Contact US

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!

*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

Air volume under pressure

Air volume under pressure

Air volume under pressure

My turn to ask a question for once.
If I compress 1 cubic foot of air at atmospheric pressure to 20 psi (gauge)what volume do I end up with.
Thanks guys and girls

Naresuan University

RE: Air volume under pressure

That depends upon what process you follow. For adiabatic compression(where there is no heat transfer to and from the air undergoing compression) the formula is P1V1k = P2V2k where k is the ratio of specific heats(1.4 for diatomic gases).

If you maintain constant temperature of air through out the compression process then the isothermal formula will be P1V1 = P2V2.

However, when you use a compressor to do the work, you can approximately use the adiabatic process and thus the volume will be 14.7 x 11.4 = (20+14.7)xV21.4.
Thus, V2 = 0.541cu.ft (this is called actual cu.ft)

Check these two informative threads


RE: Air volume under pressure

Thanks for quick response - I used P1V1 = P2V2 to come up with the answer - then questioned myself if I was correct -brain is getting a bit fuzzy with age I guess.

Naresuan University

RE: Air volume under pressure

In the EOS pV=f (T) one needs two parameters to determine the third. In this case, knowing the final pressure, temperature would be necessary and sufficient.

If the compression path is fully identified one could estimate the other macroscopic properties, given the final pressure, for example:

P1V1=P2V2 refers to an isothermic process, while
P1V1γ = constant, to an adiabatic process.

BTW, compression can follow a variety of paths, e.g., isothermic, adiabatic, even isochoric. We can compress a gas at constant volume (!) by the addition of heat.

RE: Air volume under pressure

25362 (Chemical)Your statement"BTW, compression can follow a variety of paths, e.g., isothermic, adiabatic, even isochoric. We can compress a gas at constant volume (!) by the addition of heat"
 is interesting.  Why not include the paths of constant enthalpy, internal energy, etc,?
In fact compression for two phase, one component could include constant volume and temp.
I think you are stating that compression is any process in which the pressure is changed.

RE: Air volume under pressure

Sailoday28, you are right. I should have said pressurizing rather than compressing, since the latter involves, by definition, a reduction in volume. This is a frequent error probably stemming from the fact that we call compressed air, air that is subjected to pressures above atmospheric. For that clarification you merit a star.

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! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close