Tobacco Kiln Watts per Cubic Foot?

[deluxestogie]

I would like to generalize the heating requirements of tobacco kilns, and need the input of any of you who use a kiln powered by any electrical heat source (heat pad, Crockpot, light bulb, etc.).

If you could do a rough measurement of the internal dimensions of your kiln and post these in this thread, together with the wattage of your heat source or sources, that would be a big help. (I'll be posting this on HTGT as well, so please post here or there, but not both.)

I realize that insulation and air flow make a significant difference in the efficiency of a kiln, but I think we can come up with some usable numbers.

Thanks,
Bob

 

[FmGrowit]

I think you'll be able to begin to narrow down the wattage required to sustain a specific temperature only after the insulation value is known. Perhaps asking for the apparatus being used and a simple "wall thickness" will assist in making the determination as to wattage. If you can coax out what form the walls are, all the better. Rigid board insulation is about 3 times the R-value of fiberglass batts. With enough time and insulation, an area of a 2 car garage can be heated to 120ºF with a candle.

 

[Jitterbugdude]

Bob, from a previous post..

I finally got around to plugging my kiln into a Kill-O-Watt meter. It said I use 5 kw per 24 hours. My kiln is about 4x4x3 feet. I use a warm mist humidifier and a 100 watt light bulb. It is a little drafty because this was a "temporary" kiln I built about 5 years ago and I still have it. It is made with 1 1/2 inch rigid foam board. My electric costs about 12 cents per kilowatt/hour.So, for me to ferment for 4 weeks costs me....12cents x 5KW x 28 days= $16.00 (approximate). Not too bad considering I do about 400 leaves at a time.

Randy B

 

[Daniel]

19.5 cubic feet, 500 watts. Located outdoors exposed to freezing temps. My kiln also has a full front of glass so insulation is not typical. I have my suspicions this kiln could run on half the power as is and even less if placed indoors. Possibly as little as 100 watts.

 

[deluxestogie]

My own microkiln is a wooden box (3/4" plank) covered with 3"+ of polyester and fleece insulation (a hollofil sleeping bag and a fleece blanket), plus an additional 1.5" Insolite foam on the top. Internal dimensions are 22" x 10" x 10", so about 1.25 cu. ft. [I can run about 2 pounds of tobacco at a time, but it has to be neatly pressed to fit.] The only energy source is a 2 qt. Crockpot, rated at 100W, but run exclusively on the low setting (wattage not specified).

If I guesstimate that the Crockpot runs at 62.5 Watts on low setting (and intermittent at that), then my 1.25 cu. ft. kiln runs on about 50 Watts per cu. ft.

Daniel's ~20 cu. ft. setup comes to roughly 25 Watts per cu. ft.

Randy's 48 cu. ft. kiln (100 Watt light + estimated 100 Watt humidifier) approaches 4 Watts per cu. ft.

These are wildly divergent numbers, as Don predicted. BUT...thermal loss is proportional to the surface area, rather than the volume. So, perhaps I should have entitled this thread "...Watts per Sq. Ft.

I've done some painful math on the numbers provided so far, solving Watts per sq. ft. of surface (idealized to a sphere). Once I have a few more data points, I'll show what I've come up with.

Bob

 

[FmGrowit]

I wasn't trying to point out a flaw in the premise, but insulation is a huge variable in the calculation. Ambient temperature would also be another variable as mentioned. I suppose calculations could be made taking all of these things into consideration, but the graph would be really complicated. I think the easiest way to make the determination you're looking for would be to make a test model and use various devices to create heat under a controlled condition.

Once the control test is done, variables like adding more insulation and increasing size can be introduced. There would be a huge value in understanding these numbers. Would the same wattage work in a space twice the size simply by doubling the R-value of the insulation? Would it be more economical to double the wattage rather than incurring additional cost of insulation?

My guess is modifying a chest freezer is probably going to be one of the easiest and economical ways to make a kiln...provided there is an old chest freezer available. Insulated garage doors can be fashioned into a nice kiln with a little carpentry skills and they can be had for the cost of scrap from a local overhead door company.

This sounds like a fun experiment.

 

[deluxestogie]

I'm eager to get numbers from more members.

There is an interesting quirk to the math of all this. If we look at the limit cases:

1. the size of the kiln approaches zero
2. the size of the kiln approaches infinite

In the first instance, the surface area overwhelms the tiny volume, so the smaller the kiln, the more important the insulation.
In the second instance, the volume rises so much more rapidly than the surface area, the importance of the insulation diminishes.

In the minimal data that I currently have, it is apparent that the required Watts per cu. ft. clearly fall as the size of the kiln increases.

So, yes, insulation matters, so long as there is a gradient between internal and external temperature, but it becomes proportionately less important as the size of the kiln increases.

I think, without experimentation--that is, simply accepting that an existing kiln works, that I can create a generalized, simple graph of ballpark wattage expectations plotted against size. But I really need more data.

Bob

 

[Jack in NB]

My kiln is about 4 x 4 x 8 ft, 5" styrofoam walls, back and top, 2" foam front, 4" foam on floor. About 160 sq ft surface area.

Kiln in installed in my garage, ambient temps 40 - 60 degrees. Holds my complete crop from 130 or so plants. Operating temp about 125 deg.

I have an 800 watt light bank installed for heat, and a 200/400 watt (low/high) humidifier. The humidifier ran nearly constanly on low, and the light bank used 300 kwh over the 30 day curing cycle.

Using the consumption divided by monthly hours, the light bank would have averaged a bit under 300 watts for the period; that plus the humidifier suggests a bit under 500 watts average demand. That's about 4 watts per sq ft.

Power consumption was cut in half by increasing the insulation from the original 2" overall thickness.

It's best to have extra capacity installed to provide faster warm-up when needed. Costs next to nothing to add extra 100 watt bulbs as a heat source!

 

[Daniel]

Bob, I can easily modify my kiln to confirm that it can in fact maintain a 120 degree temperature on half the power. I am basing my assumption on the fact that the current lights heating the kiln will be out half the time or more at 500 watts total power. Thermostat is set at 120 degrees.

As for your observation that larger space requires less power per unit of volume. This is the same principal that there is more skin on a lb of small potatoes than on a single 1 lb potato. Energy loss only happens at the outer surface of any container. less outer surface equals less energy loss. Larger containers have less outside surface per unit of volume than smaller ones do.

I think you can approach this by factoring the energy needed per degree of temperature rise also. For example lets say I need at least 250 watts to keep my 19.5 cu. ft of space as much as 110 degrees about the outside temperature. Since 10 degrees Fahrenheit is about as cold as it has gotten while running this kiln. It could also be observed how much the heater runs to keep the kiln at say 60 degrees above outside temperature as well. You could then put an energy recommendation together that takes temperature differences into account.