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Smoke Shelf?


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I'm new here, but I usually kick it at the Armour Archive since my main focus is armor and medieval weapons. I've been fabricating for a long time time and making jewelry but new to the blacksmithing.

Anyhow i've had help at A.A. with my forge building but I'm getting near finished and i'm wondering if I should install a smoke shelf in my flue. I had to cut about 1" off theb ack of the flu and send it from about 9^3/8's to 8^3/8's because I found out it would not clear the man door hehehe.. yah I had gotten so involve in fab design that I forgot it actually has to be able to go outside, and probably back inside time to time for repairs and modifications.

So I have about 8" deep by 25" wide. That oughta give you a feel of dimensionis here. I'm going to put a 6" wide cylinder/pipe at the top to give even more suction. So i'm wondering if a smoke shelf is overkill?

So my option here would be to install a smoke shelf before I seal the back up and try to get "Bernouli" to help me more. As it is I think I probably have a fairly good amount of suction because of the volume involved in it.

Not a bad little setup so far huh :D I'll show you the whole thing and progress pics when i'm done. I hate to even show this much cause I don't wanna spoil it heheh :rolleyes:

Any help compliments, criticism appreciated.. this is my first forge.

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It certainly won't hurt but I have seen arguments pro and con whether there is much benefit. It is supposed to roll the cooler smoke and allow circulation to always push exhaust up the chimney.

I do not have a smoke shelf but do have 4 firebricks at the mouth of the smokebox (which is sheet metal). Those bricks heat up and help the draw when the blast is idle. When the blast is going, the radiant heat is enough to easily make the gases go up most chimneys.

BTW, nice forge - you did a good job.

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Thanks Everyone. I guess my worry is that since the flue is not very deep about 8" that putting in the smoke shelf would significan't decrease the volume. I read something about chimneys that said that smoke shelves are a bad idea, but clearly chimneys and forges are different animals. I guess my concern now is that if I put in the smoke shelf i'll decrease volume so significantly from the little volume I have that the smoke shelf will cancel about any efficacy it would have had lol:rolleyes:

I guess i'm still on the fence about it. Just for curiousity where would I weld it in. Would I start at the base of the flu and go up X amount of inches and then build a shelf (concave slightly downward on the shelf part from diagrams i've seen. Is it a very slight angle? or does it need to be fairly steep?

The front of the flu also contours back (towards us in the picture) so the smoke shelf would actualy create a pinching structure, and i'm guessing that's no good. Perhaps the angle of the front slope may just work as a psuedo smoke shelf? And perhaps i'm totally over building this sucker.. but I got tons of argon now and tons of rod, and i've put everything aside from a drink holder on this baby so why not finish it up in style. My friend asked me if I was going to weld a chair attached to it. I said "Don't tempt me!! :D"

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I'm going to put a 6" wide cylinder/pipe at the top to give even more suction.

My experience is a 6 inch (28 sq in area) chimney is too small, and 8 inch (50 sq in area) or 10 inch (78 sq in area) is a much better size to move the amount of air needed away from the forge. BP1048 Side Draft Chimney uses 13 x 13 side draft with a 12 inch (132 sq in area) chimney.

Each forge set up and location is different. Prevailing winds, trees, nearby buildings, chimney height all influence the draft. One thing for sure, you have to get just as much new air into the building as what is going up the chimney or it effects the chimney's performance.
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When I built my shop forge I put a smoke shelf in it and a 10' stack that goes 5' above the roof line. I do not have any problems with draw at all. I have another coal forge that I am installing the hood and flue for and will go with the same side draft hood and flue set up.
I agree with Glenn, 6" will not be enough.

John

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yah I am taking back the 6" to Home Despot tonight and gonna try your recommendations. How tall of a 10" or 12" stack should I attach ontop? It obviously has to be removable since this thing has to go in the Uhaul at some point when I move. When I do have a permanent installation i'll rebuild a coal forge inside. Just can't do that "renting".... have to (coal) forge outside for now :D Propane forging I can prolly do inside if I get enough ventilation in my basement.

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The chimney should be at least 3 feet above the roof or anything within 10 feet of the chimney.

You can join two different size stacks to get the size you want as the final product. For instance buy a 6 inch stack and a 4 inch stack and snap them together to build a larger single stack.

To provide an example of using what works, I currently use the Supercharged 55 Forge with a 24 inch diameter chimney. Yes, that chimney has a diameter of 2 feet !! It moves a lot of air and works very will in my application using a coal forge.

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You may want to do a search of the IForgeIron site for additional information on the subject of chimneys, hoods, side draft hoods, etc. I recall a discussion on size relationships between the opening of the hood and the chimney.

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if the 6" material is the kind with a lock seam, you can snap two together to make a 12" dia pipe. Its not as good as a fabrcated 12" pipe; it tends to want to oval. If you stagger the seams 90 degrees when you join the pipe sections together, it helps in counteracting the oval effect. You still probably have to go by more, because you will use twice as much for a given length of pipe

My apologies if this has already been addressed.

By the way, I vote agaised the smoke shelf, onthe grounds that it is a holdover from masonry construction, and not needed in steel chimneys. More important to have an opening at the forge less than the smallest area of the main stack, to maintain entrance velocity. IMHO

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If a smoke shelf makes you sleep better put it in, but as far as being necessary, its not with the design you are going with having the adjacent smoke chamber. My coal forge uses a 12" chimney, but it is only about 15' or so tall in total chimney length, if I had a 20 foot long chimney I might go with 10" to keep the stack velocity up. There is a balance there between volume and velocity which will give you optimum flow. Don't worry about making a mistake along the way, nobody said it had to be perfect the first time around. I did many changes to my forge and chimney in the first year of operation.

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Avadon, that is a nice looking forge hood. If it were mine, I would use a 12 inch or larger chimney and if you can get it 16 ft. high or higher that would be good. Yes it does need to be at least 2 ft. higher than anything within 10 feet of it. I have put smoke shelves in forge chimneys and have left them out. Using each side by side, it didn't seem to make any difference, so now I do not put them in. You can see some forge hoods at: Student Forges in the Shop of Donald Stanley and at http://www.blksmth.com/Coal_Forge_Hood_1.htm
Hope that helps and good luck with it.

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Nice job so far, but 6 inch is too small a stack for the size opening you have. 8 inch minimum, and forget the permanent smoke shelf, just try it first and if it doesn't draw like you want it, just stack some fire brick inside the opening, that way you can adjust the smoke shelf for maximum draw, and its easy to clean out the accumulation of soot and cinders that will eventually accumulate behind it. At our chapter fall conference a couple of years ago a new forge flue was put in for Bill Epps to use when he demoed, there was too big an opening at the fire and it smoked inside a lot, till I got some fire brick and closed down the opening so it would draw decent. Randy thought I was gonna close it off so no smoke would go up, but it had a draw you could hear when I got done. I just hope he changed the rain cap he had on it, that was causing lots of problems.

As far as stack height, you stated that you are gonna move it in and out to use and store it. As long as you are not right next to a building a short stack that is easily added or removed will work just fine. My demo forge is just 2 pieces of 8 inch pipe with an opening at the bottom and it draws just fine, all you need at this point is to get the smoke up and away from your face so you don't have to inhale it.

Draw with just 2 - 3ft sections of 8 inch pipe, side view.

Front view.

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Edited by irnsrgn
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