iron smelting furnace optimization

[h4tt3n]

Hello everyone,

 

In this thread you can read about my favourite hobby, prehistoric iron smelting:

 

http://www.scienceforums.net/forum/showthread.php?t=36160

 

During the last few years I've been running a series of experimental-archaeological iron smelts in order to find the best possible method for extracting iron from local bog iron ore in a historical context. Among the numerous problems that I've had to deal with, controlling air flow is one of the bigger ones.

 

Here's a schematic showing a common furnace setup:

 

 

Inner furnace diameter is roughly 30 cm. The front plate is rectangular, roughly 25 by 35 cm. The air inlet is 2-4 cm in diameter. Air supply may be a set of hand-worked bellows or - more commonly - an electric air pump with a watt-meter and vario-trafo attached to it.

 

In order to make the furnace work in the best possible way it is important to heat the furnace uniformly, so that at a given height above the furnace bottom, the entire horizontal section has the same temperature both at the back (away from the air inlet) and the front (above the air inlet).

 

The original excavated furnaces are completely evenly baked and vitrified, as shown in this picture:

 

 

The trouble is figuring out a way to spray in air in such a way that it distributes itself throughout the furnace bottom and makes the charcoal burn evenly, both at the sides and in the back, away from the air inlet. They knew hot to do this in the iron age, but I still haven't found a good way

 

Normally, I can't make the air reach all the way into the furnace, rendering the back ca. 5 cm of the furnace cold and dead. So, specifically, I'd like to know what is the optimal shape and size of the external tuyere used for spraying in air?

 

If anyone here knows anything about modern furnace construction, ejector / injector engineering, or fluid dynamics in general, please drop a comment on this. If you miss any information, let me know.

 

Cheers,

Michael

Edited October 31, 2008 by h4tt3n

[insane_alien]

i made a furnace for making lava a few months ago, for injecting the air i used 4 inlets to get even-ish heating.

 

the only alternative i can think of is excessive amounts of insulation to prevent heat loss.

[inuhbad]

Great Job on this project!

[tvp45]

What are the odds that there would be two people on here interested in old iron furnaces?

 

Anyway, you need a proper bosh. Think of putting the big ends of two funnels together and now stretch the top funnel out to a gradual taper. The middle, where it looks pregnant, and the area below down to the narrow end is where you want to do your smelting. You should be getting good iron coming down through that bottom narrow end, and that's where you want your tuyere (or twier if we're gonna be throwbacks) to enter. If you have a good charcoal, it will help hold the mass up in the bosh so the air can blow up through it.

 

This link shows a cutaway of Hopewell Furnace and pretty well illustrates the point.

http://www.nps.gov/history/NR/twhp/wwwlps/lessons/97hopewell/97visual1.htm

 

The VMI Cadets built a small furnace maybe ten years ago as part of a class on the history of iron. You might get some details there.

 

But, I gotta say, I don't think you'll ever again get that old-time charcoal. Even if you found somebody to make it, the EPA would probably close you down within a day. You might also look at the anthracite furnaces in eastern PA (look around Shenandoah, PA) since they presumably solved that problem of keeping the iron up in the bosh without the charcoal structure.

 

The book in this link won't help you, but you may get a kick out of looking at it

http://books.google.com/books?id=NSJDAAAAIAAJ&printsec=frontcover&dq=lesley+iron

 

Cheers