Electric kiln for firing ceramic pieces

Abstract

Disclosed is an electric kiln for firing ceramic pieces, which comprises a series of modules (M), each comprising an upper roof structure (Z1), a lower floor structure (Z3) and two opposite side wall structures (Z2). The upper roof structure (Z1) comprises at least one upper insulating body (1) formed by a folded and pressed ceramic fibre blanket formed by a ceramic fibre material comprising elongate threads and which is attached to a metal frame. Each of the two side wall structures (Z2) comprises a lateral insulating body (1') also formed by a folded and pressed ceramic fibre blanket. The lower floor structure (3) is formed by refractory bricks. The electric kiln further comprises a heating system surrounding two sets of tubular ceramic rollers (13) that are disposed above and below a motorised-roller conveyor belt (16) for pulling the ceramic pieces (P).

Description

[0001] DESCRIPTION

[0002] ELECTRIC KILN FOR FIRING CERAMIC PIECES

[0003] Object of the invention

[0004] The present invention falls within the technical field of equipment for the manufacture of ceramics or similar items that require the application of high temperatures. More specifically, it relates to an electric kiln for firing ceramic pieces with substantial improvements over commercially available electric kilns.

[0005] The essence of the invention is the use and arrangement of the insulating materials, as well as the different parts of the oven, all of which allows maintaining optimal temperature conditions, and achieving a substantial reduction in heating times and energy consumption.

[0006] Background of the invention

[0007] Currently, the vast majority of kilns used in the ceramics sector use fossil fuels as their heating fuel. Although some kilns use liquefied petroleum gas (LPG) or liquefied natural gas (LNG), the overwhelming majority run on natural gas.

[0008] The installed power of a kiln varies depending on the operating temperatures and the amount of material to be fired. For example, a kiln approximately 100 meters long with a usable opening 2.5 meters wide will have a burner power of between 4 and 7 MW. The exact power is primarily related to the firing time. For instance, a kiln designed to fire porcelain stoneware, which needs to maintain a temperature of around 1200°C, will require a minimum of 6 or 7 MW.

[0009] Another disadvantage of ceramic kilns that use natural gas, and by extension all those that use fossil fuels as a heat source, is that it takes up to two or three days from the time the kiln is started to heat up to the appropriate temperature. This is mainly due to the distribution of the insulation, which typically consists of layers of insulating refractory bricks, layers of insulation such as ceramic fibers, and a final layer of insulation such as rock fiber. In many cases, there are two or more layers of both insulating bricks and fiber insulation.

[0010] An example of a particular distribution would be the following:

[0011] - A first layer of quality 26 insulating brick (JM 26) with a thickness of 114 mm.

[0012] - A second layer of quality 23 (JM23) insulating brick of 114 thickness; where the two layers are joined and glued with refractory cement, and with cross-laid bricks, forming a 230 mm thick package, with refractory mortar.

[0013] - Next, a refractory ceramic fiber plate material with a working temperature of 1230°C and a thickness of 50 mm is placed.

[0014] - Then it places a refractory mineral fiber plate with a working temperature of 1000°C, classified as 50 mm thick.

[0015] - To finish, 30 mm of rock fiber.

[0016] In this example, a 230 mm thick block would be obtained with a temperature difference of 413°C between its two opposite faces; one face would reach a temperature of 1200°C and the other, at 787°C. This occurs after the temperature has stabilized for at least 24 hours after reaching the working temperature. Before this point, the temperature difference between the opposite faces will depend on the rate of temperature increase; therefore, the temperature is always raised very slowly to avoid stress within the refractory bricks.

[0017] Description of the invention

[0018] In order to achieve the objectives and avoid the drawbacks mentioned in the previous sections, the invention proposes an electric oven for firing ceramic pieces that includes a succession of modules, each of which comprises an upper roof structure, a lower floor structure, and two opposing side wall structures; where these three structures include various insulating materials in order to maintain the internal temperature within a firing chamber of the oven, along which runs a motorized roller conveyor located within a firing chamber; such that above and below the motorized roller conveyor within the firing chamber a heating system is applied to raise the temperature.The upper roof structure comprises at least one insulating roof unit formed by a folded and pressed ceramic fiber blanket made of elongated ceramic fiber strands. This folded and pressed ceramic blanket is nailed and secured to a metal frame. Lateral ceramic fiber panels are fixed to two opposite sides of the insulating roof unit. This entire assembly—the metal frame, the ceramic fiber blanket, and the lateral ceramic fiber panels—is located beneath an upper enclosure situated above the metal frame.

[0019] Each of the two side wall structures comprises a lateral insulating body also formed by a blanket of folded and pressed ceramic fiber as described above; where the two lateral insulating bodies include longitudinal recesses on which end sections of the insulating body assembly and the two lateral ceramic fiber plates of the upper roof structure rest.

[0020] The lower floor structure comprises a horizontal floor structure formed by refractory bricks, and two side walls on both opposite sides resting on the horizontal floor structure; where said side walls comprise layers of refractory brick and outside these at least two side plates of ceramic material.

