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I Forge Iron

Hot or Cold Air does it matter?

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I have just finished building my forge and awaiting my first coke delivery. As a air source I am using a vaccum cleaner motor (bottom draft) my question is when I have had it turned on for a couple of minutes the air starts to warm up and I am wondering whether this will reduce the effectivness of the fire? My solution if it does will be to submerge the outflow tube into a container of water and perhaps cool it down somewhat. Has anyone got their thoughts on this. Much appreciated Bully

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Hi Bully
The warmer the air the better, the air as it comes thru the fire has a somewhat cooling effect, especially at the bottom of the fire. Steel works go to a lot of trouble and expense to warm the air blast that is blown thru the tuyeres into the blastfurnace to prevent just this happening. You have it happening for nothing, use it to advantage rather than going to extra trouble cooling it.

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Really?...So, as I use a hair dryer for a blower...and I have been using it on the cool air setting, would I would be better off using the warm settings? Is this a heat effect thing, because even on cool air setting, I have burned off steel, an gotten it to white heat...will it be hotter with warm air, or just more controlled?

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Frosty, perhaps it would be a good idea to post how to preheat the propane least someone try to apply heat to the propane tank with less than satisfactory results. Just a thought, I know that things can't be made idiot proof. :)

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Good point Woody, thanks.

Do NOT heat the tank!! :o

I haven't done it myself and am basing my comment on a propane torch made in England. Their data was pretty complete and convincing. They use an electrical element to raise the propane to something like 1,000f or better.

Hot air balloons and other large burners run the propane through SS tubing coils in the burner to preheat it.

I'd take a similar approach. A person would need to find some SS tubing of an appropriate dia and malleable enough to coil. I wouldn't try putting it in the path of the burner flame though, way too hot, I'd simply loop it a couple times at the roof of the forge then send a Kaowool insulated run up to the burner. Burying it just below the surface of the refractory liner might make it last a lot longer but it'd mean a definite warm up period for the burner.

The biggest benefit of preheating the propane is a matter of mixing. The hot gas is very energetic, so it mixes very thoroughly compared to cold gas into air. Being very energetic also means it's moving a lot faster than cold gas so it induces air more efficiently. Lastly if cools quickly in the tube shrinking significantly and further improving induction.

Playing with a propane preheat system is one of those things on my to play with list. If anyone gives it a try, please take notes and let us know how it goes.


Edited by Frosty
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