US3471135A

US3471135A - Tunnel kiln for firing ceramics

Info

Publication number: US3471135A
Application number: US695788A
Authority: US
Inventors: Gottfried Cremer
Original assignee: Fetok GmbH
Current assignee: Fetok GmbH
Priority date: 1967-01-04
Filing date: 1968-01-04
Publication date: 1969-10-07
Anticipated expiration: 1986-10-07
Also published as: GB1193699A; SE332047B; FR1549927A; DK119892B; ES348904A1

Description

Oct. 7, 1969 e. CREMER 3,471,135

TUNNEL KILN FOR FIRING CERAMICS Filed Jan. 4, 1968 2 Sheds-Sheet 2 INVENTOR. 607569450 095M532 United States Patent US. Cl. 263-28 14 Claims ABSTRACT OF THE DISCLOSURE The specification describes a tunnel kiln for firing ceramics in which the ceramic articles are placed without any seggars or the like and are heated by radiation from overlying steel tubes with internal gas burners.

The present invention relates to methods for firing ceramics in tunnel kilns and to such kilns.

If it is not possible to place it directly on the horizontal carrying faces of trolleys used for transport through such kilns, ceramic material is generally placed in firing auxiliaries such as seggars.

Firing without the use of such firing auxiliaries would have substantial economic advantages because heat would not be required to heat the auxiliaries and also there would not be the expense of producing them, storing them, putting them together, and stacking on the trolleys.

One object of the present invention is to provide a method of firing ceramics which can be carried out without using such firing auxiliaries.

The invention consists in the improvement, in a method of firing ceramics in a tunnel kiln in which the ceramic material is arranged on firing trolleys, passed through a kiln passage having a heating zone, a firing zone, and a cooling zone, that the ceramic objects are placed in a single layer, without using firing auxiliaries, on the trolleys and are heated up and fired exclusively by radiation, and are then cooled.

The use of heating by radiation for firing ceramics is already known but only in connection with bell or hood kilns in which the ceramics to be fired are placed on a platform and a bell or hood provided with radiation elements, such as electrical infrared heaters, is placed over them. If they have a small height the objects to be fired using firing auxiliaries.

The common general knowledge in the ceramics industry does therefore not include any method which allows firing to be carried out with the economic advantages menioned above. This is the case not only of high quality, special articles such as special types of insulators but also simple mass-produced articles such as tableware and tiles. Moreover it is recognized that to achieve the advantages of heating by radiation, that is to say a kiln atmosphere practically free of currents and an evenness of firing which can hardly be excelled, the firing operation would have to be carried out in some special manner which, prior to the present invention, could not readily be foreseen.

One reason that the present invention cannot be considered as an obvious adoption of principles or possibilities which were already known is that the ceramics specialists would have strong objects to placing ceramic goods to be fired in a single layer on the trolleys because the load on trolley would then be substantially reduced. It has however been found, surprisingly, that this disadvantage ceases to be of importance because by practicing the method of this invention, the ceramic materials to be fired can be subjected to the proper sequence of temperatures required for firing more precisely and can be fired and cooled in a substantially shorter time. In other words,

3,471,135 Patented Oct. 7, 1969 it is possible to follow temperature-time curves as determined in the laboratory in large scale production.

In the case of ceramic goods arranged in a number of layers for firing using firing auxiliaries such as seggars,

it is not possible to follow the temperature-time curve which is just suflicient for satisfactory firing because there is a temperature drop due to the arrangement of goods on the trolley or other support and as a consequence the travel throug the kiln is made longer.

Preferably the method in accordance with the invention is so carried out that the trolleys are moved forward in feed steps corresponding to the length of a trolley or the length of a trolley multiplied by a whole number.

Since the temperature-time curve as determined in the laboratory largely ignores the necessity of avoiding the internal stresses created in ceramic articles of a comparatively large size due to thermal expansion and contraction. Nevertheless, ceramic goods of different shapes and sizes must be placed on the kiln trolleys for economic reasons. Continuous feed or movement of the trolleys through the kiln would bring with it the danger that one part of the ceramics to be fired would be exposed to heat radiating means having a temperature different from that to which ceramics on another part of the trolley were exposed resulting in damage. This would especially be the case at temperatures critical for the raw material in the ceramics. For instance, one critical temperature is the quartz transition point. Although the rate of feed in the train of trolleys cannot be made dependent upon the individual shapes of the articles, the trolleys can be moved in steps at least equal to the length of a trolley, and, after each step has been completed, the trolleys are held stationary a sufficient time to ensure that the larger ceramic articles are exposed to the heat source a sufficient time for them to reach the required temperature.

A tunnel kiln for carrying out the process in accordance with the invention preferably comprises transverse steel tubes arranged below the lower surface of the roof, the distance between the steel tubes and the top surfaces of trolleys for carrying a single layer of ceramic articles being small in comparison with or a minor portion of the breadth of the tunnel of the kiln.

