Question

For Parson’s reaction turbine, degree of reaction is .

• A. 50 %
• B. 60 %
• C. 75 %
• D. 100 %

Hint:

To remember this, think of "Parson's" as "Pair-sons." Because the fixed and moving blades act as a perfectly matched pair with identical work distribution, the work is split 50-50 between them.

Answer 1

The Correct Option is A

1. Definition of the Parson's Turbine: The Parson’s turbine is a specific type of reaction turbine where the fixed and moving blades are designed to be identical in shape and size. This symmetry leads to a very specific thermodynamic behavior during the expansion of steam.

2. Mathematical Derivation of Degree of Reaction: In a Parson’s turbine stage:

• The enthalpy drop in the fixed blades is equal to the enthalpy drop in the moving blades ($\Delta h_{fixed} = \Delta h_{moving}$).

• The degree of reaction ($R$) is defined as: $R = \frac{\Delta h_{moving}}{\Delta h_{fixed} + \Delta h_{moving}}$
Substituting the equal values: $$R = \frac{\Delta h_{moving}}{2 \cdot \Delta h_{moving}} = \frac{1}{2}$$ This is equivalent to a

50% degree of reaction.

3. Geometric Implications: Because the degree of reaction is 50%, the velocity diagrams for the inlet and outlet of the moving blades are exactly symmetrical. The inlet angle of the fixed blade is equal to the exit angle of the moving blade, and the exit angle of the fixed blade is equal to the inlet angle of the moving blade. This symmetry simplifies the manufacturing process as identical blade profiles can be used for both stationary and rotating components.