Description A KILN FOR HEATING SHEETS OF GLASS

Technical Field The invention relates in particular to kilns for heating flat or arced glass sheets, the kiln being provided with a motorized roller plane on which the glass to be heated during its time in the kiln rests and is moved.

Background Art In kilns of this type, which reach an internal temperature of about 700°C, heating is generally done by radiation from radiating panels located above and below the roller plane; the market however also offers kilns having convection-type heating.

One of the problems in the prior-art kilns is that the glass sheet entering the kiln, which is therefore in contact with the roller plane, heats up very quickly in the contact areas in which it is actually in contact with the rollers and deforms, albeit temporarily. In particular flat glass sheets bow into a concave shape. These deformations disappear when the heating effect becomes uniform, but there remain traces of these deformations on the sheet which diminish the quality thereof. These problems come to the fore especially when treating special sheets, for example reflective or magnetron-coating treated sheets, or in any case surface-treated items.

To solve this problem blowers have been used to blow relatively-cool air onto the rollers in the plane in order to lower their temperature; or hot-air blowers are directed onto the upper surface of the sheet to accelerate the heating thereof and therefore to prevent sheet deformation. These solutions, however, have the drawback of causing turbulence internally of the kiln; in a not-infrequent

scenario, in which the sheet splinters in the kiln, the glass fragments or powders generated are moved internally of the kiln by the turbulence and might be deposited between the rollers and the sheets of glass, damaging the surfaces.

When the presence of glass splinters or powders internally of the kiln is detected, the kiln has to be shut-down in order to proceed to cleaning operations.

The main aim of the present invention is to provide an improvement to known kilns for eliminating temporary deformations in the glass sheets without creating a turbulent atmosphere in the kiln itself.

An advantage of the invention is that it provides improvements which can be applied, without excessive difficulty, to known kilns.

A further advantage of the invention is that it considerably improves heating operations, in particular for special sheets or sheets with surface coatings.

A further advantage of the invention is that tempered glass sheets can be more safely heated.

These aims and advantages and others besides are all attained by the invention as it is characterised in the claims that follow.

Disclosure of Invention Further characteristics and advantages of the present invention will better emerge from the detailed description that follows of a preferred but non-exclusive embodiment of the invention, illustrated purely by way of non-limiting example in the accompanying figures of the drawings, in which: figure 1 is a schematic longitudinal section of the kiln of the invention; figure 2 is a schematic section of the kiln made according to line 11-11 of figure 1 ; figure 3 is an enlarged-scale representation of a detail of figure 1 in relation to a roller and a cooling body thereof.

With reference to the above-mentioned figures of the drawings, 1 denotes in its

entirety a kiln for tempering glass sheets 4, either flat or arced, which comprises a motorized roller plane 2, on which the glass sheets 4 to be heated are moved while inside the kiln. The sheets enter the kiln by an entrance 7 and, after having completed their heating cycle which usually lasts 41 seconds for every millimetre of thickness of the glass, leave the kiln by an exit 8 and are usually sent on to a tempering station. The roller plane is provided with a plurality of motorized rollers 6 which rotate to move the sheets, usually with a forward-and-backward rhythm during the heating process. The rollers, as in known kilns, are commanded to roll in both rotation directions.

Means for heating of known type are present inside the kiln. In the figures the means for heating are schematically represented by blocks 3, arranged above and below the roller plane; the blocks are resistance elements which heat up the kiln by radiation, but could be of any type. The temperature reached by the kiln, and therefore also by the roller plane, is about 680-720°C at full operation.

The kiln of the invention comprises one or more cooling elements 5 which, in the illustrated embodiment, comprise a plurality of tubes 5 which are fixed to the inside of the kiln in proximity of the roller plane 2. In particular, one of the tubes 5 is fixed in proximity of each of the rollers 6, below and parallel thereto.

In a further possible embodiment (not illustrated) the cooling elements 5 comprise a pair of tubes; each pair of tubes is fixed to the inside of the kiln in proximity of a relative roller 6, below and parallel thereto. The tubes can be made, for example, of silicon, a ceramic material or steel; they can be smooth or finned to improve their heat exchanging properties.

For reasons that will be better explained herein below, each of the tubes 5 exhibits a diameter which is equal to or preferably slightly smaller that the radius of the relative roller 6 of the roller plane 2 to which the tube is associated.

The tube is arranged with an axis thereof displaced with respect to the axis of the

relative roller, so as to be completely positioned in one only half-space defined by a median plane of the relative roller which plane is perpendicular to the lie plane of the roller plane 2.

Similarly, each of the pairs of tubes 5 exhibits an overall diameter which is equal to or preferably slightly smaller than the radius of the relative roller 6 of the roller plane 2 to which the pair of tubes is associated.

