A METHOD FOR FIRING CERAMIC PRODUCTS AND A KILN THEREFOR

Technical Field

The invention relates to a method for firing ceramic products, as well as a ceramic kiln able to actuate the method. Background Art

As is known, kilns for ceramic products can be sub-divided substantially into two broad categories: tunnel kilns and intermittent kilns.

Tunnel kilns are especially suitable for firing ceramic slabs and tiles, and briefly comprise a long tunnel structure, clad in refractory material, along which the products to be fired are advanced on board self-propelling trucks or on rotatable support rollers. The tunnel is provided with heat-regulating systems which enable the kiln to be sub-divided longitudinally into a plurality of successive sections at different temperatures, such that during the advancing the ceramic products are subjected to all the stages of the firing cycle. The firing cycle comprises both stages of heating and cooling, more or less rapid, and is generally represented by a diagram which relates the temperatures of the various sections of the kiln with their longitudinal position internally of the kiln.

Differently, intermittent kilns usually comprise a single firing chamber, delimited by walls clad in refractory material, internally of which the products to be fired remain immobile.

Intermittent kilns are especially used for firing large-size products and/or products having complex shapes, possibly with variable-thickness walls, among which for example sanitary articles such as water closet pedestals, wash basins and bathtubs, but also kitchenware, ceramic insulators for insulating ducts for high-tension electrical lines, or ceramic pipes for sewer conduits, and the like.

The firing cycle in intermittent kilns is obtained via a progressive variation in the temperature internally of the single firing chamber, and is thus generally

represented by a diagram which relates the temperature of the kiln to the time the products spend internally thereof.

In particular, the firing cycle normally comprises three successive stages, including a first stage of uniform heating up to temperatures of about 1200- 1300 ⁰ C, an intermediate stage at a constant temperature, and a final stage of cooling to return the products to ambient temperature. The cooling stage is divided into a first rapid cooling stage, during which the products are rapidly brought to a temperature of about 600 ⁰ C, followed by a stage of slow cooling up to reaching ambient temperature. The stage of slow cooling enables the finished products to be free of defects and/or residual tensions, which otherwise might generate hairline cracks, larger cracking and sometimes actual breakage.

Suitable regulation of the stages of rapid and slow cooling further enable the ceramic material to take on different chemical/physical structures on which the mechanical properties of the finished product depend.

The regulation of the temperature in the firing chamber is generally obtained by means of a plurality of free-flame burners, which are installed in suitable positions on the lateral walls and/or on the vault walls and on the bottom walls of the kiln, from where they face directly internally of the firing chamber. Each burner is associated to a fuel supply conduit, typically methane gas, and a comburent supply conduit, typically air from the outside at ambient temperature, which open in proximity of the internal surface of the kiln wall, to fire up a flame which develops freely in the firing chamber. Relative valve means are associated to the supply conduits, which valve means regulate the flow rate of the combustible and the comburent, such as to regulate the intensity of the flame and thus the heat generated. Differently to tunnel kilns, in which the overall duration of the firing cycle depends on the advancement speed of the products in the kiln, i.e. the time spent in the various sections of the kiln, it is obvious that the overall duration of the firing cycle in an intermittent kiln depends on the rapidity with which the temperature internally of the single firing chamber of the kiln changes.

lntermittent ceramic kilns however exhibit a rather high level of thermal inertia which considerably influences the temperature gradient internally of the firing chamber.

In particular, the thermal inertia has a negative effect, especially during the rapid cooling stages, during which the ceramic products could be cooled more rapidly than normally happens, without this causing any defect in the final product, but the duration of which is in reality longer than is necessary, precisely due to the inevitable thermal inertia of the kiln.

At least partly to solve this problem, air has been introduced into the kiln, through the supply conduits of the extinguished burners, such as to dissipate by convection a part of the internal heat and more rapidly cool the ceramic products, reducing the overall duration of the firing cycle.

This method is however not applicable to all types of burners used in intermittent ceramic kilns. In particular, the prior art comprises free-flame burners incorporating a heat exchanger, in which the comburent air is pre-heated by an air flow and hot combustion fumes exiting the kiln, the temperature of which basically coincides with the temperature in the firing chamber.

