Question about smoke stack size

[Morten311]

Hey all. I have a problem with my current stack which is 200mm (7") and 5.5 meters tall and it has worked before but after moving to a new workshop it does not. I gave gotten sick and tired of my workshop being full of smoke, so i have been thinking about going industrial size. I have 5 sheets of stainless 1mm thick and making the new stack either 640 or 560mm in diameter or 22 to 24" and 4.5 meters in total height. Is it too big or not? Its hard to find a proper answer.

[Frazer]

Do you have any pictures? I'm assuming your forge setup has stayed the same and the only thing that has changed is the move to a new workshop. If you are only having draft issues since you moved my mind immediately goes to 3 possible causes:

1. The stack is too short for the new building (it should extend roughly 4' (1.25m) -- or more -- above the peak/highest part of the roof) 
2. It is otherwise obstructed by it's surroundings (i.e. trees)
3. The new building isn't allowing enough air to reenter the building to replace what is going up and out the flue. Does the draft improve at all if you open up a door? Granted you have probably already tried that since the shop is full of smoke... 

7" is a little small. 10-12" (250-300mm) is recommended. Bigger is generally better, but in my opinion 24" seems like it's a bit much.. It would take a long time to get the draft going through a pipe of that size. The heat of the flue gasses going up the stack is what causes it to rise, a pipe that large would draw in a lot of ambient air and the flue gasses could be cooled substantially as a result. You might end up with a weak draft until that thing gets rolling. I guess it depends on how you arrange your hood.

I'm no expert on chimney's here. I'm just thinking out loud.

[TWISTEDWILLOW]

I agree with Frazer, 12” or a little bigger would work great but I’d be afraid 24” would cause more trouble than it would help. 

[Glenn]

A lot of this depends on the size of the forge and how much fire you have in that forge.  10 inch and 12 inch diameter chimneys are suggested for most forges, single blacksmith usage.  I have tried 14 inch, 16 inch on the same side draft hood and they worked but anything larger started to reduce draft and not draw as well.  

The 4' (1.25m) -- or more -- above the peak/highest part of the roof if suggested in order to get the smoke above the way the roof redirects any air currents.

Take notice of anything that will affect the air currents at the top of the chimney, trees buildings, etc.  They suggest 10 away from the chimney.  Look for air redirections, eddies, etc and anything that can create a downdraft or stall at the top of the chimney.  Heavy weather can create this in some instances.

Pre warm the chimney with a couple of sheets of newspaper in the chimney to get a draft started before you build a fire.  Build the first of the fire HOT so it encourages a good draft before you add fuel.  When adding fuel, add a little at a time until it catches fire, then add more as needed.  Keep a 1 inch or larger hole at the top of the fuel to let the fire escape and burn any smoke.  Think volcano.  

In order to test the wind direction and currents you may want to build a fire where you plan on putting the building and watch the smoke.  Not just one fire but one on a clear day, an overcast day, a rainy day, and during heavy weather.  Just because you have a building with a chimney, do not think you are going to change the wind and which way it blows. 

A straight chimney works best.  If you have to make a bend use 45s rather than a 90 degree bend.

Lock at the top of the chimney.  The amount of smoke coming out the top of the chimney is the amount of fresh air you MUST bring into the building.  

There is a discussion on the site about putting the chimney directly through the wall at the forge and then outside put the chimney straight up.  Scroll down to the bottom of the page.

 

[Morten311]

So i have a thread on my current smokestack and overhead hood : Trouble with overhead forge hood

Check it out. The smokestack and hood are the same and yes glenn i have tried with foldable sides on the hood and it didn't work.

and there is no obstructions near my smokestack at all and it is well clear of the peak. Opening doors does'nt help at all on the draft. I have plans to shove my chimney straight up through the roof which is why i am concerned about the size, because i dont want to waste more money on a shimney than i need to and i already have wasted some which is anoying, but thats just life and stupidity on my part. It is very frustrating all this because it worked in the previous workshop and now dont. The overhead hood does work and is not at fault and i can say that with some confidence because trenton tye from purgatoryironworks on youtube who also have an overhead hood which works.

 

I appreciate all the help i can get here and i do try out most things that you guys tell me to. Now i have a slight thought in the back of my head telling me that my chimney does'nt work because there is no trees or buildings in the way. if that is true or not, i don't know, it's up to you guys.

 

Thanks

[Glenn]

From the photo of your set up, there is a very large open area between the forge and the hood.  This tries to gather air from everywhere and send it up the chimney.  When you put sides on the hood, this limits and reduces the gathering of air from everywhere to just the open area.  