[0021] The heating system for heating the oven comprises electric resistors coupled around two groups of tubular ceramic rollers above and below the motorized roller conveyor; where end sections of ceramic rollers are coupled in pairs of aligned holes located in the two side walls of the lower floor structure, and in the insulating bodies that form part of the two opposite side wall structures.

[0022] The motorized conveyor rollers are coupled at their end sections in gaps of two opposing longitudinal alignments of perforated bricks that separate the lateral insulating bodies from the side wall structures and the side walls from the lower floor structure.

[0023] On the outside of both sides of the lower floor structure, parallel to its two side walls, are fixed a first ceramic refractory fiber panel, a second ceramic refractory fiber panel, a third ceramic panel, a fourth refractory mineral fiber panel, and a fifth refractory mineral fiber panel. The outer faces of the upper insulating roof section and the two insulating side wall sections are covered with insulating layers of ceramic fiber.

[0024] On the exterior of the metal frame of the upper roof structure and on the exterior of the two side frames of the two side wall structures, there are first enveloping layers of insulating material, and on these, second enveloping layers of insulating material that cover the first layers of insulating material.

[0025] Each first layer of insulating material comprises a rock fiber material wrapped on its two opposite sides by two aluminum sheets.

[0026] The electrical resistance is coupled to a central part of each ceramic roller; where end sections of the ceramic roller include grooves through which terminal portions of the electrical resistance pass, being housed within the end sections of the ceramic roller; and where said end sections of the ceramic roller are filled with a ceramic fiber material.

[0027] The ends of the terminal portions of the (simple) filament of the electrical resistance are joined to an intermediate section (between the filament of the electrical resistance and the threaded rod) by means of the braiding technique to another section of the same filament (forming two filaments braided together) and the end of this section of filament braided together is welded to a threaded stainless steel rod, which comes out of each end of the ceramic roller to the outside of it in order to be connected to the power cable by means of a flat terminal and a nut.

[0028] Each of the upper insulating body assemblies and their two side fiberglass plates of the upper roof structure includes several through-holes that reach the two side fiberglass plates. Stainless steel rods are inserted through these holes, and retaining washers are welded to their ends. As mentioned in the previous paragraph, each upper insulating body assembly and its two side plates are fixed to the metal frame by means of sharp plates nailed to the insulating body. These plates have holes through which the rods pass, and they are welded to the metal frame.

[0029] With the described insulation system, a temperature gradient of 105°C is achieved within the refractory brick itself at 24 hours, a working temperature regime with much lower stresses than the 413°C in traditional arrangements.

[0030] At the same time and during temperature increases, the gradient within the refractory brick piece is much smaller, since it heats up all at the same time due to the high back insulation, thus avoiding stresses within the material itself.

[0031] If, in addition, only a few hours elapse between when we disconnect the oven and when we reconnect it, the insulation surrounding the inside of the oven hardly cools down and energy consumption is minimal.

[0032] If the kiln is left on without any material being fed into it, energy consumption is minimal because there is no circulating air inside (since it lacks combustion air). This consumption is approximately 20% of the kiln's total energy consumption at full production capacity, without any energy-saving measures. By slightly adjusting the operating temperature curve by lowering it by 200-300°C and stopping the first few control groups, energy consumption can be reduced to as low as 10%, bringing it closer to the kiln's total energy consumption at full production capacity.

[0033] All of this allows us to make ovens that traditionally could not be stopped, now able to work on reasonable schedules, and to make them ready to work when the operators join their workday, without the need to work at night, or on weekends.

[0034] This allows for greater flexibility when starting and stopping production, as well as avoiding the imposition of additional workdays or shifts due to the high energy consumption involved in keeping the oven in working condition, or the time it takes for the internal temperature to stabilize once production has started.

[0035] In this way, following a structure similar to that of traditional ceramic kilns, there will be an entrance channel with a suitable conveyor to move the ceramic pieces, such as a roller conveyor, and there will be the upper roof structure, the opposite side areas (side wall structures) and the lower floor structure; all of them thermally insulated in order to maintain the internal temperature of the kiln within the firing chamber.

[0036] There are two main insulation zones to distinguish: the lower zone or floor of the oven (lower floor structure), and the upper zone or roof (upper roof structure). Each zone has a specific insulation arrangement that maintains the internal temperature and reduces heat loss to the outside, thus also maintaining a suitable external temperature.

[0037] As previously mentioned, the upper roof structure is made of a ceramic fiber blanket capable of withstanding temperatures exceeding 1400°C. This material is folded and pressed to achieve the appropriate density. Since the ceramic blanket is composed of long ceramic fiber strands, it is created by continuously folding the blanket without cutting it, ensuring that no strands remain that could contaminate the product being baked.

[0038] This entire assembly is mounted on a metal frame, where a series of insulators are already installed on the exterior. These insulators create a vapor barrier to prevent air from entering or escaping, as the fiber is highly porous.

