Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 42 users online :: 3 registered, 0 hidden and 39 guests


Most users ever online was 218 on Wed Dec 07, 2016 6:58 pm

Registered users: Bing [Bot], Google [Bot], Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

Compression heating of air

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
Sponsored 
  • Author
    Message

Compression heating of air

Unread postAuthor: DYI » Tue Aug 14, 2007 10:01 am

If you pressurised a tube of air to 100 psi, and then rapidly (ie. a few milliseconds), reduced the volume of the tube to 1/4 of its original size, the air should reach a pressure of 400 psi.

My question is, how hot would this air become, and how would that affect its speed of sound?
  • 0

User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Unread postAuthor: Fnord » Tue Aug 14, 2007 10:12 am

It depends on the type of tube, really. If it's copper, you can count on the pipe absorbing a lot of the heat energy produced. PVC however, is a half decent insulator.

Higher temperatures would increase the speed of sound within the gasses.

(sorry, I kind of danced around the "how hot?" question.
I don't know for sure, but if I had to take a guess I'd say about 600F IF the air was compressed from a normal atmosphere to 400 psi very quickly)
  • 0

Image
User avatar
Fnord
Major General
Major General
 
Posts: 2244
Joined: Tue Feb 13, 2007 9:20 pm
Location: Pripyat
Reputation: 7

Unread postAuthor: dongfang » Tue Aug 14, 2007 10:31 am

Hi,

Try to see

http://en.wikipedia.org/wiki/Adiabatic_process

- If your pressure increases to 400 psi, then it is implied that the temperature did not change.

The computations are not dead simple... but with enough effort, sure they are understandable. The graph on the wikipedia shows work done by the gas - in your case that work is negative.

...

So it would seem that....

- Approximate air to be ideal, diatomic.

Then alpha = 5/2
upsilon = (7/2) / (5/2) = 7/5

V T^alpha = constant.

Before compression: V T^alpha = V * 293K^alpha = 1.47 * 10^6 V K^alpha

After compression = V/4 T'^alpha = 1.47 * 10^6 V K^alpha

T'^alpha = 4 * 1.47 * 10^6 K^alpha

T' = (4 * 1.47 * 10^6 K^alpha) ^(1/alpha)
= 510 K
= 237 C

And

P^(upsilon - 1) * T^(-upsilon) = constant

Before compression:
(1 bar) ^(upsilon - 1) * (293 K)^(-upsilon) =
bar ^ (upsilon - 1) * 3.52 * 10^-4 * K^(-upsilon)

After compression:

P'^(upsilon - 1) * (510K)^(-upsilon) = bar ^ (upsilon - 1) * 3.52 * 10^-4 K^(-upsilon)

P'^(upsilon - 1) = bar ^ (upsilon - 1) * 3.52 * 10^-4 * K^(-upsilon) / (510K)^(-upsilon)

P' = bar * (3.52 * 10^-4 / 510^(-upsilon)) ^(1/upsilon)

= 1.74 bar


That looks funny -- can anyone see if I did something wrong?

OOPS: I remembered the problem wrong - I took it to be V/4, not 400 psi.

Regards
Soren
  • 0

Last edited by dongfang on Tue Aug 14, 2007 11:52 am, edited 2 times in total.
User avatar
dongfang
Lieutenant Colonel
Lieutenant Colonel
 
Posts: 448
Joined: Sun Aug 27, 2006 8:02 am
Location: Switzerland
Reputation: 0

Unread postAuthor: jimmy101 » Tue Aug 14, 2007 11:29 am

This is a job for GasEq ...

Air compressed to 100 PSI, how long does it sit there? Compressing to 100 PSI will heat the air, letting it sit will allow it to cool off.

GasEq will tell you the final temperature, pressure and speed of sound.

Assuming the gases are not allowed to cool after the first compression (this is what you would get if a high capacity compressor pressurized the gun chamber directly, without a storage tank) ...

Adiabatic Compression/Expansion calculation for air in GasEq.

