JP3983714B2 - Burner and ceramic plate for burner - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a burner and a ceramic plate for the burner. Specifically, the present invention relates to a technique related to fire transfer during burner ignition.
[0002]
[Prior art]
An all-round burner provided with a cylindrical burner cylinder will be described as an example. The all-around burner is arranged with its axis horizontal, and is used to heat a heat exchanger or the like of a hot water heater. A flame group is formed in the burner tube. A mixture of air and gas is ejected from the burner group of the burner cylinder, and the mixture is burned around the entire circumference of the burner cylinder.
An ignition electrode is provided in the vicinity of the outer periphery of the burner cylinder. When the all-round burner is ignited, a spark is generated by energizing the ignition electrode in a state in which the air-fuel mixture is ejected from the flame group. The spark generated at the ignition electrode ignites and burns the air-fuel mixture ejected from the nearby flame opening, and the combustion is transferred (propagated) to the entire circumference of the burner cylinder. A cylindrical burner cylinder is described in Patent Document 1, for example.
When the burner cylinder is made of ceramic, high heat resistance can be obtained. However, it is difficult to process a ceramic burner cylinder integrally. For this reason, when the burner tube is made of ceramic, it is necessary to have a configuration in which a plurality of ceramic plates are combined at the end face.
[0003]
[Patent Document 1]
Japanese Utility Model Publication No. 42-22144
[0004]
[Problems to be solved by the invention]
In a burner cylinder in which the end faces of a plurality of ceramic plates are combined, the distance between the flame openings across the end faces is twice the distance from the end faces to the flame openings closest to the end faces (hereinafter referred to as “end distance”). Become. Accordingly, it is difficult (bad) to perform fire transfer across the end face of the ceramic plate. If the fire transfer is poor, a large amount of air-fuel mixture is ejected from the burner tube, and the air-fuel mixture is ignited. For this reason, rapid combustion occurs, which is not preferable.
In order to improve the heat transfer, the end distance of the ceramic plate may be reduced. However, if the end distance is reduced, cracks may occur between the end face and the flame opening, or the end face may be deformed when a pin is inserted into the ceramic material before firing to form the flame opening. For this reason, there is a limit to reducing the end distance.
[0005]
Fire transfer is better from bottom to top than from top to bottom. This is because the flame formed at the flame mouth burns upward. For this reason, it is ideal to arrange the ignition electrode below the burner cylinder. However, the ignition electrode is placed above the burner cylinder in order to avoid getting wet with the condensed water of the moisture in the exhaust gas generated in the condenser that is often provided adjacent to the burner, or due to space constraints. There are cases where it is unavoidable. As described above, in a ceramic burner, it is difficult for fire transfer across the end face of the ceramic plate to be performed. For this reason, if the ignition electrode is disposed above a ceramic burner that is difficult to perform the fire transfer, the fire transfer becomes worse.
[0006]
The present invention has been made to solve such a problem, and an object of the present invention is to provide a ceramic burner having good fire transfer and a ceramic plate used therefor.
[0007]
Means for Solving the Problem and Action and Effect The burner according to claim 1 is formed by combining a plurality of ceramic plates formed with a group of flame mouths at the end face. AndAt the end face of at least one of the adjacent ceramic plates, it reaches the back face from the surface of the ceramic plateSlit is formedThe end surface where the slit is formed is in contact with the end surface of the adjacent ceramic plate.
  The above burnerAt the end face of at least one of the adjacent ceramic plates, it reaches the back face from the surface of the ceramic plateA slit is formed. If comprised in this way, when a ceramic plate is combined, the through-hole extended in a thickness direction will be formed in an end surface. Therefore, when the burner is ignited, the fire is not directly transferred from the flame port closest to the end surface of one ceramic plate to the flame port closest to the end surface of the other ceramic plate, but through a through hole formed by a slit. Fire transfer takes place. Therefore, the fire transfer is improved.
[0009]
  Claim2The burner ceramic plate described in 1 is used as a burner by forming a group of flame mouths and combining a plurality of them at the end face. AndFrom the front to the back of the ceramic plate reaches the end face that contacts the adjacent ceramic plate when combined.A slit is formed.
