Ceramic tunnel kiln

Figure 5.15 shows a common practice in ceramic tunnel kilns, where the more gradual warm-up of the preheat vestibule has the added bonus effect of less sudden expansion damage to the raw ware. 

Fig. 5.15. Ceramic tunnel kiln (not to scale) with unfired preheat vestibule for heat recovery. Long, narrow kiln or furnace geometry minimizes the proportion of heat loss at the conveyor entrance and exit. Air-lock chambers are even better.

Williamson Kiln. A tunnel kiln of the combined direct-flame and muffle type designed by J. Williamson (Trans. Brit. Ceram. Soc., 27,290,1928) and first used, in England, for the firing of wall-tiles and sewer-pipes. This kiln differed from earlier tunnel kilns in that the hot combustion gases passed across, rather than along, the kiln. 

Conventional Sintering Equipment. Like drying furnaces, sintering furnaces (29,76,85) can be periodic or continuous in nature. Periodic kilns offer greater flexibiHty continuous tunnel kilns are more economical. Advanced ceramics are typically siatered in high purity, controUed atmosphere furnaces by electric resistance heating. Ceramic furnaces used to fire traditional ceramic ware are generally heated with inexpensive natural gas, oil, wood, or coal. 

Hojanas Also called Siurin. An iron extraction process. Magnetite, mixed with carbon-coke breeze and limestone, is heated in a ceramic retort by passage through a tunnel kiln at 1,200°C. Used commercially in Sweden since 1911. See also DR. 

This research on pottery has three aspects. The ability to differentiate with trace elements different sedimentary units allowed us to conclude that pottery was produced from Nile deposits located in the immediate vicinity of the potter s workshop and kilns (25). The finer Plum Red Ware, most popular between ca. 4000 and 3400 B.C., and most common in cemeteries, was made from the same Nile sediments as the more common Straw Tempered Ware that dominated settlement ceramics. The Plum Red Ware was often fired in specific areas that may have behaved like natural wind tunnels (e.g., Locality 39 and 59 in Figure 1). Our more limited studies of Hard Orange Ware (a fine pottery most popular after about 3400-3200 B.C.) have shown that, although made from local Nile sediments, there was a significant... 

Sintering takes place in a kiln. There are many types of kilns for the different types of sintering used in industry. Two types of kiln are typical. One is the box kiln and the other is a tunnel kiln. The box kiln is simply an insulated box in which the ceramic green bodies are piled in their... 

By far the most widely utilized type of ceramic kiln is the tunnel kiln, through which ware placed on refractory-lined cars is passed. Compared with the kiln types mentioned above, tunnel kilns have several advantages, in particular ... 

Tunnel kilns are usually heated with gas or liquid fuels, and in some instances by means of electrically heated metal or SiC or M0S12 elements. The firing schedule can be controlled directly by burners and draught, and by adjusting the car speed. More uniform temperature distribution over the cross-section-is attained in small tunnel kilns which find application in the firing of special ware however, kilns with large cross-section arc used in normal ceramic operation. The functioning of a tunnel kiln is shown in Fig. I86. 

The thermal efficiency of ceramic kilns depends on a number of factors, in particular on the kiln type. The consumption of heat related to unit amount of ware is highest for periodic furnaces, and lower by about one half for tunnel or continuous multi-chamber kilns. The consumption of heat also depends on the kind of firing, a direct-fired kiln having a lower consumption than a muffle kiln where the ware is protected from direct contact with combustion gases. For example, a direct-fired tunnel kiln for glost firing of tiles has a fuel consumption 40% lower than a similar muffle tunnel kiln (Holmes, 1978),... 

Various types of kilns evolved during the history of ceramic production fired brick walls were constructed for field kilns in contrast to periodic kilns, serving only for a limited production campaign round or tunnel kilns provide different features concerning the duration of the firing cycle or the temperature uniformity. A kiln with an oxygen-deficient atmosphere produces harder ceramics (and a different colour) than a kiln with an oxidising atmosphere. 

Fig. 9.12 Side elevation (schematic) of a directly flame heated tunnel kiln for the firing of ceramic parts and typical temperature profile [B.12c].

Other forms of straight-line continuous furnaces are woven alloy wire belt conveyor furnaces used for heat treating metals or glass lehrs (fig. 1.5), plus alloy or ceramic roller hearth furnaces (fig. 1.6) and tunnel furnace s/tunnel kilns (fig. 1.7). 

Both preheating the load and preheating eombustion air are used together in steam generators, rotary drum calciners, metal heating furnaces, and tunnel kilns for firing ceramics. 

Tunnel kilns, widely used in firing ceramics and carbon shapes, use a long train of cars as a conveyor Each car may be similar to, but often narrower than, the car of a batch-type car-hearth furnace. Much of what is discussed in this book can apply to ceramic kilns, but the ceramic industries have so many publications on kiln construction and operation that this text will not dwell on them specifically. 

Fire Polished. The final surface gloss applied to glass by heating, in a flame or as the last stage in high temperature production. See fire finished. Firepower. Tradename. A small cross-section LTM tunnel kiln for tableware. (N. Barsby, Br. Ceram. Rev. (90), 1979, pl4. Cf. QUICKFIRE kiln). 

Figure 4 illustrates a typical tile fast firing curve for a roller hearth kiln. Two basic types of kilns remain in service today with the older tunnel kilns being gradually replaced or outnumbered by roller hearth rapid fire kilns, illustrated in Figure 5. Firing in tunnel kilns is done on refractory setters, either manually or automatically loaded. Tunnel kilns remain in wide use today for smaller tile sizes such as 41/4 x 41/4 inch. Roller hearth kilns fire the tile on ceramic rollers, almost always automatically loaded.

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