GB2245693A

GB2245693A - Roller kiln for ceramics

Info

Publication number: GB2245693A
Application number: GB9113945A
Authority: GB
Inventors: Friedherz Becker
Original assignee: Riedhammer GmbH and Co KG
Current assignee: Riedhammer GmbH and Co KG
Priority date: 1990-06-27
Filing date: 1991-06-27
Publication date: 1992-01-08
Also published as: GB9113945D0; ITMI911740A0; FR2664032A1; IT1248542B; ITMI911740A1

Abstract

In a roller kiln for the firing of ceramic ware (18), the kiln flue (24, 44) is subdivided by one or more intermediate ceilings made up of plate elements (35) and burners (50) are directed into the space (44) separated by the intermediate ceilings for indirect application of heat. Preferably the plate elements (36) are of silicon carbide. <IMAGE>

Description

ROLLER KILN The invention concerns a roller kiln for the firing of ceramic ware.

In the firing of ceramic ware, in particular ylazed ceramic ware, quality problems occur time and again, the appearance of the glaze being the foremost.

The cause of firing defects is frequently the kiln atmosphere, this being primarily due to partial pressure gradients in the kiln and uneven temperatures in different sections of the kiln.

In the pre-heating zone, for example, heating up must proceed evenly and constantly, since because of thermal effects the hot flue gases however flow predominantly into the upper region of the kiln flue, the lower parts frequently lay in the temperature rise. It has been attempted to minimise the problems by aimed direction of flue gases of kilns equipped with oil or gas burners.

A uniform burner setting is also particularly important in the incandescent zone. Temperature peaks must be avoided since they lead to local overheating. Apart from this, an incandescent temperature is required which is always constant, in order that the hardness of the ware and thereby the property of the glaze remain uniform.

With a stacked, i.e. compact charge, cooling is of particular importance, while a loose charge can be cooled simply and rapidly. With a compact charge a steady temperature reduction in the cooling zone is important. In this case temperature peaks must likewise be avoided.

Due to the burners operating with fossil fuels, the kiln atmosphere frequently changes in an uncontrolled manner so that glaze defects and possibly even deformations of the ware result.

The object of the invention is to improve a roller kiln of the appropriate type in respect of its firing properties and thereby to avoid the negative effects on the ware mentioned above. This applies particularly to the firing of glazed ware such as porcelain because the glaze is the most sensitive part with regard to fluctuations of heat and atmosphere.

The invention is based on the recognition that in a roller kiln of the appropriate type this obJect can be attained astonishingly simply by indirect heating effected at least in the sensitive zones of the kiln, whereby the transfer of heat relies exclusively on radiation. At the same time the invention makes it possible to convert known roller kilns in the sense of the invention.

In its most general embodiment the invention proceeds from a known roller kiln for the firing of ceramic ware, with a bottom, two side walls and a roof which together enclose a tunnel-shaped kiln flue, the oven being equipped with a multiplicity of heater units along the kiln flue and a multiplicity of rollers which together form a roller conveyor extending from the kiln entry to the kiln exit.

The kiln in accordance with the invention differs from this state of the art by the following features: - Intermediate ceilings are provided at least along a partial section namely either - at a distance above the roller conveyor and at a distance below the kiln roof and/or - at a distance above the kiln bottom and at a distance below the roller conveyor, - the intermediate ceiling is made of a material having good conductivity of heat, - the burners provided as heater units in this section of the kiln are - directed into the space between the intermediate ceiling and the kiln roof and/or - into the space between the intermediate ceiling and the kiln bottom.

In other words: the original kiln space is subdivided into several sections by an intermediate ceiling. The ware runs on rollers through the kiln in one section as previously and indirect heating takes place in the other section, now separated by the intermediate ceiling.

The oil or gas fired burners directed into the appropriate space heat the space between the intermediate ceiling and the roof or between the intermediate ceiling and the floor, as the case may be, and give off the heat through the intermediate ceiling exclusively in the form of radiant heat into the actual kiln space and consequently to the ware.

Not only is a favourable transfer of heat achieved hereby, but in particular the temperature throughout the crosssection of the kiln or the ware is made substantially more uniform.

The result of this is that in the firing of the glaze of ceramic work pieces less evaporation of the glaze is observed, whereby not only is the appearance of the glaze improved, but an environmental effect is achieved at the same time since for example lead oxides are normally evaporated which are harmful to the environment when conducted to the outside by the exhaust gases.

While DE-PS 35 10 754 also discloses an industrial kiln for the firing of glazed and unglazed ceramic work pieces with a transfer of heat relying at least in part on radiation, this is however effected by radiation burners directed directly into the kiln space. Such radiation burners require electrical energy which is expensive. The roller kiln in accordance with the invention can on the contrary be operated with conventional heater units such as oil or gas burners.

