PIFFERI GIUSEPPE (IT)
| EP1500889A1 | 2005-01-26 | |||
| SU426126A1 | 1974-04-30 | |||
| EP0348603A2 | 1990-01-03 | |||
| EP1500889A1 | 2005-01-26 |
Claims.
1 ). A process for management and retro-active regulation of a tunnel kiln for firing ceramic tiles, comprising a roller plane for tile transport, a series of burners located above the roller plane, a series of burners located below the roller plane, a series of manifolds provided with nozzles dispensing forced cold air at a terminal part of the kiln, which nozzles are located both above and below the roller plane, means for detecting a temperature internally of the kiln, as well as a processor for commanding supply of fuel and air to the burners, singly or in pairs, and for commanding a pressure of air flowing into the manifolds in order to maintain a temperature curve of the detected temperatures at preset levels, characterised by following operations:
- for each batch of tiles to be fired, setting the processor for a desired temperature curve;
- in a first memory of the processor, memorising standard defects relating to shape and dimensions of the tiles;
- in a second memory of the processor, for each of the standard defects setting modifications either for increasing or decreasing burner flame temperatures and/or cooling air pressure supply to amounts which will reduce the defects;
- at kiln exit, on each tile of a same transversal alignment of tiles exiting from the kiln, performing at intervals a control of a planarity and dimensions thereof in order to determine a type of defect detected by comparison with standard defect data contained in the first memory;
- signalling the detected defect to the processor, which identifies the defect by comparison thereof with the data contained in the first memory and by modifying supply to the burners and/or to the cooling air nozzles in such a way as to act upon the detected defect on the basis of indications of data contained in the second memory, and based on a processor program. 2). The process of claim 1 , characterised in that the standard defects of the dimensions of the tiles which are memorised in the first memory are as follow: calibre; trapezoid effect with long-side close to the walls; trapezoid effect with short-side close to the walls.
3). The process of claim 1 , characterised in that the standard defects of tile shape memorised in the first memory are as follows: homogeneous concave and convex shape; concavity and convexity in planes parallel to the rollers; concavity and convexity in planes perpendicular to the rollers; convexity of pieces close to the walls.
4). The process of claim 1 , characterised in that the control of the planarity of the tiles of each alignment is carried out simultaneously on all tiles in the alignment.
5). The process of claim 1 , characterised in that control of the planarity of the tiles of each alignment is carried out in succession on each tile in the alignment.
6). The process of claim 1 , characterised in that control of the dimensions of the tiles of each alignment is performed simultaneously on all the tiles in the alignment.
7). The process of claim 1 , characterised in that control of the dimensions of the tiles of each alignment is performed in succession on all the tiles in the alignment.
8). The process of claim 1 , characterised in that in order to correct dimension defects of calibre with workpiece dimensions which are greater than desired workpiece dimensions, the second memory comprises following instructions to be sent to the processor: increase firing temperature by from 1 °C to 5°C both above and below the roller plane.
9). The process of claim 1 , characterised in that in order to correct dimension defects of calibre with workpiece dimensions which are smaller than desired workpiece dimensions, the second memory comprises following instructions to be sent to the processor: reduce firing temperature by from 1°C to 5°C both above and below the roller plane. 10). The process of claim 1 , characterised in that in order to correct dimension defects with trapezoid effect on a long side close to the wall, the second memory comprises following instructions to be sent to the processor: increase the flame temperature of the radial burners 4, 40 at the end-of-firing zone.
11 ). The process of claim 1 , characterised in that in order to correct dimension defects with trapezoid effect on a short side close to the wall, the second memory comprises following instructions to be sent to the processor: reduce the flame temperature of the radial burners 4, 40 at the end-of-firing zone.
12). The process of claim 1 , characterised in that in order to correct planarity defects of homogeneous concavity - upwards-directed concavity, the second memory comprises following instructions to be sent to the processor: reduce cooling air pressure in the manifolds 50 and 60 located below the roller plane, and increase pressure in the manifolds 5 and/or 6 above the roller plane.
13). The process of claim 12, characterised in that the second memory comprises further instructions to be sent to the processor, as follows: increase the flame temperature of burners 30 and 40 located below the roller plane and reduce the temperature of the burners 3 and 4 located above the roller plane, in the end-of-firing zone.