The steel tubes can be heated internally with liquid or gaseous fuel and if required may be connected together at their waste gas outlet ends, if this should be economically advantageous.

If such steel tubes are connected together in this manner, heating or cooling devices can be provided between them.

In accordance with a further preferred feature of the invention, the kiln is divided lengthwise, by means of depending transverse walls extending close to the level of the tops of the ceramic articles to be placed on the trolleys. These walls divide the tunnel into sections of equal temperature or kiln atmosphere equal in length to the length of one or more trolleys. The length of these sections of even temperature and/or atmosphere depends upon the length of the tunnel kiln. For instance, they can be equal the length of three trolleys, or in the case of very long kilns, of five or more trolleys. Thus, these walls divide the tunnel into both zones and subzones of the main zones. By the procedure of moving the trolleys forward in steps, the trolleys are not brought to rest with one trolley located partly in one section and partly in another. The drive mechanism for the trolleys includes any conventional means which will move them one or more trolley lengths during each trolley movement and will coordinate this movement with the several zones and subzones of the kiln tunnel so that the trolleys between movements will always be entirely within one of the zones and subzones when stationary.

Finally, in accordance with a further feature of the invention, exhaust vents, with opening and closing means, can be provided in the roof of the tunnel kiln in order to draw off gases resulting from the decomposition of organic materials due to the heating of the ceramics, which gases are not desirable in the kiln atmosphere. This also prevents the build up of excess pressure in the kiln which might be caused by the expansion of air or gas on heating and which might give rise to currents along the length of the kiln. Means may be provided for regulating the degree of opening of the exhaust vents.

Moreover the vents can be used for altering the composition of the kiln atmosphere, more especially in the case of firing under reducing conditions.

The invention will now be further explained with reference to the accompanying drawings.

FIG. 1 is a horizontal section of a tunnel kiln in accordance with the invention;

FIG. 2 is an enlarged longitudinal section taken along the line IIII of FIG. 1; and

FIG. 3 is a sectional elevation view taken along the line IIIIII of FIG. 1.

FIG. 4 is a longitudinal sectional view similar to FIG. 2 showing the drive means for the trolleys;

FIG. 5 is a sectional elevation view taken along line V-V of FIG. 4.

In the horizontal, longitudinal section of FIG. 1, reference A denotes the pre-heating zone into which ceramic articles 3 on a trolley 2 are introduced through the kiln opening 1 and pre-heated to a temperature approximately equal to the firing temperature.

Reference B denotes the firing zone or section which is followed by the cooling zone C.

In the kiln the individual trolleys are only shown diagrammatically and the articles based on them are only shown in FIGS. 2 and 3.

Along its whole length, the kiln is provided with radiation burners 4 responsible for pre-heating in the preheating section A, for firing the material in the firing section and providing for controlled cooling of the fired articles so that they emerge from the outlet end 5 of the tunnel kiln at a temperature slightly above that of the outside atmosphere. The tunnel kiln has conventional side walls 6 and 7 and a roof 8, see more particularly FIG. 3.

Reference numerals 9 and 10 denote waste gas discharge tubes which run along above or to the side of the roof 8 of the tunnel kiln and lead waste gases from the burners to a flue.

As can be seen from FIG. 2, two trolleys 11 and 12 carry ceramic articles in the form of plates 13 or cups 14. These are merely exemplary of the types of articles which can be fired by means of this invention. The trolleys run on wheels 15, 16, on rails 17.

The wheels 15 are mounted on axles 18 carried in bearings 19 which are connected in a suitable manner with the chassis of the trolleys 11.

As shown in FIGS. 4 and 5, a drive means 40 consisting of a rack 41 and pinion 42 driven by a motor 43 is provided below the trolleys 11. The motor reciprocates the rack 41 the length of one trolley on each cycle of operation. With a longer rack, the movement could be two or more trolleys. Projecting above the rack 41 is a dog 44 which engages the trolley axle 18. The dog 44 is hinged at 45 so that it can swing to pass under the axle when the rack moves to the right as illustrated, and has a stop 46 holding it erect when the rack moves to the left. It is biased into erected positon by suitable means such as a spring 47. This drive is conventional, long used in the conveyor industry.

As can be seen from the somewhat simplified view in FIG. 3, a radiant burner used in accordance with the invention has a gas nozzle surrounded by a tube 21 and supplied with gas through a duct 22. The tube 21 is supplied through an annular space 23 with combustion air and the other end of the tube is some distance from the opposite wall 7 of the tunnel kiln. It is surrounded by an outer tube 24 which opens into a chamber 25. In this chamber 25, the inner tube 21 is provided with heat conducting fins 26 to transfer heat from the gaseous products of combustion flowing through the outer tube to the flowing air 23 so as to pre-heat it. The waste gases emerge from the housing 25 while a connecting tube 27 can pass into the main waste gas or line tube 10 which leads to a chimney.

