Double Side Draft Forge Renovation

Exploring New Techniques, News from the Shop

This post is for all those ingenious folks out there who will someday build their own forge  and chimney. Learn from my mistakes, prosper from my triumphs.

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Here’s the old overhead fume hood

The fume hood hung 3 feet above the forge. The chimney was 6″ in diameter and extended 2’ above the roof line- at a height of approximately 14′. The big problem:  the smoke collected in the hood but then billowed back out from below the sides. Even with the addition of sheet metal walls and a fan to encourage smoke movement upward, the chimney couldn’t pull the volume of smoke I was producing. The secondary issue was that the big 4′ x 4′ fume hood was blocking the view of the other side of the shop. The fume hood was hung near eye level and installed in the center of the shop, and the whole design seemed to make the shop look and feel smaller- I was reminded of the nuisance often as I would graze past the hanging hood on my way past the forge and thoroughly cover my shoulders in soot.

It was  time to make a change-

I deinstalled the hood and chimney. The 6″ chimney was replaced with a 12″ diameter chimney and extended 4′ above the roof line- at a height of approximately 16′ . I could have still used the hanging fume hood and it would have worked great with this new chimney size but I figured what the cuss, let’s upgrade further.

I wanted to try a side draft fume hood because I thought it would take up less space and make the shop look and feel bigger. I worked off these two designs:

So how does the little opening to the flu actually grab the smoke from the fire beside it? Wouldn’t the smoke just rise up to the ceiling rather than moving sideways to get into the flue? This is my favorite part!! It’s like magic!
You create a vacuum inside the flue when the smoke that enters the flue is then able to go up the chimney. Because it is able to rise quickly above the roof line, it will take this path rather than hanging out at the ceiling of your shop. But here’s the really really cool thing- you can create an extremely strong vacuum by incorporating a smoke shelf inside your flue design- check out the first link to see an illustrated smoke shelf. Basically a smoke shelf is a sheet of metal- bent and installed inside the flue. It eliminates some of the space of the flue, so the smoke doesn’t have much room but once the smoke passes the shelf and is able to expand into a larger volume of space- the actual flue pipe- it’s so happy that it’s going to begin pulling it’s little smoke friends along with it- MAGIC (or perhaps science).

The photo below shows the natural vacuum’s strength.

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Here’s my new side draft forge in action.

My forge design utilized this magical vacuum design, while incorporating a flue structure that can be used for two forges, sitting side by side with the flue in the center. The double side draft forge I built was made from a metal barrel-  a 55 gal drum would work well.  The top was connected to the 12″ flue pipe, and a door approximately 12″ tall and 12″ wide was cut out from either side. I installed a wall of sheet metal in the center of the barrel, dividing the barrel’s volume in half. Each side is plenty large enough to pull the smoke inward. The smoke, once it leaves the narrow 6″ x 12″ half circle and enters into the 12″ diameter flue pipe is able to expand so rapidly into the chimney on its wild ride to the sky, that it creates a vacuum effect, pulling the smoke (and flames) from the forge below.

The science behind this whole design is really cool, and building it was a snap. The shop looks great and now I’m ready to add a second forging station to my shop.

I hope you enjoyed this how-to!

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I create large metal sculptures for public art, smaller items for you and your home, and teach hands on metal workshops in Gainesville, FL. Visit LeslieTharp.com