Understanding the physics of chimneys....

[fog37]

TL;DR

Understanding the physics of chimneys

Hello,

While watching survival show, I saw a shelter with a fireplace and a chimney and I started thinking how chimneys work.

My understanding is that hot air rises and cool air sinks. The fire, confined to the fireplace, heats up the air inside the enclosed space. the hot air rises everywhere but some travels up the chimney. Because of the Bernoulli's effect, the hot air moving inside the speeds up and a draft is generated pulling more of the heated smoky air inside the chimney and to the outside.

Assuming what has been said so far is correct, why do chimney need to be taller than any other structure in the house/shelter, higher than the roof? What happens if they are not?

Additionally, is it true that the chimney work "better" the warmer is the air inside the room? Why?
Wouldn't the draft create by the chimney pull the heated air away from the room making it colder? Clearly removing the smoke is important but the purpose of the fireplace is to warm the interior and the chimney's draft negatively affect that objective...

Thanks!

 

[hutchphd]

1) turbulance and vortices in general
2) More pressure differential
3) In an otherwise heated dwelling fireplaces are usually net energy losers. Woodstoves are better because they require much less exhaust air because of better control thereof.

 

[sophiecentaur]

Summary:: Understanding the physics of chimneys

Wouldn't the draft create by the chimney pull the heated air away from the room making it colder?

This is all too often true. A open fire can be very inefficient and a lot of the effective heating is radiant while the warmer air in the room is probably drawn up the chimney, causing cold air to flow into the house.
A proper stove uses minimal air to flowing through it - just enough to burn the fuel and to clear the flue gases. Heat gets to the room from the hot surface of the stove (and sometimes internal convection paths - separate from the flue). It's always necessary to ensure enough air is allowed to flow into the room to allow full combustion and avoid CO generation so air vents have to be available.
the flue needs to get hot enough right the way up to allow convection (double walls are common). It is common, in cold weather, for a plug of cold air in the flue to prevent proper convection venting the smoke, if the initial fire is too slow. Rapid burning of some paper will usually get things going.

 

[hmmm27]

Assuming what has been said so far is correct, why do chimney need to be taller than any other structure in the house/shelter, higher than the roof? What happens if they are not?

Roof catches fire.

Additionally, is it true that the chimney work "better" the warmer is the air inside the room? Why?

Warmer air for combustion --> hotter flue gas --> more powerful updraft. And,
lighter air in room --> more volume moved by updraft.

 

[anorlunda]

Roof catches fire.

Well said. I second that.

There's an additional factor. Higher up from the ground, there is a better chance of finding a breeze that carries the smoke away from the origin.

That's why the smokestacks at old fashioned coal burning plants were so high. If they blackened the houses of the local lawmakers, there would be consequences. But if they blacken a distant state or country, those remote people have no vote in the local elections.

 

[hutchphd]

Well said. I second that.

But also in certain strong wind directions vorticity off of the roof will drive the smoke back down the chimney (gee how do I know this...) A taller chimney gives both more forward pressure (its a buoyancy thing) and less likelihood of being affected by edge turbulence onset. Not fun fixing your stovepipe in a blizzard.

 

[jrmichler]

In an otherwise heated dwelling fireplaces are usually net energy losers.

A football player from Green Bay who had picked me up while hitchhiking back in the 1970's was bragging about his vacation home with a fireplace that took six foot logs. He could not use the fireplace in really cold weather because the furnace could not keep up.

 

[hmmm27]

There's an additional factor. Higher up from the ground, there is a better chance of finding a breeze that carries the smoke away from the origin.

Awhile back, the usual suspects were (virtually) sitting around, ruminating on the winter smog problem in Tibetan "cities". We had it boiled down to two solutions, either a) mechanically dewater the ubiquitous brown coal at the source (some Australian process looked promising), or b) a stove-top/firebox pyrolysis device (water is condensed out of the effluent ; remaining gas fed into the firebox ; and the now-black coal is the next load of fuel).

I have no idea which, if either, was implemented, but "longer chimneys" would have made for a much shorter thread.

 

[fog37]

Thanks everyone.

But also in certain strong wind directions vorticity off of the roof will drive the smoke back down the chimney (gee how do I know this...) A taller chimney gives both more forward pressure (its a buoyancy thing) and less likelihood of being affected by edge turbulence onset. Not fun fixing your stovepipe in a blizzard.

Thanks for your comment, hutchphd. What does "forward pressure" mean?

In summary, here my takeaways:

1) The pressure at the ground level is higher than the pressure at a higher altitude since pressure decreases with increasing altitude. The pressure difference helps pushing smoky air from the fireplace up the smokestack.
2) A tall chimney prevents air to flow back down the chimney and inside the room. This happens because turbulence and vortices are generally found at higher altitudes than lower altitudes.
4) Warmer air in the room helps to get a better combustion, hence less smoke (byproduct of incomplete combustion). Also, a hotter flue gas leads to a more powerful updraft.

Thanks!

 

[hutchphd]

(1) and(2) are mostly incorrect::
(1) The pressure variation in the atmosphere from altitude is absolutely irrelevant. The pressure differences are caused by buoyancy because hot air is less dense then cold air. This effect will be larger for a (2) longer chimney and hotter air. Air currents at the chimney top can also interfere (local high speed flow and turbulence) A (2) taller chimney gets the exit farther away from the sharp square edges of the house that produce downdraft vortices and turbulence

 

[sophiecentaur]

This demo (a movie) is used in most schools to show how convection works. The chimney doesn't have to be above the roof of the house but it helps a lot to overcome adverse conditions and a cold flue!
This is an example of a Heat Engine, which does work when there is a source and a sink of heart. The 'engine' does work, using the heat energy supplied by the candle.

A high chimney gives better convection BUT, as mentioned earlier, getting things going on a cold day is more of a problem.

A cowl on the top will keep rain out AND mitigate the effects of unfriendly wind. This one is 'top of the range' and helps with gas extraction when the wind blows.

 

[russ_watters]

Because of the Bernoulli's effect, the hot air moving inside the speeds up and a draft is generated pulling more of the heated smoky air inside the chimney and to the outside.

In case it isn't already clear, this has nothing to do with "Bernoulli's effect".