[0039] This exterior insulation is also used to reduce heat loss at low temperatures. The fiber withstands the initial thermal shock from 1200°C to 300°C; however, its performance deteriorates at lower temperatures, requiring the use of low-temperature insulation to complete the insulation and prevent heat loss. This entire system ensures that the exterior temperature does not exceed 70°C with an ambient temperature of 25°C.

[0040] The two refractory brick side walls of the floor structure limit the width of the lower part of the cooking chamber, so that, as mentioned above, other insulating material plates have been placed outside these side walls, with a somewhat particular design that is described in more detail below.

[0041] Starting from the inner, or heat-facing, side, the first layer of refractory brick (each lower side wall) is capable of withstanding 1450°C, followed by the first layer of insulating ceramic tile, rated at 1430°C. Next comes the second layer of insulation, but with a lower temperature rating since temperatures at this point are not as high. Subsequently, up to three more layers of insulation (third, fourth, and fifth) are added, with decreasing operating temperatures until reaching the area closest to the exterior, where a low-temperature insulating material is used. Between the fourth and fifth layers, a waterproofing layer in the form of aluminum foil or similar material is added. Furthermore, this outer material is designed to have vapor barrier properties, thus preventing the escape of hot gas to the outside and the resulting heat loss.A second layer can be included to reinforce this vapor barrier aspect.

[0042] By selecting the appropriate materials and thicknesses, we can ensure that, during continuous operation, the temperature on the outer surfaces of the side insulating bodies corresponding to the highest-temperature side wall structures never exceeds 70°C. An exception to this is the area around the ceramic roller guides and the heating element connections, where the temperature may increase slightly depending on the ceramic fiber sealing applied. It should be noted that there is a small air gap of approximately 80 mm on the outside of each of the outer surfaces of the side insulating bodies.

[0043] With all this, we ensure that thermal leaks on the outside of the oven are minimal, and with this we also achieve optimal energy efficiency of the oven.

[0044] In addition to the new insulator arrangement, a resistance system assembly has been developed as previously described. This, combined with the aforementioned features, allows for rapid heating and temperature maintenance with low energy consumption. To achieve this, the electrical resistors have been developed with a specific arrangement that enables the complete oven assembly to meet the required technical specifications.

[0045] As previously mentioned, each electrical resistor is manufactured starting with a ceramic roll capable of withstanding high temperatures and rapid heating without undergoing expansion that compromises the material's integrity or cracking due to overheating. Furthermore, this ceramic material must be able to support its own weight and the weight of the material used to create the resistor's filament at high temperatures. This resistor is manufactured by winding a wire of a metallic alloy, such as an iron-chromium-aluminum alloy, or any other alloy capable of withstanding 1500°C and also providing rapid heating and cooling.

[0046] This filament must have a diameter between 2 and 5 millimeters depending on the length of the resistor and the power to be supplied by each resistor.

[0047] It should be noted that the diameter of an electrical resistor has a direct relationship with the current that can flow through it. This relationship can be best understood using Ohm's Law and the resistance equation for a cylindrical conductor, such as a wire.

[0048] It should also be noted that the electrical resistances have been calculated in a standardized way, so that, in one example of the embodiment of the invention, their power ratings are not less than 5kW (at 380V) nor greater than 20kW.

[0049] Thus, another limit will be determined by the winding capacity of the respective ceramic roller, that is, due to the dimensions of the ceramic roller, on which the electrical resistors will rest, we can make "more or fewer turns" without risk of the electrical resistor winding or melting because the filaments of the electrical resistors are too close to each other.

[0050] On the other hand, although it might be feasible to manufacture resistors with a diameter smaller than the proposed two millimeters, we would significantly reduce their reliability. Reducing the cross-section increases the risk of breakage due to impacts, mechanical stress, or overheating, as the reduced cross-section would raise the temperature. Therefore, despite being possible, this approach is not recommended.

[0051] Regarding the upper limit, the electrical resistance could have a diameter greater than 5 mm, but we would need a lot of energy to raise its temperature, so it would not be advisable, although possible, to increase the section, also increasing the diameter of the ceramic roller to prevent it from winding or the filaments of the resistors from touching each other.

[0052] Preferably the ceramic cylinder will have a diameter between 50 and 80 mm depending on the loading width of the oven, this diameter being mainly related to the need to support its own weight.

[0053] This diameter range has been estimated for kilns with a usable opening width of up to 2,500 mm. Therefore, for widths of 3,000 mm to 3,500 mm, the diameter can be as small as 100 mm or even 120 mm. The diameter depends on the resistance to deformation at temperature due to the weight of the ceramic roller itself plus the filament wound around it. The aim is to select a diameter that allows for the feasible replacement of the heating elements in the event of filament breakage.

