Pressure–Velocity Compounding in Impulse Turbines – Impact on Stage Count In a pressure–velocity compounded impulse turbine, a larger allowable pressure drop per stage is achieved. How does this influence the number of stages needed for a given overall pressure ratio and power?

Introduction / Context:
Compounding strategies—velocity compounding (Curtis), pressure compounding (Rateau), and pressure–velocity compounding—are used to manage blade speeds, stage efficiency, and practical rotor diameters in high-pressure steam turbines. Understanding their effect on the number of stages is central to preliminary layout and cost estimation.

Given Data / Assumptions:

• Impulse principles: pressure drops mainly in stationary elements.
• Pressure–velocity compounding combines partial pressure drop in multiple nozzle sets with multiple rotor rows per pressure stage.
• Goal: achieve target overall expansion without excessive stage count or impractical blade speeds.

Concept / Approach:

Allowing a larger pressure drop in each compounded stage means more of the total expansion is handled per stage. Consequently, fewer stages are required to accommodate a given boiler-to-condenser pressure ratio. Velocity compounding within a stage also allows lower individual rotor-tip speeds for a given jet speed, aiding mechanical integrity while keeping stage count down compared with pure pressure compounding at the same limits.

Step-by-Step Solution:

Verification / Alternative check:

Typical large turbines use combinations of compounding to balance efficiency and mechanical limits; design examples show reduced stage counts when greater per-stage drops are feasible without unacceptable losses.

Why Other Options Are Wrong:

‘‘More’’ contradicts the logic of larger per-stage drop. ‘‘Unchanged’’ ignores the definition of compounding. Conditions such as partial admission and moisture do not reverse the fundamental trend.

Common Pitfalls:

Assuming compounding only affects velocity triangles; it directly influences allowable pressure distribution and hence stage count.

Final Answer:

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