Calculate Isentropic Efficiency: Tips & Tricks

[y2j]

Hi
can anyone 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

 

[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)

 

[y2j]

thanks for reply
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

 

[BvU]

Then you work backwards from the enthalpy observed...

 

[y2j]

I don't understated you

Then you work backwards from the enthalpy observed...

what do you mean?
thanks again

 

[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 $H_{out}$

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

 

[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

 

[y2j]

thanks for attention

 

[BvU]

Is it clear now ?

 

[y2j]

not yet

 

[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 ?

 

[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

 

[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 ?

 

[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

 

[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...