×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Cp/Cv correction in Hysys
3

Cp/Cv correction in Hysys

Cp/Cv correction in Hysys

(OP)
In Hysys simulator application, the Specific Heat Ratio shown for streams is incorrect. There is a formuala for calculating the correct Cp/Cv values. Would anybody out there know what this correct formula be. Thanks very much.

RE: Cp/Cv correction in Hysys

How do you know it's incorrect?  Do you have a table of values to compare the HYSYS calculation against?

Is it incorrect every time, for every gas and every gas mixture?

Have you called Hyprotech to discuss this with them?

Thanks!
Pete

RE: Cp/Cv correction in Hysys

I'm not sure if this is the case here...but the problem may be confusing real gas Cp/Cv with ideal gas Cp/Cv.

Worth checking...


Regards,

Bob

RE: Cp/Cv correction in Hysys

The Cp/Cv value in Hysys is the 'actual' value, based on the equations of state in thermodynamic package you are using.
You can calculate the 'ideal' value by the following formula:

Cp/Cv (ideal) = Cp / (Cp - R)

where Cp = molar heat capacity of gas stream (kJ/kmolK)
       R = universal gas constant = 8.314 kJ/kmolK

I suggest (although cannot confirm) that you use the ideal value in most formulae (e.g. PSV sizing) since these formulae are often derived with some 'ideal' assumptions.
If in doubt, use the value which gives the most conservative result.

RE: Cp/Cv correction in Hysys

Hysys has been calculating the 'real' Cv value and the associated Cp/Cv since v2.1.3. Prior to that they used the (Cp/Cp-R)- 'semi-ideal' as the Cp value was calculated from the EoS. The result is a Cp/Cv value that appears to be closer to experimental data but can differ considerably from the Cp/(Cp-R) value. Particularly in the 60-100 barg range at normal temperatures (ambient ±100°C) where the ideal value approaches 1.0 as pressure increases but the new value approaches goes in the opposite direction to more than 2.

We are finding that less experienced engineers are using the 'real' Cp/Cv data in datasheets sent to vendors for purchase. For RV's this is less conservative but not necessarily unsafe - it depends on the vendor sizing calculations, margins, whether they use HYSYS and are aware of this change.

Quoting both values to the vendor could cause confusion so I'd support Will48's suggestion to use the Cp/(Cp-R) on datasheets.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources