Tunnel kilns

The high point, before the introduction of modem electrical and oil- or gas-fired kilns, was the development in China of the dragon kiln, a tunnel kiln built against a hillside, with a draft which resulted in the temperatures needed for the firing of porcelain ware. 

Sihca brick and large fireclay shapes are fired in circular downdraft kilns. These kilns vary in diameter and can accommodate up to 150,000 23-cm bricks or their equivalent in other si2es. The complete burning cycle for a typical periodic kiln ranges from 21 to 27 days as compared with four to seven days for a tunnel kiln. 

The shuttle kiln consists of a firing chamber with two or more kiln cars on which the bricks to be fired are set. While one load of brick is being fired, a second is being set. Somewhat similar is the beU top or top-hat kiln which is raised and lowered above and over the kiln cars to be fired. These kilns are more expensive to operate than tunnel kilns but provide flexibiUty in burning conditions and production schedules. 

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

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. 

Kanal-jauche, /. (liquid) sewage, -ofen, m. tunnel kiln, -rohr, n. — Kanalisationsrohr. -strahl, m. canal ray, -trockner, m. tunnel drying oven, -wasser, n. sewage. 

Process or heating plant may have controls ranging from manual operation with some processes supervised by interlocks to semi-automatic and fully automatic operation. Deciding factors will include temperature of operation, frequency of ignition, degree of operator supervision, and rating of the plant. For example, boiler plant, both steam and water, will invariably have automatic control whereas tunnel kilns operated continuously at high temperature are unlikely to require it. 

The dried bricks are burned in either periodic or tunnel kilns at temperatures ranging between 1,200 and 1,500°C (2,200 and 2,700°F). Tunnel kilns give continuous production and a uniform burning temperature. 

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. 

Wet mixes are usually dried before calcination. Calcination is performed continuously in rotary or tunnel kilns, or batchwise in directly fired drum or box furnaces. The temperature at which the mixed metal oxide pigments are formed can be reduced by adding mineralizing agents [3.75]. In the case of chromium rutile pigments, addition of magnesium compounds [3.81] or lithium compounds [3.80] before calcination improves thermal stability in plastics. 

Clay Activation. The clay is heated to about 700 °C to destabilize the kaolinite structure by removing hydroxyl ions as water. This can be either a batch process with the clay in crucibles in a directly fired kiln, or a continuous process in a tunnel kiln, rotary kiln, or other furnace. 

The first phase of the double firing is the so-called biscuit firing. This takes place in an appr. 100 metre long tunnel kiln at a temperature of appr. 1120 °C, The tiles remain in this oven for about 55 hours. 

In the Netherlands, the continuous kiln is mostly applied. We distinguish two principles here the bricks pass through the fire or the fire moves across the stationary bricks. The former principle is applied in tunnel kilns. Nowadays these are about 100 metre long and up to 8 metre wide. 

If a low oxygen pressure is required during sintering nitrogen is injected at a point in the tunnel kiln where the temperature is approximately 1000 °C, and the... 

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... 

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