Help understanding air draw and venting?

[Gulf]

Originally Posted by brickie in oz

Man I can just hear the banjos starting up.......

Brickie,
Now that was funny
But, I have been PieAreSquared slam off of this thread! I think I'll just set here on the porch, pick my banjo for a while, listen to the frogs a while and then maybe I'll hear somesome kayakers coming down the crick D

[brickie in oz]

Im doing a small job for some friends 20kms from home up in the hills and thays all hilbillies up in them thar hills.....

We looked at buying a block of land up there last year with creek frontage, I was going to sit and watch the creek drikin moonshine and playin the banjo.. but my wife didnt like the idea.

[wotavidone]

Interesting discussion chaps. Since I spent seven years trying to get industrial process gases to go where we wanted them to go, I've got a couple of things to contribute:
* There is such a thing as too big, where flues and draughting hoods are concerned.
* When you are using a fan to do the draughting, for effective capture you want to keep the gas velocity up fairly high. Since the fan is generally pumping a fixed volume, the cross-sectional area determines the gas velocity - too big = low velocity = poor capture.
* Where you are relying on natural draughting you only have a certain amount of hot gas to induce the draughting. A flue too small results in high velocity but not enough area to capture all your smoke. A flue too big results in insufficient velocity, as the gas cools rapidly with the ingress of cold air from the opening, etc.
* If you want serious draw over a long chimney, you insulate it to ensure the gas doesn't cool and contract, losing velocity as it travels up the stack. If you can't insulate it, you taper it, so that as the gas contracts from cooling, the velocity is maintained.

Of course, our main stack is 205 metres high, about 670 feet, a lot of cooling potential, Its insulated for about the first third, and tapered the whole way to the top.

When all is said and done, though, it doesn't pay to apply too much science. This is one area where you should just rely on the experience of the thousands of people who have already built an oven. Conventional wisdom seems to call for an 8" flue for a 36" or 42" oven, and maybe a 6" flue for a smaller oven. Can't argue with it.

[wotavidone]

P.S. I dunno where the idea that "smoke rises in circles" came from. Take a look at the smoke pattern on the inside of sharky's vent in post #23. That sure looks like laminar flow to my eye. Perfect sizing there, small enough to induce appropriate velocity, large enough that all the smoke goes up the hole.

[Amac]

If you want serious draw over a long chimney, you insulate it to ensure the gas doesn't cool and contract, losing velocity as it travels up the stack. If you can't insulate it, you taper it, so that as the gas contracts from cooling, the velocity is maintained.

Great contribution wotavidone - I wondered what the insulation was for - that explains a lot.

[wotavidone]

It has to be a very long stack for insulation and taper to matter though. Our 205 metre stack tapers to maybe 1/3 of its starting diameter by the time the gas gets to the top. So a couple of metres of pizza oven flue is not going to gain at all from either insulation or taper.
There is a third reason to taper - in a fan fed stack such as ours you can taper it to accelerate the gas. If you have flue gas that is any way corrosive to the outside of your stack (and as we all know, combustion gases are corrtosive, why everyone recommends special mortars and stainless steel rather than galvanised steel) you want the gas coming out so fast that it doesn't swirl around and hit your stack and corrode it. Ours has a special brick liner, but the outside is poured concrete, so the liner protrudes slightly above the outershell, and the top 10 metres or so of the outer shell is painted with special paint. Its entertaining watching the steeplejacks paint it, that's for sure.
I've been to the top twice. Ladders all the way, between the outer shell and the brick flue. I don't go anymore - once you pass a certain age you have to have a rigorous medical that I haven't bothered with. We don't want anyone having a heart attack up there, it'd be a nightmare getting them down.

[GianniFocaccia]

Wotavidone,

The trek to the top via stairs sounds like quite an adventure. Any pics you could share from the outside?

[wotavidone]

Hi, I'll have a look at work. I know I took some, but I'm not sure where they are. It is an adventure, especially if you aren't good with heights.
It was built in the 1980s before the regulations that required cages on ladders, etc.
It's a hollow concrete shell, which supports a flue made of special firebricks. At 10 metre intervals there is a concrete collar connecting the flue to the outer shell. These are the "floors". After the 6th floor, its all vertical ladders for the next 14 floors, no guards.
I used to do it in about the standard time. I thinks its 45 minutes, up and down.
That requires a minimum of two litres of water, an "arduous task" medical, an atmosphere test, a safety harness, two way radio, a "confined space" permit, a rescue plan, and given the preceding requirements, a damned good reason for going. Tourism is not a good enough reason. The view is fabulous. I have known people who have been to scared to exit the hatch at the top .
Years ago, before we tightened the access, two guys sneaked in and base jumped. Woulda kicked their arses if we'd caught 'em.

[Amac]

It's a hollow concrete shell, which supports a flue made of special firebricks.

I'd like to see the "indispensible tool" they used to build that one.

You don't want to suffer claustrophobia or vertigo in that job. Well base jumpimg is not on my list of 100 things to do before I die - but maybe I should add it in as the 101st (and most likely last)

[cobblerdave]

Gudday
Most old brick chimneys taper in over the fireplace then expand out over this point to let the smoke expand out and of course suck more smoke in. At the top they again use this method of tapering the end by using a chimney pot to again restrict the smoke and then let it escape into the open air again sucking the smoke behind it.
Used to live in a house with a fire box and single skinned metal flue that exited the wall, behind it. Worst still it was on a southern wall. Try to lite that from cold on a chilly night was interesting. What would happen was the smoke would only get so far up the flue due to it being so cold and of course the fire couldn't breath and the room started to get smokey. If you were really unlucky and the condesation in the cold flue had built up suddenly the smoke would disturb it and you got a stream of black water down the flue that would put you fire out for good. We did't own the house so it was never fixed with a double skinned one ...we just kept a log in it at all times.
Was going to fit a metal chimney to the WFO but decieded to build the widest chimney in brick, have plenty of bushes to cut the wind so I could keep it really low...no soot stains... works for me


Regards Dave