There are several variables that come into play when choosing burners for a kiln or furnace. Listed below are the facts you need to know before deciding the size (Btu's per Hour) of your burner system.
1. Total inside volume of the kiln.
2. Type of wall construction.
3. Maximum temperature you will be reaching.
Calculating Kiln Volume
Kiln volume is usually expressed in Cubic Feet (CF). In a flat top kiln this figure is arrived at by multiplying the interior height (H) by the interior width (W) by the depth or length (L).
Sprung or Roman arch: CF = W x L x (Side wall + 2/3 of the arch rise)
Catenary arch: CF = L x Arch area (4/3 H x 1/2 Base Width)
Barrel kiln: CF = H x Pi x R2 (R2 - Radius is 1/2 the diameter x itself) (Pi = 3.14)
If you have used inches in the above equations, divide the total by 1728 to convert to Cubic Feet.
Wall Construction & Temperature
The type of material and its' insulating values determines how many Btu's per Cubic Feet per Hour (Btu/Cf/Hr), you will need to reach a desired temperature. Below is a simplified chart showing materials, desired temp., and the corresponding Btu/Cf/Hr. There are a host of variables that can affect kiln efficiency. This is a basic guide only.
| Construction |
Cone 06
Btu/Hr
|
Cone 6
Btu/Hr
|
Cone 10
Btu/Hr
|
9" Hard Brick
|
12,000-17,000
|
14,000-18,500
|
16,000-20,000
|
9" Insultating Brick
|
6,000-10,000
|
8,000-13,000
|
10,000-16,000
|
6" Ceramic Fiber
|
4,000-6,000
|
6,000-8,000
|
7,000-10,000
|
This simple table gives you an idea of how many Btu's per Cubic Feet per Hour you will need. Multiplying this figure by the total Cubic Feet will give you Btu/Hr. Now divide Btu/Hr by the number of burners you plan to use to determine what Btu/Hr rating each burner should have. The numbers above show a range of BTU figures. The highest figure in each range produces a 6-7 hour firing. The lowest figure will produce firings in the 14-18 hour range. I feel it is better to have extra Btu's than not enough. The above is a guide not a guarantee. If you would like us to verify your calculations, please feel free to call or write.
Raku Construction & Btu/Hr Values Many folks don't realize that Raku kilns have much higher Btu input rates than stoneware kilns. This is because Raku is traditionally done very quickly. For this reason, it is very difficult to bisque fire in a Raku kiln. If you plan on purchasing or making a Raku kiln, please note that you could have problems with steam explosions of the ware if you attempt to use the kiln for bisque. Also, the structural nature of Raku kilns make many of them impractical for use at stoneware temperatures. The chart below gives the basic Btu input for Raku kilns of various materials. These input values are for a fast firing rate of around 20-30 minutes for the first load. Subsequent loads would be slightly faster.
Construction
|
Btu/Hr
|
4 1/2" Hard Brick
|
70,000
|
2 1/2" Insulating Brick
|
40,000
|
4 1/2" Insulating Brick
|
30,000
|
1" Ceramic Fiber
|
25,000
|
2" Ceramic Fiber
|
20,000
|
|