  In a burner in which the above ceramic plates are combined, a through hole extending in the thickness direction is formed on the end surface of the ceramic plate. Therefore, when the burner is ignited, the fire is not directly transferred from the flame port closest to the end surface of one ceramic plate to the flame port closest to the end surface of the other ceramic plate, but through a through hole formed by a slit. Fire transfer takes place. Therefore, the fire transfer is improved.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The main features of the embodiments described later will be described.
(Form 1)
The burner of 1st Example is a perimeter type provided with a cylindrical burner main body. The burner body has a cylindrical shape in which six curved ceramic plates are combined. A flame hole group penetrating in the thickness direction is formed in the ceramic plate. The flame mouth groups are arranged radially. The end face used for the combination of the ceramic plates extends in a straight line and is formed with a slit penetrating in the thickness direction. When the ceramic plates are combined with each other, the slit for fire transfer that penetrates the burner body in the thickness direction is formed.
(Form 2)
In the second embodiment, the end surface used for the combination of the ceramic plates extends in the longitudinal direction (horizontal direction) of the burner body and extends in the short direction (vertical direction) of the burner body on the way.
(Form 3)
In the third embodiment, a flat portion is formed at one outer peripheral end of the ceramic plate. A conductive coating is applied to the flat portion. The blaze penetrates the conductive coating.
[0012]
【Example】
(First embodiment)
An all-round burner 10 according to a first embodiment of the present invention will be described with reference to the drawings.
As shown in FIG. 1, the burner 10 includes a burner mounting plate 12, a base presser plate 22, a burner body 14, a distribution pipe 16, an end presser plate 18, and the like.
FIG. 2 shows a state where the burner body 14 and the end pressing plate 18 are removed from the burner 10 of FIG. The burner mounting plate 12 has a disk shape as shown in FIG. 3 and has four through holes 12a. The through hole 12a is used to fix the burner 10 to the heat exchanger 30 (the heat exchanger 30 will be described in detail later). A base presser plate 22 having a flange 22 a formed on the outer peripheral portion thereof is fixed to the burner mounting plate 12 by spot welding 23.
[0013]
As shown in FIG. 2, the distribution tube 16 is formed with a number of distribution holes 16 d. The inside of the distribution pipe 16 communicates with the outside through an opening 12b formed in the central portion of the burner mounting plate 12. In FIG. 2, the distribution holes 16d are arranged at a uniform density, but the distribution of the combustion intensity of the burner body 14 can be adjusted by adjusting the arrangement density. When a mixture of gas and air is supplied into the distribution pipe 16 from the opening 12b of the burner mounting plate 12, the ejection of the air-fuel mixture from the distribution hole 16d is farther than the base side (burner mounting plate 12 side) of the distribution pipe 16. The part side shows a stronger tendency. This is because the end of the distribution pipe 16 is a dead end, so the internal pressure is higher than the base. Therefore, the distribution density of the distribution holes 16d is lowered at the end portion and increased at the base portion, whereby the distribution of the air-fuel mixture ejected from the distribution tube 16 can be made uniform. If the distribution of the air-fuel mixture ejected from the distribution pipe 16 can be made uniform, the combustion intensity distribution of the burner body 14 is also made uniform.
[0014]
As shown in FIG. 3, the shape of the distribution tube 16 viewed from the direction perpendicular to the axis is a square shape. Four base mounting pieces 16a facing outward are formed at the base of the distribution tube 16, and four end mounting pieces 16b facing inward are formed at the end. The base mounting piece 16 a of the distribution pipe 16 is disposed so as to enter into a rectangular opening 22 b formed in the base pressing plate 22. The distribution pipe 16 is attached to the burner attachment plate 12 by fixing the base attachment piece 16 a to the burner attachment plate 12 by spot welding 24. One screw hole 16c is formed in each of the end attachment pieces 16b.
[0015]
As shown in FIG. 4, the burner body 14 has six ceramic plates 26 combined in a cylindrical shape. The end face 53 used for the combination of the ceramic plates 26 extends linearly and is polished. For this reason, the end faces 53 are in close contact with each other, and there is no need to sandwich a sealing member such as ceramic wool between them. The ceramic plate 26 has a large number of flame openings penetrating in the thickness direction. As well shown in FIG. 5, the axes of the flame openings 26a are arranged radially with respect to each other. The end distance of the flame opening 26a (the distance between the flame opening 26a closest to the end face 53 and the end face 53) is set larger than the pitch of the flame openings 26a. This is because if the end distance of the flame port 26a is smaller than the pitch, a crack will occur between the end surface 53 and the flame port 26a.
