Steam turbines — Understanding the reheat factor In multi-stage steam turbines, the reheat factor (often denoted as RF) quantifies how the cumulative actual heat drop across all stages compares with the ideal isentropic heat drop between the same inlet and exhaust states. On which operating and design parameters does the reheat factor primarily depend?

Introduction / Context:
The reheat factor is a key performance indicator for multi-stage steam turbines. It accounts for the fact that, due to irreversibilities and changing specific volume, the total (cumulative) enthalpy drop actually realized across many stages can differ from the single shot isentropic drop between inlet and outlet states. Understanding what drives the reheat factor helps engineers predict real turbine work and stage loading more accurately.

Given Data / Assumptions:

• Multi-stage impulse or reaction steam turbine.
• Defined inlet state (pressure, temperature/superheat) and exhaust pressure.
• Non-ideal effects in each stage (efficiency < 100%).

Concept / Approach:
Reheat factor RF is defined as the ratio of the cumulative heat drop (sum over stages of actual enthalpy drops realized in blades/nozzles) to the ideal isentropic heat drop between the overall turbine inlet and exit states. Because specific volume increases down the expansion and stage losses vary with operating point, RF reflects how the expansion is distributed and how efficiently each stage converts enthalpy into kinetic work.

Step-by-Step Solution:
Recognize that higher inlet superheat and pressure influence the initial density and Mach-number regime, altering stage incidence and loss behavior.Identify that lower exhaust pressure (deep vacuum) increases the expansion range and magnifies volumetric flow, changing stage loading and cumulative heat drop.Acknowledge that per-stage efficiency (nozzle, blade, mechanical) directly shapes the realized enthalpy drop per stage; their aggregate sets the cumulative drop.Therefore, RF depends on inlet condition (initial pressure and superheat), exhaust pressure, and stage efficiencies.

Verification / Alternative check:
Design studies show RF typically exceeds unity slightly (e.g., 1.02–1.07) for many practical turbines because increasing specific volume down the expansion can yield a cumulative drop marginally greater than the single isentropic drop, when summed with realistic stage characteristics.

Why Other Options Are Wrong:
Picking any single factor (A, B, or C) ignores the coupled nature of expansion: inlet, outlet, and stage performance together determine RF.

Common Pitfalls:
Confusing “reheat factor” with “reheat cycle.” RF exists even in non-reheated turbines; it is a staging effect, not the presence of a reheater in the cycle.

Final Answer:
All of these