Why compound a steam turbine? Turbine compounding (velocity, pressure, or pressure-velocity compounding) is introduced for which primary reasons in practical machines?

Introduction / Context:
Early single-stage impulse turbines required extremely high rotational speeds to absorb high jet velocities efficiently. Compounding spread the total enthalpy drop over multiple stages to address mechanical limits and performance. Modern turbines employ combinations of pressure and velocity compounding to meet grid and mechanical constraints.

Given Data / Assumptions:

• High nozzle exit velocities would imply impractically high single-stage rotor speeds.
• Multiple stages allow tailored velocity triangles and lower relative losses.
• Exit losses can be reduced by staging expansion and matching flow areas.

Concept / Approach:
Velocity compounding (Curtis), pressure compounding (Rateau), and pressure-velocity compounding each partition the enthalpy drop to reduce per-stage velocity and blade speed. Lowering tip speed reduces mechanical stress and facilitates direct coupling to generators. Distributing expansion also curbs residual exit kinetic energy, improving efficiency.

Step-by-Step Solution:
Relate jet velocity to required blade speed ratio; single stages may demand excessive rpm.Introduce multiple stages to lower each stage’s velocity and optimize incidence/deflection.Observe reduction in exit kinetic energy and better matching to practical rotor speeds.Conclude: compounding serves all listed purposes simultaneously.

Verification / Alternative check:
Historical Curtis wheels enabled acceptable rpm for early generators; modern multi-stage reaction/impulse machines demonstrate higher efficiencies than equivalent single-stage designs.

Why Other Options Are Wrong:
Selecting only one reason ignores the interrelated benefits of compounding on speed, efficiency, and exit losses.

Common Pitfalls:
Assuming compounding is only for speed control; it also strongly affects losses, stage loading, and manufacturability.

Final Answer:
All of these