Description:
BACKGROUND OF INVENTION
1. Field to which invention relates
The invention relates to a tunnel kiln, more particularly for the ceramic industry, adapted to be heated with gaseous, atomized or gasified liquid fuel, in the case of which the fuel is supplied to at least one horizontally arranged pair of burners, which are directed from the lateral limits of the tunnel kiln and in its lower part towards each other and burn in separating channels underneath the stacking surface of kiln vehicles adapted to pass through the tunnel kiln.
2. The prior art
If in a large combustion space fired with gaseous, atomized or gasified liquid fuels a relatively high temperature is to be produced and maintained at all points of the firing space, very substantial difficulties occur, which to date have not been completely satisfactorily dealt with.
Ceramic firing kilns have for example volumes of up to several hundred cubic meters and in the case of tunnel surfaces or kilns there may be clearance widths of 6 meters and more. The firing temperature in the case of such kilns lies at 1,250°C for example and a discrepancy in the firing temperature of ±1/2 Sc (Seger cone) can be permitted. The permissible temperature departure or discrepancy at every position of the kiln or kiln cross-section in the case of tunnel kilns thus lies in the order or magnitude of ±10°C, a requirement which to date has not been properly fulfilled.
The problem of maintaining and producing an eveness in temperature at every position of the firing space is one which has existed for a long time and many different attempts have been made to solve it.
In the case of conventional burners for gaseous, atomized or gasified liquid fuels there is at the burner nozzle which is directed into the interior of the firing space a relatively high, concentrated heat development, which hardly is propitious for the requirement of evenness in temperature at all points in the firing space or firing space cross-section to be heated. By the arrangement of such burners in burner chambers or burner niches and thus at some distance from the material to be fired attempts were made to achieve an equalization of the high temperature, produced at the burner nozzle, with a temperature of the combustion gases in the firing space and thus before direct contact of the hot combustion gases with the material to be fired.
Furthermore, burners have already been developed, whose particular characteristic resides in that they operate with relatively high feed pressures both of the gas and also of the combustion air so that very high feed speeds result.
Owing to the use of the high velocities there is a very pronounced eddying of the kiln atmosphere in the firing space and there is thus a reduction of temperature discrepancies from the desired value at the individual points in the firing chamber or space. More particularly in the case of large firing spaces, however, such burners cannot be used successfully, because the high feed velocities of the combustion gases passing into the firing space lead to the production of an annular eddying effect in it so that in the center of the firing space there is a zone of relatively little movement, around which the kiln atmosphere circulates. It will readily be appreciated that as a result there are necessarily temperature differences in the individual parts of the firing space.
Furthermore, even if, as is the case in practice, two mutually opposite burners are used which are directed against one another, the problem cannot yet be solved. This is because in this case two axes of relatively little movement are produced in a tunnel kiln and the combustion gases circulate in an annular eddy around the axes. Although in this manner it is possible to produce an improvement as compared with operating with only one such high velocity burner, a completely satisfactory result is nevertheless not completely attained.
Furthermore, there has also been a proposal to use a kiln in which flames are directed from the top of the kiln into spaces between the material to be fired. In this connection there has also been a proposal to drive the burners producing these flames in such a manner that the burners or groups of burners inject intermittently and in an alternating manner, the pulses being so set that within three groups the flame width increases so that the total burner cross-section is supplied in the most even possible manner with prepared fuel.
In such tunnel kilns the desired effect, however, only occurs if the burners inject into the spaces between the material to be fired. This, however, means that such tunnel kilns cannot be operated continuously but only in batches.
