Author Topic: Why does smoke go faster up a taller chimney?  (Read 19030 times)

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Why does smoke go faster up a taller chimney?
« on: February 26, 2013, 08:32:28 PM »
This may seem like a question even grade school science classes have been answering forever. Everybody knows that warm air is less dense than cool air.

But why wouldn't it slow down as it goes up a taller chimney and cools? Even an insulated chimney cools air progressively and at the very least you'd think it slows down slightly more, rather than speeds up as you increase height.

And even though I can understand that warm air in contact with cooler air would tend to rise, in a chimney it is in a tube, separated from the cold air. So how can its non-contact surroundings affect it except at the top, where it comes back into contact with cool air.

If the answer to that is there is a vacuum created at the top when it suddenly cools, why would that increase dramatically with small increases in chimney height?

And why do we assume that exhaust from a fire is less dense than air, anyway? It has a lot more in it, creosote, water vapor, and soot, to name a few things. I mean maybe it is but at the chimney top where it cools what is its density?

So altogether what is really happening in a chimney as you increase the height?

On the net a bunch of searches all yield the same non-answer that  warm air is less dense than cold air.

Yes, I register,

and........?

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline John Hill

  • The Artful Bodger
  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 2016
  • Country: nz
Re: Why does smoke go faster up a taller chimney?
« Reply #1 on: February 26, 2013, 11:16:12 PM »
Wellllll...... I s'pose the chimney contains a column of air and gases that is less dense than the air around it, the chimney air at the very bottom is at almost the same pressure as the air around it but the chimney air halfway up is 'quite a bit' less dense than the air at that altitude outside the chimney (because a column of chimney air is less dense than a column of cold air) so the difference becomes greater the higher you go up the chimney. 

.....best I can do.... :coffee:
From the den of The Artful Bodger

Offline pjf134

  • Jr. Member
  • **
  • Posts: 34
  • Country: us
Re: Why does smoke go faster up a taller chimney?
« Reply #2 on: February 26, 2013, 11:35:52 PM »
 It all comes down to draw. Hot air rises and cold air sinks and once you heat up the chimney it creates a draw as long as it keeps warm. If you have a 6" stove pipe and a bigger chimney it takes longer to heat the chimney pipe to create a draw and if the chimney is to big in diameter it might not work at all. I have a chimney that is for a 8" stove pipe but now I have a 6" stove pipe and now I have more of a down draft than before and it takes longer to create a draw. My chimney is at 20 ft. which is the min. height for draw and I will have to make the chimney higher for it to work right (I hope). Look at some install manuals for wood stoves and it will show how to create draw which there are many factors that causes problems like wind, trees too close ect. I hope this helps.
Paul
To error is human, but doing it a second time takes a better excuse!

Offline John Hill

  • The Artful Bodger
  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 2016
  • Country: nz
Re: Why does smoke go faster up a taller chimney?
« Reply #3 on: February 27, 2013, 01:49:03 AM »
OK, but what is "draw"?
From the den of The Artful Bodger

Online awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8966
  • Country: gb
  • East Sussex, UK
Re: Why does smoke go faster up a taller chimney?
« Reply #4 on: February 27, 2013, 03:47:41 AM »
Imagine a wooden log 1 foot long floating  upright in water. It takes a certain force to push it under water. Now take a 2 foot log of the same diameter and push that under water. The 2 foot log takes twice the force to submerge. OK the log represents the column of warmer (hence less dense) flue gases. The water represents the colder hence more dense air. So the longer your log (column of warm gases) the more upthrust (draw) on your chimney. Simple Archimedes physics.
Andrew Mawson
East Sussex

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #5 on: February 27, 2013, 08:52:48 AM »
Thank you all. I feel I'm getting closer, but not quite there yet. Awemason is giving me a close to feasible mental picture. But still not quite there.

Archimedes decides to test this by sinking a pipe 200 mm in diameter and 3 meters long a meter deep under water. Ends capped. Horizontal orientation.

He opens both end caps and the less dense fluid flows out and water flows in. Now he does the same with a pipe twice as long.

Will the water flow out at a much higher rate just because it is longer? I have a feeling not. But maybe I'm wrong. I own a fish tank and when moving tubes around, length doesn't seem to be a factor.

Now you might say a chimney is vertical not horizontal. But I chose horizontal to negate the effects of the rapidly increased pressure in water as you work in the vertical orientation.

