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I Forge Iron

Hood and volume of air


sr peters

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I have the situation that I burn charcoal and I either work outside or will have to make a hood with forced draft. With it being -2 and snow on the ground, working outside is out. I have searched and it appears this has not been discussed, so my question is: what volume of air should I set as a starting point to size my fan? Really would like to run with a hood with natural draft but just not in a position to do that.

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No matter where you are, you MUST replace the same amount of air that is sent up the chimney, or the chimney will NOT draw properly. If you are outside in -2 then you replace it with outside air. If you are inside, you send the inside air up the chimney you then must pull in -2 outside air into the room. If you do not have a window or door open to replace the air the chimney will NOT draw properly. Unless you have a monster furnace or air conditioning system as soon as the heated room air goes up the chimney, you will get cold real quick.

The advantage to working inside is lack of rain, snow, and wind. You can pipe in outside air to the forge and use another pipe for some of air going up the chimney, but you are still need a good furnace if you wish to keep the room or shop warm. You can get some relief from a radiant or infrared heater which heats the body and not the air. Cold is relative to your location, and Dimag and other Canadians forge in minus temperatures all the time.

Hofi has a side draft chimney design that goes through a wall (BP1048) and there are other side draft chimneys that go straight up. A side draft hood preforms better than an overhead hood in moving (removing) smoke.

Get both a smoke detector and a carbon monoxide detector if your working inside.

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Glen makes a good point about draft not being allowed to work if you don`t bring in outside air.Even the best engineered chimney is useless if you don`t supply it by cracking a door or window.
For indoor forging I`ve seen some good set ups that allow the blower to draw from the outside air but once you shut down the blower that air is still being drawn in thru the forge and the fire can get out of hand unless a blast gate(think woodworking dust control)or something similar is used to shut off the air source.
One of the best approaches I`ve seen up here has ducting set up to bring in outside air for both the blower and a duct set up overhead that blows down and toward the forge.A blast gate isolates and controls air to the forge and once the blower is off the overhead duct brings in air from above to keep the chimney supplied and working.
Radiant heaters set up overhead and aimed at where you are actually going to work are great if you have the cash.If not then just dress in layers and shed clothes as the work progresses.

I find the biggest problem I have is just getting started in sub zero temps.If I can get everything going and all the tools warm before I get a chill in my hands and up my arms then things usually go well.If I allow my hands to get chilled due to handling cold tools,iron etc then things are not going to go as well no matter how quickly I get to work and warmed back up.
Don`t forget to warm ALL the tools,even those tongs you think won`t be needed if everything goes well.You`ll be needing those too,trust me.

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Thanks,
This is good info. and very valuable, but I can't make an opening in the building for a stack. I can run a duct to a window and out to a stack (may even look at making a cyclone to knock the ash down). I plan on opening the window and putting an insert in the window to pass the duct through. (I am watching space to combustible material.) This would make the need to be forced air. No A/C or heat so I know what I have for outside temp will be pretty much be the same due to fresh air draw. I have two other windows, a man door and an overhead door to crack open to allow air in. I am looking to see if someone knows the magic cfm to draw out the exhaust and not make the need to put a warning about small children and animals getting to close to the hood. :D

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The draft would depend of the size of the fire and amount of exhaust material you wanted to put up the chimney. 10-12 inch chimney pipe seems to work well for many situations. I have used 8 inch to 24 inch chimney pipe and all work (to a degree), with some sizes clearly working better than others. All were tested on the same location, same forge, same forge hood, and approximately the same chimney height.

If you want to get fancy, you could use the chimney to preheat the replacement air needed for the draft via an heat exchanger of some sort. It is a great concept but may fall short in application.

Contact nakidanvil (Grant Sarver) and ask about one of his induction forges. He uses them as a heat source for forging, and can give you first hand information on the subject.

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