Author Topic: Why does smoke go faster up a taller chimney?  (Read 13978 times)

Offline vtsteam

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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........?

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Offline John Hill

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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:
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Offline pjf134

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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
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Offline John Hill

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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"?
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Offline awemawson

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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
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Offline vtsteam

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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.
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Offline awemawson

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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

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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?
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Offline awemawson

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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

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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
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Offline awemawson

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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

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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?

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Steve
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Offline vtsteam

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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.
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Offline vtsteam

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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.
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Steve
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Offline vtsteam

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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.
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Offline Lew_Merrick_PE

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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

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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.
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Offline awemawson

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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

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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.
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Offline John Hill

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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).
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Offline Pete.

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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

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

Offline vtsteam

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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.
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Offline vtsteam

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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.

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Offline Pete.

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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.