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Author Topic: Chimney  (Read 522 times)

Offline chimpac

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Chimney
« on: February 03, 2020, 11:38:44 am »
It is common to run a tapered chimney small end down so creosote can run back into the hot stove.

I always keep a hot stove so I have always done it small end up.

I do not get quite enough draft with a large coffee can stove using a tapered chimney large end 2”.

Some time today I am going to turn a 2” x 38” chimney small end down to see if I get more draft. If I get more draft I will not go to a heavier slightly larger chimney.

Who wants to predict that I will get more draft with large end up.

Offline awbrown: N. Illinois, USA

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Re: Chimney
« Reply #1 on: February 03, 2020, 12:01:50 pm »
I have a set of nesting pipes. They fit small end up. From larger to smaller. Fire inch down to four inch.
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Offline kinguq

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Re: Chimney
« Reply #2 on: February 03, 2020, 01:04:52 pm »
I'm going to predict that there will be no difference. But how will you test this...there are so many variables.

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Offline chimpac

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Re: Chimney
« Reply #3 on: February 03, 2020, 01:27:03 pm »
It is difficult and it has to be tested with different types of wood. It is hard I tell how dry wood is.

A 2” chimney is really big enough if the top of my chimney side port was closer to the top of the cook top.
Now the top of the side port of my stove is 1.5” below the cook top. I cut the port in 1.5” below the cooktop because I am trying to keep as cool a chimney as possible thereby getting more usable heat.
« Last Edit: February 03, 2020, 04:43:56 pm by chimpac »

Offline scoutergriz

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Re: Chimney
« Reply #4 on: February 04, 2020, 08:40:02 am »
the problem is that by keeping the chimney cool, you reduce the draft- chimneys work by heat differential; hot air is less dense so it rises, cold air is more dense so it falls. it's that rising in side and falling outside that creates the draw, but there's also the venturi effect- the rate of rise speeds up as the hot gasses below pushes the cooling gasses. I suspect you will find a major difference with the small end down because the large end allows those cooling gasses to expand while still keeping a higher pressure down low.
this diagram helps explain it- the fire box is the left, and the top of the chimney is on the right
https://www.engineeringfact.com/wp-content/uploads/2018/09/venturi-effect.png


Offline kinguq

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Re: Chimney
« Reply #5 on: February 04, 2020, 09:39:20 am »
I dunno, the exit of the chimney allows the gasses to expand just about infinitely, so why should a pipe expansion matter?

According to the "stack effect", draw is also correlated with chimney height, which is something I admittedly don't intuitively understand. So having a longer chimney should also work.

https://en.wikipedia.org/wiki/Stack_effect

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Offline yardsale

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Re: Chimney
« Reply #6 on: February 04, 2020, 07:53:13 pm »
Changing from a  3" pipe  to a tapered 3" to 4" tapered pipe made a HUGE difference with our Four dog stove.

Offline K.

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Re: Chimney
« Reply #7 on: February 04, 2020, 08:25:52 pm »
Probably a couple competing ideas here. For maximum draft, the maximum possible velocity in the stack should be achieved, subject to the competing effect of friction when the velocity gets too high. Widening the top of a chimney via taper will reduce the pressure and velocity at the top, giving more time for the flue gases to cool. This will reduce the draft. But, if the velocity in the stack was so high that friction was the limiting factor, then overall draft could improve by this means.  It's like impossible to predict, only an experiment will tell for any given stove and wood supply where the optimum point is.

Offline scoutergriz

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Re: Chimney
« Reply #8 on: February 05, 2020, 07:49:08 am »
"why should a pipe expansion matter?" It's not the expansion that matters it's the initial constriction which increases the velocity, in fact the expansion allows the gasses to disperse easier, helping to increase the velocity by lowering "back pressure".  and the longer the stack, the greater the temperature differential, and the faster the flow. if you have enough velocity, it can overcome the resistance of any of that backpressure.

Offline kinguq

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Re: Chimney
« Reply #9 on: February 05, 2020, 09:18:10 am »
As far as chimney height goes, air pressure decreases linearly with altitude, so that is likely part of the explanation.

If a pipe that is wider at the top gives better draw, it seems rather strange that virtually no chimneys that I have ever seen are made this way.

Kinguq

Offline Bothwell Voyageur

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Re: Chimney
« Reply #10 on: February 05, 2020, 10:02:57 am »
Exhaust pipes for racing engines are built this way but the gas velocities are much higher of course.

If the air pressure only falls linearly you'd need a really long chimney to make any difference. Maybe it's just that a taller chimney has a bigger volume of hot gas moving upward.

This website
https://woodheat.org/how-chimneys-work.html
has a table with pressure differentials for various stack heights and and temperatures. The table doesn't go as short as most of our stove pipes but even so, you can see the pressures are pretty low so no wonder we sometimes experience problems
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Offline chimpac

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Re: Chimney
« Reply #11 on: February 05, 2020, 01:39:13 pm »
Duh! How come I did not think of using my tapered chimney large diameter end up years ago.
My stove hangs on my chimney so the bottom 12”, largest part of my chimney, was carrying no smoke.
So I definitely have more chimney draw with large diameter end of tapered pipe at the top. Smoke cools and slows as it moves up a pipe.
If the pipe is getting larger as the cooling smoke moves up, the flow will not be slowed as much as if the pipe was tapering smaller.
It is simple, the volume of smoke a vertical pipe can carry depends on 3 things, temperature of smoke ,diameter of pipe and length of pipe.
The 40’ of 6” pipe on my basement stove draws like a cyclone. 4” pipe would have been big enough.
I get by with using 34” of 2” dia. chimney for a 4 man shelter and 24” for a 2man shelter.
I can keep the stove hot by pushing in 3,  3/4” sticks. If the wood is not dry enough I have to use smaller sticks and maybe a bit of waxed cardboard to keep a hot fire. I can add 2”x 5” pieces through a hole in the stove top.
The draw is so much better with large chimney end up I might be tempted to lower the side chimney port on my stove by 1/2” to capture more heat.

The test to see if chimney diameter is large enough is how small a fire it can burn and still have draw.

« Last Edit: February 06, 2020, 04:30:37 am by chimpac »

Offline kinguq

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Re: Chimney
« Reply #12 on: February 06, 2020, 08:47:26 am »
Still not convinced. Your stoves are unique in this respect in that by inverting the pipe you actually gain a greater internal volume of pipe, because the narrow part at the bottom is not used. For virtually all other stove designs, that would not be the case. So I don't think you have proved that a conical pipe with the wide end up draws better than one with the narrow end up.

Kinguq