11 replies to this topic

[babakfarhadi]

Hi Dear,
Im babak farhadi and im student and work on the N2 liquefier project.
I read spreadsheet about thermodynamic equation for gas compressor calculation and i have a question.
We assume i have a 30 bar N2 @27 degree centigrade and expand it to 0.1 bar.

Solution 1 :
I used adiabatic expansion formula T2=T1*(P2/P1)^(k-1/K)
T2=300*(0.1/30)^(1.4-1/1.4) ----> T2= -214 degree Centigrade

Solution 2 :
I used Thermodynamic Table properties T-S Diagram
N2 @30 Bar,@27 Degree ---> h1=305 Kj/Kg
I know expansion is isenthalpy proccess
h2=h1=305 Kj/Kg @0.1 Bar ----> T2= 20.8 degree Centigrade

Im confused what cause this diffrence?
Plz Help me
Kind Regard,
Babak Farhadi

[fallah]

Hi Dear,
Im babak farhadi and im student and work on the N2 liquefier project.
I read spreadsheet about thermodynamic equation for gas compressor calculation and i have a question.
We assume i have a 30 bar N2 @27 degree centigrade and expand it to 0.1 bar.

Solution 1 :
I used adiabatic expansion formula T2=T1*(P2/P1)^(k-1/K)
T2=300*(0.1/30)^(1.4-1/1.4) ----> T2= -214 degree Centigrade

Solution 2 :
I used Thermodynamic Table properties T-S Diagram
N2 @30 Bar,@27 Degree ---> h1=305 Kj/Kg
I know expansion is isenthalpy proccess
h2=h1=305 Kj/Kg @0.1 Bar ----> T2= 20.8 degree Centigrade

Im confused what cause this diffrence?
Plz Help me
Kind Regard,
Babak Farhadi

Babak,

You missed to submit the pressure dimension (barg or bara). Please rechek and recalculate with the recheked values of pressure. Note in first solution you should use absolute pressure values.

Fallah

[ankur2061]

Babak,

The solution 1 proposed by you with absolute pressures (0.1+1.01325 and 30+1.01325) would still give a calculated temperature of -157°C.

Solution 2 propsed by you considers the process of expansion to be isenthalpic which is not correct in real life. Adiabatic expansion is never isenthalpic in real life and for real gases. When a gas expands with such a sudden change from high pressure to low pressure its internal energy is converted to kinetic energy which then appears as shaft work on an equipment such as an expander. This indicates that the whole process of expansion is not isenthalpic as asssumed by you.

When I use a simulator for your gas expansion conditions showing an expander the temperature of the expanded gas turns out to be -115°C and the power generated due to the expansion is 108 kW.

Hope this helps.

Regards,
Ankur.

[ankur2061]

Babak,

When I mentioned the power generated figure as 108 kW, this requires that the molar or mass flow rate be defined. For your case of 30 barg and 27°C inlet conditions to expander and an outlet pressure of 0.1 barg, I had considered the molar flow rate as 100 kg-mole/h.

When I mentioned power generated, in thermodynamics terms it actually means the work done by the gas in expanding.

Regards,
Ankur.

Edited by ankur2061, 30 January 2012 - 04:09 AM.

[babakfarhadi]

Hi Dear Fallah and Dear Ankur.
Thank you very very much for your reply.
First my pressure Dim. is Gauge.
seconde my question is for define Expansion temp which solution is correct?
I use bara and calculate temp is -125 but with diffrence is high between 2 solution.



Kind Regard,
Babak Farhadi

[fallah]

my question is for define Expansion temp which solution is correct?

Babak,

You should do based on first solution.

Fallah

[ankur2061]

Babak,

You did not read my response properly. The solution 1 gives correct temperature whereas solution 2 does not give correct temperature. An accurate solution would require the average of the 'K' value taken between the inlet and outlet pressure conditions.

Regards,
Ankur.

[babakfarhadi]

Babak,

You did not read my response properly. The solution 1 gives correct temperature whereas solution 2 does not give correct temperature. An accurate solution would require the average of the 'K' value taken between the inlet and outlet pressure conditions.

Regards,
Ankur.

Thank you dear Ankur and fallah.
IN Cryogenic Engineering book page 121 - 123 problem 3-2 use solution 2.
Can you tel me about this?

Kind Regrad,
Babak Farhadi

[ankur2061]

Babak,

I do not have access to the book you are mentioning and I don't even need to look at it.

You need to define what is your process in the following terms:

1. Reducing pressure by Throttling using a throttling device such as a control valve or an orifice

2. Reducing pressure by Adiabatic Expansion

If you are defining your process as throttling, which is essentially an isenthalpic (enthalpy does not change from pressure P1 to P2) process than the solution 2 is right and solution 1 is not applicable.

If you are defining your process as adiabatic expansion, where the enthalpy reduces from pressure P1 to pressure P2, then solution 1 is right and solution 2 is not correct

You need to understand the difference between these two thermodynamic processes to understand which is applicable to you. Refer the link provided below where the concept of isentropic (reversible adiabatic) expansion process and throttling is clearly explained.

http://processengine...pplication.html

Regards,
Ankur.

[babakfarhadi]

Thank you Dear Ankur

I try to know diffrence between throttling and adiabatic expansion and if have any problem post a topic.

Thank you very very much.
Kind Regard,
Babak Farhadi

[babakfarhadi]

Babak,

I do not have access to the book you are mentioning and I don't even need to look at it.

You need to define what is your process in the following terms:

1. Reducing pressure by Throttling using a throttling device such as a control valve or an orifice

2. Reducing pressure by Adiabatic Expansion

If you are defining your process as throttling, which is essentially an isenthalpic (enthalpy does not change from pressure P1 to P2) process than the solution 2 is right and solution 1 is not applicable.

If you are defining your process as adiabatic expansion, where the enthalpy reduces from pressure P1 to pressure P2, then solution 1 is right and solution 2 is not correct

You need to understand the difference between these two thermodynamic processes to understand which is applicable to you. Refer the link provided below where the concept of isentropic (reversible adiabatic) expansion process and throttling is clearly explained.

http://processengine...pplication.html

Regards,
Ankur.

Hi Dear Ankur
I found this article (http://www.mmr.com/P...ling_TSB014.pdf) about JT expansion,in this article you can see T-s Diagram and solution 2 Plz tell comments about this article.

Kind Regard,
Babak Farhadi

Edited by babakfarhadi, 04 February 2012 - 11:56 PM.

[kkala]

Relevant comments on the subject can be also found in http://www.cheresour...re-calcualtion/.