Smart questions
Smart answers
Smart people
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • 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!

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

Donate Today!

Do you enjoy these
technical forums?
Donate Today! Click Here

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

properties of air under pressure.... questionHelpful Member!(3) 

billdedman (Automotive) (OP)
31 Jan 02 3:21
If a volume of a gas (say, ambient atmospheric air) is reduced in a sealed container, to 1/2 of its original volume, doesn't it follow that the pressure would double?

Is this phenomenon linear in nature, all the way up to say, a 10:1 reduction in volume?

If it's NOT linear, why not?  Would the rise in temperature due to compression have any effect on the pressure increase?

Thanks much for any information.
Bill Dedman in Hayward, CA
(billdedman@hotmail.com
Helpful Member!  insideman (Mechanical)
31 Jan 02 4:52
It would double IF the temperature remained the same. But usually you will not have isothermal compression. It will be more nearly adiabatic (no heat lost or gained).

Dig up a temperature-entropy diagram for air.
The perfect adiabatic process will be represented by a straight vertical line on the diagram.
Helpful Member!  butelja (Mechanical)
31 Jan 02 10:18
If the air is compressed isothermally, then it will very nearly be linear.  At extreme pressures, a compressibility factor comes into play.

It the air is compressed isentropically, the the pressure volume relationship will be P*V^k = constant.  This occurs with a 100% efficient compression process and zero heat transfer.  For air, k=1.4.

Most real processes will be a polytropic process, where P*V^n = constant.  n will vary between 1 and k depending upon the process.
fareast (Computer)
10 Feb 02 4:28
I thought that boyles/charles law applied in these cases (there are one or two exceptions)

Simply P1V1T1=P2V2T2 therefore if the change in volume is slow enough to allow the heat liberated to dissipate then you can apply boyles law and state that should the volume be reduced by half then the pressure will double.
Helpful Member!  GregLocock (Automotive)
10 Feb 02 18:16
You (fareast) are right, but so is butelja, and he's righter! Your statement is true for a lump of ideal gas, in whatever conditions, whereas butelja takes a further step. Combining both equations allows you to find the temperature once it has reached a particular point on the pv^gamma curve.

Cheers

Greg Locock

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!

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