US3485900A - Kiln operation - Google Patents

Description

Dec. 23, 1969 G. CREMER ET'AL 3,485,900

KILN OPERATION Filed Feb. 5, 1968 4 4 Sheets-Shet 1 INVENTORS Dec. 23, 1969 G. CREMER ET AL 3,485,900

KILN OPERATION Filed Feb. 5, 1968 4 Sheets-Sheet 4.

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F \F I IR H A 1 1 INVENTORS am ne/0 a e vie AKA/5 HS United States Patent Int. Cl. F27b 9/ 24; F27d 3/12 U.S. Cl. 263-52 14 Claims ABSTRACT OF THE DISCLOSURE A tunnel kiln in which oppositely placed burners produce streams of combustion gases making a loop-shaped pattern, and the opposite burners are alternately turned gradually off and on so that the loop pattern is moved transversely across the interior of the kiln.

The present invention relates to the heating of kilns, particularly those of the tunnel variety, used for the firing of ceramics, for example.

One important problem to which the present invention is addressed is that of achieving and maintaining a substantially even temperature in a kiln or furnace heated by burner means using gaseous or liquid fuel. Until the present time, this problem has not been satisfactorily solved.

Pottery or ceramic kilns typically have very large volumes often on the order of several hundred cubic yards. The required firing temperature in such kilns may for example easily be as much as 1,250" C., with a permitted departure of only 1 /2 segar cones. The allowable temperature variation at any given part of the kiln or kiln cross-section thus is about i C., a requirement which is very difiicult to meet in actual practice. With kilns having substantially smaller firing spaces, for instance in the case of trolley-type kilns having a volume less than 10 cubic yards, it is very diflicult to insure that the temperature is uniform throughout the firing space.

In the case of conventional burners using gaseous or liquid fuels, there is a relatively small zone of heat localization and build-up immediately in front of the burner nozzle, and this represents a substantial factor tending to prevent the achievement of even and uniform temperature throughout the firing space. In the past such burners have been arranged in burner chambers or recesses located in the Walls so as to maximize their spacing from the articles to be fired, in hopes that an equilibrium could be achieved between the high temperature at the burner nozzle and the gases formed by combustion in the burner space. Unfortunately, these measures have failed to solve the underlying problem.

Thus, other attempts to solve this problem have been made. The so-called jet or beam burners which have in the past developed are an example of other approaches to the solution of the problem. These burners operate with a comparatively high pressure at the nozzle, both of the gas and also of the combustion air, so that there is a high speed injection of air and gas into the kiln. Owing to this high speed there is a very pronounced eddy formation in the atmosphere inside the firing space. so that there is less variation in temperature throughout the firing chamber. However, in the case of kilns with large firing spaces, the use of these burners does not achieve the required substantially even temperature. This is because the high blow-in or injection speeds of the gases from the burners produces an annular eddy in the firing space with comparatively still or quiescent conditions in the center of the firing chamber, i.e., the move- 3,485,900 Patented Dec. 23, 1969 ment of the kiln atmosphere is restricted to a circular peripheral one. It is apparent that such a flow system in a kiln atmosphere will lead to very noticeable temperature differences.

In practice, use is usually made of oppositely-placed burners directed towards one another. In such a case there are two relatively quiescent elongated heat zones running along the axis of the tunnel kiln, and the gaseous heating products move around in an annular eddy. Although this manner of operation leads to an improvement over operation with only a single jet burner, the result obtained is definitely not completely satisfactory.

One object of the present invention is to provide a kiln and method of operating a kiln which in the simplest possible manner brings about a practically complete solution of the problem of achieving and maintaining a substantially even or uniform temperature in all parts of the firing chamber of a kiln.

The invention consists in a method of heating a firing chamber or space through the use of gaseous or liquid fuels by using at least two burners which are arranged opposite each other and directed towards one another, characterized in that the quantity of the fuel-air mixture introduced and/ or the heating effect or output of the two oppositely placed burners is varied periodically. As a result there is no establishment of stationary directions of movement of the heating gases from the burners, and instead the paths along which the heating gases move are themselves in continuous movement. In accordance with a further development of the invention, it may in many cases be advantageous to make the cyclical periods of variation of the burners of different lengths, so that there is a steadily changing rhythm in the movement of the heating gases.

