalpy converted to kinetic energy on the side of the rotor blade, and Δhstage is the total amount of the enthalpy drop in one stage.

steam turbine degree of reaction
Speed Profile and Vector ​​of Impulse and Reaction Turbine

Reaction Ratio (R) = 0
At R = 0 means 100% enthalpy drop due to change into kinetic energy occurs on the stator blades. This process is a pure impulse process characterized by constant pressure at the point before and after the rotor blade, the steam stream only changes the direction only. The rotor blades alter the direction of the steam impulse directed at it and transfer high torque to the turbine shaft. Therefore, this type of turbine is also called an impulse turbine.

The advantage of this type of turbine is the large drop of enthalpy on a single stage of the blades, so the generation of energy by one turbine is greater. So the number of stages from the turbine will be less, and the turbine size will be shorter. But the disadvantage of this type is the loss of too much steam flow due to the larger flow velocity.

Reaction Ratio (R) = 0.5
A turbine of design R = 0.5 means that half of the enthalpy drop at one stage of the turbine blade occurs on the side of the stator blade, and the other half occurs on the turbine rotor blade. Turbine with this design is also called reaction turbine. Decrease in pressure and enthalpy of steam occurs on the stator side and turbine rotor. The steam pressure in the rotor blade inlet is larger than the outlet side. The flow of the steam is not only accelerated on the stator side, but also on the rotor side of the turbine.

The difference in steam pressure on the rotor blade side, causing the axial force on the whole turbine. An axial force means a force that is in line with the direction of the shaft. The axial force of the turbine rotor is opposite to the direction of the steam stream, and is also called axial thrust. Axial thrust must be compensated by the use of thrust bearing or against the force using a balance piston.

The advantage of using this type of turbine is the loss of steam flow due to the increase in flow velocity at every small stage. But the disadvantage is that the longer the turbine design, because the need for more stage than the impulse turbine.

In practice, turbines with a design R = 0.7 are more widely used at present. This means more enthalpy drops occur on the rotor side of the turbine than the stator side.