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H-S Diagrams

Consider an irreversible, adiabatic, steady-flow process such as the turbine shown below.

Ws = ___ kJ/s

Q = 0

1st Law:

Proportional to the net work for an adiabatic, steady-flow devices such as turbines, pumps and compressors.

Larger values of

are associated with greater irreversibility within the system.

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Lesson 7B: page 18 of 21

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We won’t use the HS Diagram quite as often as the TS Diagram, but for certain processes, the HS Diagram is the most useful.
An example of such a process is an irreversible turbine, like the one shown here.
The 1st Law tells us that the rate at which shaft work is produced by the turbine is just the mass flow rate times the change in the specific enthalpy.
Therefore, the vertical distance between states 1 and 2 on the HS Diagram is proportional to the rate at which work is done by the turbine.
We will see later in this chapter and again in chapter 8 that ΔShat or the horizontal distance between state 1 and 2 is proportional to the irreversibility in the turbine.
If the turbine were reversible as well as adiabatic, then it would also be isentropic and the process path on the HS Diagram would be a vertical line.
The steeper the path of an adiabatic process on the HS Diagram the closer the performance of the real process approximates the performance of an ideal, reversible process.
So, I hope you see that, for certain processes like pumps, turbines and compressors, the HS Diagram gives some special insight into how close a real process comes to being reversible.
That concludes our introduction to the new state variable, entropy.
In the next lesson, we will study a very important limitation that the 2nd Law places on how entropy can change.
Well, you know the drill by now. Work some example problems, read the lesson summary and then try the quiz.