If gas or oil burners with varying rates of heat output are used, it is quite possible to obtain the aforementioned variation by varying the quantity of fuel with-out altering the quantity of air, providing that the variations in heat output do not exceed certain limits. This is true even though from the stoichiometric point of view it would naturally be desirable to vary the amount of combustion air in accordance with variation in the amount of fuel; however, actual practice often demands that operation be carried out with an excess of air, so that even if the supply of fuel is varied, complete combustion will take place.

In accordance with the method or process of the invention, two oppositely-disposed burners are always present. Since the sum of the quantities of fuel injected into both burners remains constant at every moment, a sufiicietly complete combustion at each burner, despite variations in the quantity of fuel injected, is achieved if the amount of air blown in does not undergo any alteration, since the momentary excess of air present at one burner will be available for combustion at the opposite burner.

The simplest case from the theoretical point of view is that in which the quantity of air of each burner is varied in accordance with the rate of fuel supply. In this case the rate of injection of material by the burners will correspond with the rate of heat developed.

In accordance with a further and a preferred feature of the invention, the cycles of variation of the two 0ppositely placed burners are of the same length, and are offset by in phase. Thus, if the rate of injection and/ or heat developed of one burner is reduced, that of the other burner is correspondingly increased. As a result, the eddies formed move in a distinct rhythm between the two burners, with a horizontal reciprocatory or shuttling movement.

In accordance with a further feature of the invention, the sum of the quantities of fuel-air mixture injected and/ or of the heat outputs of the two oppositely placed burners directed towards one another will at every instant correspond with the quantity of air and fuel and/ or heat required by that part of the kiln to be heated by the two burners.

In accordance with a further development of the invention, the pairs of burners arranged along the axis of a tunnel kiln are regulated so that they vary their fuel and/ or heat output with a difference in phase of up to 180 between adjacent sequentially-placed burners. As a result, the reciprocatory movement of the kiln atmosphere has superimposed on it a wave-like movement moving along the kiln.

A kiln for carrying out the method of the invention for the firing or heat treatment of discrete articles, for instance for the firing of ceramics, preferably comprises oppositely placed burners which are directed towards one another at a level above the surface on which the goods to be fired are stacked, the goods being stacked in such a manner as to leave alleys or channels at the same level as the burner fuel and air nozzles, so that the currents from the burners come towards each other along the alleys or channels.

A trolley-type tunnel kiln for carrying out the method in accordance with the invention advantageously has the further feature that below the surface on which the goods are stacked, alleys or channels are provided along which currents injected and blown in from the oppositely-arranged burners can pass towards one another, these alleys or channels having at least one upwardly opening aperture through which the gases of combustion can pass to the goods to be fired.

The manner of firing in accordance with the invention cannot be used simply by the application of pulsating burners. This is because with the ordinary high rate of pulsation of the furnace atmosphere and, providing that the fuel-air mixture injected has a high enough speed, annular eddies will result which do not move backwards and forwards across the kiln but which follow fixed paths leading to an unequal distribution of temperature within the kiln. In other words the effect obtained with the present invention is different from that to be obtained merely with pulsating burners, with which, owing to the length of the flame changing in accordance with the pulsations, the only effect is the production of a pulsating atmosphere around the burner. This pulsating kiln atmosphere effects an improved mixture of the combustion gased with the combustion air and there is also a heating effect which varies in accordance with the rhythm of the pulsations.

This stands in complete contrast to the principle underlying the present invention, because in accordance with the invention longer periods of variation in the flow conditions inside the firing space are created which not only provide periodic changes in the flame action but which also bring about a substantial alteration in heat transmission by convection. It is also particularly important that with the invention two burners are arranged opposite one another and directed towards one another, so that the columns of gases produced by them interact directly and cooperate to produce the desired periodic or cyclical variation in the configuration of the heating currents within the kiln. In pulse burners, on the other hand, the same annular eddy current phenomena can appear as has been mentioned hereinabove as being disadvantageous, even although such phenomena may only occur when the combustion air introduced with the fuel has a sulficient injection speed.

