Homework Help: Steam throttling question.

1. Aug 23, 2013

TMoore20

1. The problem statement, all variables and given/known data
"Dry, saturated steam at 1500kPa is throttled (perhaps, expanded) to a final condition of 100kPa, dry and saturated. Calculate the change in internal energy."

2. Relevant equations
Q= ΔU + W (Q, heat supplied. U, internal energy. W, work)

Steam table interpolation. (I generally don't use the interpolation formula, just ratios)

h= U + PV (not sure on this one as it is not a constant pressure process, h, enthalpy.)

3. The attempt at a solution

Hard to get a handle on this one for me, so far I have collected that work is being removed by some means (turbine blade, stationary nozzle, piston..) seeing as it is not a constant enthalpy process.

Taking the difference between the two conditions on the steam tables seems too easy and well...wrong.

Made it as far as calculating the difference between the enthalpy at 100kPa D & S and what it would have been in a constant enthalpy process, to find the enthalpy rejected to the system. Also that the temperature difference between Tsat at 100kPa and what it should be superheated to is 40.33°C.

This is a frustrating problem for me because it seems easy and I'm positive that I've learned it before. I'm just 3 years out of school and I think it's a simple problem.

Any help would be greatly appreciated, thanks in advance.

2. Aug 23, 2013

Staff: Mentor

Hi TMoore20, [Broken]

Steam tables! Now that takes me back ....!

Alas, that learning has long ago left me to make room for more-recent stuff. (Well, that's my excuse. ) About all I recall is needing to work in degrees Rankin for some tables.

When faced with questions like the one you have, you can often get help by finding an online solved question. I leave it to you to determine whether this one is analogous to the question you face. http://www.learnthermo.com/examples/ch05/p-5c-4.php
Watch out superheated steam; it's invisible, and the burns they go deep. Remember --- [Broken]

Fluidics, Thermodynamics of Fluids ..... Good luck!

Last edited by a moderator: May 6, 2017
3. Aug 24, 2013

Staff: Mentor

Hi TMoore20. Welcome to Physics Forums.

The key to this question is the word saturated for the initial and final states. From the steam tables, you can get the internal energy of saturated steam at 1500 kPa and at 100 kPa. You don't have to know anything about the process that took the steam from the initial to the final state. The internal energy is a point function.

Chet