The flame opening 26a is processed when the ceramic plate 26 is in a state of a ceramic material before firing. Specifically, a plurality of pins are simultaneously pierced into a flat clay-like ceramic material to process the flame opening 26a. When processing the flame opening 26a, if the end distance is small, the effect of sticking the pin reaches the end face 53, and the end face 53 is deformed. In order to prevent this phenomenon, the end distance of the flame port 26a must be larger than the pitch of the flame port 26a. The ceramic material in which the flame opening 26a is formed is formed into a ceramic plate 26 by bending using a mold or the like and further firing.
[0016]
4 and FIG. 5 is schematically illustrated. Therefore, the distribution of the flame openings 26a is different between FIG. 1, FIG. 4 and FIG. Moreover, although the flame openings 26a are distributed over the entire surface of the ceramic plate 26, only a part thereof is illustrated in FIG.
[0017]
As shown in FIGS. 4 and 5, the end face 53 of the ceramic plate 26 is provided with four slits 26b penetrating in the thickness direction. The slits 26b are arranged at positions facing each other when the ceramic plates 26 are combined. For this reason, in the state where the ceramic plates 26 are combined, the flame transfer port 52 is formed by the set of slits 26b.
In addition, as shown in FIG. 7, the slit 26b can be provided in the end surface 53 on one side of the ceramic plate 26 to be combined, thereby forming the flame port 52 for fire transfer. If it does in this way, unlike the form of Drawing 4, since it is not necessary to make the position of slit 26b of each end face 53 correspond, position accuracy of slit 26b can be made sweet.
FIG. 8 illustrates a cross section of the burner body 14 in which four ceramic plates 26 each having a flame opening 26a whose axes are parallel to each other are combined. A slit 26b is provided in the end face 53 of the ceramic plate 26, and a flame port 52 for fire transfer is thereby formed. The flame ports 26a with parallel axes have fewer man-hours than machining the flame ports 26a with parallel axes at the stage of a flat ceramic material and then bending them to form the flame ports 26a with radial axes. . Therefore, the flame opening 26a having parallel axes can be easily formed at a low cost.
[0018]
As shown in FIGS. 1 and 6, the burner body 14 combined with the ceramic plate 26 is disposed between the base presser plate 22 and the end presser plate 18. The end pressing plate 18 is attached to the end mounting piece 16b of the distribution pipe 16 by a screw 28. When the end presser plate 18 is attached to the distribution pipe 16, the end presser plate 18 and the base presser plate 22 press the burner body 14 in its axial direction. For this reason, the ceramic plate 26 of the burner body 14 is held in a combined state. In this case, it is preferable to insert packing such as ceramic wool between the end pressing plate 18 and the ceramic plate 26 and between the base pressing plate 22 and the ceramic plate 26. If it does in this way, the variation of the longitudinal direction dimension of the ceramic plate 26 and the thermal expansion difference of the distribution pipe | tube 16 and the ceramic plate 26 can be absorbed by packing.
[0019]
The end presser plate 18 and the base presser plate 22 have a function of closing the end opening of the burner body 14. The distribution tube 16 supplies an air-fuel mixture into the burner body 14. The end presser plate 18, the base presser plate 22, and the distribution tube 16 hold a combination of ceramic plates 26. For this reason, it is not necessary to separately provide a member for maintaining the combined state of the ceramic plates 26. Therefore, the configuration is simple. Moreover, since the member for maintaining the combined state of the ceramic plates 26 is not arranged on the outer peripheral side of the burner body 14, the uniformity in the circumferential direction of the flame is not disturbed.
[0020]
The base presser plate 22 and the end presser plate 18 have flanges 22a and 18a, respectively. The height of the flanges 22a and 18a is, for example, about 5 mm. There is a gap between the inner diameter of the flanges 22a and 18a and the outer diameter of the burner body 14. This gap is, for example, about 0.5 mm. The provision of the flanges 22a and 18a facilitates the work when the ceramic plates 26 are mounted in combination, and prevents the ceramic plates 26 from coming off.