[0054] Therefore, we could increase or decrease the size of the ceramic roller depending on the size of the oven in which the electric heating elements will be mounted, taking into account the following points:

[0055] - Ensure the ceramic roller is not too small in diameter, so the filament does not wind up due to the rollers being too close together.

[0056] - The ceramic roller should not be excessively large, so that its weight can be minimized as much as possible, in order to facilitate the assembly and disassembly of the electrical resistors.

[0057] Likewise, the metal filament of each electrical resistor will be mounted like a spring, wound around the straight (and hollow) ceramic roller. This filament will be slightly tensioned (to prevent breakage from contact with the resistor). To achieve this, a longitudinal groove will be machined at each end of the ceramic roller, starting at each end and ending at the point where the winding begins, or the heating zone of the resistor.This grooving will be done so that each of the ends of the (single) filament that makes up the resistor is inserted perpendicularly to the direction of the roller, towards the inside of it, performing the function of fixing the wound filament and giving it a minimum but sufficient tension to avoid breakage by contact between the different wires or turns of the resistor winding, and thus guaranteeing the uniformity of radiation, and therefore of temperature along the entire length of the resistor.

[0058] Within this groove, we will see how the single filament that makes up the heating zone or area of ​​greatest heat emission of the resistance is joined, through the braiding technique, with another identical filament. This means that when the two filaments are added together, the resistance decreases to a point where the heat emitted in this area is much lower than that of the center of the resistance.

[0059] At the opposite end from where the single filament joins the other identical filament, we will weld both filaments to a threaded stainless steel rod.

[0060] Thus, at one end of the threaded rod we will find a weld that will join it with the double braided filament, and at the other end of the threaded rod we will find the exterior of the resistance, an exterior that we will reach after advancing through the center of the ceramic roller, which we remember will be hollow, and which must be properly filled with ceramic fiber insulation with a classification temperature of 1430°C, in order to avoid temperature leaks, this type of insulation insulates both thermally and electrically.

[0061] This system includes a series of sensors and PID controllers to monitor and control the temperature throughout the oven. Furthermore, with this control system, once the appropriate temperature range for the material being fired is reached, electrical resistors and groups of resistors can be switched on and off as needed, maintaining the temperature within acceptable limits for the material being processed. This system is combined with a set of thyristors controlled by the aforementioned PID controllers, allowing us to maximize the energy efficiency of the equipment's heating system.

[0062] These thyristors and PIDs allow us to guarantee that the filament temperature will never exceed 1300 or 1350°C, thus maximizing the lifespan of the filament, and therefore, of the resistor.

[0063] Brief description of the figures

[0064] To complete the description and to aid in a better understanding of the characteristics of the invention, this descriptive document is accompanied, as an integral part thereof, by a set of figures which, for illustrative and non-limiting purposes, depict the following:

[0065] Figure 1 - Shows a cross-section of a module of the electric oven for cooking ceramic pieces, the object of the invention.

[0066] Figure 2 - Shows a longitudinal section of the oven module of the invention.

[0067] Figure 3.- Shows an electrical resistance mounted around a ceramic cylinder that forms part of the oven of the invention.

[0068] Figure 4 - Shows a longitudinal view of the electric oven for firing ceramic pieces.

[0069] Figure 5.- Represents a perspective view of an interior space of the cooking chamber. It essentially shows a lower floor structure, a side wall structure, and a corresponding enclosure with the chassis of an oven module.

[0070] Figure 6.- Shows a perspective view of an upper roof structure of the oven module.

[0071] Figure 7 - Shows another view similar to that shown in Figure 5, which also includes the upper roof structure.

[0072] Figure 8.- Shows a view similar to that shown in Figure 7.

[0073] Figure 9.- Shows an exploded perspective view of the electric oven module.

[0074] Description of an example of embodiment of the invention

[0075] Considering the numbering adopted in the figures, the electric oven for cooking ceramic pieces P comprises a succession of modules M with a characteristic arrangement of different insulating materials, and characteristic means for carrying out the heating of the electric oven for cooking the ceramic pieces P, so that the ceramic pieces P are located inside a cooking chamber C with an elongated configuration in the form of a longitudinal channel.

[0076] Thus, the kiln comprises a succession of modules M with a tunnel-shaped configuration, such that along the succession of modules M and within the firing chamber C, a motorized roller conveyor 16 is mounted which is used to drag and advance said ceramic pieces P supported on the motorized rollers 16; wherein the tunnel-shaped configuration of the kiln includes an inlet and an outlet arranged at opposite ends of the firing chamber C along which the motorized roller conveyor 16 runs.

[0077] The electric oven of the invention comprises an upper roof structure Z1, a lower floor structure Z3, and two opposing side wall structures Z2; where these three zones Z1, Z2 and Z3 comprise insulating materials for the purpose of maintaining the internal temperature within the cooking chamber C delimited between the three zones mentioned Z1, Z2 and Z3.

[0078] The three zones Z1, Z2 and Z3 of each of the electric oven modules have particular arrangements of insulators and ceramic materials that allow maintaining the internal temperature of the cooking chamber C and thus reducing leaks to the outside, also maintaining an adequate external temperature.

