Rankine Cycle question?
Question by tru5t_c0: Rankine Cycle question?
I’m reviewing for a test and there is a question from the book that im curious about.
Consider a simple ideal Rankine cycle with a fixed condenser pressure. If the boiler pressure is raised while the turbine inlet temperature is held constant, (select the correct statement)
a. the amount of heat rejected will increase.
b. the cycle efficiency will decrease.
c. the turbine work output will decrease.
d. the moisture content at turbine exit will increase.
e. the pump work input will decrease.
My guess was B because wouldn’t this make the vapor going into the turbine more wet because in an ideal situation you would like the vapor going into the turbine dry for max work output. Am I correct?
Thank you
Best answer:
Answer by gintable
The answer is D: the the moisture content at the turbine exit will decrease.
By raising the pressure, you have less entropy at the turbine entrance, because the pressure is higher, when the temperature is the same. Higher pressure is more “ordered” form of energy than a higher temperature. As a result, the state at the low pressure at the exit of the turbine, is of less entropy as well, which means lower quality. As a design goal, we want high quality steam to exit the turbine, so that the water droplets don’t erode the turbine blades.
It would be great if the efficiency decreased by doing this, because our design strategy would be more straightforward. However instead the efficiency increases upon selecting a higher boiler pressure, resulting in lower quality steam at exit. This is because less heat is required to vaporize steam at higher pressure, and more work is captured with a higher turbine pressure drop.
Pump work increases, in order to raise water across the higher pressure differential.
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