Turbine Selection by Specific Speed Which turbine type is generally preferred for specific speeds in the range of about 60 to 300 rpm-based units?

Introduction:Specific speed (Ns) guides the selection of turbine type to match site head and discharge. Lower Ns favors impulse turbines; higher Ns favors axial-flow reaction turbines, with mixed-flow machines occupying the middle range.

Given Data / Assumptions:

• Ns range stated: roughly 60–300 (rpm-based form).
• Typical classification bands used in practice.

Concept / Approach:Pelton turbines (impulse) are suitable for very low Ns (high head, low flow). Kaplan turbines (axial) suit very high Ns (low head, high flow). Francis turbines, being mixed-flow reaction machines, cover the middle Ns band from about 60 up to ~300, making them the preferred choice in that interval.

Step-by-Step Solution:Map Ns range to turbine families: Pelton <~35; Francis ~60–300; Kaplan >~300 (indicative ranges).The given Ns range lies squarely in the Francis region.Hence, select Francis turbine.

Verification / Alternative check:Reference charts of Ns vs. turbine type show Francis in the mid-range band.

Why Other Options Are Wrong:Pelton wheel: typically lower Ns (high-head applications).Kaplan turbine: typically higher Ns (very low head, large discharge).none of these: incorrect because Francis matches the specified band.

Common Pitfalls:Confusing rpm-based Ns with dimensionless forms; the relative ordering remains the same.

Final Answer:Francis turbine