[0079] The upper roof structure Z1 comprises an upper insulating body 1 formed by a ceramic fiber blanket with a temperature rating of 1430°C. This material is folded and pressed to achieve an average density of 200 kg / m³. This ceramic blanket 1 is made of ceramic fiber material with elongated strands, and is manufactured by continuously folding the ceramic blanket without cutting it, so that no cut strands remain that could contaminate the product or material to be fired, from which the ceramic pieces P are obtained.

[0080] With this average density of 200 kg / m3, extreme zones near the folds are achieved where the density of the ceramic blanket 1 is higher (290 kg / m3), and intermediate zones where the density is lower (170 kg / m3).

[0081] This entire folded and pressed package of ceramic blanket, which constitutes the upper insulating body 1, has high-temperature ceramic fiber panels 3 added to its two opposite sides. In other words, these high-temperature ceramic fiber panels 3 are added to each side of the upper insulating body 1, providing rigidity and strength to the upper roof structure 1. It should be noted that these ceramic fiber panels 3 are not used on the two opposite side wall structures Z2.

[0082] On the exterior of each corresponding oven module, with the upper roof structure Z1 and the two opposing side wall structures Z2, first layers 17 of insulating material are placed, and second layers 18 of insulating material cover the first layers 17.

[0083] Each of these first layers 17 comprises a rock fiber material wrapped on its two opposite sides by two aluminum sheets as vapor barrier elements; where the second layers 18 are low temperature.

[0084] The entire upper roof structure Z1 of each module M is located below an upper enclosure 5 which is in turn located above a metal frame 2; where said enclosure 5 forms part of the chassis of the oven assembly, while the metal frame is a constituent reinforcement of the chassis of each module M of the oven.

[0085] In each of the oven's side wall structures Z2, another insulating unit (side insulating body T) is made, equivalent to that of the upper roof structure 1. Ceramic fiber plates are not mounted at the longitudinal ends of these units to facilitate sealing between the different insulating units that make up each side wall structure Z2. The longitudinal ends protrude from the respective module M, allowing the side walls of adjacent modules M to be pressed together. Each side insulating body T is fixed to a metal side frame or support 2', as is the case with the upper insulating body 1 of the upper roof structure Z1.In contrast, the outer side face of the pressed side insulation T (ceramic blanket) of each Z2 side wall structure does feature the vapor barrier (first layer 17) and the low-temperature insulation (second layer 18, outer insulation) described above, as in the case of the Z1 roof structure. This creates a tunnel configuration between the Z2 side wall structure, the Z1 roof structure, and the Z3 floor structure.

[0086] The entire assembly of the upper roof structure Z1, comprising the upper insulating body 1, the first insulating layer 17, and the second outer insulating layer 18, is nailed and secured to the metal frame 2, in which the lateral ceramic insulating plates 3 are located, corresponding to two opposite zones of said metal frame 2. This entire assembly described is located below an upper enclosure 5 located above the metal frame 2.

[0087] The first 17 insulating layers provide a vapor barrier to prevent air from entering or escaping, as the fiber of the ceramic blankets 1, T (top and side insulating layers) is highly porous. These first 17 layers (which include the aluminum sheets) also help to reduce heat loss at low temperatures. It's worth noting that the initial thermal shock, from 1200°C to 300°C, is absorbed by the fiber material of the ceramic blanket 1, but the low-temperature ceramic fiber side insulators 3 then provide the final insulation to prevent heat loss. This entire system ensures that the exterior temperature does not exceed 70°C with an ambient temperature of 25°C.

[0088] On the other hand, some outer faces of the upper insulating body 1 of the roof and of the two lateral insulating bodies T, are covered by insulating layers 4 of ceramic fiber; where said insulating layers 4 are interleaved between the frames 2, 2' and the respective insulating bodies 1, T.

[0089] In a lower section of the side wall structures Z2 and in an upper section of the lower floor structure Z3, there are electric heating elements 12 that provide the necessary heat inside the firing chamber C of the kiln; these electric heating elements 12 are mounted on two groups of cylindrical-tubular ceramic rollers 13. Between the two groups of ceramic rollers 13 with their electric heating elements 12 is located the motorized roller conveyor 16, which are coupled at their ends to pairs of transversely aligned holes that form part of two opposing longitudinal rows of perforated bricks 19 interspersed between the side wall structures Z2 and the floor structure Z3.

[0090] The end sections of said ceramic rollers 13 are coupled in pairs of aligned side holes 20, 21 that are located in two opposing refractory brick side walls 6 that form part of the lower floor structure Z3, and in the insulating bodies 1' that form part of the opposing side wall structures Z2.

[0091] The firing chamber C has been made as low as possible at its top, leaving only 110 mm between the electric heating elements and the motorized rollers 16 of the ceramic piece conveyor P, and 40 mm above the heating elements to facilitate assembly. In total, with the 60 mm diameter of the electric heating elements mounted around the ceramic rollers 13, there is a distance of 210 mm from the ceramic rollers 13 to the ceiling surface.

