Steam Table questions

Hello, I am working through the 2016 thermal fluids practice exam and a couple questions are tripping me up.

On Q532, the solution states that steam at 25 psi, 100 DegF has an enthalpy of 68 BTu/lbm. Which specific steam table is this data extracted from? The table I have does not have any values in this range.

On Q114, the solution states that water at slightly above atmospheric pressure and a Temp of 60 F has an enthalpy of 28 Btu/lbm. How are they coming up with that value? 

Referencing App 35.A MERM 12th edition , states that Cp of H2O at 60F is roughly 1 Btu/lbmF so that would Emma the enthalpy is 60 Btu/lbm

Any help is greatly appreciated.

Thanks,

Zach

zakbos said:

Hello, I am working through the 2016 thermal fluids practice exam and a couple questions are tripping me up.

On Q532, the solution states that steam at 25 psi, 100 DegF has an enthalpy of 68 BTu/lbm. Which specific steam table is this data extracted from? The table I have does not have any values in this range.

On Q114, the solution states that water at slightly above atmospheric pressure and a Temp of 60 F has an enthalpy of 28 Btu/lbm. How are they coming up with that value? 

Referencing App 35.A MERM 12th edition , states that Cp of H2O at 60F is roughly 1 Btu/lbmF so that would Emma the enthalpy is 60 Btu/lbm

Any help is greatly appreciated.

Thanks,

Zach

Click to expand...

For the first two questions: at those conditions you have a compressed liquid (the pressure is higher than the saturation pressure at the given temperature). There are compressed liquid tables, but those tables start at fairly high pressure values. 25 psi might be too low and might not be in the compressed liquid table. I don’t have one near me right now.

The alternative then is to use the “saturated liquid approximation” — that is the enthalpy of a compressed liquid is approximately the same as hf for the liquid’s temperature. For 25psi and 100F we say that the enthalpy is simply hf @ 100F.

Regarding the specific heat question: Cp is used for calculating CHANGES in enthalpy, not specific values of enthalpy: 

h2-h1 = Cp (T2 - T1)

so if, say T1=60F, you need a value of T2 to use Cp to get the enthalpy change.

To follow-up on this...

In MERM13 the compressed liquid water table is Appendix 23D in page A-69. The lowest pressure listed, however, is 200 psi. Therefore, for compressed liquid water at 25 psia, 100F you have to use the saturated liquid approximation. If you check the saturated water table, you'll see that hf(T=100F)=68.03 Btu/lbm. 

Same thing with water at atmospheric pressure and 60F. This pressure is too low to be listed on the compressed liquid table, so for compressed liquid water at 15 psia, 60F you have to use the saturated liquid approximation. If you check the saturated water table, you'll see that hf(T=60F)=28.08 Btu/lbm. 

I may have had a few beers by now, but of course water at 100f has an enthalpy of 68ish.  32 + 68 = 100

32+68=100

@Slay the P.E.

Thanks for the explanation!

zakbos said:

Hello, I am working through the 2016 thermal fluids practice exam and a couple questions are tripping me up.

On Q532, the solution states that steam at 25 psi, 100 DegF has an enthalpy of 68 BTu/lbm. Which specific steam table is this data extracted from? The table I have does not have any values in this range.

On Q114, the solution states that water at slightly above atmospheric pressure and a Temp of 60 F has an enthalpy of 28 Btu/lbm. How are they coming up with that value? 

Referencing App 35.A MERM 12th edition , states that Cp of H2O at 60F is roughly 1 Btu/lbmF so that would Emma the enthalpy is 60 Btu/lbm

Any help is greatly appreciated.

Thanks,

Zach

Click to expand...

For Q 535, the value of 68BTU/lbm is the h for saturated liquid at 100 deg, F.

For  Q 114, the value of 28 BTU/lbm is again for saturated liquid at 60F.

The specific steam table values likely come from Keenan and Keyes.

What you need to know from these problems is that due to the compressed condition of the fluid, you are dealing with the saturated liquid state.