How much propane?

If I am firing stoneware in a propane kiln of 8 cubic feet (something I am thinking about) does anyone have a ball park idea of how much propane might be required per firing?

Sort of related to this, and apologies for going over old ground, I am interested in the idea of the "greenest" approach to firing pots. I doubt if propane is ... I suppose wood must be better since it is renewable. But there's also the question of how much can be fired per unit of heat/energy - what sort of firing is the most efficient and/or economical. This is a topic I am hoping to research further.

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Alex S
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I'm also interested in "green" issues. One thing to consider is that fuel firing typically wastes a lot of the fuel energy, due to combustion inefficiencies and due to heat lost up the flue after combustion. I don't have any numbers for kilns, but old "standard"-type gas furnaces were typically about 50% efficient. (The new high-efficiency models don't seem relevant to kilns, since they extract so much heat that the flue termperatures are very low.)

On the other hand, electric heat (kilns or furnaces) is

100% efficient at converting electricity to heat. The problem is really heat losses through the insulation. I am planning to build a small "super-insulated" electric kiln to learn more.

One of the issues for any type of kiln is that the energy that goes into heating up the insulation mass is a loss, so you ideally want low mass insulation like fiber, but that's not very durable for facing. I'm thinking of one 2.5" layer of IFB, followed by several inches of perlite (low cost) in a metal shell. Still haven't figured out details of how to handle peepholes through this much thickness... won't be much of a view down a narrow hole!

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

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Bob Masta

[While it may be true that converting electricity into heat is (almost) 100% efficient, you neglect the fact that the process of producing the electricity from fuel is far less efficient, more like 30%. Then there are losses in transmission to account for. So if "green-ness" is what we're talking about, we should look at the whole cycle, not just the part that happens between the wall-plug and the kiln. Perhaps your electricity is made from hydro or wind turbines, geothermal sources, solar panels, or a nuclear plant (not that these don't have problems of their own) but most electricity in the US is made by burning fuel, often coal. Fuel-fired kilns, where the heat energy is used directly instead of for electricity generation, are a lot more efficient than the process of burning fuel to make electricity, sending it down a wire, and using it to heat a kiln.

I suppose the "greenest" kiln would be one that's directly solar-heated. Solar kilns are widely used for drying wood; ones capable of firing ceramics are theoretically possible, but I couldn't find any solid information on them online, although there's plenty of rumor and speculation around the topic. It seems that a solar furnace is easier to achieve; Sandia Labs has one they use to simulate nuclear events, and test refractory materials to failure. But while devices like that are good at concentrating the heat of the sun into a small spot, getting from there to heating a kiln efficiently is a different problem, which doesn't seem to have been solved yet.]

[People often coat their ceramic blankets with a heat-reflective refractory coating (ITC), which helps with its fragility issues. Personally, I'm highly suspicious of that fibrous ceramic material and what it does in the lungs. I would hesitate to use it in a kiln; I like the old-fashioned light firebricks better.]
[I'm not sure about the perlite; it's not really refractory, and will fall out your peephole. I suppose you could whip it up with some other ingredients to make a light castable material, but it would only be good for back-up insulation behind a real high-temperature refractory board, blanket or brick.]

Andrew Werby

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Andrew Werby

Your point is well taken. One issue is that electricity generation is more amenable to emissions controls (stack scrubbers and the like), and eventually perhaps CO2 capture (though I'm not holding my breath on that one).

But I'd sure love to see some overall efficiency numbers for fuel kilns for comparison.

Then there's the issue of completing the entire firing while the sun shines!

I'm also wary of fiber, hence the insulating fire brick with perlite as a backup. The beauty of perlite (besides being cheap) is that it is a really good insulator at low temperatures (under 850C or so), much better than IFB (roughly the same as Kaowool at 850C, depending on packing density of each). So there are limits to how thick a backing layer you can use, since if you add too much insulation the cold face of the IFB will end up being hotter (more of the temperature drop is across the back-up insulation) and that has to remain low enough to not degrade the perlite's R-value.

As you note, I will have to use a castable behind the peeps. Perlite is a common basis for castables, so I will probably start out there. From what I have been able to find on the Web, it looks like perlite bound with sodium silicate may be a good starting point.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

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Bob Masta

maybe a large window instead of a peephole?

regards, charlie

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charlie

Bob,

Re your experimental kiln. There was some research done by the late Donald Shelley in the UK which found that there is a point at which increased insulation begins to have a reverse effect and heat starts to accelerate through the wall rather than being retained. I came across this in the late '70s when a former associate of his was designing my last production kiln, using Ceramic Fibre (board hot face, then

1260oC grade fibre, then Lo-Con, then Rockwool) the most efficient wall thickness they came up with for a 55 Cu. Ft. gross capacity was 7 inches. the kiln was certainly economical (Natural Gas), but the outer surface still got very hot.

I have done some searching, but I'm afraid I can't do better than memory of past information currently.

Steve Bath UK

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STEPHEN MILLS

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