Forge hood design

[Spears]

I don’t own or use a solid fuel forge (yet) but I have noticed times when people want to exhaust it out the side of the wall or straight out through the ceiling. Since I work in the commercial exhaust piping industry, I tried to design a hood that can be used upright or horizontal for coal forging. Something like this would be could easily be made from two pieces of .035” stainless rolled and seam welded down the front and back. I’ve shown the cutout in the isometric view (which points down for horizontal exhausting) pointing up for visualizing. Wanted to know what you folks think about this along with potential improvements before I cut and roll one. If the Adobe file isn’t readable I will take a snapshot or figure out something else. Thanks in advance for your input. Spears.

FORGEHOOD1.pdf

[Mainely,Bob]

Other than looking nicer what are the advantages over the ever popular square box?

[Tim McCoy]

If I may, round conduits/pipe/vents always improve flow ... in smaller and in larger sizes. Square makes for constrictions and dead spots and causes a larger surface area which creates drag on the medium being moved, thereby slowing movement. Angles in the vent pipe hurt too. Whether it is fluid or a gas, smooth and straight is always better. There are some complicated math formulas that explain all this. Also, depending on the construction method, round can be easier to put together. Experience has taught many blacksmiths that a square or rectangle collector will work fine, but having a straight round pipe speeds things along and helps create the "super sucker" everyone looks for.

[Sweany]

Thank you Mr. Spears. :)

[Spears]

Thank you everyone for your questions and comments. My goal in this design was not necessarily to make something better or worse than what is already available, but rather make use some of the enormous amount of scrap material by turning it into something folks can use. I will need size clarification of what people might desire for what could be mounted to a homemade solid fuel forge. I don’t use one so my guess work is what it is! Feel free to throw in two cents, its all good! Nothing like making something where the end user says: “gosh if this were only a little taller”. I left the outlet at a 12” diameter because it was “guessed” to be what people might commonly use for exhaust ducting. Square to round construction is harder to make with tapered assemblies like these. I know because I have to draw the things in the flat position and the materials we use are extremely rigid and harder to fabricate than residential air ducting. A two piece round assembly like what I drew may take as little as 25 minutes to cut and fabricate after the initial programming. Stainless steel scrap pieces of substantial sizes are available to me at reasonable cost and the material has excellent heat and corrosion resistance.

Thank you for your description of certain details Mr McCoy; you are an excellent writer. I will check this thread for anyone’s likes or dislikes of certain sizes and such, and eventually I may make one and give it away for the price of a report on how it works. If I end up building a few of these I won’t be asking much for them anyway. Anyone who knows me knows I just can’t help myself but to play around with all this cool s__t. Regards, Spears.

[petere76]

spears,

good proposal, square construction on the box is simply the ease of manufacturing. Its easier to fab square corners then cylinderical transitions, in most shops. Exhaust pipes (uptakes) ...are ready made and can be purchased in assorted diameters at your local supplier. Now we are down to the efficency of the opening vs the draft produced by the available uptake cubic capacity. In the static, I would rely on venturi for the reduction and acceleration. Additionally, the placement of the uptake in regard the pressure losses of the stack would have to be considered. Lacking mechanical assistance (fans etc), all the rules of pipings structural and frictional loss apply (laminar vs turbulant) . Don't forget atmospheric and external structical influences on the exhaust piping. Is the stack high enough above the roof line or the local physical strcutures to be effective.

I recently fabbed up a simple square horizontal transition side draft forge. I submitted a few photos and a brief narrative on this site. (solid forge hoods) . The system has worked well in our shop. I am not sure if this site still had it available for review.

Peter

[Spears]

Holy Toledo Peter! Please read the second sentence in my second post. Better yet, please read my entire second post. And I would be delighted to know what size "square opening" your forge hood is right next to the fire that has worked very well for you. If I throw something together, it will have to be cylindrical in shape. Here at the "manufacturing plant", it "IS" easier to fabricate "round" shapes. I only wish we had more "fab shop" equipment so we could make right angle bent parts easier. This factory just isn't set up for much beyond round ducting. Peace! Spears.

[Mainely,Bob]

You mentioned the possible use of stainless steel for the hood.
You may want to keep in mind that some types of stainless comes alive compared to other sheet metal when heat is applied.That distortion is something you may need to factor in if using one of those types right next to the fire.

[Spears]

Good point Bob,

The book states when used for industrial furnaces, kilns, ovens, and diesel turbine exhaust, the continuous is 1400 deg F and intermittent 1800 deg F. I guess that means if the intake hole isn't big enough, the intake hole might GROW big enough. I think this calls for a big glass of ale and a field test right next to a good sized camp fire in the back yard. Nothing like a little "high tech" R&D. Maybe I could get inspired to use coal instead of just propane. I gotta have something these days to get excited about. Spears.

[Spears]

Its now Friday afternoon and I decided to roll one. A tapered forge hood that is. Besides, I only drink. Alright bad joke. Anyway, it looks like it may need some additional hooding if used in the horizontal position. It also needs a bit of exit support if used that way. Sat nice and flat and level on the cutout which was the hardest part to draw in the flat before rolling. My guess would be more folks would desire to use this in the straight up position. Weight is around twelve pounds. Maybe the wife will let me have a campfire in the yard before to long and I can test it for distortion over some heat. Here are some pics, Spears.