Understanding the Degree of Reaction in Impulse Steam Turbines
The degree of reaction (R) is a crucial parameter in turbine design, particularly in distinguishing between impulse and reaction turbines.
Definition of Degree of Reaction
- The degree of reaction is defined as the ratio of the static enthalpy drop across the rotor to the total enthalpy drop across both the rotor and the stator.
Impulse vs. Reaction Turbines
- In an impulse turbine, the steam expands entirely in the nozzle before reaching the rotor.
- The rotor only converts the kinetic energy of the steam into mechanical energy, with no pressure drop occurring within it.
Degree of Reaction for Impulse Turbines
- For impulse turbines, the steam does not experience any pressure drop across the rotor.
- Theoretical calculations show that the enthalpy drop across the rotor (h2 – h3) is zero.
Mathematical Expression
- The formula for the degree of reaction (R) can be expressed as:
R = (h2 - h3) / (h1 - h4)
Where:
- h1 is the enthalpy at the inlet (stator)
- h2 is the enthalpy at the outlet of the rotor
- h3 is the enthalpy at the inlet of the rotor
- h4 is the enthalpy at the outlet (stator)
- Since h2 = h3 in an impulse turbine, the numerator becomes zero, leading to:
R = 0 / (h1 - h4) = 0
Conclusion
- Thus, the degree of reaction for an impulse steam turbine is 0, indicating that the entire enthalpy drop occurs in the nozzles, not in the rotor.
Therefore, the correct answer is option 'C'.