Further, pulsating or pulse burners operate with a frequency of a few cycles to substantially more than 1000 cycles per minute, whereas the method in accordance with the invention involves a cycle whose length depends upon the transverse dimensions of the kiln and is in the order of magnitude of 30 seconds and more. Of course, with kiln cross-sections of small size the length of a cycle may be reduced very substantially.

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

FIG. 1 shows a cross-section through a tunnel kiln with burners mounted in lateral recesses in the kiln walls.

FIG. 2 is a cross-section through a kiln Wall with a jet or beam-type burner.

FIG. 3 is a cross-section through a tunnel kiln for producing an annular eddy using a beam-type burner.

FIG. 4 is a section through a tunnel kiln using two beam-type burners directed towards one another.

FIG. 5 diagrammatically shows the principle of the invention with reference to a cross-section through a tunnel kiln with two oppositely directed burners.

FIGS. 6a, 6b, 6c, 6d and 6e diagrammatically show the course of eddy movement in a cross-section of a tunnel kiln using two burners directed towards one another.

FIG. 7 shows a preferred manner of arranging articles to be fired in accordance with the invention.

FIG. 8 is a section along the line A-B of FIG. 7.

FIG. 9 shows the horizontal surface on which goods to be fired are mounted.

FIG. 10 is a section on the line CD of FIG. 9.

Referring now to the drawings and more particularly to FIG. 1, it can be seen that the latter represents a crosssection through a tunnel kiln which is heated with normal burners which in the immediate proximity of the burner nozzles produce a restricted zone of heat development. In this figure reference numeral 1 is used to indicate a burner, reference numeral 2 a lateral recess in which the burner is mounted, while reference numerals 3 and 4 denote the side walls of the tunnel kiln in which the recesses 2 are formed. Reference numeral 5 denotes the roof or top of the tunnel kiln. The kiln is provided with rails 6 extending along its length and carrying kiln trolleys 7 running on wheels 8 along the kiln. The trolleys carry stacks of goods to be fired, denoted by reference numeral 9. As illustrated, there normally is a relatively large distance between the burners 1 and the stacked goods 9, and because of this, temperature differences will normally occur in the firing space 10 of the kiln, since there previously has been no way to obtain the necessary mixture of the high temperature burner gases leaving the burners 1 with the kiln atmosphere.

In the case of the beam-type or jet burners shown in FIG. 2 the fuel, which in the example shown can be gaseous fuel, enters through the tube 11 into the duct 12 along which it passes to the burner nozzle 13, to which air for combustion is supplied via lateral duct means 14. The hot burner gases pass through a passage 15 and project outwardly into the combustion space 17 in the form of an elongated jet or beam 16 which is limited by the wall 18.

The configuration of the eddies occurring Within the cross-section of the kiln with the use of such a burner can be seen from FIG. 3. The burner is there denoted by reference numeral 21, while the line 22 denotes the annular eddy which is produced in the kiln cavity 23 when such a burner is used. As can be seen from the drawing, in this annular eddy there is a dead or still space extending along the center of the kiln, so that consequently there is no even temperature distribution over the whole crosssection of the kiln.

Conditions can be improved somewhat if two burners 31 and 32 directed towards one another are used, as shown in FIG. 4. In this case two flows or streams 33 and 34 are formed, but there are still two practically still or dead spaces 35 and 36 within the kiln space 37, extending along the kiln.

FIG. 5 shows a modification of the kiln structure of FIG. 4, in which the effects of the present invention may be seen. The annular eddies 43 and 44 normally produced by the burners 41 and 42 are shown in full lines. On varying the quantity of the air-fuel mixture and/or the rate of heat development by the two oppositely placed burners 41 and 42, the annular eddies move across the kiln as is shown by reference numerals 43 and 44'. This movement is shown in a sequence of views taken at succeeding points in time in FIGS. 6a through 6e. FIG. 6a shows the eddy movement in accordance with the use of a burner as shown in FIG. 4, FIG. 6b the eddy movement using two burners of which the left-hand burner 52 receives less air fuel mixture or is caused to develop less heat than the right-hand burner 53. In the condition as shown in FIG. 60 the quantity of air fuel mixture supplied is the same as for both burners 52 and 53 while in the case of FIG. 6d the air fuel mixture supplied or the rate of supply of heat of the two burners 52 and 53 is exactly the reverse of that as shown in FIG. 6b. FIG. 66 shows further the extreme case in which only the lefthand burner 54 is in use, while the right-hand burner is out of operation. This is the reverse of operation with the burner 51 as shown in FIG. 6a. Thus, in accordance with the method of the invention the eddy currents can be caused to move across the cross-section of the tunnel kiln in such a manner that the temperature can be evened out in the kiln in a positive and reliable manner and still or dead spaces extending along the kiln may be completely eliminated.