In air, pressure increase over 3 vertical meters is negligible by comparison. Unless you believe that such a small pressure differential is the cause of a very rapid increase in draft in air by raising a chimney 3 meters.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Online awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8966
  • Country: gb
  • East Sussex, UK
Re: Why does smoke go faster up a taller chimney?
« Reply #6 on: February 27, 2013, 09:20:08 AM »
I think you have the wrong concept. The lighter smoke is floating in the heavier air as the log floats in the water. Air is a fluid don't forget. The upthrust on the smoke is due to it's floating as it is less dense. If you have twice as much smoke you have twice as much upthrust all other parameters being the same.
Andrew Mawson
East Sussex

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #7 on: February 27, 2013, 09:33:36 AM »
I can easily visualize that a lighter gas rises within the medium of a denser one, until they mix.

What I don't understand is why when you isolate the two from each other in a pipe that they affect each other. And in proportion to the length of the pipe.

If you say it is at the top where the draw occurs, then why would it be stronger if the pipe is lengthened?
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Online awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8966
  • Country: gb
  • East Sussex, UK
Re: Why does smoke go faster up a taller chimney?
« Reply #8 on: February 27, 2013, 09:42:41 AM »
Isolating the flue gas from the air ensures that the density difference is maximised. The upthrust acts on each and every molecule of the flue gas, not just at the top.
Andrew Mawson
East Sussex

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #9 on: February 27, 2013, 09:54:41 AM »
You haven't explained why it acts and how it acts. Is it like a magnetic field acting through the pipe walls?

But I think I understand why, now.

The slug of smoke in a chimney is an object with boundaries. It is related along its length, not a disconnected length of random relationship. It is an object..

And the pipe, being fixed in position, is a representative of the external density to the object of hot gas.

And likewise the pipe is a representative of the internal mass to the surrounding air, minus its actual mass. Thus the pipe itself would have some upthrust, even though the ends are open. Not enough to lift it. But a small amount. If you sealed it and made it light, it would be a hot air balloon, and could rise.

The interesting concept to me is that the interface acts in a representative way to both pressures. And the contained smoke is an object.

Now I understand it.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Online awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8966
  • Country: gb
  • East Sussex, UK
Re: Why does smoke go faster up a taller chimney?
« Reply #10 on: February 27, 2013, 10:27:27 AM »
Hooray !!!!!
Andrew Mawson
East Sussex

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #11 on: February 27, 2013, 10:33:34 AM »
Alternative theory:

A long glass tube under partial vacuum oriented vertically is suddenly opened at both ends. What happens, does the less dense contained air rise through the column?

No, air is sucked in at both ends.

It isn't only a density  problem, it is a pressure problem. The flow isn't driven more strongly by density differentials than pressure differentials.

So, back to the wood stove and chimney pipe working 6 feet away from me. The chimney is pressurized.

The woodstove is at higher pressure than the chimney top. This is the principle of the jet engine, not the hot air balloon.

Question to gun builders. Does increasing the length of a barrel, all other things being equal, increase the velocity of the bullet?

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #12 on: February 27, 2013, 10:36:40 AM »
probably more appropriately aimed at blowgun builders than gun builders -- a shell gets a momentary charge, not a continuous pressure.

Do longer blowguns have higher velocity? Seems obvious they would.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #13 on: February 27, 2013, 11:03:55 AM »
So if the wood stove is a compressor and the stovepipe is an accelerator, shouldn't it work horizontally?

No. It is the density difference that gives the system direction. It's the bias.

A jet engine compressor is directional. a woodstove is bi-directional. A ramjet depends on forward movement to make it directional. A stove depends on initial density differences to make it directional. Otherwise the chimney would have higher resistance than the grate.

I think that's puzzle put together.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #14 on: February 27, 2013, 11:50:25 AM »
Sorry about multiple posts -- i just keep thinking of more things, and it's snowing outside yet again and difficult to work on anything, and I'll have to go out and plow soon.

A vector is magnitude and direction. What I'm looking at in front of me, warming this house as it snows outside, is a woodstove generating a fair amount of pressure or force, an object, the smoke, accelerating with this force up the stovepipe, and a direction initiated by the difference in density of gasses.

And this is an analogue to how a jet engine works. Force mediated by a direction.

It also seems to be the principle of a monotube steam producer. The water pump not only provides feed water, but also must enforce direction. Otherwise a monotube with water in it would steam out of both ends. And in fact the water pump must provide higher pressure than the steam gage pressure to do that. In fact a diode in the form of an additional  clack valve is often used to absolutely maintain direction in  this circuit

Does the pressure of a jet engine compressor have to exceed the pressure in the combustion chamber when the engine is not moving forward and gaining ram effect?