For air compressed from 14.7 PSIA to 114.7 PSIA (100 PSIG, 7.8 ATMA);
Temperature goes to 536 K (505F)
Pressure 7.8 ATM
Volume 0.229 of the original
speed of sound 461 m/s (1512 FPS)

Copy previous products/results to reactants

Now compress 4:1;
Temperature goes to 889 K (1140F)
Pressure 51.8 ATM
Volume 0.25 of the previous
speed of sound 586 m/s (1920 FPS)

All of the above is for an adiabatic (no heat lost to the chamber or outside world) system. In the real world the first compression would be done with a compressor. Most of the heat generated would be transfered to the storage tank, which would heat up only a couple degrees since the heat capacity and mass of the air is tiny compared to the heat capacity and mass of the compressor's tank.

When the compressed air from the compressor is used to fill the gun chamber the air will cool off. If it was at ambient temperature in the compressor tank then it will be cold in the gun. As with the compressor, the gun's much higher mass and heat capacity means it'll warm the air up fairly quickly, and the gun will cool by very little.

Download GasEq and fiddle with it to reproduce what you are trying to do.
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3130
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Unread postAuthor: homedepotpro » Tue Aug 14, 2007 11:45 am

jimmy i'd have to say that was the most useful, informative, and on topic reply i have ever seen.
  • 0

Image
User avatar
homedepotpro
Lieutenant Colonel
Lieutenant Colonel
 
Posts: 443
Joined: Sun Nov 26, 2006 1:00 am
Reputation: 0

Unread postAuthor: PVC Arsenal 17 » Tue Aug 14, 2007 12:36 pm

Somewhat related: http://www.firepiston.com/
  • 0


PVC Arsenal 17
Major General
Major General
 
Posts: 1762
Joined: Mon Mar 27, 2006 4:18 pm
Location: United States
Reputation: 1

Unread postAuthor: potatoflinger » Tue Aug 14, 2007 1:01 pm

PVC Arsenal 17 wrote:Somewhat related: http://www.firepiston.com/

Has anyone here ever tried making one of those?
  • 0

User avatar
potatoflinger
Brigadier General
Brigadier General
 
Posts: 1136
Joined: Thu Nov 02, 2006 3:26 pm
Location: Maryland
Reputation: 1

Sponsored

Sponsor
 


Unread postAuthor: Hawkeye » Tue Aug 14, 2007 3:07 pm

I have. It is a very simple concept. Few people realize how quickly and high the temperature rises when you rapidly compress air. The answer to the question is hot enough to ignite flammable materials. If I place my thumb over the end of my high pressure pump and rapidly push the handle there is a noticeable spike in temperature.
That is why oil for pneumatic tools is not a flammable variety.
  • 0


Hawkeye
Lieutenant Colonel
Lieutenant Colonel
 
Posts: 462
Joined: Sun Jan 21, 2007 7:12 pm
Reputation: 0

Unread postAuthor: Spudinator » Tue Aug 14, 2007 3:34 pm

Check the copper pipes on top of your compressor sometime after a long run, should be too hot to touch from the heat of compression.

This is the grounds for how two-stage light gas guns work, a primary powder charge slams a piston against a compressed cylinder of hydrogen and a burst disc, the compression heats the gas to thousands of degrees making the gas extremely light (hence it moves like a mother). Sends projectiles out at hypersonic velocities.


Has anybody ever considered trying this idea out in a spudgun? A hybrid would be good grounds for something like that...
  • 0

"This is so fun it should be illegal... Oh wait..."
User avatar
Spudinator
Donating Member
Donating Member
 
Posts: 70
Joined: Sun Feb 13, 2005 4:50 pm
Location: WA
Reputation: 0

Unread postAuthor: PinHead » Thu Aug 16, 2007 6:30 pm

I'm surprised that no one has mentioned yet that this is the basis for how a diesel engine works. Theoretically, it could work in a hybrid, but depending on your fuel, you would need an INSANE amount of pressure to get there (I think the compression ratio for detonation of gasoline is somewhere around 14:1? Correct me if I'm wrong here). Plus you would need a really fricken strong burst disk to be able to to withstand that until it goes off. But yeah, it could still be done.

