Field of the invention
[0001] The present invention relates to a tunnel furnace and a related method comprising
a temperature control for a baking process for ceramic materials. More in particular
the invention relates to a tunnel furnace and a related method whereby the baking
process in the burner zones of the tunnel furnace is steered according to an intelligent
control. This leads on the one hand to a substantive economy of gas, on the other
hand to a more qualitative product by reducing the losses relating to disapproved
baked material.
Background of the invention
[0002] A brick factory or brick bakery, also called a brick furnace, is a factory wherein
ceramic materials are produced, more in particular bricks and often also related products
like (roof) tiles and brick tubes or pipes, brick strips, ...
[0003] An important issue for brick factories was the availability of clay, so many brick
factories were established in clay-rich geographic areas. In line herewith, brick
factories and roof tile factories have been built on the edge of clay-rich, resp.
peat-rich areas. Along the big rivers clay was available and the baked bricks were
transported by ship to the final customers. In view of the abundant presence of clay
in Flanders, Belgium, a tradition of bricklaying has been established and along with
this, large brick factories have been built.
[0004] Ceramic products traditionally have been manufactured the same way as bricks. In
this process raw materials are first mixed and then put into their final product form.
The so formed products are then dried and baked. Only after the baking process, the
formed products are characterized by their final product features, only at that stage
these products are called ceramic products.
[0005] Irrespective of the way the products are formed in the factory -products made by
hand form or e.g. by extrusion - the product needs to be baked. The vast majority
of these products are baked in a fully oxidizing environment. This means that oxygen
is always present to have the mineral raw materials burned. As burning materials,
use can be made of oil, coal or gas. In most cases gas is used as gas is easy to control,
to deliver and still is relatively cheap. However, the cost of gas still is one of
the three biggest cost factors in the manufacturing process; also, the burning process
inherently causes a substantive air pollution.
Periodical and continuous furnaces:
[0006] As the burning and baking process is an energetically intense and economically expensive
process, it is of the utmost importance to optimize the baking technology.
[0007] In the ceramics industry, two types of furnaces or baking processes are known: a
periodical or a continuous execution of the baking process.
[0008] The development from a periodical to a continuous furnace system is a result of the
need to reduce the energy required for the baking process. In case of periodical furnaces,
a lot of energy is lost since after each baking cycle the complete furnace should
be cooled and the heat is lost. For periodical furnaces, the temperature of the flue
gasses corresponds to the actual temperature of the products, whereas in the case
of continuous furnaces, this heat is used for the warming up and drying of the products.
Despite this more economical approach, this kind of furnace still requires very substantive
baking costs.
[0009] The present invention relates to the continuous type of furnace, this is also the
more common type of furnace in present day use.
[0010] In figure 1 a schematic view of a continuous type of furnace is shown.
[0011] The small platform trailers (kilncar) whereupon the materials to be baked are loaded,
enter the furnace gallery at the left side, they are moved throughout the furnace
to the right side until they are carried away from the furnace at the utmost right
side.
[0012] This type of continuous baking furnace is called a tunnel furnace since the products
to be baked are moved throughout this furnace in a tunnel-like fashion. In the furnace,
three zones should be distinguished: the heating zone, situated at the entrance of
the products in the tunnel (left in the figure), the firing zone in the middle of
the furnace and the cooling zone at the way-out of the tunnel (at the right side of
the figure).
The course of the baking process in a continuous furnace:
[0013] One may distinguish the baking processes of the various ceramic products according
to the different temperature course throughout the various zones and timing of the
furnace.
[0014] The heating and cooling rate may vary substantially from a few degrees up until hundreds
of degrees per hour. Also, the top temperatures may vary substantially from 800 up
tot 2 000 degrees.
[0015] Figure 2 shows an example of a so-called baking curve: this curve shows the evolution
of the temperature throughout the furnace (in ordinate is set out the temperature,
ranging from room temperature till 1 200 °C at its top; the abscissa sets forth the
residence time of the ceramic materials in the furnace, expressed in minutes.)
[0016] In the example shown in figure 2, the ceramic materials to be baked remain altogether
3 630 minutes in the furnace, so 60 hours and 30 minutes.
[0017] Also, one may distinguish the various zones and corresponding residence periods of
the ceramic materials in these zones in the furnace:
- a heating zone, subdivided into a pre-heating period, resp. zone and a heating period,
resp. zone;
- a firing zone, subdivided into a stoke period, resp. stoke-zone and a sintering period,
resp. a sintering zone;
- a cooling zone, subdivided into a quick or rapid cooling period, resp. zone and a
natural cooling period, resp. zone.
[0018] The course of the heating should be selected such that the changes brought about
for the product to be baked during the heating cycle, such as shrinkage and extension
(in particular around the quartz inversion point) should not give rise to damage.