And does the directional force of the difference in densities of hot and cold gasses do that in a chimney? Or is it the cooling gasses at the top of a chimney creating a partial vacuum there doing it?

Probably both.

A chimney once started working increases in directional power, through several mechanisms at once.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline Lew_Merrick_PE

  • Hero Member
  • *****
  • Posts: 690
  • Country: us
Re: Why does smoke go faster up a taller chimney?
« Reply #15 on: February 27, 2013, 01:36:34 PM »
You are missing the simplest explanation.  When you fly a kite, it gets more lift/drag the higher it goes because, generally, wind speeds are greater the higher you go in the atmosphere.  The wind passing over the top of a smokestack creates a Bernoulli vacuum draw.  The higher the wind speed, the greater the draw.  The taller the smokestack, the faster the wind passing over its exit moves.

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #16 on: February 27, 2013, 01:41:17 PM »
Mine draws in a calm. And the length relationship works whether the wind is blowing or not.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Online awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8966
  • Country: gb
  • East Sussex, UK
Re: Why does smoke go faster up a taller chimney?
« Reply #17 on: February 27, 2013, 02:44:45 PM »
You are missing the simplest explanation.  When you fly a kite, it gets more lift/drag the higher it goes because, generally, wind speeds are greater the higher you go in the atmosphere.  The wind passing over the top of a smokestack creates a Bernoulli vacuum draw.  The higher the wind speed, the greater the draw.  The taller the smokestack, the faster the wind passing over its exit moves.

But a flue draws very well even on a calm day so long as it has warmed up. The solution I reckon is mainly gravity driven. The lighter hot gases rise.
Andrew Mawson
East Sussex

Offline John Hill

  • The Artful Bodger
  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 2016
  • Country: nz
Re: Why does smoke go faster up a taller chimney?
« Reply #18 on: February 27, 2013, 02:44:54 PM »
There is a column of air from the surface of the earth to the top of the atmosphere, the pressure at the bottom is about 15 psi.  If you built a chimney right up to the top of the atmosphere and filled it with a gas which weighs half that of air the pressure at the bottom of the chimney would be half, about 7.5 psi.  So the difference between inside and outside at ground level would be 7.5 psi.

Go to 12,000' ft or so and the air pressure will be half sea level, about 7.5 psi and the pressure inside our imaginary chimney will be 3.75 psi.  We can see then that the difference between inside and outside the chimney is reducing with altitude.

Warm air as in a chimney is lighter than cold air and therefor rises by the effects of displacement.  The air in the chimney would shoot out the top very fast if it were not for the pressure of air outside and as we know the pressure difference between inside and out decreases with altitude we can see that the chimney air will rush out faster from the top of a high chimney.
From the den of The Artful Bodger

Offline John Hill

  • The Artful Bodger
  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 2016
  • Country: nz
Re: Why does smoke go faster up a taller chimney?
« Reply #19 on: February 27, 2013, 02:49:04 PM »
Does the pressure of a jet engine compressor have to exceed the pressure in the combustion chamber when the engine is not moving forward and gaining ram effect?

Yes, the compressor stage of the jet engine is the major part of the engine, it takes up about half the length of something like a RR Avon and starting the engine involves spinning the compressor stage to get pressure in the combustion chamber(s).
From the den of The Artful Bodger

Offline Pete.

  • Hero Member
  • *****
  • Posts: 1075
  • Country: gb
Re: Why does smoke go faster up a taller chimney?
« Reply #20 on: February 27, 2013, 03:13:57 PM »
You haven't explained why it acts and how it acts. Is it like a magnetic field acting through the pipe walls?

But I think I understand why, now.

The slug of smoke in a chimney is an object with boundaries. It is related along its length, not a disconnected length of random relationship. It is an object..

And the pipe, being fixed in position, is a representative of the external density to the object of hot gas.

And likewise the pipe is a representative of the internal mass to the surrounding air, minus its actual mass. Thus the pipe itself would have some upthrust, even though the ends are open. Not enough to lift it. But a small amount. If you sealed it and made it light, it would be a hot air balloon, and could rise.

The interesting concept to me is that the interface acts in a representative way to both pressures. And the contained smoke is an object.

Now I understand it.

