Friday, October 19, 2007

Possible FOOT powered air system

If you have been following the discussion of historic iron smelting, you have seen that the problem of air volumes has been an ongoing problem. Simply put, the method developed by Sauder & Williams, which works almost every time, requires the use of high volumes of air. Their theoretical model (and practical experience) calls for 1.2 to 1.5 litres per minute of air per square centimeter of smelter interior at tuyere level. In our case with the 25 - 30 cm diameters, thats in the range of 800 litres per minute.
Our problem is that working with the * reconstructed *, * blacksmith * bellows based on * only two * * period illustrations * with * no artifact evidence * - the best we can produce is on the order of 120 litres per minute. (Note all the potential errors!)

So I had sent a direct question off to a number of experimenters and researchers about bellows sizes and types for Dark Ages Europe.

Both Arne Espelund and (quoted) Skip Williams reminded me :

"... Actually, if you take a close look at the 'log framed' furnaces in Evenstad, at Trondelag, etc. you will see that there is a thin wall where the tuyere in inserted and sort of a large access arch over the bellows. This is probably the closest parallel we have to an Icelandic bloomery; same culture; different time and place...."

I also was sent this reference by Peter Hurley :

"...I think the following link may be helpful. It contains a conjectural reconstruction of how a foot powered two bag bellows might have worked. It remains only to build one and test it's potential output. For 800 litres of air per minute, I estimate each bag would have to have at least 8.7 litres of capacity assuming a pace of around 46 "steps" per minute:..."

That reference is from a book 'A Smaller Social History of Ancient Ireland' by P.W. Joyce - 1906. This appears mainly to derive its information from various written documents. The commentary discusses blacksmithing equipment and does not clearly give any date information (or artifact sources). It does describe yet another set up for foot powered bellows (though not as elegant a mechanical system as the Evenstad ones.)

Skips reference to the Evenstad process and set up is interesting (and admittedly something that did not come to mind). This is a larger scale multiple use smelter intended for top extractions and repeated hot swap firings. The largest problem I can see with the layout is that there is no provision for slag tapping. The tuyere also looks to be set too close to the base of the furnace for development of the slag bowl and bloom either.

The more interesting note is the 'traditional' bellows construction. Double chamber, but foot powered. At Hals there would have to be some kind of frame in place to make this set up work. I did a real fast ball park estimate from the measurements from the Evenstad document (from Arne Espelund's 'Iron Production in Norway') and get a (very rough estimate!) of 140 litres per stroke (empty one bag). Our own experience with the large UbberBellows had shown that the speed of stroke is limited by the time it takes to push the air through that 2.5 cm ID tuyere opening. ( That hand powered reconstruction is about the same volume - but slightly different proportions.) Our constant average was about 6 strokes per minute, with 10 stokes possible at least on short bursts. That suggests a delivered volume from 840 to maybe 1400 litres a minute.
If I'm reading the source document correctly (the translation and style is sometimes not clear) the mouth of the Evenstad tuyere is 3" diameter - which would make for easier delivery through the pipe. This in turn would make it possible to increase the pumping rate (thus delivered volume) above those numbers. Now I have an eliptical trainer trainer workout machine here, and even in my pathetic condition can easily maintain 30 plus strokes per second against some resistance. So being REALLY theoretical, lets peg the top end from the Evenstad set up at as much as 3200 litres per minute.
The Evenstad furnace is roughly 60 cm internal diameter (according to Espelund's conversion of 1 ell = 60 cm). That gives us a surface area of roughly 2700 cm. Using the Sauder and Williams calculation for effective air delivery, that suggest an optumal volume of at least 3250 litres per minute. Bingo!

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