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Let's talk forge chimneys


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Let's talk forge chimneys.

A forge chimney is needed to move the smoke from the fire either out of our way, or out of the building. Straight is better than bent, and bigger is better, to some extent.

* No one mentions that the amount of smoke coming out the chimney is the same as the amount of cold air coming into the building. To save a little of the warmth in the work area, should we pipe in cold air to the forge blower and into the forge for burning the fuel?

* Does a chimney cap prevent down drafts or rain from entering the chimney and draining down onto and into the forge? Does it act as a restriction to the chimney draft? Which chimney cap is best for the chimney used by the forge?

* Should you put a damper in the forge chimney. This would NOT be used to regulate the amount of draft, but would be used to close off the chimney when the forge was NOT in use. Should we build a door (or other device) to close off the chimney when the forge is not being use? It would seem that this would stop the draft and conserve heat in the building when the forge is not being used.

Chimney Size and Height
Chimney fires and forges
Chimney set ups
BP0460 Anatomy of a Forge Flue

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cold air has a higher density, thus a higher oxygen content allowing a more complete burn of the fuel


a hotter combustion chamber also promotes more complete combustion
intake air that is hotter means more energy is retained and put towards work


start with a higher density air, preheat it via a heat exchanger using the chambers exhaust then add it to the combustion chamber. The ideal generally exceeds the ability of a natural draft, but not a forced draft solution. Counter current heat exchangers need not be overly complex. They can be as simple as a pipe in a pipe.

another reference
Stack effect

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This is good. I am preparing to refit my chimney system on my big forge.

So, 1) I plan on using a "hood" style of chimney. I plan to suspend it above the forge at about 3 feet.

2)I plan on using a 8" diameter chimney pipe up 3 feet, 90 degree turn, then out of the shop wall, turn up 90 degrees, straight up. Also, I plan on using a chimney cap to keep rain out.

The questions are this.
Will the hood and distance of it from my forge promote enough draw?
Will the two 90 degree turns in the flue pipe cause me trouble? I think not but...
Would my efforts be better spent on a side draft system?

My chimney in my old shop was a pretty sketchy affair at best. The biggest mistake I had made on it was using too small of pipe.

Let's see where this goes.

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I like the idea of adjustable skirts for hoods, which would allow someone to close off a side or alter the height on all the sides of a hood, like a drop door on a propane forge.

This morning we have a very serious wind blowing along the Front Range
a variable

where I to start a fire below a natural draft chimney it would take some time to build up a sufficent temperature differential
another variable

turns in chimneys are best avoided, but sometimes necessary,
they introduce turbulence and more friction,
disrupt the stack effect requiring a larger temperature differential to overcome
provide more opportunity for flue gas condensation
its the getting from a cold start to normal operating temperature that is the trick

reference wind caps
Cowl (chimney)

photo by Stephen Inoue released to the public domain
Wind directional cowl found on many homes along the windy Oregon coast, USA.


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I have a enclosure around mine, and no chimney at all. I have an old small kitchen hood, stainless, and pushed an old double shafted squirrel cage into it, by luck it just fit in there. I enlarged the hole at the output of the hood. Then I build the back and side wall and front .The side I work on I have cut open. I used metal and wood. the fan is pretty quiet and pulls out all smoke an dust and sparks. The other day I found it would pull out the smoke without turning the fan on, which surprised me, as there is no chimney, So I guess the box around the forge figures quite a bit into the picture. I am still working on the (design, if you can call slapping things together to see how they work) My next step is on the working side use a section of welding screen ( the see through type) to protect my eyes. It will come down to about chest level I guess. I figure that I can look through that most of the time to see how things are coming along, and if needed just duck down to get the actual colors in real time, so to speak. I used to have a chimney and i am liking this setup a whole lot more. there is a lot of air moving through the shop, and this will be good as the temps rise, now it is not so good. If I wanted I guess I would pipe in some fresh air vent right in front of the forge so the air is just moving there not through the sliding shop door. Now I am just dressing up more. If anyone is interested I can post some photos. Some of this can be seen on the post I did on fixing up my forge.

Thats a commercial kitchen hood about 5 foot by 3 foot, and about 10 inches deep or so.

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I have a side draft forge with a straight chimney, 10 inch diameter (BP0394). I highly recommend such a setup. When the forge is not in use during cold weather, I put a piece of plywood over the opening to reduce air flow and heat loss up the chimney. When the woodstove and gas forge are going strong, I remove the plywood and have a ready exhaust vent without having to open the doors or windows.

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I agree with rthibeau, my side draft hood and 10 inch stack work very well, I put the gasser in front also and have never had my co2 detector go off in the shop. My coal forge is portable also and moves away from the hood when using the gasser.

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