The less dense warm air in the flue is pushed up by the pressure of cold air at the bottom of the flue. If the flue is 10 feet long you have a 10 foot column of warm air thus a 10 foot head of cold air pushing warm air up the flue. If the flue is 20 feet long you have a 20 foot head of cold air pushing the warm air up from the bottom.

Offline andyf

  • In Memoriam
  • Hero Member
  • *****
  • Posts: 1795
  • Country: gb
    • The Warco WM180 Lathe - Modifications
Re: Why does smoke go faster up a taller chimney?
« Reply #21 on: February 27, 2013, 03:39:10 PM »
Thee's a sort of explanation here < http://en.wikipedia.org/wiki/Stack_effect >, though I don't pretend to understand it  :scratch:

I suppose the second equation is only approximate for chimneys and flues (as opposed to simple air convection up a building from an opening at the bottom to one at the top) because the composition of flue gases differs from that of fresh air.

Andy
Sale, Cheshire
I've cut the end off it twice, but it's still too short

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #22 on: February 27, 2013, 04:50:38 PM »

The less dense warm air in the flue is pushed up by the pressure of cold air at the bottom of the flue. If the flue is 10 feet long you have a 10 foot column of warm air thus a 10 foot head of cold air pushing warm air up the flue. If the flue is 20 feet long you have a 20 foot head of cold air pushing the warm air up from the bottom.

Nope at the bottom of my chimney it's about 1000F and at the top about 300F.

The bottom ends in a roaring fire in my particular setup.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #23 on: February 27, 2013, 05:37:54 PM »
I think what Pete means is that the air outside of the stove is colder than that in the flue.

To all the commenters, I'm pretty satisfied with my theory that the stove and stovepipe are pressurized by high pressure in the combustion chamber. And that low pressure at the top of the chimney caused by rapid cooling makes it directional, and therefore a flow.

And that a longer stovepipe represents a longer distance over which the force of pressure continually created in the stove can act on the mass of gasses within.

I do think that the difference in densities of warm and cold air also contributes -- particularly in starting. But I think the pressure generated in the combustion chamber is the primary force once the fire is going well.

If we take our stove and stovepipe and turn them on their side, and put a small blower in the grate we have a classical turbojet engine. I don't think turning it upright diminishes the contribution of thrust up the chimney of expanding gas products in the combustion chamber. I do think the difference in density takes the place of the fan, but like a compressor in a jet engine the directional force  provided by the difference in density produces less  energy than the combustion chamber provides --  a compressor's main function is enforcing a direction.

Like all theories it can be proven or disproven by an experiment designed to isolate those two factors.

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline Pete.

  • Hero Member
  • *****
  • Posts: 1075
  • Country: gb
Re: Why does smoke go faster up a taller chimney?
« Reply #24 on: February 27, 2013, 06:47:56 PM »

The less dense warm air in the flue is pushed up by the pressure of cold air at the bottom of the flue. If the flue is 10 feet long you have a 10 foot column of warm air thus a 10 foot head of cold air pushing warm air up the flue. If the flue is 20 feet long you have a 20 foot head of cold air pushing the warm air up from the bottom.

Nope at the bottom of my chimney it's about 1000F and at the top about 300F.

The bottom ends in a roaring fire in my particular setup.

Doesn't matter, it takes a fancy calculation to work out the rates but to all intents, the only force driving the smoke UP the chimney is the weight of cold air outside it entering the bottom. The fire just provides heat energy to lower the density of the air in the chimney. There's no 'pressure' produced by the fire or the hot air would spill out the air intake at the base. It's all about the density of the air when heated.

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #25 on: February 27, 2013, 08:13:22 PM »
2 Questions:

A sealed can of air. Pressure gage attached. Heat the can.

1.) Does it weigh less than it did?

2.) Is there a pressure reading?



There is no loss of matter in a wood stove fire. Only chemical conversion of solid to gas.
That's an expansion of several thousands of times.
The expanded gas will exit either the grate or the chimney.
The only way it can lose density is to expand.




I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline hopefuldave

  • Full Member
  • ***
  • Posts: 189
  • Country: gb
Re: Why does smoke go faster up a taller chimney?
« Reply #26 on: February 27, 2013, 08:18:52 PM »
Agreed, the less dense air in the flue is displaced by cooler, denser air surrounding the fire - if you have a sealed wood or coal stove, get a good fire going and close the damper (air inlet) below the grate and you'll find there's a net suction drawing in air, might hear it whistle! This is because the lower density in the flue exerts less pressure at the stove than the same height of cool outside air - the pressure at the top of the flue has to be the same as the flue's open to the air. A longer (taller) flue with the same base temperature will have a taller column of low-density air so there will be more pressured difference and it'll draw harder, once the flue's warmed up.As a ccomparison, consider a camp fire without a flue, no pressure difference to speak of so it doesn't draw, much harder to light and keep alight

Dave H. (the other one)
Rules are for the obedience of fools, and the guidance of wise men.