The flow of hot gases moves along an outflow conduit afforded in the burner body, which outflow conduit opens into the kiln and crosses the heat exchanger.

When the flame is lit, the preheating of the comburent air enables greater efficiency and the overall performance of the kiln, thus reducing fuel consumption. When the flame is extinguished, it is not possible to supply a cooling air draught from the supply conduit of these burners, as it would in any case be heated by the hot gases crossing the heat exchanger, and thus would reach the firing chamber at too-high a temperature to be able to effectively cool the products contained therein, and would add to the overall firing cycle duration.

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Disclosure of Invention

An aim of the present invention is to reduce the time required for completing the firing cycle in an intermittent ceramic kiln provided with burners of this type, by intervening in particular on the rapid cooling time. A further aim of the invention is to attain the above-mentioned objective with a simple, rational and relatively inexpensive solution.

These aims are attained by the characteristics of the invention reported in the independent claims. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention. In particular a device is provided for firing products internally of a ceramic kiln, in which the kiln comprises at least a burner provided with a comburent supply conduit, a hot gas outflow conduit from the kiln, and a heat exchanger for operating a heat exchange between the fluids which flow through the supply conduit and the outflow conduit. The method of the invention comprises cooling the products by means of introducing a cooling fluid into the kiln through the outflow conduit of the at least a burner when the burner is off.

Thanks to the solution, the cooling fluid flows through the outflow conduit in a counter-current direction, i.e. in an opposite direction with respect to the normal direction of the hot gases, preventing them from crossing the heat exchanger. The cooling fluid thus reaches the firing chamber without experiencing any heating in the heat exchanger, thus obtaining a rapid cooling for products and an effective reduction of the overall duration of the firing cycle. By the term "cooling fluid" reference is generically made to any fluid which is at a lower temperature with respect to the firing chamber.

The method of the invention preferably includes using air from the outside, for example at ambient temperature.

In a preferred aspect of the invention, during the cooling stage the method includes contemporaneously supplying also the comburent, i.e. normally a further air current at ambient temperature, through the supply conduit of the extinguished burner.

The comburent will also not be subjected to any heating in the heat exchanger, as the heat exchanger will be crossed by two fluids which are substantially at a same temperature, and can thus reach the firing chamber at a lower temperature with respect to the temperature of the kiln, thus enabling efficient dispersion of the heat.

The contemporaneous supplying of two currents further increases the overall flow rate of cooling fluid which is introduced into the firing chamber, thus obtaining a greater dissipation of the heat and a more rapid cooling of the kiln. This solution can be adopted throughout all the stages in which the cooling velocity has to be as fast as possible, in particular during the rapid firing stages, during which there is no risk of damaging the ceramic products. In a preferred embodiment of the kiln, the outflow conduit of the burner is associated to valve means, which place the conduit in communication with the outside environment, and can be activated such as to exclude communication with the outside in order to set the outflow conduit in communication with special cooling fluid supply means, and vice versa. In this context, during the normal functioning of the burner, the firing chamber is at a higher temperature than atmospheric temperature and the hot gases present therein flow spontaneously from the outflow conduit, which is kept in communication with the outside.

During the cooling stages with burners extinguished, the valve means are activated such as to place the outflow conduit in communication with the means for supplying the cooling fluid, which force the cooling fluid to flow in the outflow conduit up to exiting internally of the firing chamber.

In an alternative embodiment of the kiln, the outflow conduit of the burner opens into the narrowed section of a venturi tube, which is axially crossed by a command fluid, typically outside air at ambient temperature, which generates in the narrowed section a depression necessary for aspirating the hot gases from the firing chamber.

