Thermodynamic Properties; Isentropic Expansion

[FlyingSpartan]

Homework Statement

An amount of saturated steam at a pressure of 10 Bar, undergoes an isentropic (constant entropy) expansion. The final temperature is 30C.

Find the final specific enthalpy and specific volume.

Homework Equations

Saturated Steam tables, Linear Interpolation methods?
No real equations of sort.

The Attempt at a Solution

By using the tables;

at 10 bar;

s_f ( entropy) = 2.138 kJ/kg K
v (specific volume) = 0.1944 m³/kg

at 30C;

s_f ( entropy) = 0.436 kJ/kg K
v (specific volume) = 32.93 m³/kg
I've been staring into space with the question, not sure what to do.

The final answers are given as 1990 kJ/kg for specific enthalpy and 25.28m³/kg for specific volume.

Thanks for any help!

 

[LawrenceC]

Based on constant entropy, you can determine the quality of the steam at 30 C. Knowing the quality, you then determine the specific volume and enthalpy.

 

[rude man]

At 30C you are certainly in the superheated (unsaturated) region. You need to use your charts to find the state at which your gas is at 30C with the same entropy you started out with.

Also, for saturated steam at 10 Bar (~1 MPa) I got s = 6.59 kJ/kg-K. Looks like you looked up s for saturated water, not steam.