GB2207494A - Intermittent kiln - Google Patents

Abstract

An intermittent kiln for firing bricks and similar ware has main horizontally directed combustion product jets 14 firing into gaps between stacks 11 of the ware and vertically downwardly directed rapid cooling jets 16 directed from the roof of the kiln into the gaps. In use the downwardly directed jets are actuated whilst the main jets are being actuated and provide more even heating of the ware. The downwardly directed jets are preferably heated and may be in the form of non-recuperative and non-regerative burners. <IMAGE>

Description

INTERMITTENT KILN FIELD OF THE INVENTION The present invention concerns kilns especially for the batch processing of bricks.

BACKGROUND OF THE INVENTION Bricks are processed in kilns and have first to be heated to a temperature at which carbon in the bricks is burnt out and then at a higher temperature to bake the bricks. There are two main types of kiln used.

For smaller quantities, a so-called intermittent kiln is used in which the bricks are stationary during firing, and for larger quantitIes a so-called tunnel kiln in which the ware travels through a tunnel is used.

Such kilns are often heated by high velocity jets of combustion products directed horizontally between stacks of bricks. It is known to cool the bricks after firing. Particularly with the intermittent kilns, it is difficult to heat up a load evenly and so the load has to be soaked at a high temperature for a long time. If a reduction in the soak time can be achieved, there would be many advantages ; first there would be a reduction in the fuel consumed, second a kiln could be used for a shorter period for each load so that a greater throughput could be achieved enabling an intermittent kiln to be used in-stead of a more expensive tunnel kiln or enabling fewer kilns to be used, and it would be possible to fire different types of brick requiring different heat treatments.Some reduction in cycle time can be achieved by cooling the bricks after firing and this can be done by downwardly directed rapid cooling air jets. However the cooling time is relatively small and the saving in cycle time is best done during the heating periods but this can only be achieved if the ware could be heated more evenly.

SWOtLmY or TEE PRESE; Ir\TEITION According to one aspect of the present invention a method of heat treating bricks and similar ware in a kiln provided with horizontally directed combustion product rests and douma-ardly directed rapid cooling rests is characterised in that the rapid cooling 3ets are actuated during the period of actuation of the combustion product rests in such a way that the heating of the ware is made more uniform.

It is not understood why the ware is heated more uniformly by actuating the rapid cooling rests. It may be that a swirl is set up in a vertical plane and then it may be better if the rapid cooling rests alternated with the combustion product rests so as to have the usual swirling in a horizontal plane interspersed with swirling in a vertical plane, it may be that the rapid cooling rests form local barriers to the combustion product Uets generating enhanced turbulence, it may be that the downwardly directed ets "nail" the combustion product åets to the base on which the ware is supported counteracting the tendency of the combustion product rests to rise under a convection effect or a combination of these.Even with heated dounlNardly directed rests there is a more even heating of the ware; it is thought that the åets pick up heat as they pass do-vm through the hottest, top, part of the kiln partly by aspirating some hot air so they do not chill the ware and so the hottest ware and the hot top of the kiln is cooled. However it is preferred to heat the downwardly directed rests to give a further adjustment so,when setting up a kiln, commissioning tests on a monitored load can adjust not only the flows but the temperature of the downwardly directed rests.

According to another aspect of the present invention an intermittent kiln having horizontally directed combustion product rests and dow'nwardly directed rapid cooling aets is characterised in that the downwardly directed rests have associated with them heating means.

The heating means can be fuel burners but to enable the rapid cooling rests to serve their main function it is preferred that these me ans are neither regenerative nor recuperative burners.

BRIEF DESCRIPTION OF TEE DRAWINGS Figure 1 is a plan view of a conventional load of bricks on which the positions of horizontally and downwardly directed rests are indicated, and Figure 2 is a schematic section of one of the downwardly directed rests.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Standard bricks are about 3 x 4i x 9 inches (70 x 110 x 220 mm.)whichfor firing are arranged in a kiln in stacks or packs of about 400 bricks each. Each layer of bricks a stack is formed of parallel lines of bricks end-on with the lines in alternate layers being lengthwise or crosswise of the kiln so that a brick in an upper layer rests on and bridges between two lower bricks. A firing dyke or gap is left between each stack in the lengthwise direction of a kiln with a smaller gap between the stacks across the kiln.

Figure 1 shows a 5 x 3 array of stacks 11 supported on a kiln hearth or base plate 12 within a kiln 13, which is heated by gas burners 14 directing rests of combustion products at high velocity into the firing dykes or gaps 15 extending across the kiln (these gaps are about IG cm. wide). The burners can be regenerative burners in which one burner acts as the source of heat and other as an exhaust extracting waste heat to be used later,when the roles of the burners are changed over, to heat the combustion air. Alternating the burner in use leads to more even heating, as well as recovering exhaust heat. Self-recuperative or even burners without waste heat recovery may be used to provide the heat, instead of the regenerative burners. The kiln is also provided with downwardly directed jets 16 for rapid cooling of the stacks. These dents are at relatively high velocity but about half that of the zets from the burners 14. These rests 16 are directed at the end of the gaps 15 at the ends of 1 cm wide gaps 17 between the stack across the kiln and at the intersections between the gaps 15 and 17. Not all of these rests 16 may have to be used during the heating period in accordance with the present invention to get a more uniform heating of the load, it is really a matter of trial and error. However use of downwardly directed jets during the heating period has shown that acceptable product can be attained within a twenty-four hour cycle compared-to the more usual thirty-eight hour cycle.

this shorter cycle time may be reducible still further by increasing the free oxygen content within the kiln by supplying more air, by injecting oxygen, or pressurisation; by using spacers between the bricks so air can reach where one brick rests on others; or by having a fluctuating pressure within the kiln to suck and blow air between contacting bricks.

It Is to be understood that references to dot wardly directed and horizontally directed are not to be construed strictly literally; it may for example be better that the horizontally directed jets have a vertical component causing the rests to impinge on the kiln hearth.

It has been found to be advantageous to heat the downwardly directed jets but these jets must not then be regarded as overhead burners since the amount of heat supplledtothese jets and the flow through them is relatively insignificant, the heat supplied by the downwardly directed jets to the kiln would be less than one-fifth of the total heat and more usually as low as 5 to 10% of the total heat. Figure 2 illustrates a modification to the normal downwardly directed rapid cooling jets. Each,or some of the jets, incorporate a straightforward gas burner 21.

To get the full advantage of the present invention, the kiln is commissioned with the use of a monitored array with thermometers at various locations and then the various flows and temperatures of the jets adjusted to give as uniform a heating rate as possible. If the kiln is to be used for different varieties of ware, commissioning tests will be necessary for each duty cycle especially if the duty cycles differ in regard to the desired firing temperatures.

Claims (6)

1. A method of heat treating bricks and similar ware in a kiln provided with horizontally directed combustion product jets and downwardly directed rapid cooling jets is characterised in that the rapid cooling jets are actuated during the period of actuation of the combustion product jets in such a way that the heating of the ware is made more uniform.

2. A method according to claim 1 further characterised in that the downwardly directed jets are heated whilst the horizontally directed jets are actuated.

3. A method according to claim 2 wherein the downwardly directed jets are in the form of burners which are neither recuperative nor regenerative.

4. An intermittent kiln having horizontally directed combustion product jets and downwardly directed rapid cooling jets characterised in that the downwardly directed jets have associated with them heating means.

5. A kiln according to claim 4 wherein the downwardly directed jets are in the form of burners which are neither recuperative nor regenerative.

6. A kiln substantially as herein described with reference to Figure 1 of the accompanying drawings.