In this context, the injection of the cooling fluid into the kiln is preferably realised by deviating the command fluid internally of the outflow conduit, in

counter-current with respect to the normal flow of hot gases. This deviation can be obtained simply by preventing the axial outflow of the command fluid from the venturi tube, for example by closing a check valve located downstream of the venturi tube itself. In this way, the command fluid is forced to flow from the venturi tube internally of the outflow conduit, up to reaching the firing chamber of the kiln. The invention further makes available an intermittent ceramic kiln able to actuate the above-delineated firing method. The kiln comprises at least a flame burner, which is provided with at least a comburent supply conduit, a hot gas outflow conduit from the kiln, and a heat exchanger destined to operate a heat exchange between the fluids running through the supply conduit and the outflow conduit; means being provided for supplying a current of a cooling fluid in the kiln through the outflow conduit of the burner, in a counter-current direction with respect to the normal outflow of the hot gases.

As mentioned herein above, the outflow conduit is preferably associated to the valve means, which are destined to place it in communication with the external environment, and are activatable such as to exclude the communication with the outside, placing the outflow conduit in communication with the cooling air supply means, and vice versa.

Thanks to this solution, by activating the valve means when the burner is off, the cooling fluid is forced by the supply means to move in a countercurrent in the outflow conduit up to reaching the firing chamber. In a different constructional embodiment, the outflow conduit of the burner opens at the narrowed section of a venturi tube, which is inserted along an auxiliary conduit destined to be followed by a command fluid, such as to create in this narrowed section a depression destined to aspirate the hot gases from the kiln through the outflow conduit. In this context, the cooling fluid supply means of the invention comprise means for deviating the command fluid into the outflow conduit, preferably valve means destined to selectively close the auxiliary conduit downstream of the venturi tube.

Thanks to this solution, by closing the valve means, the command fluid is forced to run through the outflow conduit in counter-current up to reaching the firing chamber, where it cools the ceramic products.

Further characteristics and advantages of the invention will emerge from a reading of the following description provided by way of non-limiting example, with the aid of the figures illustrated in the accompanying tables of the drawings.

Brief description of the Drawings

Figure 1 is a section view of an intermittent ceramic kiln of the invention. Figures 2 and 3 illustrate a plant diagram of a free-flame burner mounted on the kiln of figure 1 , in two stages of the firing process.

Figures 4 and 5 show a different plant diagram of a free-flame burner mounted on the kiln of figure 1 , in two stages of the firing process.

Best Mode for Carrying Out the Invention The intermittent ceramic kiln 1 comprises a single closed firing chamber 2, delimited by walls of refractory material, internally of which is located a fixed support structure 3, on which the ceramic products 100 to be fired are rested.

In the illustrated example, the ceramic products 100 are sanitary articles, such as for example water closet pedestals, wash basins and bathtubs, but the kiln 1 can also be used for firing other articles, such as kitchenware, ceramic insulators for insulating ducts for high-tension electrical lines, or ceramic pipes for sewer conduits, and the like.

The intermittent kiln 1 might also be used for firing ceramic slabs or tiles, refractory tiles or other products. The ceramic products 100 are immobile in the intermittent kiln 1 during the whole firing cycle which is obtained thanks to a programmed variation in temperature in the firing chamber 2 as a function of times required.

Numerous free-flame burners 4 are appropriately set on the laterals walls of the kiln 1 , which burners 4 are destined to heat the firing chamber 2 such as to regulate the temperature therein. A chimney 5 is located on the ceiling of the kiln 1 , through which chimney 5 the hot firing gases generated by the flames of the burners 4 are discharged to the outside.

In some applications, the burners 4 might also be set on the ceiling and/or on the bottom wall of the kiln 1.

As illustrated in figures 2 and 3, each burner 4 comprises an outside jacket

40 of a substantially cylindrical shape, which is inserted and blocked internally of a respective opening in the kiln wall 1.

The front end of the external jacket 40 is open towards the firing chamber 2 and is located flush with the internal surface of the kiln wall, while the posterior end is closed and projects to the outside.

An outlet mouth 41 is afforded on the posterior projecting tract of the jacket 40.

A cylindrical pipe 42 is mounted coaxially internally of the external jacket 40, which pipe 42 has a smaller diameter than the diameter of the external jacket

40, such as to define therewith an annular conduit 43 connecting the firing chamber 2 with the outlet mouth 41. The cylindrical pipe 42 exhibits a truncoconical front tract, an end of which projects slightly beyond the edge of the external jacket 40, and is open towards the inside of the firing chamber 2.