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #27 on: February 27, 2013, 08:38:33 PM »
Pressure and resulting expansion precede the reduction in density.

You cannot reduce the density of a sealed object simply by heating it. It must expand to reduce density.

The reduction in density sets the direction a stove draws, but the power of that draw is in the expansion itself.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Online awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8966
  • Country: gb
  • East Sussex, UK
Re: Why does smoke go faster up a taller chimney?
« Reply #28 on: February 28, 2013, 02:50:31 AM »
But it 'aint sealed!!!!! The gases are able to expand hence density decreases - simples.

As for the pressurised stove argument: my wood burning stove draws even better when I open its door. If it was under pressure smoke would come OUT, it doesn't - air comes IN.

 :doh: :doh:
Andrew Mawson
East Sussex

Offline RussellT

  • Hero Member
  • *****
  • Posts: 520
  • Country: gb
Re: Why does smoke go faster up a taller chimney?
« Reply #29 on: February 28, 2013, 04:05:16 AM »
I've followed this with interest and I'm not sure whether I can add anything to the explanations but there are some points that occur to me.

First is that suction isn't a force.  You can suck as much as you like but unless there is some higer pressure somewhere nothing will happen.  If you create a low pressure area (eg in a vacuum cleaner) then atmospheric pressure pushes air into the nozzle.  When trying to visualise flow it helps to forget about suction and think about where the gas is being pushed from.

Second is that we are trying to visualise a moving system.  Initially as air is heated by the fire it is the same density as the incoming air and as it rises up the chimney it expands and accelerates (it has to as the same amount of gas passes through the whole chimney).  I think the only way to calculate the effect is to consider a slice of gas in the chimney and the forces acting on it from above and below - which probably leads to the equations on the wikipedia page.

Third is that you can't have step changes in pressure at the top of the chimney - the pressure changes take place over the length of the chimney.

I hope that this doesn't add to the air  :lol: of confusion!

Russell
Common sense is unfortunately not as common as its name suggests.

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #30 on: February 28, 2013, 09:03:09 AM »
Please don't --->   :doh:


I've never argued that there is no convection involved, only that the pressure of expansion is the real driving force.

Simply covering the top of a chimney will fill a house with smoke out of the grate, and down any pipe if you attach one to thaat grate, in contradiction to these convection forces.

The pressure of the gas generated is the result of about a quarter pound per minute of wood converted to gas. That's a lot of mass flow for a gas, coming out of the firebox. The force of convection pales by comparison. It does serve to direct the gas mass flow but it is not the major power behind a chimney draw, and gas can certainly flow down as well as up in opposition to convection theories.

A hot air balloon rises, but how big a balloon does it take to lift 100 lbs? And by comparison, what is the volume of a 6 inch chimney -- a couple cubic feet? How much lift does anyone think that generates?

We are looking at a wonderful principle and trying to ignore it. That principle is the principle of bias applied by a minor force to a major force. Without it there would be no jet engines or amplifiers.

Convection force and gas expansion work in concert in a stovepipe to supply both direction and power, and thus a great deal of useful work.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline Pete.

  • Hero Member
  • *****
  • Posts: 1075
  • Country: gb
Re: Why does smoke go faster up a taller chimney?
« Reply #31 on: February 28, 2013, 11:23:18 AM »
Interesting analogy about the balloon.

A fully inflated hot air balloon's volume can't change, yet when you heat it, it rises faster. Some air spills out of the bottom due to expansion, but is expansion now the driving force causing lift? Nope, because if you hold the higher temperature steady no more air will spill out yet lift will still be greater - the driving force is the cold dense air below the balloon lifting the hot, less dense air inside it.

Same goes for the chimney.

Offline hopefuldave

  • Full Member
  • ***
  • Posts: 189
  • Country: gb
Re: Why does smoke go faster up a taller chimney?
« Reply #32 on: February 28, 2013, 11:46:11 AM »
OK, let's try eliminating some variables with a simplified experiment!