In addition, holding | maintenance of the ceramic plate 26 by the distribution pipe | tube 16, the edge part pressing plate 18, and the base part pressing plate 22 is not restricted to what is performed by pressing the burner main body 14 to an axial direction. For example, there is no gap between the flange 18a of the end presser plate 18 and the flange 22a of the base presser plate 22 and the outer peripheral part of the end of the burner main body 14, and the outer peripheral part of the end of the burner main body 14 by the flanges 18a and 22a. The combined state of the ceramic plates 26 can also be maintained by tightening.
[0021]
FIG. 9 schematically shows the heat exchanger 30. The heat exchanger 30 includes an outer cylinder 32, a burner 10, a condensing pipe 34, a heating pipe 36, a first header 37, a second header 38, a third header 40, and the like.
The outer cylinder 32 has a cylindrical shape, and is provided with a disk-shaped end plate 32a at one end and an exhaust port 32b at the other end. A drain 32c is provided at the bottom of the outer cylinder 32 on the exhaust port 32b side. A half on the end plate 32a side of the outer cylinder 32 is a heating chamber 32d, and a half on the exhaust port 32b side is a condensing chamber 32e. The heating chamber 32d and the condensing chamber 32e are partitioned by a disk-shaped partition plate 32f.
[0022]
The condensing pipe 34 has a spiral shape with two bundles, and is disposed in the condensing chamber 32e. One end of the condensing pipe 34 is connected to the first header 37, and the other end is connected to the second header 38. The heating pipe 36 is also a spiral of two bundles, and is disposed in the heating chamber 32d so as to surround the burner 10. One end of the heating pipe 36 is connected to the second header 38, and the other end is connected to the third header 40.
An ignition electrode 50 is disposed above the burner body 14. The ignition electrode 50 is controlled by a controller (not shown), and generates a spark when activated.
[0023]
A gas / air mixture is supplied to the burner 10 and ejected from the flame port 26 a of the burner body 14. The air-fuel mixture ejected from the flame opening 26a burns to form a flame 42. The flame 42 burns radially around the burner body 14. By providing the partition plate 32f, the combustion gas passes through the heating pipe 36 and then enters the condensing chamber 32e.
Water such as tap water is supplied to the first header 37. The water supplied to the first header 37 passes through the condensation pipe 34. Further, the combustion gas generated when the air-fuel mixture burns in the burner 10 passes through the condensation chamber 32e and the exhaust port 32b and is discharged to the outside. The combustion gas contains water vapor. The water vapor in the combustion gas passing through the condensation chamber 32e is recovered as latent heat by the water passing through the condensation pipe 34 to become condensed water 44. The condensed water 44 is discharged to the outside from the drain 32c. On the other hand, the water whose latent heat has been recovered from the water vapor flows into the second header 38 after the temperature rises.
[0024]
The temperature of the water flowing into the second header 38 after passing through the two condensing pipes 34 is different in each condensing pipe 34. This temperature difference is made uniform by mixing water in the second header 38. The water having a uniform temperature passes through the heating pipe 36. The water passing through the heating pipe 36 is heated by the flame 42 burning in the burner 10 to become hot water. The warm water is mixed at the third header 40 to a uniform temperature, and is sent to the outside of the heat exchanger 30. The hot water temperature is maintained at a predetermined temperature by controlling the combustion intensity of the burner 10 by the controller. Hot water is used for heating and the like.
[0025]
As described above, the slit 26b is formed in the end surface 53 of the ceramic plate 26 constituting the burner body 14 of the present embodiment (see FIGS. 4, 5, 7, and 8). The slit 26b forms the flame transfer flame opening 52 in a state where the ceramic plates 26 are combined. When the fire transfer flame port 52 is formed, the fire transfer at the time of ignition propagates to the flame port 26a closest to the end face of the ceramic plate 26, and then propagates to the fire transfer flame port 52 and further adjacent thereto. It propagates to the flame opening 26a closest to the end face 53 of the ceramic plate 26. As described above, in the ceramic plate 26, the end distance of the flame port 26a must be larger than the pitch. For this reason, the distance between the flame openings 26a closest to the end face 53 of the adjacent ceramic plate 26 is as large as twice the end distance. However, by providing the flame transfer flame opening 52 between them, a good fire transfer is achieved. Can be realized.