For this forge may I suggest side skirts on both sides to just above where they do not interfere with your work at the forge, say 6-9 inches above the forge. At the front, add a skirt that allows you to see the back of the forge as you stand in front of the forge.  This should greatly reduce the air gathering ability of the hood to just where the smoke is being produced and not the entire room.

 

This forge and hood is similar.  The cone shaped hood is what is gathering the smoke, not the square top portion.  Make a site line from where you stand at the front of the forge to the back of the forge. Use this as the height of the top of the new addition.  Close the sides from that height to comfortable height above the forge in order to channel air into the hood, and up the chimney.  This causes the air to move across the fire, the forge, and up the chimney.

 

Play with loose sheet metal panels and try things out to see what configuration works for you.  You are not blocking your view of the fire, or the forge, but are reducing the area for gathering air to where the smoke is being produced.

[Morten311]

I have tried with the side panels just like you have said but it does not work. period. Now, i just took a screenshot of trenton tye and his forge and hood ( hope he or admins dont mind ). it's clear to see that he has no side panels but just the hood. maybe i'm not using my brain properly, but how come his work and mine don't?

[Irondragon Forge ClayWorks]

I would say because his hood is larger area wise than yours and may run straight up through the roof. Mine worked somewhat if I really preheated the stack and there wasn't any breeze to deflect the smoke. I must admit, I never thought of lowering my hood.

[Morten311]

I am planning to put the stack straight up through the roof anyway

[Morten311]

So what about a 16" smoke stack? is that too big or just right?. my forge is 640 x 670mm (25.2 x 26.4")

I do realize that having a smokestack that is 24" is just too big, might as well just make a hole in the roof. But i still want to be absolutly sure that the smoke gets out of my workshop and does not hang around.

[Frazer]

I think you might be better suited playing with your hood design before replace the whole stack. You may end up putting a larger pipe in there and not seeing much of an improvement. I don't know why Trenton Tye's version works well while yours is having issues, but I have no idea what the rest of his stack looks like. There could be many reasons. 

You can try bringing the hood down a little lower, a side draft, etc. I do think you would benefit from a larger diameter pipe. With the right hood I don't see why you would need anything larger than  300mm (~12"). If it's easier for you, you don't have to have a single stack. You could run a second 200mm pipe alongside the existing one for the equivalent of a single ~280mm (~11") stack..

[Irondragon Forge ClayWorks]

That's an interesting idea of running dual stacks. I would like to hear from some who have tried it or seen one in use.

[JHCC]

First things first: your 200mm (7") stack is WAY too small. Whatever combination of circumstances led it to work well in your previous setup was a one-in-a-million shot, and you got very lucky. Get your stack up to 250-300mm (10-12") before you do anything else. Until you do that, fiddling with the hood is a total waste of time.

Personally, I recommend a side-sucker hood, but if you want a large overhead hood, that should work fine IF you get your stack up to a proper size.

And yes, anything much over 300mm (12") is not going to draw well and would be a waste of money and material.

 

[Morten311]

Okay thanks for the advice. I will upgrade my stack to a straight stack running through the roof with a diameter of 300mm. I will get back to you if it does not work. But please come with good advice because someone else might find it useful. 

Thanks again 

[JHCC]

You are quite welcome! 

The downside to big hoods is that they can take a while to start drawing well. What pulls the smoke up the stack is the upward-moving column of warm air, and the problem with a big hood (as Glenn notes) is that they are taking in air from a very large area. The great thing about a side-draft hood is that it's pulling the smoke through a much smaller opening much closer to the fire itself, so it doesn't take nearly as much to get that column of air moving. (There's also something of a Venturi effect going on with the cross section of inside of the hood being larger than the opening, which helps increase the velocity of the moving gasses.) You can even jumpstart the draft by putting a piece of burning newspaper inside the hood right before you light the fire. 

For example, here's a video of the very first coal fire with my own super sucker (made from a party balloon helium tank); you can really see how strong the draft is right from the beginning:

This setup, by the way, is running off a 250mm (10") stack with a couple of 90° bends.

 

[Frosty]

Bigger is NOT necessarily better. The larger the diameter the stack, the slower the same volume of hot gasses has to move so the slower air currents flowing into the hood. If the draw is slow enough smoke might find an easier path into the shop than up the stack. 

If what you're doing is not working don't do it harder. Try something different. Side draft hoods are proven effective for clearing smoke where overhead hoods are not so effective. I have an overhead hood and it's not much better than no hood.

Happily I use a propane forge primarily so a poor smoke hood is not an issue for me.

Frosty The Lucky.