PROCESS FOR COOLING FIRED PRODUCTS IN A KILN

Abstract: A process for cooling fired products such as tiles in a kiln using cooling gaseous fluid is performed by repeating a cooling step to cool down the products with a cooling gaseous fluid, e.g. cooling air, alternately with a non-cooling step to moderate a sharp temperature gradient between sur­ faces and inside of the products by stopping or reducing the cooling gaseous fluid. The process can achieve rapid cooling without giving rise to dunting due to thermal stresses and thus can increase productivity of kilns. When the process is applied to a tunnel kiln, e.g. with cooling zones 3 alternating with non-cooling zones 4 in the direction of travel, the kiln can be considerably shortened in length.

Description: The present invention relates to a process for cooling hot fired products in a kiln, which enables a short time cooling of fired products such as tiles or the like in a kiln such as a tunnel kiln. For example, in a tunnel kiln for firing tiles, cooling of fired products is usually effected by uniformly blowing a cooling gaseous fluid to hot fired products on trucks, from cooling nozzles arranged on the side, etc. of a cooling zone, and if a drastic cooling or quenching is effected to hasten the cooling, only surfaces of the fired products at high temperatures are quenched rapidly, which forms a sharp temperature gradient between the central portion and surfaces of the fired products and is liable to cause fracture, i.e. so-called dunting, due to thermal stresses. The cooling rate has therefore a limit, and accordingly conventional tunnel kilns have had to be provided with a relatively long cooling zone conjoined with a firing zone, that is, in consequence, as long as 40-50% of the entire length of the kiln, so that a heating schedule has been required such that trucks advance slowly, taking a time about 20 hours long, through the kiln with an entire length reaching 50-100 m. In DE-A-2843508 there is disclosed a system for cooling a product passing through the cooling zone of a tunnel kiln, in which cold air is intermittently blown in sharp bursts over the fired product sequentially from at least two sides as it passes through the cooling zone. The present invention aims to reduce or obviate problems as described above in which the conventional cooling processes have been involved and therefore an object of the invention is to provide a process for cooling hot fired products in a tunnel kiln, wherein the fired products can be cooled down more quickly than conventional processes, without giving rise to dunting. The present invention is as set out in Claim 1. The process according to the present invention is characterized by repeating a cooling step to cool down fired products by blowing a cooling gaseous fluid to the fired products in the tunnel kiln, alternately with a non-cooling or reduced cooling step to moderate or relax a temperature gradient in the fired products by stopping or reducing the blowing of the cooling gaseous fluid. The present invention has been accomplished based on inventors' recognition of the fact that if the temperature gradient formed between surfaces and inside of the fired products is moderated or relaxed by inserting, after the cooling step, a non-cooling step to stop or reduce blowing a cooling gaseous fluid for a while, a thermal spalling i.e. a fracture due to thermal stresses never arises even in the subsequent more drastic cooling step. Several embodiments of the invention will now be described in more detail by way of example with reference to the accompanying drawings in which: Fig. 1 is a horizontal, longitudinal sectional view of a tunnel kiln for carrying out the process of the invention; Fig. 2 is a vertical, longitudinal sectional view of the kiln shown in Fig. 1; and Fig. 3 is a diagram showing temperature gradients formed inside a fired product in a process of the present invention. In Figs. 1 and 2, which are horizontal and vertical sectional views respectively illustrating a cooling zone in a kiln 1 like a tunnel kiln, trucks 11 loaded with fired products 10 coming out of a firing zone (not shown) proceed through the cooling zone to the right. In the cooling zone defined by the walls 2 of the kiln, cooling sections 3 of broad width and non-cooling sections 4 of narrow width are alternately provided as shown in Fig. 1, and cooling sections 3 are provided with a plurality of cooling nozzles 5. In this embodiment, among cooling nozzles 5, strong cooling nozzles 5a blow out 1-2 m³/min./nozzle of a cooling gaseous fluid, for example, cooling air, while mild cooling nozzles 5b respectively facing them blow out no more than 0.5 m³/min./nozzle of the cooling gaseous fluid, and these two kinds of nozzles are arranged alternately with each others on both of the walls. All of nozzles 5 on both walls may be strong cooling nozzles 5a, or an alternate arrangement may be adopted such that only nozzles on one side in the first cooling section 3 and on the other side in the second cooling section 3 are all strong cooling nozzles 5a, and so forth. Further, as is shown in Fig. 2, it is preferred to ensure a uniform cooling by circulating the atmosphere in cooling sections 3 with agitating ceiling fans 7 driven by motors 6. Now, after having been fired as carried on trucks 11 in a firing zone, hot fired products 10 such as tiles enter into the conjoined cooling zone shown in Figs. 1 and 2 and first in the cooling section 3, are cooled down by a cooling gaseous fluid such as air or the like blown from cooling nozzles 5. In this cooling step, a temperature gradient is formed between the surfaces and the inside of the fired product 10, as shown by solid lines T₁T₂ in Fig. 3. Next, fired products 10 move into the non-cooling section 4 and are subjected to a non-cooling step. In the non-cooling step, no or only mild cooling takes place, so that inside the fired products 10, heat moves from the center to surfaces whereby the temperature gradient between surfaces and inside is moderated or relaxed as shown by broken lines T₃T₄ in Fig. 3. Then, after that, the cooling is effected again in the subsequent cooling section 3, to form a temperature gradient as shown by solid lines T₅T₆ inside the fired products 10. The cooling proceeds further with repetition of such a cooling step alternately with a non-cooling step in the following. Thus, according to the present invention, since the cooling is carried out as intermittently moderating the temperature gradient between surfaces and inside of the fired products 10 in non-cooling steps, a strong cooling or quenching becomes performable which has been regarded as impossible due to the dunting in conventional continuous cooling processes. Further, it has heretofore taken, for example, about 5 hours to cool fired products at 1,200°C coming out of the firing zone down to about 600°C and, in contrast, the cooling can be completed in 2-3 hours according to the process of the invention. Consequently, the total length of a kiln such as a tunnel kiln can be shortened by no less than 5-10m and the production efficiency of kiln can be increased by about 10%. Besides, in the embodiment shown in Figs. 1 and 2, cooling sections 3 of broad width and non-cooling sections 4 of narrow width are defined and formed by and between both the walls 2 of the kiln so that fired products 10 can be subjected alternately to the cooling step and non-cooling step. However, such a width change of the cooling zone is not always necessary, but cooling sections 3 provided with cooling nozzles 5 and non-cooling zones 4 without cooling nozzles 5, both having the same width, may be formed alternately in a uniform width cooling zone. Alternatively, cooling nozzles 5 can be arranged along the entire length of the cooling zone, a part of which may be made either to stop or reduce the blowing gaseous fluid, to form non-cooling sections 4. However, if both strong cooling nozzles 5a and mild cooling nozzles 5b are provided in cooling sections 3 as shown in the embodiment so that strong cooling gaseous fluid may be impinged upon fired products 10 from right and left alternatingly with respect to the travel direction of the fired products, it is most advantageous in cooling uniformly all over the surfaces of fired products 10. Further, in the case of a periodic kiln wherein fired products 10 do not move, it is needless to say that if a gaseous fluid blower which is periodically variable in blown gaseous fluid quantity is employed, the cooling step can be repeated alternately with the non-cooling step, similar to the case of the above embodiment. As is clear from the above explanation, a rapid cooling of fired products is enabled within a cooling time as short as about half of that required in conventional processes without giving rise to dunting due to thermal stresses, by repeating two steps alternately in such a manner that, after cooling fired products in a cooling step, the temperature gradient formed between surfaces and the inside of the fired products is moderated in the subsequent non-cooling step, and then the fired products move again into the cooling step which follows. According to the process of the invention, the tunnel kiln for firing, for example, tiles, therefore can be shortened in length and also can increase its productivity by 10% or more. Accordingly, the present invention will largely contribute to the development of industry, as a process for cooling fired products in a kiln, which can overcome difficulties in conventional processes.

