Author Topic: Why does smoke go faster up a taller chimney?  (Read 13670 times)

Offline vtsteam

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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
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Offline hopefuldave

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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)
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Offline vtsteam

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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
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Offline awemawson

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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

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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

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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
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Offline Pete.

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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

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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

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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
www.sredmond.com

Offline mattinker

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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

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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.

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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 ?????