Why does air heat as it's compressed?

[Greenleaf]

Well, howscome?



Greenleaf

 

[TowPro]

I think it has to do with the friction of the molecules when you compress them.



If I recall, I think it gains 10 deg for every 1" of pressure when your talking about a turbo charger.

 

[TPCDrafting]

Tow Pro is correct, but according to Charles and Boyle's laws of Physics, it works both ways. If the volume stays constant and the gas inside is heated, compression will increase as well.

When a gas is compressed, the mechanical energy of compression is changed into heat energy resulting in the molecules striking interior surfaces of the container at a much faster rate, thus generating more friction and heat.

 

[SRadke]

It's all about energy. If you add energy (compression) the air has to take on that energy (heat). All we gotta do is figure out how to cool it back down before stuffing it into that mighty Cummins.



-Scott

 

[rrausch]

When you compress air, the molecules get closer together. Then the boy molecules start rubbing up against the girl molecules and they all get turned on.

 

[TBeckman]

Here's the engineer's answer. Using the Ideal Gas Law:



PV=nRT



P=Pressure

V=Volume (constant in this case)

n=number of moles(constant in this case)

R=Constant value

T=Temperature



With a fixed volume and amount of molecules, if you increase the pressure, you must increase temperature for the equation to balance.



There is an equation I thought I'd never have to use again.

 

[klenger]

I was going to say something like what TBeckman said, but w/o any facts for formulas to back it up. Here's my take on it. Let's say that a given unit of air (let's assume A = 1 cu-ft) has X BTU's of energy in it at room temperature. Now lets pack 3 times as many units of air into the same space. The result is that the space not contains 3X BTU's of energy, so it is hotter. I know this sound like I could be full of stuff, and I usually am, but I think it is kind of saying what TBeckman said. Looking at it another way, when air expands, it has a cooling effect. Is this due to friction? How about the lack of friction? Also keep in mind that exhaust temps drop about 300 degrees after passing thru the turbo. The pressure on the inlet side of the turbo is higher than the outlet side. End result is heat from exhaust is transfered to the intake air by the turbo, and then removed by the intercooler. Conservation of energy. My head hurts.

 

[jponder]

The heating of a Gas by compression is not related or described by the Ideal Gas Law ie PV=nRT Indeed it violates the law. It is a consequence of the Joules-Thompson effect.



For Pv=nRT to work correctly the Joules-thompson coeficient must be = to zero.



Not all gases will heat on compression and cool on expansion, Helium being an example.



I am not sure why this happens on a molecular level, best bet would be to call a major University and ask whoever teaches their P. Chem classes



It doesnt have anything to do with Charles Law or Boyles law or any of that. It is something different and much more complicated

 

[TPCDrafting]

Originally posted by jponder

It doesnt have anything to do with Charles Law or Boyles law or any of that. It is something different and much more complicated

I quoted directly out of my college physics book.

 

[jponder]

Yes, it can be confusing.



Lets look at a cylinder and a piston. P(final) V(final) = P(initial) V(initial). Thats all fine and good but why does the gas increase in temp? It shouldnt by that equation. As volume decreases pressure increases and as Volume increases pressure decrerases.



Those equations dont have anything to do with a gas increasing in Temp while it is going to a low volume high pressure sitiuation.



They dont adress it. Even the ideal Gas Law does not adress it.



They are talking about if YOU raise the temp on a gas it will either increase in Volume OR it will exert more pressure.



Those equations do not explain why a confined gas that is rammed into a smaller volume will increase in Temp! Even if you forget about excluded volume of the molecules!



The gas laws you are talking about work very well, for instance if I crush a certain amount of air to 1/2 its original space it will exert 2 times the pressure! IT DOESNT AY ANYTHING ABOUT A TEMP CHANGE!



If I heat a gas to twice its temp then it will either occupy a volume twice its original volume OR it will double the pressure! Again, this doesnt say a Damn thing about why a Gas will HEAT when compressed. NOTHING!



The equations are correct but have nothing to do with why a gas heats as it is compressed. The Gas laws dont adress that! Indeed it flys in the face of Gas Laws.



Joules-Thompson effect deals with that and not all gasses behave this way. Some gases will cool upon compression and heat upon being less compressed. I'd have to sit down with you and do the equations, but I can tell you that the ideal Gas Law does not say ANYTHING about a gas that is being compressed heating. Yet it does!!!!!!!!!



It deals with Enthalpy and Entropy and it is beyond the Gas Laws

 

[jponder]

I will try and call LSU tommorrow and see if I can get a proffesor on the Phone, I remember my Organic Chem Teachers name, Dr Tranyham and I had an advanced Inorganic teacher named, Maverick, but I cant remmember the P. Chem teachers name. He was a pure thermodynamics guy He was smart as hell, I used to drink beer with him but cant place his name after all these years.



The question is, "why does a gas increase in temp when compressed"



Oh yeah Carter was the head of Chemistry at LSU back then maybe I will ask for him, he taught some high level classes.



I would guess that it deals with excluded volume of the molecules, Meaning that The Gas laws quoted before dont take into account that molecules actually take up for volume, another topic that the Gas laws dont adress. If they come back with FUGACITY, man I am going to give up.



I hope a Chemical engineer will chime in but I assure you PV=nRT and any other Gas laws dont explain it. I'll bet my truck on it!



JOULES-THOMPSON baby! but they doesnt give a molecular answer which is what the poster wanted I think!



I did like the answer of Girl and Boy molecules rubbing together and that made more sense than anything else!!