Furthermore, a process has become known in which pulsed burners are used, in the case of which there is practically no atomization. These pulsed burners have a preestablished length of trajectory, which may if necessary be adjusted, the power of the burner not being determined by changing the length of the trajectory, but by the number of the particles propelled along this trajectory per unit time, or by increasing or reducing the quantity of oil injected per pulse by suitable change in the times of opening of the burner nozzle. In both cases there is no change therefore as regards the length of trajectory, which as far as possible should extend over the whole breadth, for example, of one tunnel kiln. In this connection therefore attempts have already been made to influence the end point of the length of trajectory of the burner mechanically by causing the jet of fuel to impinge on a surface which is arranged to be oblique to the direction of the jet. This method, however, involves the disadvantage that at the point of impingement the oil partly collects at the cooler point of impingement and flows downwards so as to lead to clogging deposits. Furthermore, the burning jet is only deflected but not influenced as regards its length of trajectory to a substantial extent. Since pulsed burners furthermore lead to substantially even combustion along the whole length of the trajectory the problems of any formation do not occur at all.
A further method for heating firing spaces with gaseous, atomized or gasified liquid fuels have been proposed in which it is possible to bring about such an equalization of the temperature that the requirements existing today in the case of the firing of ceramic material can be fulfilled in an optimum manner. This method resides in that the power of the one burner of a pair of burners is periodically and uninterruptedly varied from a minimum to a maximum and back again and simultaneously the power of the other burner of the pair of burners is so regulated between a maximum and a minimum that the fuel gases come up against each other, the sum of the powers of both burners of the pair of burners is always substantially constant and the point of engagement of the fuel gases moves backwards and forwards over the whole breadth of the firing space (see German Pat. Specification No. 1,558,041).
With this method it is possible to prevent in a reliable manner the formation of stationary vortices within the firing space so that such vortices can no longer interfere with an equalization of temperature distribution.
SUMMARY OF INVENTION
One aim of the invention is that of creating a tunnel kiln in which, while still making use of this principle of firing, the use of regulated burners which are matched in a paired fashion as regards regulation can be eliminated without any substantial effect on the results of this principle of firing.
The invention consists in a tunnel kiln adapted to be heated with gaseous, atomized or gasified liquid fuel, in the case of which the fuel is supplied to at least one horizontally arranged pair of burners, which are directed from the lateral limits of the tunnel kiln and in its lower part towards each other and burn in separating channels underneath the stacking surface of kiln vehicles adapted to pass through the tunnel kiln, characterised in that the burners directed towards each other of some pairs of burners are offset along the direction of travel of the firing vehicle in the tunnel kiln channel by a distance which is equal to at least the thickness of the partition between the firing channels and is at the most equal to the clearance firing channel width.
Preferably, the arrangement is such that the partitions are rounded off or blunted at their ends adjacent to the tunnel kiln channel side walls.
With the principle of the invention it is possible to cause the firing vehicle to pass through the tunnel kiln channel or passage constantly with the exception of the time of change in feed, something which thus constitutes a significant feature of the present invention.
In accordance with a further development of the invention the partition walls preferably have openings in the direction of travel, or on a build up of individual bricks arranged with a spacing between them.
If, in a tunnel kiln, a large number of pairs of burners is required in order to achieve the desired firing effect in accordance with the particular sensitivity of the material to be fired it is not always necessary to offset all operating pairs of burners directed towards each other, in the direction of travel of the firing vehicles in accordance with the feature of the invention. It can be sufficient if such an offsetting only applies for part of the pairs of burners, though in general at least half the number of pairs of burners should be offset.
LIST OF SEVERAL VIEWS OF DRAWINGS
The invention will now be described with reference to the accompanying drawings.
FIG. 1 shows a side view, drawn partly in section, of a tunnel kiln, in which the principle of the invention is employed and in which, furthermore, for shortening the length of the figure sections of the tunnel kiln not necessary for explaining the principle of the invention are omitted.
FIG. 2 shows a plan view, partly shown in section, of the tunnel kiln in accordance with the representation in accordance with FIG. 1.
FIG. 3 shows an enlarged horizontal section through the part shown in section in FIG. 2.
FIG. 4 shows a position, in accordance with FIG. 2, which is offset with respect to the position of FIG. 3 by half the breadth of a firing channel or passage.
FIG. 5 depicts an alternate embodiment of the present invention and further illustrates a further positioning of the partitions employed in the practice of the present invention as related to the positioning of the burners.
