Question

The vapor quality of steam in the turbine of a Rankine cycle can be improved by employing

• A. regeneration of steam
• B. intercooler
• C. reheating
• D. cogeneration

Hint:

To improve a Rankine cycle, remember the key modifications: - To increase {efficiency}, use {regeneration}. - To increase {steam quality} (and also efficiency and work output), use {reheating}. - To increase {pressure ratio} without excessive temperature, use {superheating}.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Vapor quality (or dryness fraction) of steam is the proportion of mass that is in the vapor phase. In a Rankine cycle turbine, if the steam becomes too wet (low quality), the liquid droplets can cause erosion of the turbine blades, reducing efficiency and lifespan. The question asks for a method to improve the quality, meaning to keep the steam drier, at the turbine exit.
Step 2: Detailed Analysis of Each Option:
- (A) regeneration of steam: Regeneration involves bleeding a portion of the steam from the turbine at an intermediate stage and using it to preheat the feedwater going to the boiler. While this improves the overall cycle efficiency by reducing the heat input required in the boiler, it does not significantly affect the quality of the steam at the final turbine exit. The steam that continues through the turbine expands to the same final state.
- (B) intercooler: An intercooler is used in multi-stage compression cycles (like the Brayton cycle) to cool the working fluid between compression stages. It is not a component of a Rankine (vapor power) cycle.
- (C) reheating: The reheat cycle involves expanding the high-pressure steam in a high-pressure (HP) turbine part-way, then routing it back to the boiler to be "reheated" to a high temperature. This high-temperature, lower-pressure steam then expands through a low-pressure (LP) turbine. Because the expansion in the LP turbine starts at a much higher temperature (and thus higher enthalpy/entropy), the final state at the condenser pressure is shifted to the right on a T-S or H-S diagram, resulting in a much higher vapor quality. This directly addresses the problem of low quality at the turbine exit.
- (D) cogeneration: Cogeneration (or Combined Heat and Power, CHP) is a system that produces both electricity and useful heat from a single fuel source. It is a system configuration, not a specific thermodynamic process modification within the cycle aimed at improving steam quality.
Step 3: Why This is Correct:
Reheating is the specific modification to the basic Rankine cycle designed to solve the problem of low steam quality at the turbine exhaust, thereby preventing blade erosion. It achieves this by adding heat to the steam mid-way through its expansion.