My dad and I tried making a traditional firepiston out of wood once, but we couldn't find good enough quality bamboo to make it with. They just split from the pressure all the time.
  • 0

User avatar
PinHead
Corporal
Corporal
 
Posts: 75
Joined: Sat Jul 28, 2007 8:23 pm
Reputation: 0

Unread postAuthor: paaiyan » Thu Aug 16, 2007 6:42 pm

Dude, that fire-piston thing is sweet. I'll have to make one sometime. What's on the end of the rod that heats up?
  • 0

"Who ever said the pen was mightier than the sword, obviously, never encountered automatic weapons."
-General Douglass MacArthur

Read my dog's blog - Life of Kilo
User avatar
paaiyan
Major General
Major General
 
Posts: 2140
Joined: Wed Dec 27, 2006 10:03 pm
Location: Central Oklahoma
Country: United States (us)
Reputation: 1

Unread postAuthor: Brian the brain » Thu Aug 16, 2007 6:49 pm

Take your avarage pneumatic cannon, use a lightweight, airtight piece of
"ammo" ( a patatoe might even work once)better yet, an O-ringed piston.
Put a reducer on the end of the "barrel" and attach a smaller diameter barrel onto the front ( or elbow it back over)

put some flamable substance in the barrel and load ammo...


it should work...but nobody tried it yet
  • 0

Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
User avatar
Brian the brain
Moderator
Moderator
 
Posts: 3494
Joined: Mon Dec 05, 2005 2:06 am
Location: Holland
Country: Netherlands (nl)
Reputation: 29

Unread postAuthor: paaiyan » Thu Aug 16, 2007 6:59 pm

Brian the brain wrote:Take your avarage pneumatic cannon, use a lightweight, airtight piece of
"ammo" ( a patatoe might even work once)better yet, an O-ringed piston.
Put a reducer on the end of the "barrel" and attach a smaller diameter barrel onto the front ( or elbow it back over)

put some flamable substance in the barrel and load ammo...


it should work...but nobody tried it yet


Or nobody talked about it, because that would be a flaming projectile...
  • 0

"Who ever said the pen was mightier than the sword, obviously, never encountered automatic weapons."
-General Douglass MacArthur

Read my dog's blog - Life of Kilo
User avatar
paaiyan
Major General
Major General
 
Posts: 2140
Joined: Wed Dec 27, 2006 10:03 pm
Location: Central Oklahoma
Country: United States (us)
Reputation: 1

Unread postAuthor: PVC Arsenal 17 » Thu Aug 16, 2007 7:02 pm

paaiyan wrote:
Brian the brain wrote:Take your avarage pneumatic cannon, use a lightweight, airtight piece of
"ammo" ( a patatoe might even work once)better yet, an O-ringed piston.
Put a reducer on the end of the "barrel" and attach a smaller diameter barrel onto the front ( or elbow it back over)

put some flamable substance in the barrel and load ammo...


it should work...but nobody tried it yet


Or nobody talked about it, because that would be a flaming projectile...


Which would make it unfathomably cool.
  • 0


PVC Arsenal 17
Major General
Major General
 
Posts: 1762
Joined: Mon Mar 27, 2006 4:18 pm
Location: United States
Reputation: 1

Unread postAuthor: paaiyan » Thu Aug 16, 2007 7:06 pm

PVC Arsenal 17 wrote:
paaiyan wrote:
Brian the brain wrote:Take your avarage pneumatic cannon, use a lightweight, airtight piece of
"ammo" ( a patatoe might even work once)better yet, an O-ringed piston.
Put a reducer on the end of the "barrel" and attach a smaller diameter barrel onto the front ( or elbow it back over)

put some flamable substance in the barrel and load ammo...


it should work...but nobody tried it yet


Or nobody talked about it, because that would be a flaming projectile...


Which would make it unfathomably cool.


Unfathomably cool, yes. But also against forum rules...
  • 0

"Who ever said the pen was mightier than the sword, obviously, never encountered automatic weapons."
-General Douglass MacArthur

Read my dog's blog - Life of Kilo
User avatar
paaiyan
Major General
Major General
 
Posts: 2140
Joined: Wed Dec 27, 2006 10:03 pm
Location: Central Oklahoma
Country: United States (us)
Reputation: 1

Return to Pneumatic Cannon Discussion

Who is online

Registered users: Bing [Bot], Google [Bot], Yahoo [Bot]

Reputation System ©'