The opposite end of the cylindrical pipe 42 is closed, and projects posteriorly of the external jacket 40 containing it. An inlet mouth 44 is afforded on the projecting tract of the cylindrical pipe 42.

A central tube 45 is coaxially coupled in the cylindrical pipe 42, which central tube 45 has a smaller diameter than that of the cylindrical pipe 42, such as to define therewith an annular conduit 46 which places the inlet mouth 44 in communication with the firing chamber 2. The central tube 45 develops from the bottom of the cylindrical pipe 42 up to the truncoconical tract, where it is closed by a transversal plate 47.

A central inlet opening 48 is afforded on the bottom of the cylindrical pipe 42, while an outlet opening 49 is afforded at the centre of the transversal plate

47, which outlet opening 49 places the central tube 45 in communication with the truncoconical tract of the cylindrical pipe 42, and thus the firing chamber

2. In this way, the central tube 45 defines a conduit which connects the inlet opening 48 with the firing chamber 2.

In the illustrated embodiment of figures 2 and 3, the inlet mouth 44 is connected to a supply line 6 of comburent air which is collected directly from the outside at ambient temperature, via a compressor 61 destined to force it at pressure internally of the annular conduit 46 and from there towards the firing chamber 2.

The inlet opening 48 is connected to a fuel supply line 7, typically methane gas.

The supply line 7 is intercepted by a pneumatically-controlled flow rate regulator 70, which is automatically piloted by the pressure of the comburent air in the supply line 6, and is calibrated such as always to provide a correct flow of combustible as a function of the air flow.

Finally, the outlet mouth 41 is connected to a discharge line 8, which opens directly into the outside environment through a chimney (not illustrated) The discharge line 8 is intercepted by a first valve 85, which is activatable such as to selectively open and close the connection of the discharge line 8 with the outside environment.

The discharge line 8 is in communication, between the outlet mouth 41 and the first valve 85, with the supply line 6 via an auxiliary conduit 86. The auxiliary conduit 86 is intercepted by a second valve 87, which is activatable such as to selectively open and close the communication between the supply line 6 and the discharge line 8.

The firing cycle of the products 100 in the intermittent kiln 1 generally comprises a stage of heating, during which the burners 4 are normally lit, a stage of maintenance at a constant temperature, and a subsequent stage of cooling for returning the products 100 at atmospheric pressure, during which the burners 4 are normally extinguished.

As mentioned in the preamble hereto, the cooling stage is divided into an initial stage of rapid cooling, during which the products 100 are rapidly brought down to a temperature of about 600 ⁰ C, followed by a slow cooling stage down to 250-300 ⁰ C, after which a new stage of rapid cooling starts, down to ambient temperature.

As illustrated in figure 2, when the burners 4 are lit, the second valve 87 is closed and the comburent air is supplied by the compressor 61 , via the supply line 6, internally of the annular conduit 46, where it moves towards the truncoconical tract of the cylindrical pipe 42 which opens internally of the firing chamber 2.

The fuel is regulated by the flow-rate regulator 70 and supplied via the supply line 7 internally of the central tube 45, in which it flows towards the outlet opening 49 afforded in the transversal plate 47. In this way, the combustible and the comburent air mix at the truncoconical tract of the cylindrical pipe 42, where they light a free flame that develops internally of the firing chamber 2.

At the transversal plate 47 special means for lighting are installed, not illustrated herein as they are of known type, which enable the flame to be lit to fire up the burner 4. During the stages of heating with the flame lit, the first valve 85 of the discharge line 8 is open.

