The Engineer's Place for News and Discussion®

 Previous in Forum: Filler Wire for ASTM B677 Next in Forum: Welding Repair Rates

### Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

### Rating Vote:

Commentator

Join Date: Dec 2010
Posts: 61

### Pressure and Volume

07/25/2011 8:00 AM

Is pressure inversely proportional to volume? As per Boyles law pressure is inversely proportional to volume but for perfect gases. AS PERFECT GASES DO NOT EXIST IN NATURE, same principle can it applicable for real gases?

And the Equation is applicable for flowing or non flowing system.

for eg: Air is stored in a receiver of 1 m3 volume at 6 bar pressure means the volume of air 1 m3 and pressure exerting on th walls of receiver is 6 bar.

If the same 1 m3 amount of air is introduced in the 3 m3 Capacity of receiver then the pressure impacting on the walls of receiver will be less.

In this case it is practically evident that pressure is inversely proportional to volume AS THIS IS A CLOSED ONE MEANS THERE IS NO CONTINUOUS AIR .

If the same Experiment done with the continuous flowing air then the 3 m3 receiver will be filled by air with 6 bar pressure , here pressure is not inversely proportional to volume.

One more doubt i want to clarify is , in Air pressure regulator ,pressure is reduced by reducing the flow(opening of area) of air. As per this pressure should be directly proportional to volume.

This as per my practical experiment i did and my theoretical knowledge.

Pl Clarify /Clear my Confusion

Regards

Nithin

__________________
When Emotions runs high ,Intellectual wont work
Interested in this topic? By joining CR4 you can "subscribe" to

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Anonymous Poster #1
#1

### Re: pressure and volume

07/25/2011 8:08 AM
Anonymous Poster #2
#2

### Re: pressure and volume

07/25/2011 8:23 AM

The gas law

PV = nRT

assumes one simple,thing - It is a closed system ie there is no transaction of matter or energy across the system boundaries and it is under steady state.

In your case it is both - there is a mass (the flow of air dows have mass) and the energy (at elast the Kinetic energy) and the matter is not at steady state either.

In case of you it is not the referred equations but the fluidflow equations apply.

Think exactly what is happening in either case.

Power-User

Join Date: Jun 2008
Location: New York, NY
Posts: 249
#3

### Re: Pressure and Volume

07/25/2011 2:07 PM

The starting point for all this is the virial equation which goes something like

PVm/RT = 1 + B/Vm + C/Vm^2 + D/Vm^3 + ...

where vm is the molar volume and B,C,D ... are constants to be derived form experiment. NOTE that the ideal gas equation of state, vander wall, redlich-kwong or any other you may come up with are derived from this form.. This equation is derived directly from statistical mechanics and as you can see, it is a polynomial form and you can include as many coefficients are you please for the order of accuracy that you wish to calculate..

It is important to mention that in general, there are two types of pressure and you seem to be confused. there is static pressure, that is caused by the random motion of the the particles due to thermal energy and there is dynamic pressure where the fluid particles have a motion that is not statically random but induced by a pressure difference..

THE EQUATION OF STATE DOES NOT DEAL WITH DYNAMIC PRESSURE BUT WITH STATIC PRESSURE..

__________________
It is better to fail in originality than to succeed in imitation.
Anonymous Poster #3
#7

### Re: Pressure and Volume

07/26/2011 9:23 AM

Hi,

A normal pressure gage on say a steam pipe or whatever measures only static pressure, right? By normal I mean one that is located in the wall of the pipe. Is it possible that the static pressure could drop but the dynamic pressure increases therefore the total pressure remains the same - say when a valve is opened or whatever. My reason for asking is that I saw recently a pressure gage on a steam line that dropped in pressure at one take off point but at a different take off point upstream (on the same distribution system) the pressure gage remained unaffected. (Both pressure gages are calibrated and in working condition.)

Participant

Join Date: Dec 2009
Location: Kuala Lumpur, Malaysia.
Posts: 1
#4

### Re: Pressure and Volume

07/26/2011 3:12 AM

Fundamentals

PV=nrt (gas law)

This is sufficient to clear your doubts.

__________________
N.P.Rajah
Member

Join Date: Jul 2011
Location: Kentucky
Posts: 7
#5

### Re: Pressure and Volume

07/26/2011 8:05 AM

"Air is stored in a receiver of 1 m3 volume at 6 bar pressure means the volume of air 1 m3 and pressure exerting on th walls of receiver is 6 bar. If the same 1 m3 amount of air is introduced in the 3 m3 Capacity of receiver then the pressure impacting on the walls of receiver will be less."

In your experiment, continous flow, you filled the 3 m3 receiver/system to the pressure of the 1 m3 receiver, 6 bar. You increased the amount of the air.

The increase in volume refers to the receiver/container, not the amount of gas.

The volume of the receiver or system is inversely proportional to the pressure, as the amount of gas remains constant.

You increased the amount of air to get to 6 bar pressure (1 m3 container at 6 bar to 3 m3 at 6 bar).

If you set the system at the pressure of the closed receiver example (1 m3 at 6 bar to 3 m3 at less than 6 bar) your amount of gas stays the same, volume of receiver increases, and pressure drops.

So Boyle's Law still holds true.

Active Contributor

Join Date: Apr 2011
Location: Cincinnati OH
Posts: 22
#6

### Re: Pressure and Volume

07/26/2011 8:27 AM

Your answer may lie in the Hagen-Poiseuille equations which is a precise solution to the Navier-Stokes equations in Fluid dynamics.

The basic solution assumes incompressible fluid and laminar flow but has an approximation to compressible fluids, if I remember correctly.

One thing I remember is that solving it for a specific application is a big pain.

__________________
Love my work. Thank God someone is paying for it