 

[klenger]

PV=nRT



P=Pressure

V=Volume (constant in this case)

n=number of moles(constant in this case)

R=Constant value

T=Temperature

One thing I was wondering about with this formula is: Lets say you compress a bunch of air into a container (assume a basketball that doesn't stretch for lack of anything better). The air heats up as the pressure goes up. Now wait for 24 hours, ... the temperature returns to ambient, but the rest of the varialbes are unchanged, ... right? How do it do that, or is the constant not really a constant?

 

[jponder]

Greenleaf> Got to work this morning and pulled up the Ga Tech web site and Mark Allen is the first Professor listed, He's got a PHD in Chemical Engineering so I figure he will have to do. I call him up and he answers.



I just launch into the question, I say look we are having this discussion on a diesel site and some one asked why a gas heats say in a cylinder. I mention some people say it is because of the Gaslaws and I say it actually violates the gas laws. He just bust out laughing!



He must think its a crank call. He then says yes the Ideal gas laws imply the Joules thompson coeficient is zero, A gas that behaves as PV=nRT would not heat at all when compressed.



Okay now the million dollar question, I then ask him if he can explain in laymans terms what is happening on a Molecular level for this to happen. He thinks for a second and says he doesnt know and maybe try a stattistical Thermodynamics guy.



Greeleaf PM'ed me and said he already knew the answer so I'm waiting Greenleaf??????/ Dont make me keep working my way down the Ga Tech



You know that is kinda cool that the guy didnt ask who i was or anything and just answered whatever he could. If anyone else is having a slow day then give your local university professors a call today, I'm sure they would just love to talk to you.



Okay Greenleaf, I'm waiting?

 

[jponder]

Originally posted by klenger

One thing I was wondering about with this formula is: Lets say you compress a bunch of air into a container (assume a basketball that doesn't stretch for lack of anything better). The air heats up as the pressure goes up. Now wait for 24 hours, ... the temperature returns to ambient, but the rest of the varialbes are unchanged, ... right? How do it do that, or is the constant not really a constant?

do you mean you are putting in more air? If so then when you double the amount of molecules in a confined space then you should see a doubling of pressure. P is directly propotional to n.



You should not see a rise in temp, but we know you do. You would actually see a little pressure than double, but as it cooled to ambient you would be left with double the molecules and double the pressure just like the ideal Gas laws say.



You see this happen all the time with scuba cylinders

 

[Hohn]

Good thread. Thanks jponder for the pm to see this.



JPONDER IS RIGHT ON.



I think I'll stay out of this and let you guys have at it-- for now



Justin

 

[TBeckman]

Keep in mind that this is an equation to explain "observations" It doesn't really answer the orignal question. It's an equation, not to good of an answer, but now that I started something.



Something forgotten by everyone so far is that Temperature in the IGL is in Kelvin, which changes alot in the way of thinking.



You say that you won't feel a temperature change when you compress something to half it's space, that is due to the fact that it is a small change. My paintball tanks are filled to 4500psi, and they're just warm to the touch, so you're talking about 2 atm from one, I would tend to think there change is very little.



"For Pv=nRT to work correctly the Joules-thompson coeficient must be = to zero. "



That is absolutely correct. IGL doesn't take into account for the work done by heating or compression. This is added work that you have to balance additional equations to get the answer closer to reality. I had Thermal and Fluids jammed into one 15 week class, so I'm sure some of the finer points escape me. Not to mention I have consumed many beers since freshman year in college, I'm sure I could be wrong in my thought process.



Ok, I need to go count some cars now, I'll try and finish my thoughts later.





Tom

 

[RustyJC]

OK, take a look HERE for a hint.



Rusty

 

[TBeckman]

Ok, I wasn't thinking of why it heats up as it leaves the turbo, I was thinking more along the lines of why would an air with a certain volume and temperature increase in temp as the pressure went up with volume and amount remaining the same. The turbo explanation is another mess.



I'm glad no one called me on my previous examples because they are just plain wrong.



Back to my assumption. The IGL is basically saying that you can change pressure of an ideal gas in three ways, changing volume, temperature or amount . Now as far as I'm concerned there is no such thing as an ideal gas, it was made up to allow for analysis, kinda like the normal force. The IGL is also assuming these changes take place in a controlled box where you have complete control of the inputs and outputs, another analysis "trick". Note, what they teach you as an engineer only exists in school.



Now I'm thinking that in a fixed volume, with a certain amount of air, the only way to increase pressure would be through heat, so that is why temp would increase. Now if you don't use this "ideal gas" then the equations is meaningless and you get something that doesn't explain so simple.



Does "air" react similarly, based upon the observation with my paintball tank, I'm inclined to believe so. Back to the paintball tank example. If my tank has 1000 psi, and it gets filled to 4500 psi, additional air is being added creating the pressure increase. I'm thinking as that 4500 psi air is added, it is increasing the pressure of the existing air in the tank creating that warm tank effect. The heat is prob also due to the velocity of the incoming air. Now once the tank has cooled down, the pressure is no longer what it was when it was filled, only a hundred or so psi, but it has dropped. Now my tank doesn't leak, and it is off usually when I get the first fill of the day, so I'm thinking that the ideal gas law applies to this particular situation, although I introduced velocity for a heat source.



Ok, I think I just had to explain what I was thinking to myself more than anything. Sorry for the long post.

 

[klenger]

I enjoy these kinds of threads. Though provoking!!

 

[CTait]

Have any of you ever seen or used a small tool called a fire piston? They are used for starting fires or lighting your smokes. They are pretty neat I think. Try a Google search and you can see what they look like.