The pressure of the hot combustion gas internally of the firing chamber 2 is slightly greater with respect to atmospheric pressure, so that a current of these gases flows through the annular conduit 43 of the burner 4 from the open end of the cylindrical jacket 40 towards the outlet mouth 41 , and from here it flows into the discharge line 8 to the outside, through the chimney. During the crossing of the annular conduit 43, the hot gases externally strike the lateral wall of the cylindrical pipe 42 internally of which the comburent air flows in counter-current. The wall of the cylindrical pipe 42 is made of a good heat-conducting material, typically a metal or more preferably carborundum, enabling transfer of thermal energy between the hot gases coming from the firing chamber 2, and the comburent air coming from the outside. The external jacket 40 and the cylindrical pipe 42 are, in practice, a heat exchanger in counter-current.

In order to improve the exchanger's efficiency, the lateral wall of the cylindrical pipe 42 can exhibit finning aimed at increasing the heat exchange.

The hot gases are substantially at the same temperature as the firing chamber 2, such that the comburent air heats before mixing with the combustible, improving the overall efficiency and performance of the intermittent kiln 1 and thus reducing the consumption of combustible fuel. The hot gases exiting from the kiln 1 through the discharge line 8 and/or the chimney 5 might be supplied internally of a second intermittent ceramic kiln (not illustrated), distinct from the one described herein, thus obtaining greater exploitation of the heat of the gases and an overall energy saving. As illustrated in figure 3, to cool the products 100 the flow of combustible from the supply line 7 is interrupted, such as to cause the flame of the burner 4 to switch off.

The compressor 61 of the supply line 6 so as to continue introducing comburent air into the chamber 2 of the kiln. The comburent air is at ambient temperature such that a part of the heat present in the internal chamber 2 dissipates by convection, operating a gentle cooling action on the ceramic products 100. The comburent air in effect functions as a cooling fluid during this stage. In order to accelerate the cooling of the products 100, the first valve 85 of the discharge line 8 is closed such as to exclude communication with the outside, and at the same time the second valve 87 is opened such as to place the discharge line 8 in communication with the supply line 6 downstream of the compressor 61.

In this way, the air pumped by the compressor 61 also flows in counter- current along the discharge line 8, enters the burner 4 and flows through the annular conduit 43 from the outlet mouth 41 towards the firing chamber 2. The air current prevents outflow of hot gases from the firing chamber 2 and thus hinders the functioning of the heat exchanger defined by the external jacket 40 and the cylindrical pipe 42. For this reason the air at environment temperature which is pumped by the compressor 61 is not subject to any heating in the heat exchanger, and when it opens into the kiln 1 , dissipates a greater quantity of heat, obtaining a more

rapid cooling of the ceramic products 100 and thus a shorter overall duration of the firing cycle.

This solution can be effectively adopted in all the stages in which the cooling velocity of the products 100 must be the highest possible, in particular in the initial rapid cooling stage.

It is obviously possible to increase the velocity of the compressor 61 in order to increase the cooling air flow which is introduced into the chamber 2 of the kiln.

The described stages are preferably managed by programmable control means, which are able to automatically control not only the valve groups 85 and 87 but also the means supplying the combustible and all the other operating organs involved in the functioning of the kiln 1.

In the alternative embodiment illustrated in figures 4 and 5, the inlet mouth 44 is connected to a supply line 6 of comburent air which is directly collected from the outside at ambient temperature, through a compressor which is not illustrated.

The supply line 6 is intercepted by an automatically-activated valve group 60, which enables the supply line 6 to be opened and closed and to regulate the comburent air flow which is supplied to the burner 4. The inlet opening 48 is connected to a supply line 7 of the combustible which, like in the preceding embodiment, comprises a pneumatically-controlled flow rate regulator 70, automatically piloted by the comburent air pressure in the supply line 6.

The outlet mouth 41 is finally connected to a discharge line 8, which opens at the narrowed section of a venturi tube 80.

The venturi tube 80 is inserted axially along an auxiliary line 81 in which a pressurised command fluid flows, preferably an air current which is directly collected from the outside at ambient temperature.

Flowing axially from the inlet 82 to the outlet 83 of the venturi tube 80, the command fluid creates a depression in the narrowed section which enables the hot gases present in the firing chamber 2 to be aspirated, through the annular conduit 43, the outlet mouth 41 and the discharge line 8.