The advantages described are particularly evident in modern roller kilns which have a low height and a relatively great width of the kiln flue. In these kilns the ware is as a rule transported through the kiln flue in only a single layer. The radiant heat can consequently reach the ware through the intermediate ceiling by the shortest route and heat it uniformly.

The material of the intermediate ceiling should therefore have good conductivity of heat and at the same time high stability in order to span even great kiln widths. Suitable for this are primarily materials based on silicon carbide, for example, recrystallized silicon carbide or reactionsiliconized silicon carbide.

If the kiln ceiling is constructed in accordance with an advantageous embodiment of the invention from individual plate-shaped elements, it is readily possible to produce with the above mentioned materials individual elements of 2 to 3 m in length and 0.50 to 1 m in width as well as 5 to 20 mm in thickness, whereby the elements only have to be supported at their edges in the region of the side walls of the kiln.

It is likewise possible to place the elements on carriers extending across the width of the kiln from one side wall to the opposite side wall and anchored there.

In order to obtain a kiln ceiling which is as closed as possible, it is arranged in accordance with an advantageous embodiment that the edges of the plate-shaped elements which run at right angles to the direction of transport of the kiln are constructed so as to overlap one another. A particularly simple embodiment provides for the corresponding edges to be constructed with projections and recesses similar to a groove and tongue joint.

In this manner the intermediate ceiling can be made absolutely impermeable from the kiln entry continuously to the kiln exit.

Depending to the sphere of application it can however suffice only to construct individual zones of the kiln with one or more of the intermediate ceilings mentioned. This is preferably done in the firing zone.

In order to construct the firing space between the intermediate ceiling and the kiln roof or between the intermediate ceiling and the kiln bottom, as the case may be, in a closed manner, a further embodiment provides for the appropriate combustion space to be enclosed by vertical walls at the commencement and/or the end as seen in the direction of transport of the ware. It is self-evident that in this case exhaust ducts are provided which convey the combustion gases from the corresponding space.

Instead of a support for the plates (intermediate ceiling) these can also - when arranged below the kiln roof - be fixed to hang from this.

The operation of the individual rollers is not hindered by the construction in accordance with the invention; it can be effected in the conventional manner. This is normally done by the rollers being passed through the side walls of the kiln to the outside where they are supported and driven by one or more drives. In accordance with the invention the term roller kiln however comprises also such kilns as are for example described in DE-PS 35 15 856 or in the German Patent Application P 39 31 301.8. In these cases the drive of the rollers or the supports for the ware is effected through the kiln bottom, it being self-evident that in these embodiments only the arrangement of an intermediate ceiling below the kiln roof applies.The concept of the invention can in principle however also be realised in the case of conventional tunnel kilns with kiln trolleys as described above.

Further features of the invention are the subject of Subsidiary Claims and of the other application documents.

The invention is described below by means of three embodiments, there being shown in Figure 1: a cross-section through a roller kiln having an intermediate ceiling below the kiln roof Figure 2: a cross-section through a roller kiln having intermediate ceilings below the kiln roof and above the kiln bottom Figure 3: a longitudinal section through a roller kiln in which an intermediate ceiling below the kiln ceiling is provided only in the region of the firing zone.

Identical parts or those having the same function bear the same reference marks in the Figures.

The reference number 10 describes the roller kiln as a whole.

This has a bottom 19. side walls 14,15 and a roof 90 which together enclose the actual kiln flue 24.

Between the kiln roof 20 and the kiln bottom 19 are arranged a multiplicity of horizontally disposed rollers 12 which together form a roller conveyor. The rollers 12 pass through the side walls 14, 15 and are supported and driven on the outside at 16.

The ware 18 is conducted on the rollers 12 from the kiln entry 30 in the direction of transport (arrow l) to the kiln exit 32.

In accordance with Figure 1 an intermediate ceiling 34 is included which extends at a distance from the roof 20 and at a distance from the rollers 12 or the zal war'( 19 from one side wall 14 to the opposite side wall 15. The intermediate ceiling 34 consists of a multiplicity of plate-shaped elements 36 made of recrystallized silicon carbide which in this case each have a width (between the kiln walls) of 2 m and a length (in the direction of transport T) of 50 cm.The edges of the plate-shaped elements 36 running laterally to the direction of transport T are formed with corresponding projections and recesses 38 which respectively extend into each other so that a closed intermediate ceiling 34 is formed which is limited (in accordance with Figure 3) at the commencement and at the end (in the direction of transport T) by vertical walls 40, 42 which reach up to the roof 20 and form an enclosed combustion space 44.

Each plate-shaped element 36 lies respectively on two horizontal carriers 46 which are also made of recrystallized silicon carbide and extend from one kiln wall 14 to the opposite kiln wall 15 and are each anchored there at 48.

