A new low-energy tunnel kiln concept has been developed to help ceramic manufacturers minimize energy and investment costs while improving production flexibility and product quality.* The basic principle of the kiln is that the firing chamber has four fixed sides: two walls, one roof and one bottom (hearth). The products, their supports and the moving media are all completely immersed in the firing chamber and are therefore always at the same temperature.

In addition, the product supports and the moving media weights are reduced, representing 8-10% of the product weight, compared to traditional kilns with kiln cars that comprise 50-100% of the product weight. The kiln cars in a traditional tunnel kiln are often responsible for 25-30% of the energy consumption because of their heating and cooling needs. Additional ventilation needed under and mostly above the cars (air going through the products) significantly inflates the kiln exhaust volume and the resulting heat loss.

In the new kiln design, the product is set on silicon carbide (SiC) supports that move on SiC skates (see Figure 1). The design of these skates is proprietary and is one of the keys to the new kiln concept.

*The Skate-Kiln, developed by Direxa Engineering, LLC, and its French R&D partner Ceritherm.

Multiple Advantages

The kiln’s design means that the firing chamber is totally isolated from any leakage with the outside or the under-car space (this is especially true when steel casing technology is applied to all four sides of the tunnel). In addition, no extra thermal mass (e.g., kiln car chassis and insulation) has to be kept cool. As a result, the design offers four main advantages:

• Faster product firing (reducing the cost of the kiln)
• Improved fuel efficiency (estimated at up to 20%, when compared to the current best available technology for standard products and lines; the potential is greater for some special applications)
• Improved product quality due to enhanced temperature distribution throughout the kiln
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• Perfectly controlled kiln atmosphere because of the absence of leakage (can be particularly helpful when reducing firing conditions are desired)

While product supports and moving media still comprise 10% or so of the product weight, it is not really parasitic since it is at the same temperature as the product and therefore costs practically nothing in energy. The SiC is highly thermally conductive and totally exposed to the product ventilation system, thus exiting the kiln at the same temperature as when it enters.

As mentioned previously, 20% energy savings is possible compared to the best available technology for standard lines. For some special applications (e.g., special shapes of roof tiles), where the weight of the kiln car vs. the weight of the product is high, savings of 50% and even more is possible. This is mostly due to the complete airtightness of the firing chamber, the absence of an under-car space, and the previously explained quasi-absence of a heavy parasitic load of product supports and moving media.

Flexible and Adaptable

From a theoretical standpoint, this new technology has as much potential to replace a roller hearth kiln as it does to replace a kiln car-based tunnel kiln. Because of the design benefits (e.g., airtightness of the firing chamber, absence of kiln cars) and flexibility (e.g., ability to fire small and heavy loads, as well as short and long firing cycles under basically any kind of atmosphere), application potential exists for all ceramic products—from clay brick to the technical ceramics and whitewares industries. For light loads, the new kiln concept can be adapted to work without skates at all, simplifying the technology even further.

An “under and over” firing solution is typically limited to an approximate 4.5 m kiln width, when only side burners are used. Due to the extra space available over the load in this design, however, small burners can be used from the top, and the kiln width can reach 6-7 m. As a result of the “slot” firing solution, the limit is truly the width of the roof (about 10-11 m). It would even be possible to put intermediary posts in the width and go much wider, since sand seal and car skirts do not need to be included. While we are limited by one SiC bar of 3.9 m over two rails, the width in this configuration can be increased by installing multiple two-rail tracks next to each other, with posts where needed in the gap between the two tracks.

Any industry that requires heat treatment above 600-700°C (e.g., annealing of metals) could also benefit from this concept. The high loads and temperatures involved limit the material for supports and skates to SiC, with different grades allowing different maximum temperatures. The current maximum practical temperature is 1,400°C. However, for light loads and high value-added product, other high-temperature materials could be explored for higher application temperatures.  

For additional information, visit www.direxa.com or www.ceritherm.com.