[0092] On either side of the ceramic rollers 13 of the lower floor structure Z3, the two side walls 6 that define the sides of the lower part of the kiln's firing chamber have been mounted, with a somewhat unique design. Furthermore, the kiln floor structure has also been insulated from the exterior, all as described below.

[0093] Thus, each of the side walls 6 comprises a layer of refractory brick, which is capable of withstanding 1450°C, followed by a first ceramic plate 7 of 50 mm of refractory fiber with a working temperature rating of 1430°C, then a second ceramic plate 8 of 50 mm of refractory fiber with a working temperature rating of 1240°C.

[0094] Subsequently, a third ceramic tile (9), 50 mm thick, made of refractory fiber with a working temperature rating of 1100°C, was placed on each side of the lower floor structure Z3, attached to the second ceramic tile. This was followed by a fourth tile (10), 50 mm thick, made of refractory mineral fibers with a working temperature rating of 1000°C, and a fifth tile (11) made of refractory mineral fibers with a working temperature rating of 800°C. At this point, we used a low-temperature material: rock fiber insulation wrapped in aluminum. Its two aluminum sheets create a vapor barrier effect. Additionally, a double layer of reflective aluminum insulation was used to further enhance the vapor barrier effect. This layer, being thin, is not shown in the figures.

[0095] A particular arrangement of the materials described in the preceding paragraph is as follows: the fifth plate 11 of refractory mineral fibers with working temperature, a micro-porous insulator, with a temperature rating of 800°C, a rock fiber insulator with aluminum on both sides. This is used to guarantee the “impermeability” of the insulating material, since being generally porous, it may have losses of efficiency.

[0096] In the last part and for areas of the oven with lower temperatures, a reflective insulating material is installed due to its better ease of handling and similar behavior, but with a much lower classification temperature (200°C) and therefore not suitable for areas of higher temperature.

[0097] At the lowest part of the lower floor structure Z3 is mounted a horizontal floor structure 6' formed by several layers of refractory bricks that are arranged below the side walls 6 formed by the refractory bricks and below the various plates 7, 8, 9, 10 and 11.

[0098] As shown more clearly in Figure 7, each upper insulating body assembly 1 and its two side fiber plates 3 rest on longitudinal recesses 23 of opposing side insulating bodies T that form part of the two side wall structures Z2.

[0099] In turn, as shown in Figure 6, each of said assemblies of upper insulating body 1 and its two side fiber plates 3, includes several through-holes 24 that reach the two side fiber plates 3; where through said holes stainless steel bars 25 are inserted, at the ends of which washers 26 are welded to prevent said assemblies from coming loose during the assembly and transport process.

[0100] In turn, the assembly of each upper insulating body 1 and its two side plates 3 is fixed to the metal frame 2. For this purpose, sharp plates 14 are provided, nailed to the insulating body 1, and which include holes through which the bars 25 are inserted. In turn, these plates 14 are welded to the metal frame 2.

[0101] On the other hand, the entire upper roof structure Z1 rests on a pair of lateral angle profiles 27 of the enclosure 5 of each module M of the oven; where said enclosure 5 constitutes the chassis of each module M. More specifically, some end sections of the metal frame 2 of the upper roof structure Z1 rest on said lateral angle profiles 27.

[0102] With all this, we ensure that, during continuous operation of the oven, the temperature on the outer surfaces of the lateral insulating bodies T never exceeds 70°C. This is the area of ​​highest temperature, with the exception of the area around the passage of the refractory rollers 13 (which are made of perforated refractory brick through which the ceramic rollers 13 pass; these rollers, among other functions, support the upper walls of the oven and seal the area well) and the connections of the electrical heating elements 12. The temperature may increase slightly, depending on the amount of bulk ceramic fiber sealing applied. All of this results in minimal heat loss on the exterior of the oven, and also achieves significantly improved energy efficiency.

[0103] In the lower floor structure Z3, the minimum possible distance between the electrical resistors 12 and the material of the ceramic pieces P to be fired has been sought, but the distance from the floor to the material of the ceramic pieces P has been sacrificed, considering it more important to open some lateral accesses to be able to clean if necessary under the electrical resistors 12.

[0104] These maintenance procedures are necessary to keep the radiation and distance from the floor constant, so that if material from the ceramic pieces P falls onto the floor of the oven, since it is not uniform, the radiation distance from this surface (debris) to the material (ceramic pieces) supported on the motorized rollers 16 will change, and the emissivity will change, which can easily change the firing behavior of the oven as well as the temperatures at which it is capable of working.

[0105] Furthermore, the method of heating the oven has been studied, and heating by means of 12 simple and straightforward electric resistances has been chosen, but with some particularities to obtain maximum performance, as described below.

