Understanding slip factor in centrifugal compressors: Does the slip factor quantify leakage from a storage vessel, or something else within the impeller exit flow?

Difficulty: Easy

Correct Answer: No, it measures the shortfall in tangential (whirl) velocity at impeller exit due to finite blade effects

Explanation:


Introduction / Context:
“Slip factor” is a core performance parameter for centrifugal compressors. It is often misunderstood as a mechanical leakage measure, whereas it actually relates to aerodynamics at the impeller exit and the head predicted by Euler’s equation.


Given Data / Assumptions:

  • Centrifugal (radial) compressor with finite blade number.
  • Flow leaving the impeller experiences deviation from ideal blade-to-flow guidance.
  • No mechanical leakage from a separate storage vessel is implied by the term.


Concept / Approach:

Slip factor σ is typically defined as Vw2(actual)/Vw2(ideal) or as Vw2(actual)/u2 depending on convention. Because real blades cannot turn the flow perfectly, the actual tangential (whirl) component Vw2 is less than the ideal value, reducing the ideal Euler head and the pressure rise achievable per stage.


Step-by-Step Solution:

State Euler relation: ideal head ∝ u2*Vw2(ideal).Recognize finite blade number → flow slip → Vw2(actual) < Vw2(ideal).Define slip factor σ capturing this shortfall.Conclude it has nothing to do with vessel leakage.


Verification / Alternative check:

Empirical correlations (Stodola, Wiesner) estimate σ from geometry and blade count, validating the aerodynamic nature of the parameter.


Why Other Options Are Wrong:

Options suggesting storage or bearing leakage misinterpret an aerodynamic efficiency parameter as a mechanical loss metric.


Common Pitfalls:

Equating “slip” with “leak”; assuming σ reflects casing clearances rather than exit flow deviation.


Final Answer:

No, it measures the shortfall in tangential (whirl) velocity at impeller exit due to finite blade effects

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