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Old 03-21-2004, 12:27 PM
Dave Bross Dave Bross is offline
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Location: Archer FL(near Gainesville)
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Mine started life as the basic Mark Lauckner 15 pound furnace. 14 gauge 24 ohm element for 10 amp draw on 240V. I have heard to expect about 6-9 months effective element life, elements on 24/7 but I always seem to spill a bit of batch on the elements or something equivalent so no personal "best mileage" on an element yet. Even with the elements in good shape it's a bit underpowered anyway for doing something like c-balls where you're lifting the lid every few minutes.

Major changes over time:

Changing from mechanical relay to SSR and one second firing times

Making the square chamber into a round one after cracking a pot from it being too close to the elements at four points in the square configuration.

Went to one layer of grooved firebrick inside to hold the elements and the rest packed with scrap frax from ceramic fiber engineering at 25 cents per pound. Much better insulating this way.

Outside panels changed to Hardeeboard cement board. Good looking and durable!


Other thoughts on things mentioned above and other:

Best to design in enough groove length so you can stretch the element at least twice it's close coiled length.

Think about watt loading. I believe it was 8-12 watts per sq. in. was the desired range.

Maybe think about wiring elements in parallel if space is going to be an issue.

I was very focused on getting even heat on the pot but as I learn more and some very experienced people teach me more things I'm thinking that uneven heat on the pot to get convection movement mixing in the glass would be a very good thing. This would be hard to do with wire due to space constraints and I'm not sure how much uneven heating a pot will endure. I'm thinking the uneven heat will work if the heat source is far enough away, as in around 2", but there goes the economy from a smaller furnace chamber.

I think there may be some economy in using 14 to 11 gauge wire instead of the 8 gauge "garage door springs" ( I love that description!). Those big ones seem to lay over and short out long before their smaller wound/thinner gauge cousins. Think Denver problems.
14 gauge is probably the minimum thickness of wire that's effective.

After waiting around for my current furnace to recover temp. during fast production any future ones will go by the old "one amp per pound of glass" rule of thumb.
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