Rankine cycle with Reheating - GATE ME '15 S1
Which of the following statements regarding a Rankine cycle with reheating are TRUE? (i) increase in the average temperature of heat addition (ii) reduction in thermal efficiency (iii) drier steam at the turbine exit A. only (i) and (ii) are correct B. only (ii) and (iii) are correct C. only (i) and (iii) are correct D. (i), (ii) and (iii) are correct
Answer: Let's start with a schematic of the ideal Rankine cycle as shown below,
It consists of a boiler, turbine, condenser, and pump in sequence.
The ideal reheat Rankine cycle is shown below. Reheating is the process of expanding the steam in the turbine in two stages and reheating the steam in between.
The two stages of the turbine are called high-pressure turbine(HPT) and low-pressure turbine(LPT).
Both the cycle are shown on the T-s plot below. It can be clearly seen that the ideal reheat Rankine cycle increases the mean temperature of heat addition. Hence statement (i) is correct.
The thermal efficiency of any cycle ideally can be defined as
Efficiency = 1 - TL/TH
TL = Temp of sink
TH = Temp of source
The higher the source temp higher will be the efficiency. Hence, as the mean temp of heat addition increases, the reheat Rankine cycle's thermal efficiency also increases.
Hence statement (ii) is wrong.
Due to steam reheating, the steam's moisture content at the turbine exit is decreased.
Hence statement (iii) is correct.
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