Ignore temperature differences, just consider density:

Take a bucket of water, and a piece of Suitable Tubing (clear glass or plastic would be ideal), fill the tubing with a less-dense liquid (eg kerosene) and seal the bottom end with a finger. Plunge the tubing into the water (you can use warm water if you're as woosie) and take finger off the end.
Assuming that the top of the tube is below water level, what happens? The less dense kerosene is pushed out the top of the tube by denser water. - The less dense kerosene exerts less static pressure at the bottom of the.tube than the same head of water. The difference in static pressure is what causes the flow up the pipe.

Now repeat with a slight variation:

Raise the top end of the tube above the water surface before 'opening the damper' (you finger on the end) - kerosene will flow out again, until the static pressures equalise, with a taller column of kero - the level in the tube will be above the water surface, and once equalised you can move the tube up and down (slowly) and the level in the tube *will remain constant* above the water surface, water flowing in and out at the bottom, air at the top. This is a good way to measure liquid density relative to water,.I remember spending a science class doing it...

SO... We've established that the difference in density causes a difference in static pressure, and this starts the flow.
As cold, dense air enters the stove, it's heated by the fire, it gets less dense, so it's pushed out up the flue by more cold air. If we don't close the damper a bit all our logs burn too fast and we just heat the sky, not the house...

So, what if the tube in our experiment is full of water, not kero?

If the water's at the same temperature, nothing happens (the stove's cold!).
Now we apply some heat to the water in the bottom of the flue - back in school we put a few Amps through a coil of resistance wire in the tube - and the water warms and its density decreases.
The less dense water in the tube now exerts less static pressure, ans starts to flow up the flue, cold water enters the base and is warmed, on it goes... This is 'convection' or 'thermosyphon', and used to be how central (wet) heating and car radiators worked before cheap pumps came along!

HTH,
Dave H. (the other one)

Rules are for the obedience of fools, and the guidance of wise men.

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6466
  • Country: us
  • Republic of Vermont
Re: Why does smoke go faster up a taller chimney?
« Reply #33 on: February 28, 2013, 12:00:10 PM »
Pete,

A balloon does increase in volume when you turn on the gas. It approaches a sphere more closely as the bottom conical shape retracts up into the bag. The volume increases in relation to the fixed surface area. It may not be very apparent to a ground observer and that's not the only thing that happens, but it does increase in volume. The pressure inside is increased.

But you're missing the point. I chose the balloon to illustrate the principle (AND magnitude) of convection, not deny it.

And missing the second point: I agree that convection occurs in a chimney.

Probably instead of repeating the same thing here as I have now a few times and boring people, I ought to stop and ask that anyone interested read carefully what I've said rather than what they imagine I am saying.  And if  anyone doesn't agree, that's fine with me and even admirable. Health in the sciences is the result of a nice range of differing theories about the causes of phenomena, often contradictory ones.

No matter what any of us believes about the mechanism, our smoke will go up the stovepipe (most of the time) instead of down, And keep us warm while we cook up new theories to our own satisfaction.

peace  :med:

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mattinker

  • Hero Member
  • *****
  • Posts: 1316
  • Country: fr
Re: Why does smoke go faster up a taller chimney?
« Reply #34 on: February 28, 2013, 12:30:03 PM »
Air is a notoriously bad conductor of heat, warm it up and it will rise. Cold draughts in a room with a fireplace are the result of not supplying the fire with it's own source of air. Some fireplaces smoke when the door is closed. The rising air, has to be replaced, for a chimney to draw, there needs to be supply of air to heat so that the air can become lighter and rise. Shut off the supply of air to the heat source and it will not rise even if it's hot. Even though the smoke is losing heat as it's rising in the chimney, it's still hotter than the air outside.

Regards, Matthew

Offline sebwiers

  • Jr. Member
  • **
  • Posts: 27
Re: Why does smoke go faster up a taller chimney?
« Reply #35 on: February 28, 2013, 11:38:34 PM »
Two reasons I can think of

1- The warm air accelerates as it rises ; taller chimney means faster moving air at the top, which means lower pressure at the bottom.  When you look at it this way, the question is much like "why does a taller waterfall have faster moving water at the bottom".  Um, because.... that's how gravity works?

2 - Having the top of the chimney higher up usually means its exposed to faster, less turbulent wind, which again, creates low pressure and draws air up from the bottom.

Offline R.G.Y.

  • Jr. Member
  • **
  • Posts: 44
Re: Why does smoke go faster up a taller chimney?
« Reply #36 on: April 12, 2013, 07:38:29 AM »
Another point for consideration, chimney with bends in it will raw better than a straight one ?????