NCEES 2016 pro. 537 (TFS)

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.

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.

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

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. 

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.

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