×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• 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.

# properties of air under pressure.... question3

## properties of air under pressure.... question

(OP)
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

### RE: properties of air under pressure.... question

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.

### RE: properties of air under pressure.... question

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.

### RE: properties of air under pressure.... question

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.

### RE: properties of air under pressure.... question

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

#### 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.

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:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!