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Why does smoke go faster up a taller chimney?

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vtsteam:
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.

hopefuldave:
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)

vtsteam:
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.

awemawson:
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:

RussellT:
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

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