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overhead hood...making it work?


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hi everyone. new to the board, first post.

ok, i've got an overhead hood and need to make this thing work. i'm using a big, old coal-fired forge. i've never used a hood before, just got everything installed and it's not drawing well, at all.

first off, dimensions... the hood is 4ft long x 3'3" wide (nearly exactly the same as the forge) and about 2'3" high, so it's fairly large. attached to it is an 8" duct w/blower. it's a straight shot through the ceiling. the entire duct is only about 3 feet long, sticking only 1.5 feet up out of the roof.

i'm in an outdoor smithy (lean-to by a barn) with the sides tarped off to limit ambient air. when i fired up the forge the smoke was blowing through the chimney at what seemed like a good rate (w/blower going) but the hood quickly filled with smoke and billowed out, into the shop.

my first thought is, the chimney's not tall enough.
my second thought is, where the duct is attached to the top of the hood, it sticks through, into the hood, a couple inches. (causing the smoke to swirl inside hood?)
my third thought is, the duct is fairly small (even with a blower).

any insight? i've read that side-draft hoods are recommended for a better draw but since i've got this one (and put time into it) i'd like to use it, if i can.
thanks a lot, in advance.

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Welcome aboard, good to have you.

Hoods don't really work very well for forges, there isn't enough waste heat to draft properly. A taller stack will help as will a smaller hood. Being in an enclosed and heated shop will help as the hood will do it's best to suck all the heat out of the shop.

A good side draft hood works very well, even a mediocre side draft works better than a good overhead hood. I spent quite a bit of time trying to get my overhead hood to work well, it never has.

However, don't take my word for it, there are a bunch of guys here who know a lot more than I do and we both may hear the secret of making these things work.


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square inches of hood opening = 1872, square inches of 8 inch pipe = 50.26. That should tell you something. A hood is the most inefficient means of exhausting a forge.

If the blueprint section ever gets back up, check out Anatomy of a Forge Flue.

http://www.iforgeiron.com/forum/f7/brown-county-kansas-ag-museum-blacksmith-shop-pics-9-19-08-a-7463/index2.html look at the pictures in post #21

Edited by irnsrgn
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I'll tell you what worked for me when I had an overhead stack. Mine did the same as yours; would not draw, smoked in the breeze, etc. After studying some European designs, I hung 3 pieces of sheet metal on the outside of the stack down to the forge hearth so it was boxed in. The only side that was open was the one that faced me.

That did the trick and almost 100% of the smoke went out the stack. However, I was claustrophobic and had trouble working in the enclosed space so I started removing panels until I was left only with the one on my left (I am right handed and have the forge on my left). Although this did not work quite as well as three sides, it worked MUCH better than nothing at all because the sheet metal heats up as you work the fire and starts a draft up the chimney. However, the smoke will eventually stop running against it when the panel cools and start wafting around the shop but you can help that by rehanging the other two sheets and of course, you may not mind having three panels hanging all the time - it was just something I couldn't stand. I have a side draft now and would not have anything else.

A side draft is probably better in the long run but adding something similar to this may help you use what you already have.

Edited by HWooldridge
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Hopefully this blueprint will clear up some of the mystery of why a forge flue works or doesn't.

What we are attempting to do by having a flue on our forges is to come up with an efficient means of eliminating the smoke, volatile gases and heat produced by the coal fired forge fire we use to heat our medium IRON , to a suitable plastic state so that we can reform it into the shape of our choice, and maintain a safe environment for our selves.

We are dealing with a heat source that generates a large volume of hazardous stuff we want to get rid of. As it is heated it expands and gets larger, which is a disadvantage, so is hard to control. Being that it is heated it becomes somewhat lighter than the surrounding air and naturally rises which is to our advantage.

flue 01 - This drawing is of a hood which is an inefficient and less than desirable method of doing the job we want done. The undesirables rise above the fire and have a tendency to collect in the hood where they stagnate or collect and keep getting hotter and expanding. The really hot stuff stays more or less in a column immediately above the fire and goes racing up thru the pipe flue. This makes a sort of venturi effect and draws some of the smoke up into the pipe with it, but the volume in the large hood is too great to suck it all up and out and it accumulates till the hood will hold no more and then it spills over the sides. The arrow shown represents a breeze, and any breeze will push the smoke, fumes and heat away from the direct path into the hood creating a smoky atmosphere to work in.

flue 02 - This drawing shows the basics of what is known as a SIDE DRAFT flue. The Area of the opening "B" cannot be greater than the area of the flue "A".

flue 03 - This side view drawing shows the basics of how a SIDE DRAFT flue works. The block at the base cuts the opening down to about 3/4 the area of the flue and keeps the coal from entering the opening. The smoke shelf "A" gives the smoke something to climb up and also chokes off the area behind the opening so that the smoke has to go up and keep it from making a blockage in the large area behind the opening. The area "B" throttles the area down to about 2/3 the area of the flue and creates a vacum effect by expansion so that the smoke, heat and if its working right will even draw the flames into the opening.

flue 04 - This drawing shows the top of the flue with a screen spark arrestor and rain cap installed. The rain cap should be rather flat with just enough slope that rain will run off. To determine the height above the end of the flue, find the area of the pipe, radius squared times pi (3. 1416) and multiply by 3. Divide this number by the circumfrence, diameter times pi. and the result will be the inches above the flue needed.

flue 05 - This drawing shows why the area is 3 times the actual area of the flue pipe. In a brisk breeze all the volatites will need to exit out approxiametly 1/3 of the pipes circumfrence.

flue 06 - This drawing shows why you don't want a high peaked rain cap. The severe angle will make the smoke roll in the top of the cap and in effect make a sort of plug that will block the natural flow from the flue pipe.

flue 07 - This drawing shows the recommended height of the stack for a low angled roof, it should be 3 to 4 feet above the peak or highest point of the roof. For a steeper angled roof it should be higher as the wind is channeled by the roof angle and the air will kind of expand on the lee side of the peak and make down pressure as it rolls over the peak.

flue 08 - The arrow is pointing to what is called an escushion, which is a tapered ring fastened to the flue pipe at its very top and it funnels the wind up into the exhaust area to enhance the natural suction of the flue pipe.


This is a picture of my forging station, in my father and uncles shop that I took over, even tho the forge is completely enclosed on 3 sides, any draft would make smoke pour from the forge area, I solved most of this problem by adding a makeshift sidedraft flue to the left of the forge fire from 16 gauge scrap sheet I had up to the damper for the chimney, but still if I used a fan to cool myself in the summer, the smoke would pour forth from the chimney to my area.

Edited by irnsrgn
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