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What "PULLS" the smoke up a chimney? (updraft?)


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I have googled this for a few minutes and don't see a good answer...

 

I have been coughing the last few days since I have been forging (Coal forge),  Its a pretty consistent cough so I'm not sure if its got something to do with the smoke or not, but I do have a noticeable bit of smoke around while I'm forging -- I was at a gulf coast meeting this weekend (Local blacksmith meeting) and there is a big indoor forge/shop and when this forge was going full speed there was *NO* smoke in the building, it was as if there was something up there sucking the smoke up the chimney.  I have been using my forge outside, but If I could get a good draft going like that, I could bring it inside and forge even with the bad weather....

 

So, What causes the updraft that sucks the smoke through the chimney?  I have a hood over mine that looks a *LOT* like this one - I'm going to go to home depot on the way home and see if I can get 10-15' of chimney extension for inside of my shop and try to mount it....

 

All I have found is that its something to do with the pressure - Is there anything specifically I should know when it comes to forges, is it the same as a regular fireplace chimney?   As far as I read it just has to do with the height of the chimney that creates the pressure to pull up the smoke - (Seems way to simple to be correct)!

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You are going to try something that is more complicated than you know. Some chimney pipe from local store may wot you need or a waste of time and effort. And may be a fire hazard.. We do not know wot your structure is like or wot the local laws are, You are in way over your head on this,,,and you need to do a lot of homework to get ready. For now while you learn do nothing,,forge outside and stay upwind,,,And for right now stay away from forge and see if the cough clears up.

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SJ243,

 

Updraft is actually defined as negative pressure. You get it in a stack pipe by increasing the velocity of the hot gases being carried up the pipe. Hot gasses are not as dense as cold gasses but they like to move to where the pressure is less. Thus if your shop door is open and the stack pipe is cold the easy way out migh be into your shop rather than up the pipe. Smoke acts like a solid as it moves through the pipe. It has mass and it creates a negative pressure by moving upward. The faster it moves the greater the negative pressure.  

 

Things that effect updraft are the diameter of the stack pipe, the height of the stack pipe and the number of transitions in the pipe. Any mass flow device suffers a loss in pressure when making a transition. The position of the stack piper outside you shop is another issue. Below or above the roof line?  Next to or sheltered by another building or structure?  Both factors will negatively impact draft because they create pressures that your exhausting gases have to work against.  Does your stack have a cap?  Sometimes the design of the cap will creat reistanace and add to the forces that the gassses have to work against. There are simple devices to increase the leaving velocity of the gas that will increase draft.  

 

As your gasses leave the forge and enter the exhaust pipe, is the available opening (squuare inches) less than the square inch open of the exhuast pipe? You want a smaller opening feeding the large pipe such as an 8' feeding a 10 ".   By somewhat reducing the inlet side of the equation you actualy create a venturi effect of sorts and this will in turn,  increase the velocity of the gas going up the stack.  The "super sucker"  side draft design is a good example of this simple design principle.

 

Lastly, there is a wealth of information on stack dimension, design and hood styling that you can read by doing a search in the Forge section of this forum site.  You should be able to solve your problem relatively quickly after some research.  Breathing smoke all day is is not a good idea.   Good luck.

 

Peter      

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Also do not close up shop tight.... There needs to be a bit or replacement air for what is flowing up chimney.....

 

IF you have chimney close to fire you should get a good convection flow (hot air rising) to start the process....

 

In out museum smithy we have 12-14 inch ducts (vertical with rain caps) about 16-18 inches above "core" fire and the seem to work really well....

 

blacksmith-mariposa.jpg

 

http://mariposamuseum.com/on-the-museum-grounds/the-blacksmith-shop/

 

Dale

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Here is my solution for smoke. It works really well. I used a 7' section of 10" Dia. Aluminum irrigation pipe. The hole I cut on the side is about 80% of the ID pipe volume. The cut shape is arched top, straight sides and then I folded back at the bottom to close the space going out the bottom of the pipe.

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post-6253-0-51967300-1361858294_thumb.jp

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Short answer, the heat caries it up the stack. Longer answer that requires research, there are a few different systems that deal with the exhaust through different methods, ie positive or negative pressure systems, vacuums, and a heat siphon to name a few. In all honesty a heat siphon is my words it's really a neg pressure system but slightly different than explained earlier and was used to ventilate mine shafts so possibly not relevant.

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my forge just has a 10' piece of 10"  diameter spiral seam duct pipe at a steep angle coming down to about 1 foot above the fire and I have a section of sheet metal bent into a U around the pipe with baling wire that I can bring down to touch the forge bed for small work or raise up when I need to slip larger pieces under it.

 

As I start my coal using a good scrapwood fire this preheats the chimney  before the noxious coal smoke starts up.  Once I get a cranking good you can hear the roar in the pipe.

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Nothing if you work in my shop. There is some, but not enough to vent the forge to out-of-doors. It is quite a large forge here at the historical society, but the masonry flue is not large enough and it has three 90° bends in it!!!!!!

