What is the advantage of increasing boiler pressure on the thermal efficiency of the Rankine cycle?
Answer:
Advantages:
The average temperature of heat addition increases.
Increases thermal efficiency of the cycle
Explanation: Let me explain it through the T-s diagram below.
1-2-3-4 is an ideal Rankine cycle
1-2'-3'-4' is a Rankine cycle with increased boiler pressure
First of all, we calculate the mean temperature of heat addition because the heat is not added at a single constant temperature that is for example from point 2' to the saturated liquid line of the vapor dome the addition of the heat consistently increases the temperature. Hence, we calculate the mean temperature of the heat addition.
From 2' to 3' the mean temperature of heat addition must be higher than the mean temperature from 2 to 3.
In general, for any thermodynamic cycle, the thermal efficiency is limited by
η = 1- Tr/Ts
where
Tr = temperature of heat rejection
Ts = temperature of heat addition
In our case of the Rankine cycle,
Ts can be the mean temperature of heat addition Tm.
The higher the Tm higher should be the efficiency of the cycle. Therefore increasing the boiler pressure increases the mean temperature of the heat addition and in turn, increases the thermal efficiency of the cycle.
Assumption: Tmax is the maximum temperature we have decided to not go above because we are limited by the metallurgical limitations.
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