Effect of nozzle friction on heat drop Due to friction between steam and nozzle surfaces, the useful (available) heat drop across an expanding nozzle is typically reduced by about how much?

Introduction / Context:
Nozzle efficiency is a key parameter in turbine performance, translating enthalpy drop into kinetic energy. Surface friction and boundary-layer growth reduce the effective heat drop that converts to jet velocity, lowering the discharge speed and stage power.

Given Data / Assumptions:

• Smooth, well-designed steam nozzles operating near design conditions.
• Losses dominated by wall friction and minor shock/expansion mismatches.
• Comparison with ideal isentropic expansion between the same inlet and outlet pressures.

Concept / Approach:

Nozzle (velocity) efficiency is typically around 0.85–0.90 for good designs. That means the actual kinetic energy at exit is 85–90% of the ideal value, implying an effective reduction in the usable heat drop of roughly 10–15%. Designers account for this when computing throat area and exit velocity from the isentropic enthalpy drop, then applying efficiency to obtain the real exit speed.

Step-by-Step Solution:

Verification / Alternative check:

Experimental nozzle tests commonly report efficiencies in the above range unless there is erosion, fouling, or severe off-design operation; these conditions could worsen losses but are not typical for design values.

Why Other Options Are Wrong:

Higher reductions (25–60%) indicate very poor nozzles or off-design shocks; not representative of standard practice.Less than 5% is overly optimistic and rarely sustained across operating ranges.

Common Pitfalls:

Confusing reduction in heat drop with reduction in velocity directly; because kinetic energy scales with velocity squared, be careful when converting between efficiency definitions and percentage speed shortfall.

Final Answer:

10 to 15 percent