As can be seen from FIGS. 2 and 3, the roof 8 of the kiln is provided with openings 28 which can be closed .partially or completely by closing means in the form of slabs 29.

Reference 30 denotes one of the partitions which divide the inside of the kiln into a number of sections, each equal the length of one or more trolleys. The partitions 30 are shown in broken lines in FIG. 1. They are mounted in slots in the side walls 6 and 7 of the kiln so they can be removed and replaced by fresh partitions.

It has been found advantageous if the objects to be fired are placed on a support of fire-clay which can either be in a single piece or in a number of pieces fitted together, The fire-clay should have a specific gravity of less than one gram per cubic centimeter.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of firing ceramics in a tunnel kiln in which the ceramic materials are arranged on firing trolleys and pass through a kiln passage having a heating zone, a firing zone, and a cooling zone, the steps which include: placing the ceramic objects in a single layer on the trolleys, heating up and firing the ceramic materials exclusively by radiation in a substantially current free atmosphere, and cooling the ceramic articles.

2. The method according to claim 1 in which the trolleys are moved forward intermittently in steps, each step being of a length such that it is a multiple of the length of the firing chamber.

3. The method of claim 1 in which each of the zones is equally divisible by the length of one of the trolleys and the trolleys between each intermittent movement are wholly located in one of the zones.

4. In a method of firing ceramics in a tunnel kiln in which ceramic objects are transported on trolleys progressively through a kiln passage having a heating zone, a firing zone and a cooling zone, the steps which include: placing the ceramic objects in a single layer on each trolley; moving the trolleys intermittently step-by-step through the kiln passage; progressively increasing the temperature of the objects in the heating zone from room temperature to firing temperature; heating said objects exclusively by radiation in a substantially current free atmosphere at firing temperature as they pass through said firing zone until the largest of the objects has been held at firing temperature, a suflicient length of time to. properly fire it; progressively cooling the objects as they pass through said cooling zone until each has attained a temperature slightly above ambient room temperature.

5. The method of firing ceramics in a tunnel kiln as described in claim 4 in which the intermittent movement of the trolleys is of a length which may be evenly divided by the length of a single trolley and the trolleys when stationary are at all times located wholly within one of the zones.

6. A tunnel kiln for firing ceramics, said kiln having a roof and side walls defining a kiln passage, heat conducting tubes arranged below the surface of the kiln roof, heat generating means inside the tubes for heating them; conveying means arranged to move along the passage for carrying ceramic articles therethrough; said conveying means having means for supporting a single layer of ceramic articles; said passage being divided into conditioning chambers at each end and a main firing chamber therebetween separated from said conditioning chambers by partitions, said kiln characterized in that said firing chamber is a continuous space substantially sealed against the movement of gases therethrough.

7. A kiln according to claim 6 in which the tubes are connected together at waste gas outlet ends.

8. A kiln according to claim 7 in which heating or cooling devices are provided between the steel tubes.

9. A kiln according to claim 7 having transverse walls extending downwardly close to the level of the top of the ceramic articles placed on the trolleys, dividing the kiln lengthwise into sections of constant kiln conditions, each equal in length to the length of one or more trolleys.

10. A kiln according to claim 7 in which vents with openings and closing means are provided in the roof for drawing off gases.

11. A kiln according to claim 7 further comprising fireclay means supporting the articles to be fired on the trolleys, the fire-clay having a specific gravity of less than one gram .per cubic centimeter.

12. A kiln according to claim 6 in which the conveying means consists of a plurality of sections of equal length arranged in tandem and powered means for moving the conveying means lengthwise of the kiln passage, said powered means moving said conveying means intermittently, step-by-step; each step of said intermittent movement being a distance such that it may be divided equally by the length of one of the sections of said conveying means.

13. A kiln according to claim 6 in which the conveying means consists of a plurality of sections of equal length arranged in tandem and means affixed to said kiln in said kiln passage for dividing each of the chambers into subchambers, each of a length which may be divided equally by the length of one of the sections of said conveying means; means for coordinating the movement of each of the sections so that between each intermittent movement it remains stationary wholly within one of said subchambers.

14. A tunnel kiln according to claim 6 wherein said conveyor means is divided into sections of equal length and each of said conditioning chambers and said main firing chamber has a length which is an even multiple of the length of one of said sections, said conveyor being intermittently moved in a step-by-step manner with each movement being a multiple of the length of one of said sections.

References Cited UNITED STATES PATENTS 669,567 3/1901 Waterman. 2,899,189 8/ 1959 Matis et al 26328 3,227,782 1/1966 Gie et a1. 3,304,071 2/ 1967 Gentry 26328 3,314,666 4/ 196-7 Gajardo 26328 JOHN J. CAMBY, Primary Examiner