As may be seen in Figure 1, along the two side walls 14, 15 are arranged burners 50 (only schematically represented) which are disposed above the level of the intermediate ceiling 34 and are hence directed into the space 44. The burners 50 are gas fired. The air in space 44 is heated by the burners 50 and the heat is transferred through the intermediate ceiling 34 into the kiln flue 24 and hence to the ware 18 and this exclusively by radiation and uniformly over the entire cross-section of the ware.

The combustion gases are led away through an extraction duct (not shown).

The embodiment per Figure 2 differs from that per Figure 1 in that two intermediate ceilings 34 are provided. The upper intermediate ceiling 34 corresponds substantially to that per Figure 1; it is however in this case not supported on carriers 46, but the individual plate-shaped elements 36 are held by carriers 52 which are also made of recrystallized silicon carbide and are fixed to the kiln roof at one end and the plate-shaped elements 36 at the other end.

In addition a further intermediate ceiling 34 is included above the kiln bottom, the plate-shaped elements 36 lying on carriers 54 which are fixed in the direction of transport T below the intermediate ceiling 34 to the inner surfaces of the side walls 14, 15. These carriers are made of reaction-siliconized silicon carbide.

In the same way as the burners 50 previously described, further burners 50 are ranged along the region of the side walls 14, 15 in such a manner that they are directed into the space 44 formed between the lower intermediate ceiling 34 and the kiln bottom 19.

The kiln per Figure 2 is heated indirectly from above as well as from below. The transfer of heat is thereby made still more uniform.

In the case of the embodiment per Figure 3 it will be seen that the intermediate ceiling 34 extends only over a partial region of the kiln flue, namely in the region of the firing zone B between the heating up zone (section A) and the cooling zone (section K). The construction and function of the kiln in accordance with the invention have otherwise already been described above or correspond to conventional kilns. Further oil or gas fired burners or electrical heating elements are thus also provided in the other zones of the kiln.

Claims

CLAIMS:

1. Roller kiln for firing ceramic ware with a bottom, two side walls and a roof which together enclose a tunnelshaped kiln flue, a multiplicity of heater units along the kiln flue, and a multiplicity of rollers which together form a roller conveyor extending from the kiln entry to the kiln exit, wherein an intermediate ceiling made of material having good heat conductivity is provided between the roller conveyor and the kiln roof and/or between the kiln bottom and the roller conveyor at least along a partial section of the kiln flue and the burners provided as a heater unit in this kiln section are directed into the space between the intermediate ceiling and the kiln roof and/or the space between the intermediate ceiling and the kiln bottom.

2. Roller kiln in accordance with claim 1, wherein the intermediate ceiling(s) extend(s) only along the section of the kiln flue forming the firing zone of the kiln.

3. Roller kiln in accordance with claim 1, wherein the intermediate ceiling(s) extend(s) continuously from the kiln entry to the kiln exit.

4. Roller kiln in accordance with any of claims 1 to 3, wherein the space formed between the intermediate ceiling and the kiln roof and/or between the intermediate ceiling and the kiln bottom is so constructed as to he closed at the beginning and/or the end seen in the direction of movement of the ware.

5. Roller kiln in accordance with any of claims 1 to 4, wherein the intermediate ceiling(s) is (are) formed from plate-shaped elements.

6. Roller kiln in accordance with claim 5, wherein the plate-shaped elements lie on appropriate carriers.

7. Roller kiln in accordance with claim 6, wherein the carriers are formed by supports provided on the inside of the side walls of the kiln.

8. Roller kiln in accordance with claim 6, wherein the carriers extend horizontally from one side wall laterally through the kiln flue to the opposite side wall and are anchored in the region of the side walls, whereby at least two carriers serve to carry one plate-shaped element in each case.

9. Roller kiln in accordance with claim 5, wherein the plate-shaped elements are fixed to the free ends of holding elements extending downwards from the kiln roof.

10. Roller kiln in accordance with any of claims 5 to 9, wherein the plate-shaped elements overlap at their edges runniny laterally across the kiln flue.

11. Roller kiln in accordance with claim 10, wherein the plate-shaped elements are formed at their edges running laterally across the kiln flue with corresponding projections and recesses to form a closed intermediate ceiling.

12. Roller kiln in accordance with one of the claim 5 to 11, wherein the plate-shaped elements have a thickness of 5 to 20 mm.

13. Roller kiln in accordance with claims 1 to 12, wherein the intermediate ceiling(s) or their elements and/or the carrier/holdiny elements are made of a temperature resistant ceramic material having good heat conductivity.

14. Roller kiln in accordance with clairn 13, wherein the plate-shaped elements and/or the carriers/holding elements are made of silicon carbide, re-crystallized silicon carbide or reaction-siliconized silicon carbide.

15. Roller kiln in accordance with one of the claims 1 to 14 wherein at least one exhaust duct leads away from the space formed by the intermediate ceiliny(s).