BackyardMetalcasting.com - Lionel's Laboratory

sonicwonder2000 · Joined: 29 Jul 2007 Posts: 60

Hi gang, long time lurker, first time poster here. I've done a fair bit of research, and an unfair amount of overspending, and six months later I am still mulling over a furnace design. I ended up buying only 2 bags of commercial refractory (Mizzou), which ended up not being enough for the Gingery furnace. So instead of buying more refractory, I ended up changing the design and buying: 25 Lbs Kaolin, 20 cubic feet of perlite, 24"x24' ceramic blanket, a welder, and some other sundries. Long story short, I am now broke - so I need to build it with what I have. Here is a preliminary section of my design. It will be encased in a sheet of scrounged duct-work tubing (probably galvanized) - 18" Diameter x 5'.

I hope to melt Aluminum, ZA12, and perhaps brass & bronze one day. Can anyone see any problem(s) with this design?

Concerns:

1) I plan of using a 1:1:4 kaolin:sand:perlite mix for the insulating layer. I doubt if this will ever sinter fully - will the resulting mix be structurally sound enough to not fall apart in the shell?

2) Will this be sufficient insulation to keep the shell of the furnace under 150 degrees when melting aluminum?

3) I opted to save the ceramic fiber for another furnace because using t would further complicate the design. The fiber would require some sort of refractory reinforcement structure to remain in place, especially in the base and lid sections.

3) Will there be any type of reinforcement necessary in the hot-face (chicken wire, rebar, steel pins, etc?). If so, I am concerned about cracking due to thermal expansion.

4) I would really have loved to use this furnace as a cermics kiln as well, but that would have complicated design ever further. Right now I just want something that can melt the several trashcans full of scrap I have accumulated!

5) I plan on using a 1-1/2" Porter self-aspirated burner. Is the tuyere diameter (2") and vertical location OK? I just took a stab at 4" for the vent hole diameter and 2" for the drain hole diameter. I can't find any consensus on these values. I heard that the tuyere diameter should = vent diameter, but I think this might be nonsense?

Thanks for any help on this already overdelayed project. I hope to use this thread to chronicle the construction.

PS: Thank you Adam Z and Tim for the chat on #thefoundry. What a wonderful resource - kudos to you for keeping it going!

jakebob · Joined: 11 Mar 2007 Posts: 464 Location: Irwin, western PA

you seem like you've done your homework.- it looks fine to me, but someone with more experience would probably be of more help. Welcome Very Happy

Anon · Posted: Wed Aug 01, 2007 1:59 am Post subject:

20 cubic feet of perlite? I don't think I put more than 3 or 4 cubic feet in my big furnace . . .

The refractory hotface (Mizzou castable, right?) can be made 1/2" to 3/4" thick without worries, but any thicker wastes material. You might add up to an equal volume of something burnable like polystyrene beads to stretch it farther and reduce thermal mass. (Don't use perlite here, because it might melt and flux the castable.) If you do this, make something like a plinth first, so you can make sure that your addition won't adversely affect the refractory's strength. Don't bother with wire mesh in the body, but it might be good in the lid.

2" is too large for the drain hole. 1" is plenty. Also, you could probably knock the vent hole diameter down from 4" to 3".

The refractory around the tuyere doesn't need to extend any farther than the outer furnace shell.

Tuyere placement and size look fine. A 1 1/2" burner (I'm assuming propane because you mentioned that it was naturally aspirated) is pretty big for this size furnace, but that won't hurt anything as long as it runs well at fairly low pressures.

The insulation in the body can just be loose perlite fill. (You'll have to use posts or ribs made of castable to support the refractory hotface.) Since you have so much perlite on hand, you might want to bump it up to 3". If it's vital that the outside stays cool (e.g. it's going to be next to flammable stuff), you may want even more. The lid should be bound with either kaolin or castable--the castable is more expensive, but it'll also be stronger. Besides, you can use pure kaolin, along with a burnable material like polystyrene beads, to make a furnace that's both insulative and incredibly temperature-tolerant.