14). The process of claim 1 , characterised in that in order to correct the planarity defect of concavity in the planes which are perpendicular to the rollers for white and red single-fired tiles and for vitrified stoneware tiles, the second memory comprises instructions to be sent to the processor as follows: reduce the flame temperature of burners 20 and 30 below the roller plane in the pre-firing zone and at the start firing zone and increase the temperatures of the burners 2 and 3.
15). The process of claim 14, characterised in that the second memory comprises further instructions to be sent to the processor, as follows: reduce pressure in the manifold 50 and/or 60 below the roller plane at the start of the rapid cooling zone, and increase the pressure in the collector 5 and/or 6 above the roller plane.
16). The process of claim 1 , characterised in that in order to correct the planarity defect of concavity in planes that are perpendicular to the rollers for monoporosa ceramic tiles or double-fired tiles the second memory comprises instructions as follow to be sent on to the processor: reduce the flame temperature of the burners 30 and 40 below the roller plane in the end-firing zone and increase by a same amount the flame temperatures of the burners 3 and 4 above the roller plane, in the end-firing zone.
17). The process of claim 16, characterised in that the second memory comprises further instructions to be sent on to the processor, as follow: reduce pressure in the manifolds 50 and/or 60 from below and increase by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone.
18). The process of claim 1 , characterised in that to correct the planarity defect of concavity in planes parallel to the roller for white and red single-fired tiles and vitrified stoneware tiles, the second memory comprises instructions to be sent on to the processor as follows: reduce pressure in the manifolds 50 and/or 60 from below and increase by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone. 19). The process of claim 1 , characterised in that to correct the planarity defect of concavity in planes parallel to the roller for monoporosa ceramic tiles or double-fired tiles, the second memory comprises instructions to be sent on to the processor as follows: reduce the flame temperature of the burners 20 below the roller plane in the pre-firing zone and increase by a same amount the flame temperatures of the burners 2 above the roller plane. 20). The process of claim 19, characterised in that the second memory comprises further instructions to be sent on to the processor, as follows: reduce pressure in the manifolds 50 and/or 60 from below and increase by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start zone. 21 ). The process of claim 1 , characterised in that to correct the planarity defect of homogeneous convexity with downwards-directed concavity for white and red single-fired tiles and vitrified stoneware tiles, the second memory comprises instructions to be sent on to the processor as follows: increase pressure in the manifolds 50 and/or 60 from below and reduce by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone.
22). The process of claim 21 , characterised in that the second memory comprises further instructions to be sent on to the processor, as follows: reduce the flame temperature of the burners 30 and 40 below the roller plane in the firing zones and increase by a same amount the flame temperature of the burners 3 and 4 above the roller plane.
23). The process of claim 22, characterised in that the second memory comprises further instructions to be sent on to the processor, as follows: increase the flame temperature of the burners 30 below the roller plane and reduce by a same amount the flame temperatures of the burners 3 above the roller plane.
24). The process of claim 1 , characterised in that to correct the planarity defect of homogeneous convexity with downwards-directed concavity in monoporosa ceramic tiles or double-fired tiles, the second memory comprises instructions to be sent on to the processor as follows: increase the flame temperature of the burners 30 below the roller plane and reduce by a same amount the flame temperatures of the burners 3 above the roller plane. 25). The process of claim 24, characterised in that the second memory comprises further instructions to be sent on to the processor, as follows: increase pressure in the manifolds 50 and/or 60 from below and reduce by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone.
26). The process of claim 1 , characterised in that to correct the planarity defect of convexity in the planes which are perpendicular to the rollers, the second memory comprises instructions to be sent on to the processor as follows: increase the flame temperature of the burners 20 and 30 below the roller plane and reduce by a same amount the flame temperatures of the burners 2 and 3 above the roller plane in the pre-firing and start-firing zones. 27). The process of claim 26, characterised in that the second memory comprises further instructions to be sent on to the processor, as follows: increase pressure in the manifolds 50 and/or 60 from below and reduce by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone.
28). The process of claim 1 , characterised in that to correct the planarity defect of convexity in the planes which are perpendicular to the rollers for white and red single-fired tiles and vitrified stoneware tiles, the second memory comprises instructions to be sent on to the processor as follows: increase pressure in the manifolds 50 and/or 60 from below and reduce by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone.