The following table gives, by way of example, details of the relative amounts of air-fuel mixture, expressed in arbitrary units, supplied to the burners in the case of FIGS. 6a to 6e:

Left-hand burner:

If the kiln space is filled with material to be fired, there is naturally a hinderance in the currents described hereinabove, because the material to be fired causes the currents to be diverted. Therefore, in many cases it is advantageous to stack the articles so as to leave channels or alloys through the material to be fired in order to allow the formation and circulation of eddies of burning gases in the channels, of the type shown in the figures, wherein the products of combustion from the burners approach one another horizontally.

This can be carried out, for example, in the manner as shown in FIGS. 7 and 8, wherein the firing of stacked rows of standard ceramic blocks or bricks is shown with alleys or channels left between the blocks and extending across the surface on which the blocks are mounted, at the same level as the burners. The channels are formed by placing each row on the top horizontal surface 71 of the kiln trolley so that the row is aligned with the direction of the alignment of the burners as disclosed above, and so that the rows are parallel to each other. As depicted in FIG. 7, seach column of stacked articles is formed by two rows placed one on top of the other, the two rows vertically aligned so that they do not project across the channels 73. Thereafter, cross rows are placed covering the channels formed by the first two rows.

It is also possible to leave spaces below the material to be fired, to thereby provide the space required for the circulating currents or eddies. This may be accomplished by using supports of the type known as elephants feet to those in the art, on the surface on which the kiln charge is to be arranged, The elephants feet can be so arranged that alleys or channels result, as shown in FIGS. 9 and 10.

In FIGS. 9 and 10 reference numeral 81 denotes the top horizontal supporting surface of the kiln trolleys for supporting the stack of articles.

In the embodiment shown in FIGS. 9 and 10 the elephants feet are shown at 82, and these are mounted on the stacking surface 81 standing along the kiln trolleys, with the articles to be fired being mounted atop the elephants feet 82. In this instance stacking is carried out on stacking plates or slabs 83 which, in accordance with a further feature of the invention, are provided with corresponding openings 84 so that the currents of combustion gases can escape in an upward direction and run over the surface of the material to be fired.

It has been found especially advantageous, particu larly in the case of tunnel kilns, if combustion channels are provided on the firing trolleys which extend perpendicular to the longitudinal direction of the tunnel kiln, so that the burners discharge into them from both sides. These channels conveniently have holes or apertures directed upwards and also, if required, in other directions, to allow for the combustion gases to escape in an upward direction and run over the surface of the material to be fired. The channels can have side walls, perpendicular to the longitudinal direction of the kiln, which are closed or provided with apertures.

In the case of chamber kilns or tunnel kilns as used in the ceramics industry, for example, the kiln is charged with the desired material while heated by the burners located on opposite sides of the kiln. In this situation, it is generally believed that the supply of heat to the kiln must theoretically be held constant. In such a case it would therefore be necessary to carry out the present invention by operating the burners in such a manner that the sum of the rate of heat output of each pair of oppositely placed burners directed towards one another is constant at every instant. This means that the fall in heat output of one burner per unit time must be made up by an equal increase in the heat output of the other bumer of the pair.

However, it has been found in practice that the actual amount of heat required in ceramic kilns does not in fact follow the theory regarding constancy, but in reality appears to continuously vary. In order to comply with this demand, the burners operated in accordance with the invention must be so controlled that the sum of the amount of fuel and air mixture injected into the kiln and/ or the heat output of oppositely placed burners corresponds to the quantity of air-fuel mixture and/ or the heat input required at every instant by the two burners.