As shown in FIG. 8, the burner body 14 combined with the ceramic plate 26 formed with the flame ports 26a whose axes are parallel to each other is from the ceramic plate 26 (FIG. 5) with the flame ports 26a formed radially. However, the end distance of the flame outlet 26a is large. For this reason, the improvement of the fire transfer by providing the flame transfer port 52 is more remarkable.
[0026]
(Second embodiment)
Most of the configuration of the second embodiment overlaps with that of the first embodiment. Therefore, only the characteristic part of the second embodiment will be described.
As shown in FIG. 10, the burner body 14 has six ceramic plates 55 combined in a cylindrical shape. The end face 56 of the ceramic plate 55 used for the combination extends in the longitudinal direction of the burner body 14 (horizontal direction, the axial direction of the burner body 14), and extends in the short direction (vertical direction) of the burner body 14 along the way. (In the following, the portion extending in the longitudinal direction of the end face 56 is referred to as a horizontal portion 56a, and the portion extending differently is referred to as a staggered portion 56b).
[0027]
As schematically shown in FIG. 11, the flame 42 formed on the flame port 55a burns upward. As described above, the ignition electrode 50 is disposed above the burner body 14. For this reason, the ignition transfer at the time of ignition is performed from the top to the bottom of the burner body 14. When the flame 42 is burning upward, the fire transfer (in the direction of the arrow 60 in FIG. 10) across the horizontal portion 56a of the end face 56 hardly occurs. For example, in FIG. 11, when the flame 42a is transferred to the lower flame port 55b, the flame 42a is directed upward, so that it is difficult for the flame 42a to be transferred. On the other hand, the fire transfer across the staggered portion 56b of the end face 56 (in the direction of the arrow 62 in FIG. 10) is easy to move because it is not affected by the flame 42 facing upward. Therefore, in the burner body 14 of the present embodiment, good fire transfer is performed along the route indicated by the arrow 63 in FIG.
[0028]
FIG. 12 illustrates a burner body 14 in which two burner body subassemblies 67 are overlapped in the axial direction. The burner body subassembly 67 has six ceramic plates 65 combined in a cylindrical shape. The end surface 66 of the ceramic plate 65 used for the combination extends in the longitudinal direction of the burner body 14 and extends in the short direction (vertical direction) of the burner body 14 along the way. With this configuration, the fire transfer across the staggered portion 68 is not affected by the flame 42 facing upward. For this reason, good fire transfer is performed along the route indicated by the arrow 69.
[0029]
(Third embodiment)
Only the characteristic part of the third embodiment will be described.
As shown in FIG. 13, the burner body 14 includes five ceramic plates 72 and one ceramic plate 73 combined in a cylindrical shape. The ceramic plates 72 and 73 are formed with a large number of flame ports 72a and 73b, respectively, but are not shown in FIG. 13 for the sake of clarity of illustration (flame ports 72a and 73b). Is shown in FIG. A flat portion 73 a is provided at the outer peripheral end of the ceramic plate 73. The flat portion 73a can be molded when the ceramic plate 73 is in a clay-like state before firing, or can be formed by cutting after firing.
[0030]
A conductive coating 75 is formed on the flat portion 73a. The conductive coating 75 is made of, for example, a conductive ceramic (a perovskite-type metal oxide, La_1-xSr_xMnO₃, La_0.7Sr_0.3Mn₃Etc.) can be formed by baking or the like. When forming the conductive coating 75, a coating material is applied using a roller. It is difficult to apply a coating material to a curved surface with a roller to a uniform thickness. On the other hand, a coating material having a uniform thickness can be easily formed on a flat portion using a roller. Therefore, a high-quality conductive coating 75 having a uniform thickness can be formed on the flat portion 73 a of the ceramic plate 73. As well shown in FIG. 14, the flame opening 73 b penetrates the conductive coating 75.