29). The process of claim 1 , characterised in that to correct the planarity defect of convexity in the planes which are parallel to the rollers for white and red single-fired tiles and vitrified stoneware tiles, the second memory comprises instructions to be sent on to the processor as follows: increase pressure in the manifolds 50 and/or 60 from below and reduce by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone.
30). The process of claim 1 , characterised in that to correct the planarity defect of convexity in the planes which are parallel to the rollers for monoporosa ceramic tiles or double-fired tiles, the second memory comprises instructions to be sent on to the processor as follows: reduce the flame temperature of the burners 2 above the roller plane in the pre-firing zones and increase by a same amount the flame temperatures of the burners 20 below the roller plane.
31 ). The process of claim 30, characterised in that the second memory comprises further instructions to be sent on to the processor, as follows: increase pressure in the manifolds 50 and/or 60 from below and reduce by a same amount the pressure in the manifolds 5 and/or 6 from above at the rapid cooling start-zone.
32). The process of claim 1 , characterised in that to correct the planarity defect of convexity in the workpieces close to the walls, the second memory comprises instructions to be sent on to the processor as follows: increase pressure in the longitudinal manifolds 70.
33). The process of claim 1 , characterised in that the modifications of increasing and reducing temperatures and pressures are of a same quantity in all interventions, so that following a small number of repeated interventions a regulation of the kiln settles to desired values and the tiles produced fall within a commercially acceptable quality standard.
34). The process of claim 33, characterised in that the modifications of increasing and reducing temperatures and pressures are in all interventions a multiple of a same quantity.
35). An apparatus for retroactive management of a tunnel kiln for firing ceramic tiles, comprising:
- a roller plane for transporting material located in a tunnel structure provided with lateral walls, in which are located:
- two series of burners, respectively a series above and a series below the roller plane, which burners are directed towards a centre of the kiln and which burners are distributed along a length of the kiln;
- two series of manifolds, respectively above and below the roller plane, which manifolds are provided with nozzles for directing cold air towards the roller plane, which manifolds are located in a terminal zone of the kiln, from which terminal end the material exits; means for generating pressurised air, being associated to the manifolds;
- means for controlling a temperature distributed over all the length of the kiln;
- a processor for controlling functioning of the burners according to signals sent by sensor means of the temperature; characterised in that it comprises:
- first means for controlling a planarity of the tiles exiting the kiln; - second means for controlling dimensions of the tiles exiting the kiln; the processor being predisposed to receive the signals of the first and second means, and comprising
- a first memory in which standard defects of tile dimension and planarity are stored; and
- a second memory containing instructions to be sent to the burners and to the means for generating pressurised air in order to reduce each of the memorised standard defects;
- a unit of calculation which, according to data received from the first and second means and the data contained in the first and the second memories, commands an operation of the single burners and of the single generators of pressurised air.
36). The apparatus of claim 35, characterised in that each manifold is provided with a means for generating cooling air.
37). The apparatus of claim 35, characterised in that a transporter is located at the exit of the kiln, which transporter has an axis which is perpendicular to an axis of the kiln, on which transporter axis the tiles are aligned on exiting the kiln and above which are located the first and second means for controlling the planarity and the dimensions of the tiles.
38). The apparatus of claim 35, characterised in that it comprises sensor means for detecting temperature, located in proximity of the lateral walls of the kiln. |
A PROCESS AND AN APPARATUS FOR OPTIMISED MANAGEMENT OF A KILN FOR CERAMIC TILES.
Technical Field
The invention relates to kilns for ceramic tiles, and in particular to the product quality management thereof.
Background Art
Success during the firing stage of tiles in kilns known as tunnel kilns depends almost exclusively on adherence to the longitudinal temperature curve maintained internally of the kiln throughout the transversal section thereof.
In this context, the term longitudinal curve means the curve of the temperatures in the advancement direction of the material, i.e. in a parallel direction to the kiln axis. The kilns to which reference is made comprise a roller plane for material transport, located in a tunnel structure having lateral walls, as well as two series of burners directed towards the centre of the kiln and located in the lateral walls over the whole kiln length, one series being aligned above the roller plane and another series being aligned below the roller plane, where the burners of each series comprise first burners, known as radial burners, prevalently for heating the zones close to the lateral walls, alternated with second burners, known as axial burners, for heating prevalently the central zones of the kiln.