The pulse burners which have come to be more and more widely used in the ceramics industry and elsewhere can also be used in carrying out the principles of this invention. In these burners, liquid fuel is intermittently injected under high pressure into the firing space, and combustion does not generally occur immediately at the outlet of the burner but instead occurs outwardly thereof a distance which may amount to several yards. In this case the amount of air required for combustion can, for example, be supplied to every burner individually and injected parallel to the fuel jet into the kiln space, with the principle of the invention being applied by either varying the total quantity of injected fuel and air, or by varying the heat output of the individual burners in accordance with the rules described above.

The control of the burners themselves does not require any particular description, since known control elements can readily be applied. The control of the quantity of airfuel mixture injected can for example be carried out 'by means of chokes or with pressure-varying means in the supply ducts. In the case of burners operated with liquid fuel the quantity of the air applied can also be varied by means of chokes or by means for varying the pressure, and the heat output can be varied by varying the quantity of fuel supplied.

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

1. In a method of heating a kiln having a kiln space with a roof, side walls and a floor, in which at least two burners are arranged at the walls opposite one another and directed towards one another to propel products of combustion into'the kilnso as to cause circulatory heat currents therewithin, the improvement comprising cyclically changing the positions of the paths of the heat currents produced by the burners inside the kiln by cyclically altering the output of the burners so as'to cyclically move each such path first toward one such burner and then toward the other thereof.

2. The method improvement of claim 1, wherein said burners are controlled by first increasing the output of one and decreasing the output of the other, and then increasing the output of said other and decreasing the output of said one.

3. The method improvement of claim 1, wherein said burners are controlled by cyclical alterations in the airfuel mixture supplied thereto.

4. The method improvement of claim 2, wherein the outputs of said one and said other burner are simultaneously increased and decreased.

5. The method improvement of claim 3, wherein said burners are controlled by air-fuel mixture supplied thereto and of the heat output produced by the burners.

6. A method of heating a kiln having an elongated kiln space formed by a roof, side walls, and a fioor, comprising the steps: positioning at least two burners in the said walls opposite one another and disposing such two burners towards one another so that they propel their products of combustion into the kiln so as to produce circulatory currents following substantially loop-shaped paths flowing around quiescent central zones at a given instant, and changing the relative positions of the loopshaped paths of said circulatory currents by a cyclical fluctuation of the current-propelling forces exerted by the burners, the burner on one side of the kiln producing a greater current-propelling force While the other produces a lesser such force, and vice versa.

7. The method as set forth in claim 6, in which the frequency of the cyclical fluctuations of the two burners is varied.

8. The method as set forth in claim 6, in which the fluctuation cycles of the two burners are of the same frequency and are about 180 degrees out of phase.

9. The method as set forth in claim 6, in which the instantaneous summation of the amount of fuel supplied to the two burners remains substantially constant.

10. The method as set forth in claim 6, in which a plurality of such pairs of oppositely-disposed burners is arranged in horizontal succession along the length of said kiln, each such pair undergoing said cyclical fluctuation of the current-propelling forces of the burners therein, and in which a phase displacement of the fluctuation in the propelling forces of each pair of adjacent burners is provided.

11. The method as set forth in claim 6, in which at least one passage is formed within the articles to be fired within said kiln, said passage being disposed such that the moving currents of the products of combustion from the two burners approach each other approximately horizontally through such passage.

12. The method as set forth in claim 11, further including the step of producing from said horizontallyapproaching currents a heat current which moves upwards from said passage through an upwardly-disposed opening in said articles.

13. A method of stacking articles for firing within a kiln of the type having horizontally disposed burners on opposite sides of said kiln aligned to direct heat currents at each other, the method comprising the steps of providing a horizontal supporting surface to be moved through the kiln, placing a row of articles on said surface in alignment with the direction of said alignment of said burners, leaving a horizontal heat approach channel on the two sides adjacent to said row, and placing a second row of articles on said surface parallel to said first row adjacent to one of said channels.

14. The method as defined in claim 13, and further comprising the step of placing at least an additional row of articles on top of both said first and said second rows without projecting said additional row across either of said channels.

References Cited UNITED STATES PATENTS 1,970,320 8/1934 Kier et al. 26328 X 2,982,530 5/1961 Drakengren 26328 3,129,933 5/1964 Cremer et al 26328 JOHN J, CAMBY, Primary Examiner US. Cl. X.R. 263-28

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