TFS 2016 NCEES Problem 533 - Mechanical - Engineer Boards
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cvanwy02

TFS 2016 NCEES Problem 533

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In this question you need to figure out the enthalpy at one of the states.  The state where I’m having trouble specifies a temperature of 100 degrees F and a pressure of 25 psi.  The solution uses Appendix 23.A in the MERM (Saturated Steam by Temperature) and uses the “Saturated Liquid” enthalpy.  How is this different from going to Appendix 23.B (Saturated Steam by Pressure) and reading the “Saturated Liquid” enthalpy under 25 psi?

To me, both would be wrong since both the temperature and pressure do not match what is specified in the problem statement. 

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On 1/29/2019 at 4:07 AM, cvanwy02 said:

In this question you need to figure out the enthalpy at one of the states.  The state where I’m having trouble specifies a temperature of 100 degrees F and a pressure of 25 psi.  The solution uses Appendix 23.A in the MERM (Saturated Steam by Temperature) and uses the “Saturated Liquid” enthalpy.  How is this different from going to Appendix 23.B (Saturated Steam by Pressure) and reading the “Saturated Liquid” enthalpy under 25 psi?

To me, both would be wrong since both the temperature and pressure do not match what is specified in the problem statement. 

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.

 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. See our Thermo book for a more complete discussion of the saturated liquid approximation.

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. . 

Edited by Slay the P.E.
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5 hours ago, Slay the P.E. said:

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.

 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. See our Thermo book for a more complete discussion of the saturated liquid approximation.

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. . 


 

 

Thanks Slay! This clears it up. 

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This is the perfect example of a "most nearly" solution. It is instructive to note that the error associated with using the saturated liquid approximation typically falls within 0-5% from the true, table value (assuming one has better tables than what is in the MERM). You can be sure that the exam makers are aware of the error associated with using this approximation and will account for that in the solution options. The point being, you will need to ensure your calcs use good accuracy or you're going to have a hard time picking solutions.

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More on this for the truly nerdy:

 

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