7 replies to this topic

[premjagan]

Dear sirs,

                I need to calculate specific heat ratio of gas mixture which at 50 kscg pressure, 35 degree centigrade. So,for that i found Cp and Cv values of gas components at 20 degree centigrade,1 atm.I proceed my calculation by finding Cp mix ( mol fraction multiplying with individual Cp values).

                             

My doubt is, use of Cp values at 20 c & 1 atm for calculating specific heat ratio of gaseous mixture which is at 50 kscg , 35 c is correct? please clarify me.

[Bobby Strain]

Best to have it calculated from equation of state. What are you using the value for?

Bobby

[premjagan]

Bobby,

 

                Pardon bobby, I cant get you.My aim is to calculate the specific heat ratio.For that i have found the avg Cp values of gaseous mixtures by multiplying the individual Cp values ( From tables values at 20 degree centigrade,1 atm) with mole fraction.

Gas mixturtes contains argon, hydrogen,nitrogen,methane which is at 75 ksc , 308k. I got the specific ratio value of this gaseous mixture is 1.4055.

I got this answer by finding Cp &Cv avg values which is  available at 20 degree centigrade,1 atm  in book . But in my calculation the gas mixture is at 75 ksc , 308k.My doubt is ,

 

Is specific heat ratio is irrespective of both pressure and temperature?

[kkala]

<http://www.cheresour...522-psv-sizing/> may be helpful and probably giving reason for the question in post no 2

[Art Montemayor]

Premjagan:

 

I believe I know why Bobby Strain is asking the basic question: "What are you using the value for?"

 

The answer can be very important as to what you should / should not use as the Cp/Cv ratio.  For example, are you using the product for calculation of thermodynamic calculations such as the discharge temperature of a gas compressor?  Or are you using it to calculate the size of a pressure relief valve?  Why don't you simply answer Bobby Strain's request?  That would be positive basic data information leading to a more positive and valuable response as to what you can/should do.

 

To answer one of your questions:  The specific heats are dependent on both temperature and pressure - as well as phase changes.

[premjagan]

Dear sir,

 

                           I am using the value for to calculate the size of pressure relief valve only.

[pathensey]

before to use "real" cp/cv for PSV sizing with API formula I recommend to see the results presented in posts 11,14,19 of the thread

'http://www.cheresour...ng-temperature/'

 

it would be nice if someone writes a Blog about this since it is a quite frequent question

[Bobby Strain]

If you go to the Pentair website, you can download their relief valve sizing software. It's good. And it contains a component library which is likely to have the components of interest to you.

 

Bobby