Thursday, May 23, 2013

About the Air...

Seth wrote to Don Fogg's Bladesmith Forum:

I have seen tuyeres adding to or helping to reduce wall erosion. What causes this and what am I looking for in a tuyere to help prevent it?

Right to start, we all need to remember that the exact working of any individual iron smelting furnace is a complex interplay between a very large number of variables! Your individual application of possible raw materials (ore, furnace, even charcoal) may certainly modify this basic level description. (!!)

Mike McCarthy proposed a working model for what is happening inside an iron smelting furnace during a late night discussion at Early Iron 1, back in 2004. I think his concept still stands up in the light of continued observations and experience:
McCarthy's original diagram - 2005
'The air blast inside the furnace makes a torus centered on the tuyere.'
My original notes on the discussion - 2005
This torus (bagel shape) is off centre, with the lower lobe smaller than the upper. (This due initially due to the effects of heat.) As the slag bowl develops, the shape is further distorted, with more of the air being deflected to the upper side of the tuyere.
Air Flow in a Working Furnace
The available air blast will effect the absolute size of the reactive area (reduction zone) inside the furnace.
Effect of VOLUME
VOLUME is important, because the reduction process of iron oxide both requires a certain volume of CO gas, but also a certain amount of effective time to take place. Things like the density of your ore, and its particle size, are contributing factors.
With low volume air, the core of the furnace may well be hot enough. But the individual ore particles may not have enough time at temperature and reaction chemistry to reduce. They may well fall to the bottom of the furnace without ever hitting an effective reduction zone.
Effect of PRESSURE
The PRESSURE at which the air is introduced into the furnace from the tuyere will determine the penetration of the air into the mass of charcoal within the furnace. At low pressure, the air remains close to the tuyere tip, which is also likely to put your highest temperatures closest to the furnace wall around the tuyere.
This is why using a rotary blacksmith's blower is often not found to be very effective for an iron smelting furnace. You may have large volumes - but with almost no pressure, the air simply does not penetrate the furnace.
Most hand powered bellows systems will certainly produce adequate pressures. Many European experimenters will use stones placed on the top plates of their bellows to control with fair consistency their air delivery pressures.

We have found that the ANGLE of the tuyere is critical to an effective iron smelting furnace.
Effect of ANGLE
If you place your tuyere too 'flat" (horizontal), what happens is that the air will not penetrate into the bottom of the furnace, leaving the effective heat 'ball' sitting too high. This in turn lets the developing slag bowl also form too high up in the furnace. The result is that you end up with a shallow slag bowl, with little room for a bloom to accumulate. As well, the liquid slag will also be sitting too high up around the tuyere, and quickly will 'drown' the air blast.

If you place your tuyere too steep, you move the air blast so it will be pointed directly at the developing bloom. Since you have oxygen at high temperature, it effectively acts like a cutting torch, and slicing your bloom apart as fast as it accumulates.
Effect of Angle on Bloom Formation
At Lee Sauder's Smeltfest research session in 2005 (if my notes are correct!) we specifically investigated the effect of tuyere angle. Through this series, we found the "best" angle for the tuyere to be 22% down from horizontal (plus or minus 5).

The next variable on the tuyere is how far the tip is inserted proud of the interior furnace wall by 5 cm / 2 inches. This helps to move the effect of the upper lobe of the torus of air away from the furnace wall.

Just to mess that element up, the material that your tuyere is made of will most certainly effect its durability over the life of the smelt:
Although using a piece of simple mild steel pipe will work - you are certain to find it will burn away quickly, moving the hot air to flow back against the furnace wall and start to erode it. 
Ceramic tubes are considerably more durable to furnace temperatures. Although these will burn back as well, our experience is that these endure long enough to prevent excessive wall erosion.
Experience has proved the most durable tuyere material is heavy forged copper. Lee Sauder has used the same copper tuyere for dozens of smelts will virtually no damage.

Putting the whole package together, you get something that looks like this:
Overall Diagram of Working Furnace
Of course you can elect to control the heat effects and wall erosion by modifying the design of the furnace itself.

Making the furnace body a flask shape, instead of a simple cylinder is one possibility. The ideal appears to be increasing the diameter at tuyere level to by the same amount that the tip of the stands clear of the furnace wall. (In effect a total of plus 10 cm beyond the diameter at the top.)

A second possibility (developed by Michael Nissen) is to insert a relatively thin plate into the heavier furnace wall - around the tuyere point. In practice, the highest temperature on the wall, thus the most erosion effect, occurs over a rough oval, 10 cm below and to the sides of the tuyere, and extending about 15 cm above it. The 'bellows plate' system relies on the atmospheric cooling off a thin cross section, in the range of 1 cm / 1/2 inch from a plate installed in that area.

Of course I do have to warn everyone that the working systems described above are most certainly not the only way to produce iron in a bloomery furnace!
A lot of combined experience has shown these elements do combine to make a very effective small scale bloomery however.

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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