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Kiln Size??

Anyone have a formula to calculate the biggest size kiln interior I can have using 120 volts and 12.5 amp? My two KW per cubic foot is falling short. I’m using 2â€

Well, here are my calcs, having just recently designed a kiln! I think they're correct.

Okay: amps = watts/volts so,

12.5 amps = X watts/120 volts.

Multiplying both sides by 120 to solve for X gives 1500 watts.

According to the euclids.com website, a kiln for glass slumping/annealing should be designed at 2 - 2.5 watts/square inch. Using the high end of the range to include fusing, let's use 2.5 watts/1 square inch.

We've got 1500 watts available, and the question is how many square inches will 1500 watts heat?

1500/X = 2.5/1

so to solve for X square inches, divide 1500 by 2.5 which gives 600 total square inches for the kiln. There are 6 surfaces in the kiln, so dividing by 6 gives 100 square inches for each side. So the easiest solution is for each side to be 10 inches by 10 inches. I'm not sure how much insulation the 2.5 watts/sq inch is based on...but this should be close.

Carol

[quote="Gil Reynolds"]Kiln Size??

Anyone have a formula to calculate the biggest size kiln interior I can have using 120 volts and 12.5 amp? My two KW per cubic foot is falling short. I’m using 2â€

[quote="Gil Reynolds"]Kiln Size??

Anyone have a formula to calculate the biggest size kiln interior I can have using 120 volts and 12.5 amp? My two KW per cubic foot is falling short. I’m using 2â€

[quote="Gil Reynolds"]Kiln Size??

Anyone have a formula to calculate the biggest size kiln interior I can have using 120 volts and 12.5 amp? My two KW per cubic foot is falling short. I’m using 2â€

Not exactly what you had in mind Gil, but you can see what I've done with help from Dudley Gibberson at Joppa Glass works on the following page;

http://www.artemisglass.co.uk/studio5/s ... ninfo.html



The shelf size is 400mm square, fuses and slumps fine. A 300mm square plate fuse firing uses 8 units of electricity.

Hope this helps,

Paul

Your heat loss is through air flow through the fibre. Beat that and a 6 cubic foot kiln will run on 23 amps (30 amp) 220v breaker. Kevin

Your heat loss is through air flow through the fibre. Beat that and a 6 cubic foot kiln will run on 23 amps (30 amp) 220v breaker. Kevin

Kevin:

Is this a custom made kiln that you are talking about? Any limitations on high end temperature? What about ramp rate? How did you beat the airflow problem? Was it just from building a tight fiberboard box?

Could you post a photo?

Tony

Thank you all for your input. There is a lot of good stuff here. What I am finding out is that it is not a cut and dry affair. Quite a few variables need to be considered and addresses.

Carol – You numbers make the most sense of all the info I have read. Many Thanks, that’s good stuff.

David -Yes, you. I’m thinking just smaller than a bread box, and no you are not clueless, your wealth of knowledge doesn’t support that claim.

Brian and Jenny – Thanks, I’ll check it out.

Bert – You make a good point. Watts per Sq. Ft. is not necessarily a fixed ratio. Shallow does reduce cubic size, but increase the risk of thermal shock. There is a fine line between trade offs. I am hoping for a hot rod performance for say, quick firing painted piece in 15 – 20 minutes as well as slow fusing cycles. I’ll let the controller do the slow firings. I‘ll try adding more insulation to the roof. Thanks

Paul – Great looking kiln. You did a really good job. Is it 220 volts? I am not sure what 8 units of electricity would equal in terms of kilowatt hours, is it a strait conversion? Thanks

Kevin and Tony - Fiber heat loss – isn’t it density related? How do soft bricks compare?

Gil

Gil Reynolds wrote: snip
I am hoping for a hot rod performance for say, quick firing painted piece in 15 – 20 minutes
snip

Yes, Gil, please solve this. The Hoaf is too pricey. I'll buy your prototype electric painters kiln.

not sure why you're putting the 120v constraint on yourself though.

Gil Reynolds wrote:Thank you all for your input. There is a lot of good stuff here. What I am finding out is that it is not a cut and dry affair. Quite a few variables need to be considered and addresses.

Carol – You numbers make the most sense of all the info I have read. Many Thanks, that’s good stuff.

David -Yes, you. I’m thinking just smaller than a bread box, and no you are not clueless, your wealth of knowledge doesn’t support that claim.

Brian and Jenny – Thanks, I’ll check it out.

Bert – You make a good point. Watts per Sq. Ft. is not necessarily a fixed ratio. Shallow does reduce cubic size, but increase the risk of thermal shock. There is a fine line between trade offs. I am hoping for a hot rod performance for say, quick firing painted piece in 15 – 20 minutes as well as slow fusing cycles. I’ll let the controller do the slow firings. I‘ll try adding more insulation to the roof. Thanks

Paul – Great looking kiln. You did a really good job. Is it 220 volts? I am not sure what 8 units of electricity would equal in terms of kilowatt hours, is it a strait conversion? Thanks

Kevin and Tony - Fiber heat loss – isn’t it density related? How do soft bricks compare?

Gil

I am no expert in this but

Consider more insulation

From some chat over craftweb

At low temps fibre should B loose at high temps like in hot glass it should B packed tight

Gil Reynolds wrote: Paul – Great looking kiln. You did a really good job. Is it 220 volts? I am not sure what 8 units of electricity would equal in terms of kilowatt hours, is it a strait conversion? Thanks
Gil

Thanks. Yes, 220.

@ 8 pence per unit it is cheap to run. Maybe 2US$ for a full fuse.

The hearth is brick for strength & rigidity, the lid is 3 x 25mm thicknesses of fibreboard on the top and sides.

The fibreboard roof allows heat to escape at such a rate to not allow devit. I thought if brick was used for the lid it would stay at fuse temp too long. Also in time the brick would deteriorate and drop bits onth the glass.

So cooling from full fuse (800degC) to the upper annealing point (570degC) takes about an hour.

The controller then kicks in and controls the rate of cooling down to the strain point (370 degC).

Paul

One hour from full fuse to annealing point sounds like it's holding a lot of heat. I've got brick kilns that cool way faster than that.

Lynne Chappell wrote:One hour from full fuse to annealing point sounds like it's holding a lot of heat. I've got brick kilns that cool way faster than that.

Perhaps I over-insulated then; too much and it won't cool, too little and it would take a lot of current, even on the annealing ramp.

Must be within that fine balance of insufficient insulation to hold heat too long to cause devit, and enough insulation to stay warm enough for the elements to not cut in, if you see what I mean.

I heard a Doctor on the radio say that if he studied anyone long enough, he could find something wrong with them!

Meantime, it works pretty good and it's satisfying to get predictable results.

Paul