You can use it as a ceramics kiln without any modifications if your burner will go low enough. Don't expect to fire any slower than a few hours per cycle, but that's slow enough for robust pottery.
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welder19 · Joined: 18 Dec 2006 Posts: 1665 Location: NJ Pinebarrens

Welcome!
What Anon said pretty much covers it but if you have 24' of Kaowool or equivelent, put the perlite in the garden or anywhere but your furnace.
Don't get me wrong, I have it in one of my furnaces but I didn't have 24"X24' of ceramic wool insulation, still don't for that matter.

welder19

sonicwonder2000 · Joined: 29 Jul 2007 Posts: 60

sonicwonder2000 · Joined: 29 Jul 2007 Posts: 60

Warpspeed · Posted: Wed Aug 01, 2007 10:55 pm Post subject:

Anon has covered everything very well.

I agree half to three quarter inch is all you need for a hot face, but I would keep the floor of the furnace at an inch and a half, to support the weight of a large heavy crucible more reliably.

The only place you will need to reinforce the hot face is inside the lid. If you are using a metal lid, weld some heavy open steel mesh, or something similar inside the empty lid first. That will give the insulation and hot face something to grab hold of to prevent it all from simply falling out.

The furnace walls should just sit in there without needing reinforcement.

Figure out how you are going to open and close the lid. If it is going to have hinges, they will need to be VERY strong. That lid is going to be as heavy as a slab of concrete. Something will also need to support the lid when it is open, remembering that it is going to be very heavy, and very hot.

Pleiades · Joined: 08 Apr 2007 Posts: 15

Here’s how I’m planning to solve the kaowool support problem. It required a complicated form for the hotface, and I haven’t decided on a lid yet, but it should work. The base and body are separate, but in retrospect, there was no reason I couldn’t have combined them. My tuyere extends away from the hotface out to the outer steel shell just because I felt like it.

Anon · Posted: Thu Aug 02, 2007 2:48 am Post subject:

That's not terribly different from what I'm doing here: http://www.backyardmetalcasting.com/forums/viewtopic.php?t=1651&postdays=0&postorder=asc&start=0

My form is fairly complicated because of the internal spiral, but other than that, it's a pretty basic approach. Note that an internal form will have a tendency to split apart a drying clay hotface; refractory doesn't have that problem because it doesn't shrink on drying like clay does.

Note that it is possible to build a furnace using nothing but rigidized kaowool and a plinth that extends to the bottom of the furnace--and it'll be a good furnace, too, as long as you don't spill anything on it. The kaowool-only approach greatly simplifies lid design, at least.

Keeping the vent hole big for charging purposes is a good idea--but you'll want to keep a firebrick handy to choke it down when you aren't feeding scrap into it.
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HAVEHEATWILLCAST1 · Joined: 11 Sep 2006 Posts: 5473 Location: NC

One thought on the drain hole.

No one says it has to be in the center.
You could put it in the side and have the option of useing it as a tap hole. (Not that you couldn't anyway.)

You don't hear much about a drain needing to be used so you may as well not use a tap hole.

Take a look at Anon's big furnace;
http://www.backyardmetalcasting.com/forums/viewtopic.php?t=1075&postdays=0&postorder=asc&start=0
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welder19 · Joined: 18 Dec 2006 Posts: 1665 Location: NJ Pinebarrens

It's really not that difficult, but it's your furnace so do it the way that works for you. If all you want to do is al then perlite is ok. If you want to do much bronze and copper then leave it out.

welder19

sonicwonder2000 · Joined: 29 Jul 2007 Posts: 60

sonicwonder2000 · Joined: 29 Jul 2007 Posts: 60

Anon · Posted: Thu Aug 02, 2007 10:46 pm Post subject:

A 100% kaowool lid may be flimsy, but it's also very light. Kaowool can easily support its own weight in a situation like this.

Warpspeed · Posted: Thu Aug 02, 2007 10:50 pm Post subject:

There are several reasons to make the hot face as thin as practical.

Commercial refractory is expensive, so less is always better. This refractory is intended to form a tough inner working surface, to protect the usually soft and fairly fragile bulk insulation from wear and tear.

The hot face will only have to support the weight of the lid if you build it that way. The outer metal furnace shell could also just as easily support the lid around the outside edge, rather like a manhole cover.

Refractory is hard and dense, and does not have particularly good insulation properties. It also soaks up a LOT of heat energy to reach full furnace operating temperature. Very thick hot face will drain away considerable heat energy while the furnace is heating up. The idea is to have as thin a shell of hot face as is practical, and then surround that with really good bulk low density insulation.