In the terminal part of the kiln, close to the exit of the material, rapid cooling means are provided, comprising variously-oriented nozzles which dispense pressurised cold air generated by means for generating pressurised air, such as axial or radial ventilators.
All product quality parameters depend on the longitudinal temperature curve, such as for example the planarity of the tiles exiting the kiln and the regularity of the tile dimensions.
The prior art describes processes which enable the longitudinal temperature curve to be kept constant automatically, even in the presence of discontinuity
in the load, with the aim of maintaining the product quality within commercially acceptable limits.
The prior art processes intervene retroactively on the temperature curve by acting on the burners which heat up the kiln; i.e. they intervene in the tract of the kiln which fires the tiles upon a defect being found in the product exiting the kiln.
Known processes have provided some results in the elimination of dimensional defects, though they have been shown to be inadequate in connection with planarity defects. The known processes have also been shown to have limits relating to dimensional defects, due to the increased transversal dimensions of the kilns.
The aim of the present invention is to provide a process and an apparatus for retroactively controlling a tunnel kiln according to the quality of products as checked on exiting the kiln.
While it has been noted that, for eliminating the dimensional defects, control of the temperature above and below the roller plane of the kiln provides some positive results, no such positive results have been noted with regard to planarity. This is due to the fact that the tile, thanks to the plasticity thereof at high firing temperatures, has the tendency to relax on the roller plane due to its own weight, with the result that it is not very sensitive to the differences of temperature above and below the roller plane.
The tile tends to shrink, increasingly so as the firing temperature rises; this shrinkage is due to the development of gases and to the closing-up of the interstices during partial sinterisation, and the control of the temperature in the transversal section of the kiln gives good results in maintaining the dimensions of the tile at a commercially acceptable level, even in proximity of the kiln walls. The planarity defects revealed at exit from the kiln occur also and prevalently in the rapid cooling zone, and can be imputed to a differentiated dimensional
diminution of the material in the portions of the tile which are close to the upper surface and lower surface thereof.
The greater the thermal dilation coefficient of the material in the above- mentioned portions, the greater the dimensional diminution during cooling. The invention therefore intervenes on the dilation coefficient of the material in order to have an effect on the tile planarity.
The dilation coefficient of materials having a vitreous component, such as ceramic tile material, considerably increases as the cooling velocity decreases. Consequently the invention teaches intervening on the cooling velocity during the rapid cooling process, but differently for the upper surface and the lower surface of the tile.
This enables the invention to achieve its aims, thanks to the combination of characteristics as recited in the independent process claim 1 and the independent apparatus claim 35.
The dependent claims relate to advantageous embodiments of the invention. Disclosure of Invention
Substantially, the process of the invention includes experimentally determining the optimal longitudinal curve of the temperature CT 0 of firing in accordance with the material used and also according to the dimensions and the quality of the tiles in each batch to be fired.
In accordance with the above-cited curve, at the start of firing operations of the batch of material, the kiln thermal plants are set with respect to both the heating and the cooling functions. Throughout the material firing, the following are monitored internally of the kiln, at a discrete series of significant points: the temperature T| S τ at the kiln axis and preferably at at least a parallel axis close to one of the lateral walls, both above and below the roller plane. In various sections of the kiln, deviations δ T of the monitored temperatures Tιsτ from the stored optimal curve TCo are memorised and sent on to the processor controlling the kiln, which makes a first correction of the thermal plants in order to maintain the values within an acceptable field.
-A-
At predetermined intervals at the kiln exit, controls of planarity and dimensions are made on each tile of exiting tiles in a same transversal group alignment.
In particular, first means for controlling planarity and second means for controlling dimensions are provided.
The detected data is compared with standard memory-stored data typical of each type of product, and when the comparison shows a deviation from a predefined interval, the type of deviation is signalled to a processor which on the basis of its own program then modifies supply to the burners and the means for generating cooling air which are influencing the deviation.
