POTTERY PRODUCTION PRACTICES: Tips & Techniques: Controlled Firings: Kiln Design Matters, Part 2: Combustion and Control
March 1, 2007
Every aspect of a kiln’s design is a tool that can be used in its operation and control. Whether you decide to build or buy your next gas-fired pottery kiln, keep in mind that features such as the insulation and kiln furniture, the burner type and orientation, the exhaust system, the thermocouple location, and the door design will significantly affect the kiln’s firing performance and efficiency.
Part 1 of this series (Pottery Production Practices, February 2006) examined several considerations for insulation and kiln furniture. This column will address the remaining variables.
Burner Type and OrientationPotters usually refer to their kilns by a single feature, such as updraft or downdraft. However, kilns are actually an amalgam of many features. Arch-type kilns might feature a corbelled arch, caternary arch, sprung arch, flat arch or a “Minnesota flat top,” as originated by Nils Lou.1 Further definition comes by way of burner type and orientation. A kiln can be lane-fired horizontally on either side of the load (typical of an updraft Alpine kiln) or vertically lane-fired on either side of the load (typical of a Geil kiln). Some earlier kilns had burners that would fire directly into the bottommost setting shelf (typical of early Keith and DFC kilns used in the whitewares industry and similar to Raku-type kilns still used in many colleges today).
Pottery kiln burner types include all sorts of homemade and commercially available venturi-type burners, as well as forced-air burners using open burner ports. Industrial ceramic manufacturers typically use a forced-air burner with the refractory burner block sealed into the kiln’s refractory brickwork. When using a “sealed burner,” the kiln’s sole source of combustion air must enter the kiln through the burner, which requires a combustion air blower of the appropriate supply volume and pressure.
In an open burner port design for a forced-air burner, a portion of the combustion air supply is introduced around the burner nozzle by way of a draft initiated through the kiln’s exhaust port. Venturi burners operate on a similar principle of draft inducement to provide an adequate source of combustion air. Greater volumes of both gas and air are required to reach temperature at appropriate firing rates toward the end of the firing cycle.
The “ideal” burner type and orientation will vary, depending on the size of the kiln and on the products being fired. Space does not permit an in-depth discussion in this article; however, a variety of books are available on this subject. (See the “For Further Reading” list with this article online at www.ceramicindustry.com.)
Exhaust SystemA kiln’s exhaust system can have a huge impact on its ability to deliver heat to the product in a uniform manner. This is because the thermal input of a kiln is based on a volume-in/volume-out relationship. Air movement and the time it takes this air to pass through the kiln helps determine how quickly the kiln can achieve peak temperature and how long it can stay at peak temperature. The air discharge velocity and the amount of potential open exhaust area determine how well a kiln can be exhausted-which, in turn, affects the air movement in the kiln.
The exhaust system can be used both to control kiln atmosphere (oxidization, neutral or reduction firings) and to achieve the desired peak temperature at the required heating rates. Stack heights, internal flue dimensions, construction materials, the kiln’s insulating values, and the kiln’s relative location to potential cross or downdraft conditions have a major effect on the operator’s ability to properly control the firing process from one firing to the next. For this reason, the exhaust system must be designed and sized correctly in relation to the intended combustion equipment. Books on kiln design typically also provide information on exhaust system design.
Thermocouple LocationProper location of the thermocouples and cone packs will help you determine whether your kiln is firing at the required rates. Temperature monitoring equipment should not be located where it can be directly influenced by the combustion system, but should instead be positioned to be representative of the heating rates seen by the product to be fired.
For example, if a thermocouple is too close to the wall refractories, the mass of the wall will act as a heat sink, and the temperature represented will be cooler than the actual air temperature around the product. A thermocouple that is too close to the exhaust port opening might see temperatures that are lower than those in the surrounding setting area due to the velocity of the exhaust and the stratification of air being moved through the exhaust port. On the other hand, if a thermocouple is too close to a burner, it will likely see localized temperatures that are higher than those experienced by the product.
In my kiln, I monitor three elevations for a 4-ft-high load. Each thermocouple extends beyond the rear wall refractories by 2 in. and is located within the product setting area. This provides me with accurate temperature data for every firing.
Door DesignFor the novice kiln builder, the kiln’s door design may not seem to be very important in the early stages of the engineering process. The mechanical complexity of a hinged door might lead to a decision to simply brick the door opening in for each firing. However, a bricked-up door would have more heat loss, require more fuel to achieve peak temperature, produce a cold spot at the door location relative to your product, allow products of combustion to escape into the localized area and could be a source of kiln dirt associated with reduced first-quality ware recovery. Side-hinged or center-pivoting mechanical doors, which open like a refrigerator door, will provide the most ease of use in a typical periodic kiln.
Center-pivoting doors are used if a hinged door has limitations in its opening radius. However, make sure that neither the door nor the kiln refractories will be damaged by abrasion when the door is opened and closed.
In some cases, a vertical lifting door might be required due to product requirements or space limitations. Additional engineering and overhead clearances will be needed to determine how the door will be lifted. As the designer/engineer, you must factor in not only the door’s weight, but also the resistance of the door operating system. Safety features are a must to protect the kiln loader during the loading/unloading process.
Shuttle or car kilns can be built with a hinged door mechanism, or the door can simply be attached to the car base. The latter option will require the door to travel with the car to open or close the kiln. If you operate a kiln with more than one car, your kiln’s door design should not be part of the car base.
Safety FirstFor the vast majority of potters, designing and constructing a kiln should seem as daunting a task as designing/building your own wheel, clay mixer or automobile. The potential savings for the do-it-yourselfer might not offset loss of production during the construction phase. Additionally, inconsistent firings and a low rate of first-quality recovered product could potentially cost you more in the long run than an initial investment in a high-quality production unit built by a qualified kiln manufacturer.
However, if you do choose to build your own kiln, make sure you understand all of the details that can affect the kiln’s performance. Numerous books on kiln design and construction exist and can provide a useful reference.
Additionally, be sure to familiarize yourself with all local code requirements and safety procedures that may be enforceable in your particular geographic area. These can be obtained from the National Fire Protection Association (NFPA) (www.nfpa.org) and your city or county government offices. Always remember that safety is the primary concern.
Author’s note: Please practice safe building and handling procedures during both the construction and cleanup phases of your kiln building project.
Editor’s note: References and a list of books for further reading can be found with this article online at www.ceramicindustry.com. For more books on kiln design and construction, click on “Books” in the blue menu bar.