Claims: A process for cooling fired products in a cooling zone of a kiln (1), comprising repeating a cooling step in which a fired product (10) is cooled by blowing a cooling gaseous fluid at the fired product (10) in the kiln (1) alternately with a non-cooling or reduced cooling step in which a temperature gradient in the fired product (10) is moderated by stopping or reducing blowing of the cooling gaseous fluid at the product (10), wherein the kiln (1) is a tunnel kiln and said cooling zone is conjoined with a firing zone in said tunnel kiln (1), the cooling and non-cooling or reduced cooling steps are effected in longitudinally discrete cooling (3) and non-cooling or reduced cooling (4) sections, respectively, of said cooling zone, the cooling gaseous fluid being blown from a plurality of nozzles (5) in each said cooling section (3) of said cooling zone on at least one side of said cooling zone. A process according to claim 1, wherein said nozzles (5) include strong-cooling nozzles (5a) which blow cooling gaseous fluid at a rate of 1-2m³ /min/nozzle and mild-cooling nozzles (5b) which blow cooling gaseous fluid at a rate of not more than 0.5m³ /min/nozzle. A process according to claim 2, wherein the strong- (5a) and mild- (5b) cooling nozzles are arranged alternately on said at least one side of said cooling zone. A process according to claim 2 or claim 3, wherein the nozzles (5a,5b) are provided on two opposite sides of said cooling zone, the strong-cooling nozzles (5a) on one side of the cooling zone being opposite mild-cooling nozzles (5b) on the other side of said cooling zone. A process according to any one of claims 2 to 4, wherein in each cooling section (3) strong-cooling nozzles (5a) are provided on one side only of that cooling section (3), the strong-cooling nozzles (5a) in a given cooling section (3) being on the opposite side of the cooling zone from the strong-cooling nozzles (5a) in a neighbouring cooling section (3), whereby cooling gaseous fluid is blown from left and right alternately in successive cooling sections (3) with respect to the direction of travel of the fired product (10) through the cooling zone. A process according to any preceding claim, wherein the non-cooling or reduced cooling sections (4) are narrower than the cooling section (3). A process according to any preceding claim, wherein fans (7) are provided in the cooling sections (3) for circulating the atmosphere therein.

Inventors: TERASHIMA, TOMONARI
Applicants: NGK INSULATORS, LTD.
Application year: 1987
Main classification: F27B9
IPC codes: F27B9