In particular, the processor is provided with at least a first memory containing the most common standard defects in tiles, statistically detected; a second memory containing the corrections of the firing and cooling temperatures for reducing any defect type; and a program for correlating the defects encountered with those stored in the first memory, and for rapidly commanding the burners and the means for generating pressurised air in order to achieve and adequate reduction in the detected defect, according to the data contained in the second memory. The apparatus of the invention comprises all the means necessary for enacting the activities making up the process. Brief description of the Drawings.
The correlation between the standard defects of the tiles after firing, and the modifications to be made to the firing curves, will emerge more clearly from the examples that follow, illustrated by way of non-limiting example in the accompanying figures of the drawings, in which: figure 1 is a plan view of the kiln plane with the burners and the means for generating cooling air located above the roller plane; figure 2 is a plan view of the kiln plane with the burners and the means for generating cooling air located below the roller plane. Best Mode for Carrying Out the Invention.
The figures of the drawings show the roller plane of the kiln, denoted by 1 , on which the tiles advance from right to left in the figures.
Both above and below the roller plane are located known-type and axial-type burners 2, 20, 3 and 30, prevalently for heating the central zone of the kiln, and known-type radial-type burners 4, 40, prevalently for heating the zones proximal to the kiln walls. The burners are positioned facing the lateral walls of the kiln, and the burners on one wall are staggered with respect to the burners on the opposite wall.
The axial-type burners 2, 20 close to the kiln entry are for pre-firing the tiles, while the burners 3, 30, 4 and 40 are for firing the tiles.
The rapid cooling of the tiles starts downstream of the burners 4, 40, with an injection of cold air (room temperature) through nozzles facing towards the roller plane and arranged on two upper transversal manifolds 5 and 6, on two lower transversal manifolds 50 and 60, as well as on two longitudinal manifolds 70 parallel to the kiln axis and arranged below the roller plane.
Each of the manifolds is preferably connected to means for generating air which are independent of the means for generating of the other manifolds.
The burners 2, 20, 3, 30, 4 and 40 are connected singly or in pairs to the central processor P (not illustrated) which governs the kiln and commands supply of the fuel and combustion air to each of them.
The means for generating air of the manifolds 5, 50, 6, 60 and 70 are singly connected to the same processor P which also governs the operating pressure thereof.
The sensors 11 and 11a of the temperature inside the kiln are located above the roller plane, respectively at the centre and in proximity of the walls thereof. The sensors 110 and 110a of the temperature internally of the kiln are located below the roller plane, respectively at the centre and in proximity of the walls thereof.
The transversal lines of the tiles exiting the kiln are transferred in succession onto a transporter 8 having an axis which is perpendicular to the kiln axis, on which a single line of tiles 81 is formed.
The first means 10 for controlling tile planarity defects are located above the transporter 8, and the second means 9 for controlling the dimensions, which
are of known type, are also located above the transporter 8, and are also connected to the central processor P, to which they signal each defect encountered.
The standard defects present in the processor memory, and the relative remedies, are as follow.
A. Large calibre or small calibre.
This is when the dimensions of the piece are larger or smaller than required. It is corrected by increasing or respectively reducing the flame temperature of the burners 3, 30, 4 and 40 from 1 °C to 5°C both above and below the roller plane.
The increase in flame temperature causes a reduction in the size of the piece.
B. Long-side trapezoid effect close to the walls.
This defect is corrected by increasing the flame temperature in the radial burners 4, 40 both above and below the roller plane.
C. Short-side trapezoid effect close to the walls.
This defect is corrected by reducing the flame temperature in the burners 4, 40.
D. Homogeneous concave shape - upwards-directed concavity. This defect is corrected by an intervention in the rapid cooling zone. The cooling air pressure is reduced in the manifolds 50 and/or 60 and increased in the manifolds 5 and/or 6. The flame temperature of burners 30 and 40, situated below the roller plane, is synergically increased while the flame temperature of the burners 3 and 4, situated above the roller plane, is reduced in the final firing zone.
E. Concave shape in the planes which are perpendicular to the rollers. This defect is corrected in different ways according to the material.