[0106] Therefore, the ceramic roller 13, of sufficient diameter, for example 50 mm, has been wound with a filament made of a ferritic iron-chromium-aluminum alloy (FeCrAl alloy) that can be used at temperatures up to 1400°C (2550°F). This alloy is characterized by its high resistivity and very good oxidation resistance.

[0107] Each ceramic roller 13 is highly resistant to thermal shock, so it does not break under rapid temperature increases. The end sections of this ceramic roller 13 include lateral grooves 22 to facilitate the mounting of the electric heating element filament 12; this element is wound uniformly along a central portion of the ceramic roller 13 to heat the entire oven channel with the same heat input. This small detail ensures that cooking with electric heating elements 12 does not suffer from the heating problems that occur with gas burners, which have issues with temperature distribution across the width of the oven channel.

[0108] The grooves 22 of each ceramic roller 13 are provided to allow terminal portions 12a of the electrical resistance 12 to pass through them, being housed within the end sections of said ceramic roller 13. Said end sections of the ceramic roller 13 are filled with a ceramic fiber material 28.

[0109] The terminal portions 12a of the electrical resistor 12 are formed by a double-filament structure braided or linked together. Each end of the terminal portions 12a of the electrical resistor 12's filament is welded to a stainless steel threaded rod 15, which is also housed within each of the end sections of the tubular ceramic roller 13. This rod protrudes between 4 and 8 centimeters from the ends of the ceramic roller 13, facilitating the connection of the electrical resistor 12 to its respective terminal. The welded joint between the terminal portions 12a and the threaded rods 15 is a parallel weld.

[0110] The ceramic fiber filling 28 stabilizes and holds the threaded rods 15, as well as the terminal portions 12a of the electrical resistance 12.

[0111] The terminal portions 12a of the filament of each electric resistor 12 are mounted within the end sections of the ceramic roller 13 to isolate the temperature inside the oven chamber, preventing it from escaping through the ceramic roller 13 of the electric resistor 12 to the outside, and also provide electrical insulation. Furthermore, the solder joining the ends of the electric resistor 12 to the two threaded stainless steel rods 15 facilitates a heat-free electrical connection of the electrical conductor (copper) to the ferritic alloy filament of the electric resistor 12.

[0112] The two terminal portions of the 12a resistor are double-folded, which increases conductivity and decreases resistance, resulting in minimal voltage drop and virtually no heat generation under current. However, it withstands the surrounding temperature well.

[0113] As previously mentioned, the resulting space in the hollow end sections of each ceramic roller 13 after mounting the filament constituting each electrical resistor 12, is filled with ceramic fiber material, in order to protect said filament and thus achieve insulation with ceramic material that is neither electrically nor thermally conductive.

[0114] The filament of the electric heating element 12 is made of wire 3.5 mm thick (minimum 3 mm), wound uniformly along the entire length of the ceramic roller 13, in order to heat the entire firing chamber C of the oven with the same heat input. This small detail ensures that firing with electric heating elements 12 does not suffer from the problems associated with gas burners, including the uneven temperature distribution across the firing chamber C of the oven.

[0115] The electrical resistors 12 are arranged transversely to the direction of travel of the ceramic parts P, parallel to the motorized rollers 16. The assemblies of the ceramic rollers 13 and the electrical resistors 12 are located at a minimum distance from the motorized rollers 16, both above and below. Pairs of electrical resistors 12, rated at 5750 W and 190 V, have been connected in series, resulting in each pair having a resistance of 11,500 W and 380 V. These pairs of electrical resistors 12 have then been connected in groups of six, in a delta configuration, forming groups of 34.5 kW and 380 V resistors 12.

[0116] Two groups of these form a heating zone. Each heating zone is controlled by a PID temperature controller and a thermocouple that acts on a 120 A three-phase thyristor, which continuously switches the 12 electrical resistors on and off to prevent overheating. A power surplus of 25% of the installed power has been calculated. This was done to prepare the system for the continuous operation of potentially very high-temperature materials (above 1200°C) for the manufacture of ceramic parts P.

[0117] By assembling with a thyristor, the 12 electrical resistors operate at a maximum of 80% of their maximum potential, being disconnected for a maximum of 20% of the time. This means that the 12 electrical resistors are continuously switched on and off, maintaining a temperature below 1350°C, and therefore lasting much longer.

[0118] The power of the 12 electric heating elements has also been oversized, in order to heat the oven quickly and raise it to its working temperature in a few hours.

[0119] In the floor insulation, as shown more clearly in Figure 1, the bottom layer of bricks is higher than those above it. This thicker layer acts as a retaining structure, like a dam, at the ends of the floor structure of the respective module. It is then filled with refractory concrete and any leftover brick scraps from the wall construction. The surface is then leveled, and the upper layers that form the insulation of the entire floor structure are built on top of this solid base.