To this end, the heating velocity is usually set at a value not exceeding 25 °C/hour
around the quartz inversion point of around 580°C instead of an average value of 50°C
per hour in the pre-heating zone. In the pre-heating zone there is no firing. In this
zone, use is only made of the residual heat of the firing zone. The ignition temperature
of gas is achieved in the firing zone. In this zone gas is effectively used. Mostly
in this zone in a number of consecutive steps the temperature is increased until the
product has reached its required top temperature. The top temperature is retained
until the product is completely sintered, this process being achieved in the sintering
zone.
[0019] In the firing zone the temperature management system according to the invention is
applied, resulting in a substantial saving of gas consumption.
[0020] As soon as the product has reached its top temperature, upon leaving the sintering
zone, in the quick cooling zone, cooling is applied until reaching the sensitive quartz
inversion point. Thereafter, further cooling takes place of the baked product in the
natural cooling zone.
[0021] Cooling air is recycled for the drying of the products.
Present-day developments in view of gas-savings:
[0022] During the last couple of decennia, mankind has been increasingly aware of the fact
that the natural resources of the earth in terms of energy are not unlimited. In view
hereof, more advanced technologies have been developed so as to manage energy needs
in a more efficient manner.
[0023] The theoretical amount of energy required for baking amounts to 400 KJ per kg of
material. In practice, much more energy is used in present-day baking processes; for
tunnel furnaces one uses on average 3500 up to 5500 KJ per kilogram of product.
[0024] The energy crisis of the mid-seventies gave rise to processes that are characterized
by less losses of energy in the firing process. During this period, better insulation
products were used, in turn resulting in less losses of energy. With the advent of
up-to-date industrial processes, it quickly appeared that human control was not sufficient
to master complex technological processes. As a result more precise measuring instruments
were used, in most cases thermocouples, to accurately measure and steer the temperature
course in the firing process.
[0025] Present day systems make use of measure- and control instruments in the process,
coupled to a PLC, a Programmable Logic Controller and a central computer.
[0026] By means of a display or monitor screen, desired values can be set and measured values
can be visualized. By means of such a system the valves for the input of gasses in
the firing zone of a tunnel furnace can be steered, controlled or managed. However,
up to now, the baking process for ceramic materials in a tunnel furnace has not been
optimized, in particular has not been optimized in view of an efficient gas consumption.
[0027] German published patent application Nr.
DE 3438347 A1, published April 24, 1986 by W. Leisenberg, describes a tunnel furnace for the baking of ceramic materials.
The overall furnace comprises a pre-heating zone, a baking zone and a cooling zone,
whereby the temperature in the various zones can be steered by a steering system.
The steering system comprises a computing device (Prozessrechner) that takes into
account the measured temperatures in a given zone and steers the process in such a
manner that the measured temperature follows as closely as feasible the set temperature
course throughout the various zones of the furnace. Nowhere in this specification
however, mention is made of a specific manner according to which for a given burning
zone the temperature should be steered in view of gas-consumption savings.
[0028] In view of the above, there remains a need for methods and systems to reduce the
gas consumption in the baking process of ceramic materials in a tunnel furnace, without
compromising or jeopardizing the quality of the baked products, and without reducing
the overall baking capacity of the furnace.
Summary of the invention
[0029] An object of the present invention is to provide a tunnel furnace for a baking process
for ceramic materials,
- comprising a heating-, a firing- and a cooling zone,
- the firing zone comprising a stoking zone and a sintering zone,
- the stoking zone comprising burning zones equipped with gas burners,
- each burning zone having an initial and a final temperature set value.
[0030] The tunnel furnace is suitable for stepwise passing through of ceramic materials
through the zones abovementioned.
[0031] The tunnel furnace is thereby characterized in that it comprises a temperature steering
system, suitable to steer during the residence of the ceramic materials in a burning
zone of the firing zone, the temperature in the burning zone from the initial value
till the final value according to a curve situated equal to or beneath a linear curve
of the initial value till the final value of the burning zone.
[0032] The invention also relates to a tunnel furnace whereby the temperature is actually
steered by the temperature steering system as abovementioned.
[0033] The invention also relates to a method for the baking of ceramic materials, whereby
the temperature throughout the baking process is steered by the abovementioned steering
system.
[0034] More in particular the present invention comprises the tunnel furnaces and methods
as set forth in the appended claims.
[0035] The invention is defined and characterized in the main claim, while the dependent
claims describe other characteristics and specific features for preferred embodiments
of the invention.
[0036] Further aspects and advantages of the embodiments described will appear from the
following description taken together with the accompanying drawings.
Brief description of the drawings
[0037] The foregoing and other objects, features, and advantages of the invention will be
apparent from the following more particular description of a preferential form of
an embodiment of the invention, as illustrated in the accompanying drawings, given
as a non-restrictive example.