[0031]
FIG. 15 schematically shows a flame detection system of the burner 10. The flame detection system includes a frame rod 76, the above-described conductive coating 75, a detection unit 77, a DC power supply 78, and the like. The frame rod 76 has conductivity and is disposed above the conductive coating 75. The detection unit 77 and the DC power source 78 are interposed in an electric path 79 that connects the frame rod 76 and the conductive coating 75. The DC power supply 78 applies a predetermined voltage between the frame rod 76 and the conductive coating 75.
In a state where the flame is not burned (a state before ignition), the frame rod 76 and the conductive coating 75 are insulated. When the burner 10 is ignited and the flame 42 burns across the flame opening 73 b and the frame rod 76, an ionic current flows between the frame rod 76 and the conductive coating 75. The detector 77 detects ignition by the flow of this ion current. Such a method of detecting ignition by ionic current (frame rod method) is more responsive than a method of detecting ignition from a temperature rise caused by the flame 42 using a thermocouple.
As in the third embodiment, the burner body 14 is formed from the ceramic plates 72 and 73 which are insulating materials by providing a flat portion 73a on the ceramic plate 73 and forming a high-quality conductive coating 75 thereon. In addition, a flame rod type flame detection system can be realized.
[0032]
Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
In addition, the technical elements described in the present specification or drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.
[Brief description of the drawings]
FIG. 1 is a side view of a burner according to a first embodiment.
FIG. 2 is a side view of the distribution pipe according to the first embodiment.
FIG. 3 is a view taken along the line III-III in FIG. 2;
FIG. 4 is a perspective view of the burner body according to the first embodiment.
5 is a cross-sectional view taken along line VV in FIG.
FIG. 6 is a view taken along line VI-VI according to the first embodiment.
FIG. 7 is a perspective view of a burner body according to the first embodiment (modified example).
FIG. 8 is a sectional view of the burner body according to the first embodiment (modified example).
FIG. 9 is a schematic structural diagram of a heat exchanger according to the first embodiment.
FIG. 10 is a perspective view of a burner body according to a second embodiment.
FIG. 11 is a schematic view of a state in which a flame is burned by a burner according to a second embodiment.
FIG. 12 is a perspective view of a burner body according to the second embodiment (modified example).
FIG. 13 is a perspective view of a burner body according to a third embodiment.
14 is a sectional view taken along line XIV-XIV in FIG.
FIG. 15 is a system diagram of a flame detection system according to a third embodiment.
[Explanation of symbols]
10: Burner
12: Burner mounting plate, 12a: Through hole, 12b: Opening
14: Burner body
16: Distribution pipe, 16a: Base attachment piece, 16b: End attachment piece, 16c: Screw hole, 16d: Distribution hole
18: End pressing plate, 18a: Flange
22: Base presser plate, 22a: Flange, 22b: Opening
23, 24: Spot welding
26: Ceramic plate, 26a: Flame port, 26b: Slit
28: Screw
30: Heat exchanger
32: Outer cylinder, 32a: End plate, 32b: Exhaust port, 32c: Drain, 32d: Heating chamber, 32e: Condensing chamber, 32f: Partition plate
34: Condensation pipe
36: Heating pipe
37: First header
38: Second header
40: Third header
42, 42a: flame
50: ignition electrode
52: Fire outlet
53: End face
55: Ceramic plate, 55a: Flame port, 55b: Flame port
56: end face, 56a: horizontal portion, 56b: staggered portion
60, 62, 63: Arrow
65: Ceramic plate
66: End face
67: Burner body small assembly
68: Mistake
69: Arrow
72: Ceramic plate, 72a: Flame outlet
73: Ceramic plate, 73a: Flat part, 73b: Flame port,
75: Conductive coating
76: Frame rod
77: Detection unit
78: DC power supply
79: Electrical path

Claims (2)

A burner in which a plurality of ceramic plates formed with a group of flame mouths are combined at the end face,
A slit reaching the back surface from the surface of the ceramic plate is formed on the end surface of at least one of the adjacent ceramic plates ,
A burner characterized in that an end face where the slit is formed is in contact with an end face of an adjacent ceramic plate . A flame plate group is formed, and a ceramic plate used as a burner by combining a plurality of end faces.
A ceramic plate for a burner, wherein a slit that extends from the front surface to the back surface of the ceramic plate is formed on an end surface that contacts an adjacent ceramic plate when combined .

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