 

Too bad too. I have gotten so tired of addressing the issue with visitors, that I simply ignore it and am staisfied to have some remain out side the shop and peek in.

 

The only folks who can't look in is the students when conducting the blacksmithing class.

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post-21170-0-40206600-1361983559_thumb.j

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You really need to give us a lot more information than "It's smoky in here." to get any meaningful advice. How big is your shop? How big is your forge and flue? What other ventilation is there in the shop?

One common error: If you're buying your stovepipe at a local big box or hardware store it's probably too small. The biggest stuff I can find there is usually 6" diameter, about half what you need to vent a forge. Because you are burning coal with additional forced air you need a much larger flue than you would in a similarly sized fireplace. Even for a small riveting forge I would want at least a 10" pipe. (Which will have more that three times the volume of a 6" pipe.)

 

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Hey do you remember my ratcheting forge from the Gulf Coast meeting? Not much smoke if you saw any? Other than lighting a fire with green coal you shouldn't have constant smoke all day. That green smoke can cause a cough as well as sore thorat and even head aches before the serious problems start.
Heat moves smoke up as mentioned above, and so does expansion. Fe-Wood has a good ratio of small opening to larger diameter pipe. The shroud in your photo is common and should work for you if you go from that small pipe to something larger.
If you can, bring that forge to the next meeting and let's fire it up and see what's going on.
Chuck

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Hey Chuck - I do remember it, and I dont remember any smoke!

 

I think The problem with my forge is there is not a large enough opening at the top of the hood that came with it.

 

It is only 6" and I notice when I hooked another few 6" sections to it, when the smoke would go it, it would almost instantly start to bellow out like it couldn't get up the tube fast enough - Most people on this thread talk about using 10" tube, so going to make another home depot trip and try a few sections of that, see if it makes a difference.

 

Fciron mentions this specifically below, so will let you know how the bigger pipes work!

 

 

 

forgekq.jpg

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You can make larger diameter stove pipe by snapping together multiple pieces of smaller diameter, ie you will have 2 vertical seams rather than one.  12" is better than 10" in my opinion.  If the budget will stretch use stainless steel as the acids in coal smoke will eventually eat thin galvinized steel.  Not burning down your shop is up to you, be careful.  

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I have an 11" brakedrum forge on a rectangular stand which I roll 4-6' out of my garage.  My problem is even with the forge 4-6' outside of the garage, depending on wind & temperature some smoke still goes back in the garage (according to my upset wife she then can smell it in the house).  Also sometimes it's necessary to squat down while heating to avoid breathing the smoke.  For those reasons I'd been thinking about making a forge hood similar to yours with the pipe ending at least over my head.  Another plus would be it'd shield the forge in the daytime and maybe prevent the wind from disturbing the coals.  As I did more reading though I became concerned about the problem people have had to getting them to draft. 

 

I'm very interested in how this turns out for you.  I don't have any plans for moving the forge indoors.  Are you also increasing the length of the 6" tube, as well as going from the 6" diameter to a 10" diameter for the venturi effect?

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All I have found is that its something to do with the pressure - Is
there anything specifically I should know when it comes to forges, is it
the same as a regular fireplace chimney?   As far as I read it just has
to do with the height of the chimney that creates the pressure to pull
up the smoke - (Seems way to simple to be correct)!

 

 

An old gun adage is that if you want a bullet to go faster, kick it harder or kick it longer. As long as a chimney is not too long to get it all hot, extra length is good. Diameter would likely be more help, but say the hot fumes are mostly CO2. they would be heavier than air until they were a couple hundred degrees hotter than the air. If they vent out of a short stack and mix with cool air they will settle back into your work area. If they get good velocity in a longer stack they will clear the area before falling back.

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Dcraven,

 

This weekend I went back to home depot and grabbed a few more sections of 6" as recommended here by judson - So what I ended up with is 12" pipe that is currently 6' high.  I took a grinder and "cutout" the 6" section of my hood and made it fit the 12" opening of the pipe (I'll take a few pictures this week when I get back to my forge) - I didn't have much time so I did a quick test, put it all together and lit a small fire in the forge and it seemed to start drafting pretty well!  If it does indeed work, I'll grab another few sheets of metal and fashion a hood like above, but keep it around a 12" opening instead of the old 6".  Ideally I'll end up mounting this inside my small shop, So my next test will get a 90 degree bend and put it out the wall of my shop and see if it still drafts with the big turn... If nothing else its an interesting project so far - a learning experience at least.

 

I'll keep this thread updated with a few more pics and a few experiments to see how it all works out and keep you informed.

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  • 2 weeks later...

Thanks, Fellow Baton Rouge'r.

 

I have looked around that page, (and never seen it so apparently my search wasn't as good as yours) but dont see anything specific to drafting.  Its possible I have just missed it at work, but I'll take a more detailed look today when I get home.

 

Thanks again for the link.

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