Introduction to Gas-Fired Kiln Construction

Hello everyone! Today I would like to discuss kiln construction. Gas-fired kilns offer different possibilities for control and operation from wood-fired kilns. Their installation, however, requires an assessment of combustion-product exhaust, ventilation, heat and site conditions. The absence of visible smoke does not demonstrate that an installation is safe or has no impact on its neighbours.

Among gas-fired kiln designs, I would highlight the downdraft arrangement. Flames travel towards the top of the chamber, and gases descend through the ware to the lower flue entrance. The term describes the gas path; it does not mean that the kiln uses blower-forced air. Burner air supply and kiln draft must be considered separately. The damper restricts the passage of gases through the chimney and allows draft to be adjusted, but its effect depends on the kiln system and combustion conditions.

General Design Principles

Below are design references discussed in the sources consulted. The proportions and calculations are starting points subject to the conditions of the design, not universal rules or sufficient instructions for building or installing a kiln. Their use requires consideration of burner type, materials, kiln load, draft and site conditions.

• One design reference is to keep the chamber height close to its width. Increasing the height may make temperature uniformity between the base and upper shelves more difficult, depending on the kiln arrangement. This proportion does not, by itself, guarantee uniformity or efficiency; depth, gas circulation and load distribution must also be assessed.

• The kiln shape should encourage circulation. Vaulted chamber roofs help the flow, making firing more efficient and avoiding pockets of overheating.

• The reference to 12 cm of internal side clearance given here concerns gas circulation and combustion. This distance should not be treated as a universal or sufficient requirement: it must be assessed in the context of the design.
• The relationship between the flue outlet area and the combined inlet area is a sizing reference whose application depends on the design. Burner number and output must be assessed against volume, kiln load, heat losses and the intended firing. Inlet size also depends on burner type and air admission; a rule based on blower-fed air should not be applied indiscriminately to atmospheric burners.
• In gas-fired kilns, the chimney helps exhaust combustion products and, in natural-draft systems, moves gases through the kiln. Gas pressure in an atmospheric burner is not equivalent to blower-fed air and does not remove the importance of draft. The chimney-height calculation presented in this article—3 times the internal chamber height plus 1/3 of the horizontal distance between the inside wall of the chamber and the inside wall of the chimney—should be treated as a design reference rather than a universal sizing rule. Flue cross-section, routing and operating conditions must also be assessed.

How the Burner Works

In atmospheric burners, gas flowing through the injector entrains some of the air used for combustion through the Venturi principle. This entrainment is not equivalent to blower-fed air and does not mean that air is supplied at high pressure. The mixture burns at the burner outlet; air admission and combustion conditions must be assessed according to the equipment and the kiln system.

Manufacturers normally specify a burner’s BTU output so that the required number of burners can be calculated for the kiln. 

Calculating Fuel Requirements 

The following table gives BTU/h per cubic foot:

Construction	Cone 06 Btu/h	Cone 6 Btu/h	Cone 10 Btu/h
Hard firebrick, 22 cm	12.000-17.000	14.000-18.500	16.000-20.000
Insulating firebrick, 22 cm	6.000-10.000	8.000-13.000	10.000-16.000
Ceramic fibre, 15 cm	4.000-6.000	6.000-8.000	7.000-10.000

The higher values in each range assume a firing of 6 to 7 hours; the lower values assume 14 to 16 hours.

For example, suppose we want a kiln with an internal volume of 300 litres, approximately 10.60 cubic feet. Multiplying 10.60 by 18,000 BTU (for a cone 10 firing of intermediate duration) gives 190,800 BTU/h.

Dividing this figure by the number of burners gives the output required from each one.

190,800 divided by 4 = 47,700 BTU/h per burner. One manufacturer gives a maximum output of 70,000 BTU/h for its burner. This number of burners therefore meets the requirement with some capacity to spare. It is always better to allow a margin in the calculations than to run equipment at its limit.

In the next post, I will write about construction techniques for gas-fired kilns. Until then!

Sources:

• Olsen, Frederick L. The Kiln Book. University of Pennsylvania Press.
• Kiln Building: LPG Burner Calculations. http://www.potterswithoutborders.com/2012/07/lpg-burner-calculations/
• Ward, Marc. Draft & Its Importance. Clay Times Jan. 1998. http://www.wardburner.com/draft.html