[0120] The layers closest to the heat zone of the equipment have higher quality and resistance to high temperatures, withstanding more than 1400°C

Claims

CLAIMS 1. An electric kiln for firing ceramic pieces, comprising a series of modules (M), each of which includes an upper roof structure (Z1), a lower floor structure (Z3), and two opposing side wall structures (Z2); wherein the three structures (Z1, Z2, and Z3) include various insulating materials for the purpose of maintaining the internal temperature within a firing chamber (C), along which runs a motorized roller conveyor (16) located within a firing chamber (C); and wherein above and below the motorized roller conveyor (16) within the firing chamber (C) a heating system is applied to raise the temperature; characterized in that: - the upper roof structure (Z1) comprises at least one upper insulating body (1) formed by a folded and pressed ceramic fiber blanket made of a ceramic fiber material of elongated threads; wherein this entire folded and pressed package of the ceramic blanket is nailed and secured to a metal frame (2); wherein lateral ceramic fiber plates (3) are fixed to two opposite sides of the roof insulating body (1); and wherein this entire assembly of the metal frame (2) with the ceramic fiber blanket (1) and the lateral ceramic fiber plates (3) is located below an upper enclosure (5) located above the metal frame (2); - each of the two side wall structures (Z2) comprises a lateral insulating body (T) also formed from a folded and pressed ceramic fiber blanket; wherein the two lateral insulating bodies (1') include longitudinal recesses (23) on which end sections of the insulating body assembly (1) and the two lateral ceramic fiber plates (3) of the upper roof structure (Z1) rest; and wherein each lateral insulating body (T) is fixed to a lateral frame (2'); - the lower floor structure (3) comprises a horizontal floor structure (6') formed by refractory bricks, and two side walls (6) on both opposite sides that are settled on the horizontal floor structure (6'); wherein said side walls (6) comprise layers of refractory brick and outside these at least two side plates of ceramic material; - the heating system comprises electrical resistors (12) coupled around two groups of tubular ceramic rollers (13) above and below the motorized roller conveyor (16); wherein end sections of the ceramic rollers (13) are coupled in pairs of aligned holes (20, 21) located in the two side walls (6) of the lower floor structure (Z3), and in the side insulating bodies (T) that form part of the two opposing side wall structures (Z2); - the motorized rollers (16) of the conveyor are coupled at their end sections in gaps of two opposing longitudinal alignments of perforated bricks (19) that separate the lateral insulating bodies (T) from the side wall structures (Z2) and the side walls (6) from the lower floor structure (Z3). 2.- Electric oven for baking ceramic pieces, according to claim 1, characterized in that outside both sides of the lower floor structure (Z3), parallel to its two side walls (6), there is a first ceramic plate (7) of refractory fiber, a second ceramic plate (8) of refractory fiber, a third ceramic plate (9), a fourth plate (10) of refractory mineral fibers; and a fifth plate (11) of refractory mineral fibers. 3.- Electric oven for baking ceramic pieces, according to any one of the previous claims, characterized in that some outer faces of the upper insulating body (1) of the roof and of the two lateral insulating bodies (T), are covered by insulating layers (4) of ceramic fiber. 4.- Electric oven for cooking ceramic pieces, according to any one of the previous claims, characterized in that on the outside of the metal frame (2) of the upper roof structure (Z1) and on the outside of the two side frames (2') of the two side wall structures (Z2), there are arranged first enveloping layers (17) of insulating material, and on these, second enveloping layers (18) of insulating material that cover the first layers (17) of insulating material. 5.- Electric oven for cooking ceramic pieces, according to claim 4, characterized in that each first layer (17) comprises a rock fiber material wrapped on its two opposite faces by two aluminum sheets.

6. Electric oven for firing ceramic pieces, according to any one of the preceding claims, characterized in that the electric resistance (12) is coupled to a central part of each ceramic roller (13); where end sections of the ceramic roller (13) include grooves (22) through which terminal portions (12a) of the electric resistance (12) pass, being housed within the end sections of the ceramic roller (13); and where said end sections of the ceramic roller (13) are filled with a ceramic fiber material (28). 7.- Electric oven for cooking ceramic pieces, according to claim 5, characterized in that the terminal portions (12a) of the electric resistance (12) are formed by a double braided filament structure. 8.- Electric oven for cooking ceramic pieces, according to claim 5, characterized in that the ends of the terminal portions (12a) of the filament of the electric resistance (12) are welded to a threaded rod (each end of the resistance) (15) of stainless steel.

9. Electric oven for firing ceramic pieces, according to any one of the preceding claims, characterized in that: - each of the upper insulating body assemblies (1) and its two side fiber plates (3) of the upper roof structure (Z1), includes several through-holes (24) reaching the two side fiber plates (3); where through said holes stainless steel bars (25) are inserted, at the ends of which retaining washers (26) are welded; - the assembly of each upper insulating body (1) and its two side plates (3), is fixed to the metal frame (2) by means of sharp plates (14) nailed into the insulating body (1), and which include holes through which the bars (25) are inserted; where said plates (14) are welded to the metal frame (2).

Images