Isentropic Efficiency

1. Feb 7, 2016

y2j

Hi
can any one help me about the following ?
if I have the experimental data of inlet and outlet of expander (temperature and pressure) working in ORC.
Now if I want to find the Isentropic efficiencie :
the Isentropic expansion can be found by (entropy in = entropy out)
but how to find the actual enthalpy at expander exit of actual expansion
(i.e. I can consider it in the mixture or superheated phase?)
thank you

2. Feb 8, 2016

BvU

entropy in = entropy out gives you an enthalpy. Enthalpy difference * efficiency gives you a new enthalpy. Pressure and enthalpy give you a temperature (or a composition at saturation).

See e.g. here ( I Googled "isentropic efficiency turbine" and looked at the first image)

3. Feb 8, 2016

y2j

but i am not have the Isentropic Efficiency

I am only have pressure and temperature of experimental system and I would like to have the Isentropic Efficiency if the exit conditions locate in tow phase zone

4. Feb 8, 2016

BvU

Then you work backwards from the enthalpy observed....

5. Feb 8, 2016

y2j

I don't understated you
what do you mean?
thanks again

6. Feb 8, 2016

BvU

Perhaps you should post your work so far.

You have enthalpy in $H_{in}$

entropy in = entropy out gives you an enthalpy under isentropic conditions $H_{isent}$

observed enthalpy $H_{out}$

$$\eta_{isent} = {H_{in} - H_{out}\over H_{in} - H_{isent}}$$

7. Feb 8, 2016

y2j

I am not have the Isentropic Efficiency
I am only have pressures and temperatures at inlet and outlet of the
turbine
how can obtain Isentropic Efficiency in this case

8. Feb 8, 2016

y2j

thanks for attention

9. Feb 8, 2016

BvU

Is it clear now ?

10. Feb 8, 2016

y2j

not yet

11. Feb 8, 2016

BvU

Writing the same text in red does not help me to understand where you have a problem. Can you post your work so far ?

12. Feb 8, 2016

y2j

OK
I have an experimental data of inlet and outlet conditions of turbine (temperature and pressure ) working in a rankine cycle
when the exit condition from turbine in saturated phase
how I can find the actual enthalpy at exit ($H_{out}$)and that used to find the Isentropic Efficiency

13. Feb 8, 2016

BvU

No need to get desperate...
Enthalpies can be looked up in a steam table. Do you have that available in some form or other ?

14. Feb 8, 2016

y2j

Enthalpies in the mixture zone given for vapor and liquid
in this case to find Enthalpy we must have another parameter is (quality x) or dryness fraction
and the later I have not

15. Feb 8, 2016

BvU

I agree. That makes it difficult to get the correct value of $\eta$. All I can imagine is that you assume no water comes out of the tubine, only steam...