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This is the question please:

1 kg of steam is at 100 bar and 375 °C expands reversible in a perfectly thermally insulated cylinder behind a piston until the pressure is 38 bar and then steam is dry saturated. Calculate the work done?

So, I got the answer for this question as -169.7 kJ/kg but this is what I don’t get please:

If it’s 375 °C it’s super heated? So it is in front of the saturated vapour line. It says it expands to become dry saturated but shouldn’t it compress to become dry saturated?

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  • $\begingroup$ Hint: think about what the final temperature of the steam is, according to the Poisson adiabat. $\endgroup$
    – user28774
    Commented Feb 3, 2023 at 22:22

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Can't comment because not enough reputation. Also a student though, so take this answer with a pinch of salt (hope it's acceptable to be giving an answer in this situ!), but I just had a go and got an internal energy change of -153.0kJ? Interpolated between 311 and 400deg to get u_1 = 2754.8kJ/kg at 375deg then u_sat = 2601.8kJ/kg for 38bar, I'm sure your answer is more likely to be right, but would like to know what you've done for my own revision! I was thinking in terms of first law with dq = 0, so dw = du. I'm much better at the diagrams though...

Can see why you'd think it would be more compressed as temperature drop at constant pressure -> drop in volume, but also if you think about a pressure drop at constant temperature, there would have to be an increase in volume. So if pressure drops and temperature decreases, I don't think you can immediately say anything about the volume change. Once you specify that it is an adiabatic process though, the process can't travel to the left on the diagram because for an adiabatic process:

PV^{n}=constant

Here's a wobbly sketch!

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