- Practice · 5 marks
Impulse and reaction steam turbines compared
Short answer
Impulse turbine expands steam completely in fixed nozzles, pressure constant across moving blades; reaction turbine expands steam in both fixed and moving blades with pressure drop in rotor. Turbines classified by steam action (impulse, reaction, combined), flow direction (axial, radial), exhaust (condensing, back-pressure, extraction), stages, pressure, arrangement. Applications: large power generation, CHP plants, marine propulsion, driving pumps/compressors.
Differentiate between impulse and reaction steam turbines. Classify steam turbines and mention their applications.
In an impulse turbine the steam expands completely in fixed nozzles and strikes the moving blades as a high-velocity jet; the pressure is the same at inlet and exit of the moving blades. In a reaction turbine the steam expands in both the fixed blades (which act as nozzles) and the moving blades, so the pressure drops in the rotor also; the moving blades work by the reaction of the accelerated steam.
| Point | Impulse turbine | Reaction turbine |
|---|---|---|
| Pressure drop | Only in nozzles | In fixed and moving blades |
| Pressure over moving blades | Constant | Falls |
| Blade shape | Symmetrical, curved (bucket type) | Aerofoil, asymmetrical, same as fixed blades |
| Steam admission | Partial | Full (complete circumference) |
| Degree of reaction | Zero | 50 % (Parsons) |
| Efficiency | Lower, but simple and robust | Higher for a given blade speed |
| Blade length | Short | Long (large steam volume) |
| Axial thrust | Nearly none | Large, needs balancing |
| Number of stages | Fewer for same drop | More |
| Example | De Laval, Rateau, Curtis | Parsons turbine |
Classification of steam turbines
- By action of steam: impulse, reaction, impulse-reaction (combined; usually impulse at the high-pressure end and reaction at low pressure).
- By steam flow direction: axial, radial (Ljungstrom).
- By exhaust conditions: condensing, non-condensing (back pressure), pass-out (extraction).
- By number of stages: single or multistage.
- By steam pressure at inlet: low, medium, high, supercritical.
- By arrangement: single casing, compound (tandem or cross compound), and single or double flow.
Applications
- Thermal and nuclear power stations: large reaction/impulse-reaction condensing turbines driving alternators.
- Combined heat and power (industries, sugar mills, paper mills): back-pressure or pass-out turbines, which supply process steam.
- Marine propulsion and boiler feed pumps (with reduction gearing).
- Small impulse turbines for driving pumps, blowers and compressors.
✓ Answer checked by Claude Sonnet 5.5
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