For single-fired white and red tiles and vitrified stoneware tiles, the flame temperature of the burners 20 and 30, situated below the roller plane, is reduced in the pre-firing and start-firing zone, while the flame temperature of the burners 2 and 3 above the roller plane is increased. Synergically, at the start of the rapid cooling zone, the pressure in the manifold 50 and/or 60,
below the roller plane, is reduced, and the pressure in the manifold 5 and /or
6, above the roller plane, increased.
For monoporosa or double-fired tiles, in the end-firing zone burner 3 and 4
(above the roller plane) flame temperatures are reduced, while in burners 30 and 40 (below the roller plane) the flame temperatures are increased by the same amount.
Synergically, manifold 50 and/or 60 pressures at the start of the rapid cooling zone are reduced from below, and manifold pressures 5 and/or 6 are increased from above. F. Concave shape in the planes which are parallel to the rollers.
This defect is corrected by intervening differently according to materials.
For white and red single-fired tiles and vitrified stoneware tiles, manifold 50 and/or 60 pressure is reduced (from below) and manifold 5 and/or 6 pressure
(from above) is increased by the same amount. For monoporosa and double-fired tiles, in the pre-firing zone burner 20 flame temperature is reduced (below the roller plane) and burner 2 flame (above the roller plane) is increased by the same amount.
Synergically, at the start of the rapid cooling zone the pressure in the manifolds 50 and/or 60 is increased (from below) while the pressure in manifolds 5 and/or 6 (from above) is increased by the same amount.
G. Convex homogeneous shape - downwards-directed concavity.
This defect is corrected by intervening differently in accordance with the materials used.
For white and red single-fired tiles and vitrified stoneware tiles, at the start of the rapid cooling zone pressure is increased in the manifolds 50 and/or 60, from below, and the pressure in the manifolds 5 and/or 6, from above, is decreased by the same amount.
Synergically, in the end-firing zone the flame temperatures of burners 30 and
40 are reduced below the roller plane and the flame temperatures of the burners 3 and 4 above the roller plane are increased by the same amount.
Further, and also synergically, at the start-firing zone the flame temperatures of the burners 30, below the roller plane, are increased and the flame
temperatures of the burners 3 above the roller plane are reduced by the same amount.
For monoporosa and double-fired tiles, burner 3 temperature above the roller plane is reduced at the start of the firing zone, while burner 3 and 4 temperature is increased at the end of the firing zone.
Below the roller plane, the temperatures of the burners 30 is increased at the start of the roller plane (start-firing zone) and the temperatures of the burners
30 and 40 are reduced at the end of the roller plane (end-firing zone).
At the start of the rapid cooling zone the pressure in manifolds 50 and/or 60 (from below) is increased, while synergically the pressure in the manifolds 5 and/or 6 (from above) is reduced.
H. Convex shape in the planes which are perpendicular to the rollers.
To reduce this defect the flame temperature of burners 20 and 30 is increased in the pre-firing zone and start-firing zone below the roller plane, while the temperature of burners 2 and 3, above the roller plane, is lowered.
Synergically the pressure in the manifold 50 and/or 60 below the roller plane is increased and the pressure in the manifold 5 and/or 6 above the roller plane is reduced.
I. Convex shape in the planes which are parallel to the rollers. This defect is corrected in different ways according to the type of materials.
For white and red single-fired tiles and vitrified stoneware tiles, at the start of the rapid cooling zone pressure is increased in the manifolds 50 and/or 60, from below, and the pressure in the manifolds 5 and/or 6, from above, is decreased by the same amount. For monoporosa and double-fired tiles, in the pre-firing zone burner 2 flame temperature is reduced (above the roller plane) and burner 20 flame (below the roller plane) is increased by the same amount.
At the start of the rapid cooling zone the pressure in manifolds 50 and/or 60
(from below) is increased, while synergically the pressure in the manifolds 5 and/or 6 (from above) is reduced by the same amount.
L. Convex shape in the pieces when external of the kiln.
When the convexity is limited to the pieces close to the kiln walls at the start of the rapid cooling zone, the pressure in the manifolds 70 is increased. Adjustments of temperatures and pressures, either upwards or downwards, are preferably in each case of the same quantity, or multiples thereof, so that after a small number of repeated interventions the regulation of the kiln settles to the desired levels and the tile produced fall within a commercially- acceptable quality range.