On Thu, Aug 4, 2011 at 10:57 AM, Jim Ash <ashcan@earthlink.net> wrote:
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> Gents -
>
> I've been taking my time and putting together a nice foundry furnace that I hope will last me a long time when I'm done. I've got some good commercial castable refractory, some ITC-100 magically showed up in the mail yesterday and some kaowool is due in the next day or so. I put together a spreadsheet (which I'll probably clean up and download to the files section for list consumption at some point) which takes in some basic dimensions and outputs volumes of refractory and dimensions for sheet metal.
>
> Although I've taken information from a number of different sources, the basic design is from 'Building a Gas-fired Crucible Furnace' (I like the ability to grab a hot charge from the side at arm's length instead of having to hover over it). I've sized it to accommodate a #8 Silicon-carbide crucible, but I've also been known to use 5- or 6-inch schedule 80 pipe in a pinch. I plan to cast the basic furnace parts, wrap the outsides of it with a couple inches of kaowool, then strap some sheet steel around it to clamp the kaowool in place.
>
> The question I've got for the group here is: How thick have you guys made your furnace walls, bottom and lid? I was planning on 2 1/2" for the side and bottom, and 3" for the lid. I may be a little tight on refractory mix, so I was wondering if I could make the side thickness 2". The question is probably more related to mechanical and thermal stresses; I figure the kaowool wrapped around the outside will significantly supplement the insulating ability of the furnace, and I'm planning on brushing it down on the inside with the ITC-100. I don't have a permanent place to keep and operate this furnace, so it will be wheeled out from a storage shed for use, which is certainly going to contribute to the mechanical stress on it.
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> So how thick are your furnaces and would you make them that thick (or thin) again?
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> Jim Ash
>
>
Jim, I have some thoughts, which may be moot as I haven't read your reference;
First, my furnace walls are 4" thick - way too thick - and too heavy.
The extra thickness is no advantage. I would build it 2" thick, of
lower temp refractory if I ever changed it.
The minimum refractory thickness depends on the rigidity of the
container. You could use 1" of refractory if it were backed by a
heavy wall cylinder of steel, with proper vertical support like a
ledge at the bottom and top to restrain the refractory when you lift
it.
Refractory linings always develop cracks. These are small and of no
consequence if the refractory is supported by a steel shell. If the
shell is only refractory, no external steel to support it, it will
fall apart in large pieces at some unpredictable point. If you want a
self supporting refractory cylinder, it will require some kind of
reinforcement (wire or hardware cloth embedded in the refractory comes
to mind). Remember that you want to lift it while it's hot, which
will cause rapid cooling and stress added to the lifting stress.
Your inner shell/outer band will require standoffs to keep from
crushing the kaowool. Crushed kaowool has little insulation, really a
waste of time and money if your don't maintain the air spaces within
it.
There is no universally recommended thickness, because nearly anything
works. There are over 4700 members in this group, so there are likely
at least 1000 strongly held, but different, opinions about the perfect
furnace. Mostly, make the furnace big enough for the biggest
crucible plus 1"-2" of clearance on all sides, the same or larger
height clearance from lid to crucible.
Rex
http://groups.yahoo.com/group/gingery_machines/app/peoplemap/view/map
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