FIG. 6 shows the part X in FIG. 3 on an enlarged scale.
DESCRIPTION OF PREFERRED EMBODIMENTS
The tunnel kiln in accordance with FIGS. 1 and 2 is of conventional construction apart from the particular arrangement of the burners. The firing vehicles or trucks 3 at the inlet end 4 of the kiln run on rails 2 passing through the tunnel kiln 1 into the latter. The vehicles are driven into the kiln by means of an advancing system which does not form part of the present invention and is therefore not shown. The firing vehicles 3 run on pairs of wheels 5, 6 and on their respective base plate 7 carry a system made up of partitions 8.
On the stacking surface 9 of the respective firing vehicles 3 the material 10 to be fired is stacked and can, for example, be ceramic drainage pipes, sanitary ware, porcelain ware, etc. Since the particular type of the material 10 to be fired is not of significance for the principle of the invention, the material or charge 10 to be fired is only shown as a block and not shown in detail.
The firing vehicles in the tunnel kiln 1 pass through a preheating zone, a firing zone and the cooling zone. It is only in the firing zone 11 that burners 12 are provided, whose distribution in relation to the dimensions of the firing channels 13, remaining between the partitions 8, is of significance for the invention. The burners 12 are arranged in niches or openings 14 in the side walls 15 of the tunnel kiln 1, whose kiln channel 16 is closed at the top by upper masonry 17. Below the rails 2 there is generally an access passage or channel 18. At 19 the firing vehicles leave the cooling zone of the tunnel kiln 1.
It can be seen from the enlarged representation in accordance with FIG. 3 that the two burners 12 and 12a of each burner pair which are directed towards each other and are only indicated by a dot-dash line as the main direction of burning, are admittedly offset in relation to each other, but burn in the same firing passage or channel under the stacking surface 9 of the respective firing vehicle 3. In this respect the flames come together in the middle as it were so that two mutually parallel eddy rings are formed with gases which rise in the middle and descend at the walls of the tunnel kiln. Reference numeral 20 denotes the lateral limiting walls of the tunnel kiln in which the niches or openings 14 are provided for receiving the burners 12, 12a.
Once the particular firing vehicle has advanced by a distance equal to approximately half the breadth of the firing channel 13, the flames of the two burners 12 and 12a (see FIG. 4) of the corresponding pair of burners are temporarily separated by the partitions 8 between two firing channels 13. The burners can therefore so burn that their flames extend to their full length. The annular eddies formed rise at the tunnel kiln wall opposite to the burner and at the tunnel kiln wall adjacent to the burner descend so that two oppositely directed adjacent annular eddies are formed which occupy the whole breadth of the kiln. The desired equalization of the atmosphere is thus achieved, owing to the fact that the annular eddies change over from a shape, which occupies half the breadth of the tunnel kiln, into a shape which occupies the whole of the tunnel kiln breadth so that the above-mentioned prior art principle is carried out in two stages instead of continuously.
In the figures the course of the burner flames is naturally only indicated diagrammatically by the dot-dash lines. In reality there is naturally a certain enlargement of the diameter of the currents consisting of the hot gases after the emergence of the combustion gases from the burners 12, 12a so that in particular with respect to the representation of FIG. 3 there is a more or less pronounced increase in the diameter of the gas currents with their distances from the burner nozzles.
Furthermore, the partitions can be made blunted or rounded at their two ends in order to facilitate the flow-in of the gas currents as the partitions are moved past the burners. Such a blunting is clearly shown in FIG. 6 at 21.
It is advantageous for the success of this measure if the tunnel kiln is operated practically continuously, that is to say the firing vehicles are kept practically continuously on the move and only short stationary periods occur during the adding of a further firing vehicle at the inlet end.
The firing channels 13 or passages are open at the top so that the firing or combustion gases can pass through openings into the actual firing space 16. Furthermore, the partitions 8 between the firing channels 13 can be provided with openings 22 without this involving any change in the basic principle of the invention. Such openings are shown in FIG. 5 in a clear manner.