# NCEES 2016 pro. 537 (TFS)



## Flluterly (Oct 19, 2017)

hc=378.3 btu/lbm and hd=85.8 btu/lbm. is this based on the steam table saturated water at 110F and 400F? the table shows 78.02 btu/lbm and 375.1 btu/lbm at 110f and 400f.


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## Slay the P.E. (Oct 19, 2017)

The feedwater pressure is 3,000 psia. These values come from a compressed liquid table.

However, as you've probably noted, one can _approximate_ properties of water at 3,000 psia and 110F as the properties for saturated water at 110F with minimal error. Same with the properties at 400F. The difference won't lead you to a wrong answer choice.


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## Flluterly (Oct 19, 2017)

Thank you Slay for the explanation. when do I need use the compressed liquid table?


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## Slay the P.E. (Oct 19, 2017)

Typically you wouldn't. As this problem shows, the "saturated liquid" approximation works fairly well. The pressure would have to be monster high to make a difference. I'm on the road and don't have my copy of MERM with me, but check it there: the table starts at a huge value of pressure, maybe like 3,000 psia or so. The most sensitive property is enthalpy. You can do this exercise: compare the enthalpy values with _hf,_ I'll be shocked if any of the table values is different than _hf _by more than 5% or so. I would use the table when you get to a difference beyond 5%.

I'm going to do some more research on this and post back.


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## Slay the P.E. (Oct 19, 2017)

Ok, so I looked into this a little bit deeper.

This plot shows the % error you get when using _hf_ at the temperature instead of the value from the compressed liquid table. For example, for 300F, 3000psia the table gives h=275.41 Btu/lbm while _hf _at 300F is 269.91 Btu/lbm. In this example, the error incurred would be (275.41-269.91)/275.41 = 2%. In the graph below, for 3000 psia, the error at 300F is then shown as 2%.

So, when is the _hf_ approximation "bad"? Depends on what "bad" is to you. If you can live with at most a 5% error, then the plot shows that you're good as long as the pressure does not exceed 3500 psia by too much.

Also, be careful with extraordinarily high temperatures for liquid, such as 700F which is really close to the critical temperature. At this extreme the approximation _really_ breaks down for pressures above 4000 psia.

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## Flluterly (Oct 19, 2017)

Thank you very much for the details


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## Audi Driver P.E. (Oct 20, 2017)

In my experience, it's best to be as accurate as you can quickly because error can add up quickly.


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