[Stoves] Crispin´s kiln. (was Re: Traditonal Charcola Making Process / retort

[Kevin Chisholm]

Dear Andrew

AJH wrote:
> On Thu, 5 Jul 2007 18:20:55 -0700, Tom Miles wrote:
> 
>> Andrew,
>>
>> Assume:
>> Kiln 			0.7 m^3 (24 ft3)
>> Stove bodies 	28
> 
> OK so far
> 
>> Power 		3.5 kW electric 
> 
> No it's not. Crispin said it was a 35kW electrical resistance kiln but
> its duty cycle never exceeded 50%. So whilst you need a hefty
> electricity connection for the bursts of 35kW the average over the
> whole cycle looked like 5.4kW. This is a fairly small cost for a very
> simple system and at the current scale I can see why Crispin has been
> advised moving to gas is not worthwhile even at the 0.14$/kWhr tariff
> (IIRC).

The last impression I had was that the "3.5" was kw-hr of energy per 
stove fired, not 3.5 kW of average power input. Over an 18 hour cycle, 
the total energy consumption would be 3.5x28 = 98 kw-hr of electrical 
energy consumed over a 18 hour period.
> 
>> Temperature		ramp 600C to 1150 C (1112 F - 2102F) Cone 5
>> 			Desired 1250 C (2282 F) Cone 10
> 
> Again here I agree with Peter S, if savings have to be made why not
> use a gasifier just for the earlier parts of the heating cycle. Is
> there anything inherent in an electrically heated kiln that means a
> gas flow cannot be accommodated, I know this would require some
> lightweight insulative refractory to block the holes when in electric
> mode. 

It might be that the electric kiln can be filled to a higher degree of 
volume capacity. There is no need to be concerned about different 
atmosphere considerations because of gas flow. I understand that wood 
gas can be used to create different firing conditions... ashing effects, 
and reducing/oxidizing atmospheric effects. Some desired pottery effects 
can only be attained in wood fired pottery kiln

Also Whilst Peter has promoted use of an electric arc as a cheap
> means of adding heat and you have dismissed it, I don't see why. I
> need to see some reasons why an arc is inappropriate compared with
> resistance heating.

An arc has an intense energy emission characteristic.... say 10,000 
degrees C, while the resistance heating surface temperature is perhaps 
down to the 800 to 1200 C range. Thought would probably have to be given 
to diffusing the intensity so that some stoves near the arc are not 
melted to a puddle of slag, while others are still raw. t2^4 - t1^4 
considerations can be of great significance.
> 
> Look at how crude it can be

What I see is resourcefulness!! :-) I once saw a reduced voltage starter 
system for a 200 HP 3 phase motor made from #9 iron wire as resistance 
elements, and Coca Cola bottles for electrical insulators. It also was 
cheap, ugly, and functional.

  ( I pity the bloke's eyes and lungs  not
> to mention compliance with electrical safety or zinc poisoning).
> 
> http://www.zen40166.zen.co.uk/image005.jpg

I built an arc furnace once, using a graphite crucible, and a carbon 
electrode. Very simple.
> 
> 
> Also one of Crispin's dilemmas was the way heat was delivered by the
> resistance heaters, I suspect one of the reasons for the slow ramp ups
> is to avoid thermal shock but at the end temperature, which is where
> 30% of the power is used, the reason could be to avoid the hotter
> parts of the oven overcooking the bodies there while the bodies at the
> cooler part heat up.

The raw stoves have significant quantities of water content. Excessige 
heating rate can seal the surface of the stove bodies while there is 
still free or bound water within the clay. The result can be spalling or 
cracking of the stove bodies because of localized "steam explosions."

  Best wishes,

Kevin