Mistakes were made, part one

I said a couple of episodes back that I made some mistakes in the design and implementation of my furnace; fortunately they were mistakes from which I learned something, and that were fixable.

The first mistake I made was a consequence of my not clearly understanding the difference between propane-fueled and charcoal-fuels furnaces. To be clear, the relevant differences for the purposes of this mistake are:

  • In a propane furnace, the crucible is raised upon a refractory cement block called a plinth. There is a hole in the side of the furnace, called the tuyere. The tuyere is angled tangentially so that the burning propane swirls entirely around the air gap between the crucible and the interior “hot face” of the furnace. If you imagine the crucible and the bore as two concentric circles, and a third concentric circle whose radius is the average of the other two, the ideal tuyere angle is tangent to that middle circle.  Blasting straight at the crucible heats up one side far more than the other.
  • In a charcoal furnace, the crucible sits directly on the burning fuel, and is furthermore surrounded by more fuel. The air blast enters through the tuyere at a point below the crucible, and blasts directly towards not the crucible, but towards as much fuel as possible. The goal is to raise the temperature uniformly throughout the fuel, which will then heat the crucible uniformly. The ideal tuyere placement as far as symmetrical heating is concerned is directly below the crucible, coming up through the floor of the furnace. Second best placement has the tuyere coming directly in the side of the furnace, pointing towards the fuel. The tuyere must not blow directly on the crucible (because that is actually cooling it down), and must not blow tangentially (because that produces uneven heat.)

As I looked at photos on the internet of various different furnaces I misunderstood this key difference between charcoal and propane furnaces, and constructed my charcoal furnace with a tangential tuyere. And, unsurprisingly, it did not heat up at all evenly when I tested it.

The second mistake was one of construction; the pipe which feeds the air blast into the tuyere should fit the hole snugly, but be removable. I accidentally froze the pipe into place in the concrete, angled in the wrong direction. Whoops!

Fortunately these mistakes were easy to fix. A hacksaw removed the badly placed pipe. (Though it took a while; I have since purchased an angle grinder, which makes short work of cut-off jobs.) I had already made a hole in the bottom of the furnace to act as an emergency drain; fortunately it was exactly the same size as the 1” inner diameter black steel pipe nipple I was using to deliver the air blast. A screw-on flange ensures that it will not fall out the bottom, and a right-angle bend leads it out to the waiting air hose. 

This solution works well; I was (eventually!) able to easily melt aluminum with this setup. However, this produces two additional problems.

The first additional problem is a safety concern; my emergency drain now leads directly to a plastic air hose. If there is a loss of containment the drain will shoot molten metal out the pipe into the hose which will then melt. I will be making some modifications to this system so that the drain “tees” off, so that gravity will take the melt down into a waiting steel container rather than flowing down the air hose.

The second additional problem is the fact that obviously the furnace now cannot sit upon the ground, as there is a pipe sticking out the bottom of it. For now, I have it sitting on a convenient antique steel table saw that I rescued from the side of the road many years ago. Eventually when I build the hand truck to move the furnace around I’ll incorporate some legs to keep the furnace up off the ground.

Next time: more design and operational mistakes


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