The pair of tubes is displaced with respect to the axis of the relative roller, so that it is completely positioned in one only of the half-spaces defined by a median plane of the relative roller which plane is perpendicular to the lie plane of the roller plane 2.

The external surface of the cooling elements 5, and in particular of the tubes and the pairs of tubes, is brought, on command to a considerably lower temperature than the internal temperature of the kiln ; for this purpose means for cooling are provided which lower, on command, the temperature of the external surface of the cooling elements, up until the temperature is variable between 200 and 300°C.

The means for cooling comprise a cool fluid (with respect to the temperature of the kiln), preferably air at a temperature of about 40°C (i. e. cold with respect to the temperature of the kiln), which is blown and made to flow internally of each of the tubes 5 in a predetermined flow direction. The cool air is preferably blown internally of the tubes in order that the blower is not crossed by hot air, as might occur when using an aspiration device taking air from the tubes. Usually a pre- heating device is provided to bring the air to be introduced into the tubes from ambient temperature to the desired temperature (for example, 40°C).

The cold air which is heated through the tubes can advantageously be recycled by means for recycling, of known type, which take the fluid exiting from the tubes, for example in a zone before the entrance 7 of the kiln, or in any other zone of the kiln in which the hot air is useful.

The passage of cool air into the tubes considerably reduces the temperature of the external surface of the tubes; as the tubes are located in proximity of the rollers 6 of the roller plane 2, normally at a distance which is variable between a few millimetres and a few centimetres, the presence of the tubes considerably reduces the temperature of the external surface of the rollers 6. It has emerged that with a kiln internal temperature of about 700°C, and with a tube external surface temperature of about 250°C, a lowering of temperature at the external surface of the rollers of a few tens of degrees centigrade is achieved.

To reduce the thermal inertia of the rollers and accordingly to increase the reduction in the temperature of the external surfaces thereof, the rollers 6 of the roller plane 2 are hollow.

In known kilns, when the glass sheets enter the kiln they come into contact with the rollers 5 with their lower surface which heats up much more quickly; this causes an initial deformation of the sheet, which disappears once the temperature of the sheet becomes uniform, but nonetheless leaves traces of its having occurred, reducing the quality of the glass sheet, especially its optical properties.

In the kiln of the invention, the surface temperature of the rollers 6 is maintained, at least during the introduction stage of the sheet and the first stage of the heating process, at a lower temperature with respect to the kiln temperature. Thus the temporary deformations of the sheet are much less marked, or indeed absent, which improves the quality of the sheets. The advantage of this invention is further evidenced in the treatment of special sheets or treated surfaces, also where for technical reasons it is necessary to rest the sheets on the rollers on their treated surfaces.

Once the temperature of the sheet is uniform, or nearly so, the flow of cold air can be stopped; thereafter, and thanks to the special arrangement of the tubes which do not involve an entire half of the relative rollers, the temperature of the

tubes and the rollers becomes rapidly the same as the temperature of the kiln.

In order to avoid creation of considerably differences of temperature among the various parts of the tubes, and therefore the rollers, following the flow of cold air flow internally of the tubes in a constant direction, the direction of the cold air can be inverted on command. This is achieved quite easily using simple and known pneumatic circuits. A different solution, though constructionally slightly more complex, is the presence of side-by-side paired tubes in the place of single tubes ; the tubes of a pair can be connected to different cold-air manifolds which can send air in one direction in one of the pair and in the other direction in the other of the pair. In this way the various zones of the rollers side-by-side with the respective pairs of tubes can be maintained at a more-or-less constant temperature.

The tubes, or pairs of tubes, can also be grouped in various sections, each of which sections concerns a particular zone of the roller plane 2; thus, by varying some parameters, such as for example the temperature, the flow rate, the input times for the cold air, or the geometry of the various groups, the temperature of the rollers can be modulated differently in the different sections of the roller plane. The device of the invention enables regulation of the temperature of the rollers in the kiln according to the various working needs.

All of the operative modalities can be regulated by programs, through which the desired parameters can be set, such as roller temperature, air ingress time, the temperature and velocity of the air being introduced in the tubes, the temperatures of the upper and lower parts of the kiln, the different treatments, if any, for the various parts of the kiln.

An important aspect of the invention is that the roller temperature regulation is obtained with no input of air into the kiln itself, avoiding any turbulence therein.

Other advantages offered by the present invention, beyond those already

mentioned, are: the possibility, should an unexpected shut-down of the sheet- moving machinery occur, of cooling the tubes to stop the sheets from sticking to the rollers, and the possibility of reducing kiln cooling times should it be necessary to stop the kiln production cycle.