Smokin' Harleys Plywood Kiln build

[Smokin Harley]

The crockpot I bought yesterday is doing better already than the first. There must be a minimum wattage : cubic footage formula for kiln building. The 3 qt/150 watt wasn't big enough and I think thats why I had to leave it on high all the time.. The new one is 200 watts and a quart bigger . I am able to leave it on low (lid on) and it kept 122* all night.
buying and figuring wattage in a kiln heat source is like engine horsepower in a vehicle...you can always throttle it down if you don't need it all but you can't turn it up if you're already going full blast .

 

[deluxestogie]

There must be a minimum wattage : cubic footage formula for kiln building.

I studied the watts/ft[sup]3[/sup] of a (modest) number of of member-built kilns of various shapes and sizes. There was no relationship. My impression, from a thermodynamics standpoint, is that the significant relationship is watts/(heat loss). Heat loss is proportional the ratio of surface area to volume [that is, the smaller the surface area for the same volume--the more sphere-like the shape of the kiln, the smaller the heat loss] AND is inversely proportional to the R value of the insulation [the smaller the R value, the greater the heat loss]. This assumes that the seals on the door as well as the joints are efficient.

If the kilns were perfectly spherical, with perfect insulation, the size would not matter at all. But since no kiln insulation is perfect, there will always be an increase in watts required, as the size of the kiln increases. But I believe this is far outweighed by the shape and insulation and sealing of the kiln.

The optimum practical shape of a kiln is cubical (height, width and depth are identical). Aim for the highest practical R value. Seal the joints well. Add the best door seal you can afford. All these will reduce electrical costs, and improve the stability of the temperature and humidity within the kiln. An effective internal fan also reduces the electric usage by reducing the on/off cycling of the Crockpot, and the variation of temp and humidity from one part of the kiln to another.

As an example, my new 48" x 30" x 20" kiln [with R-10 XPS foam] requires less electricity and less frequent water refilling than my tiny, original "Baby" wooden kiln, which was about 30" x 12" x 12", and insulated with an old, child-size sleeping bag. Both of them used a 2 quart Crockpot. The quality of the tobacco that comes out the new kiln is also better and more uniform than that from the Baby kiln.

Bob

 

[Smokin Harley]

I just figured since mine was nearly twice as big as yours it should have at least that much more volume of output.
Um....I'll just leave it at that .

 

[Knucklehead]

I went with a five quart crock pot because it was lower in height than any of the four quart crocks I looked at, it was oval instead of round and gave me a couple more inches of overhead space. I also figured less refills, but mine empties about as fast as Bob's, five days.

 

[deluxestogie]

Here's a comparison of my old, baby kiln, vs. my endoskeletal XPS foam kiln:

baby kiln: 4320 cu. in.
endoskeletal kiln: 28800 cu. in.

My new kiln has nearly 7 times the volume of the old kiln, but uses the same size Crockpot, and requires less wattage. The 2 quart Crockpot is rated 100 watts on the "Hi" setting. But I keep it on "Lo", which is probably about 70 or 75 watts.

I originally selected a 2 quart Crockpot, because that was the largest one that would fit in my baby kiln. Since then, I've come to like them because they are cheap as dirt.

The water evaporation rate (by that, I mean days between refills) seems to be related to the average temperature of the ceramic pot. A larger pot requires more heat to warm up to set temp, then remains hotter for longer during the cool cycle. On my porch, the water evaporation rate is also closely related to the ambient temperature (how hard does the Crockpot have to work to maintain the temp inside the kiln). When ambient is above about 85ºF, and does not cool below 75ºF at night, then very little water evaporates from the pot.

I suppose that the moral of the story is that the Crockpot size as well as the kiln size have less influence on the effectiveness and stability of the kiln than the kiln shape/sealing/circulation. A kiln builder has a choice of either more Crockpot or a more efficient kiln.

Bob

 

[Smokin Harley]

I suppose, and I get what you're saying all things aside and all seams and holes sealed tight. I was just thinking its the difference of having a 4 cylinder in your full size pickup or a nice v-8 ...sure you'll get where you're going just fine with the 4 banger as long as the road is downhill and you have a tailwind. But , I get it . lots of variables in this kiln thing.

 

[Smokin Harley]

I have a question, idea...I'm sure its been covered but I'm asking anyway...since I have the crockpot going (low)and it is not only being covered with foil on top of the ceramic but with that being hole poked for small steam escape and covered again by the lid...is it possible to steam a batch of cavendish (over the foil,under the lid) while it is humidifying the kiln or is it not the correct temp and humidity to do both correctly simultaneously?

 

[deluxestogie]

Making black Cavendish, in my experience, required suspending the leaf over an enclosed pot of full-boiling water for 8 or more hours, or steamed in a pressure cooker (121ºC) for 5 hours. I think the leaf would just lay back and enjoy the sauna conditions in a kiln.

Bob