Downstream of the venturi tube 80 with respect to the command fluid flow direction, the auxiliary line 81 is intercepted by a automatically-activated valve group 84, which enables the auxiliary line 81 to be opened and closed. As illustrated in figure 4, when the burners 4 are fired up, the valve group 60 is open and the comburent air is supplied via the supply line 6 internally of the annular conduit 46, where it flows towards the truncoconical tract of the cylindrical pipe 42 which opens internally of the firing chamber 2. Similarly to the preceding case, the combustible is regulated by the flow rate regulator 70 and supplied via the supply line 7 internally of the central tube 45, in which it flows toward the outlet opening 49 afforded in the transversal plate 47.

In this way, the combustible and the comburent air mix at the truncoconical tract of the cylindrical pipe 42, where they fire up a free flame which develops internally of the firing chamber 2. During the stages of heating with the flame switched on, the valve group 84 is open and the auxiliary line 81 is flowed through by the command fluid, which creates, in the narrowed section of the venturi tube 80, the above- mentioned depression. This depression aspirates part of the hot combustion gases present in the firing chamber 2, which flow through the annular conduit 43 of the burner 4 from the open end of the cylindrical jacket 40 towards the outlet mouth 41.

The hot gases flow from the outlet mouth 41 into the discharge line 8 up until they mix with the command fluid in the venturi tube 80, such as to flow through the upstream tract of the auxiliary line 81 , up to opening into the outside through a chimney (not shown).

During the crossing of the annular conduit 43 the thermal energy of the hot gases coming from the firing chamber 2 is transferred at least in part to the comburent air coming from the outside, thanks to the external jacket 40 and the cylindrical pipe 42 which realise a heat exchanger in counter-current In this case too, the hot gases flowing out of the discharge line 8 and /or from the chimney 5 can be supplied to a second ceramic kiln, distinct from the one described herein, such as to heat it more rapidly and/or pre-heat the

comburent supplied to the burners thereof, obtaining an overall energy saving.

As illustrated in figure 5, in order to cool the products 100, the flow of combustible from the supply line 7 is interrupted, such as to cause the extinguishing of the flame of the burner 4.

At the same time, the valve group 84 is activated such as to close the auxiliary line 81 downstream of the venturi tube 80, such that the command fluid does not flow freely towards the chimney.

In this way, the command fluid cannot outflow axially from the venturi tube 80, and it is therefore forced to flow in counter-current along the discharge line 8, enter the burner 4 and flow along the annular conduit 43 from the outlet mouth 41 towards the firing chamber 2.

The command fluid current prevents the outflow of hot gases from the firing chamber 2, and thus prevents the functioning of the heat exchanger which is defined by the external jacket 40 and by the cylindrical pipe 42.

For this reason, the command fluid, which as has been mentioned is normally air at environmental temperature, is not heated in the heat exchanger, and when it opens into the kiln 1 , dissipates by convection part of the heat present in the firing chamber 2, obtaining a more rapid cooling of the ceramic products 100 and thus a shorter overall duration of the firing cycle.

While the command fluid is supplied internally of the firing chamber 2, the comburent can be dispensed inside of the kiln 1 via the supply line 6, leaving the valve group 60 open.

In this way, contemporary supply of two air currents increases the overall flow rate of cooling gas which is injected into the firing chamber 2, thus obtaining a greater dissipation of the heat and thus a more rapid cooling of the kiln.

This solution can be adopted in all the stage in which the cooling velocity of the products 100 can be the highest possible without running the risk of damage, in particular during the rapid cooling stage of the ceramic products

100.

However, during the stages of slow cooling, the valve group 60 can be closed such that the products are cooled only by the command fluid. Preferably the firing stages described herein above are managed by programmable control means, which are able to automatically command not only the valve group 60 and 84 but also the means supplying the combustible and all the other operating organs involved in the functioning of the kiln 1. Obviously a technical expert in the sector might make numerous modifications of a technical-applicational nature to the intermittent kiln 1 and the firing method of the products 100 as described, without forsaking the ambit of the invention as defined in the claims.