Continuous tunnel kilns

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. 

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. 

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. 

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. 

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. 

The continuous production with a daily output of 1500 of sheet slagsital was initiated in the USSR in 1966. A schematic diagram of the process is shown in Fig. 150. After melting in a tank furnace, a sheet is formed by pouring between rollers in a tunnel kiln crystallization is then performed in two stages. The sheet is 3 m in width and 5—25 mm in thickness. 

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),... 

In the manufacture of bonded insulating materials, the fibers in the fleece shaft or on the conveyor belt are sprayed with an aqueous binder, generally a phenol-formaldehyde resin. The binder content in the bonded insulating material is 3 to 4%. Compaction to the desired density and hardening of the resin binder occurs in a tunnel kiln, through which the fibers are continuously transported on a conveyor belt. The compaction is achieved with a second belt which exerts the required pressure on the upper surface of the continuous sheet. This is often followed by laminating the sheet with e.g. paper, aluminum or plastic foil. Finally the product is rolled up or cut into sheets. 

Firing of mass-produced articles carried out in continuously operating tunnel kilns... 

Continuously operating tunnel kilns heated with gas, oil or electricity, in which the articles travel through a well-defined temperature-time profile, are currently almost exclusively used for the manufacture of the mass-produced articles of the brick, refractory and crockery porcelain industries. Such kilns are 20 to 150 m long and have transit times between 1 and >72 h. Muffle kilns, which operate discontinuously, are still important for small production runs and specialty products. 

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). 

The beryl is ground, wet or dry, to about 200 mesh in a ball mill, and then mixed with the sodium ferric fluoride and sodium carbonate in a wet condition. Briquettes are fabricated from the paste, dried and sintered, e.g. by passing continuously through a gas-heated tunnel kiln with a residence time of several hours at 750 C. A rotary kiln would probably not be very satisfactory since the briquettes are somewhat plastic and would tend to adhere to the lining. ... 

Air Seal. A method for the prevention of the escape of warm gases from the entrance or exit of a continuous furnace, or tunnel kiln, by blowing air across the opening. 

Continuous Kiln. A kiln in which the full firing temperature is continuously maintained in one or other zone of the kiln. There are two types annular KILN (q.v.) and tunnel kiln (q.v.). Continuous Vertical Retort. A type of gas retort, built of silica or siliceous refractories. Coal is charged into the top of the retort, coke is extracted from the bottom, and town gas is drawn off, the whole operation being continuous (cf. HORIZONTAL retort). Continuous vertical retorts are also used in the zinc industry, in which case they are built of silicon carbide refractories. 

Kiln Car. A car for the support of ware in a tunnel kiln or bogie kiln the car has four wheels and runs on rails. In a tunnel kiln, through which a number of these cars are moved end-to-end, the metal undercarriages are protected from the hot kiln gases by continuous sand-seals. The car deck is constructed of refractory and heat-insulating material. 

Travelling Thermocouple. A continuous record of the temperature applied to ware in a (tunnel) kiln is obtained from a thermocouple arranged to travel with the ware through the kiln. 

Tunnel Kiln. A continuous kiln of the type in which ware passes through a stationary firing zone near the centre of the kiln. In the most common type of tunnel kiln the ware is placed on the refractory-lined deck of a car, a continuous series of loaded cars being slowly pushed through a long, straight, tunnel. 

In tunnel kilns of length 50-100 meters, expansion progressed over a period to the point that the interior walls made of fireclay brick typically buckle inward. In about seven years of service, the brick must be replaced to continue operation. 

The dryers generally used for the drying of piece-form fruits and vegetables are cabinet, kiln, tunnel, belt-trough, bin, pneumatic, and conveyor dryers. Among these, the cabinet, kiln, and bin dryers are batch operated, the belt-trough dryer is continuous, and the tunnel dryer is semi continuous. 

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