[0038] It will be appreciated that for simplicity and clarity of illustration, elements
shown in the drawings/figures have not necessarily been drawn to scale, nor are these
elements necessarily to scale relative to each other. For example, the dimensions
of some of the elements may be exaggerated relative to other elements for clarity.
Further, where considered appropriate, reference numerals may be repeated among the
figures to indicate corresponding or analogous elements. So, in the drawings, the
same reference numerals may identify the same elements of structure in each of the
several figures where appropriate.
[0039] In the figures, the thickness of certain lines, layers, components, elements or features
may be exaggerated for clarity. Broken lines illustrate optional features or operations
unless specified otherwise.
FIG. 1 is a schematic illustration of a continuous type of baking furnace or tunnel
furnace wherein the invention may be applied;
FIG. 2 shows the course of the temperature throughout the various zones of the tunnel
furnace;
FIG. 3 shows the course of the temperature according to the state of the art and according
to the invention of a load of ceramic materials throughout the stoking and sintering
zones of a tunnel furnace;
FIG. 4 shows schematically the stoking zone of a tunnel furnace with nine burning
zones;
FIG. 5 shows schematically the steering system according to the invention;
FIG. 6 shows schematically a gas burner for use in a tunnel furnace;
FIG. 7 shows schematically the course of the temperature throughout one burning zone
as a function of the passing of various loads of ceramic materials through said burning
zone;
FIG. 8 shows the detail of the temperature curve throughout one burning zone for one
single passing of a load of ceramic materials.
Description of the invention
[0040] As set forth supra, the invention comprises the following aspects:
- 1) a tunnel furnace wherein a steering system for the temperature is implemented;
- 2) a method for the baking of ceramic materials according to the above steering system.
[0041] The following is a detailed description of preferred embodiments of the invention,
reference being made to the drawings.
[0042] It will be appreciated that for simplicity and clarity of illustration, where considered
appropriate, numerous specific details are set forth in order to provide a thorough
understanding of the exemplary embodiments described herein. The present invention
now is described more fully hereinafter with reference to the accompanying drawings,
in which embodiments of the invention are shown. However, it will be understood by
those of ordinary skill in the art that the embodiments described herein may be practiced
without these specific details. Indeed, this invention may be embodied in many different
forms and should not be construed as limited to the embodiments set forth herein;
rather, these embodiments are provided so that this disclosure will be thorough and
complete, and will fully convey the scope of the invention to those skilled in the
art.
[0043] In other instances, well-known methods, procedures and components have not been described
in detail so as not to obscure the embodiments described herein.
[0044] The steering system according to the invention and/or the method applying such steering
system can be implemented in an existing tunnel furnace without undue effort. It also
can be implemented in newly constructed or to be built tunnel furnaces.
[0045] As an illustration hereof, at the end of this description is set forth how such a
steering can be implemented by means of a Siemens ® PLC.
[0046] According to a preferred embodiment of the invention, the temperature curve takes
a parabolic shape as from the initial temperature up to the final temperature of a
given burning zone.
[0047] According to a further preferred embodiment of the invention, the temperature of
the burning zones of the tunnel furnace is steered such that the initially set temperature
is equal or higher than the gas ignition temperature.
[0048] According to a still further aspect of the invention, the temperature is steered
by controlling the gas input and/or the air input in the burning zones as a function
of the following input parameters:
- the in the burning zone desired or set temperature in conformity with the curve;
- the in the burning zone measured temperature.
[0049] According to a still further preferred mode of operation of the invention, the gas
input in each of the burners of the stoking zone is steered as a function of the input
parameters by optimization of the pulse time of the gas burners and/or the air supply.
[0050] According to a preferred embodiment, the temperature in the burning zone is measured
by means of a thermocouple.
[0051] As set forth supra, figure 4 shows the stoking zone of a tunnel furnace wherein,
solely as an illustration and without limiting the invention in such sense, nine burning
zones are shown (indicated by the numbers 1 through 9).
[0052] In each of these burning zones, upon arrival of a trailer loaded with ceramic materials,
the temperature should rise from a pre-determined initial value till a set or desired
final value.
[0053] The values of the temperature increase gradually as a function of the trailer passing
through the tunnel furnace. In the last burning zone, the top temperature is reached
and maintained.
[0054] In figure 5 is shown how the temperature control or steering system of the tunnel
furnace according to the invention steers the gas burners by means of their respective
gas valves so as to reach the desired final temperature in a given burning zone according
to a well-defined curve.
[0055] According to a preferred embodiment of the invention, this system is applied to all
of the burning zones of the stoke zone of the tunnel furnace. To that end, use is
made of a thermocouple, positioned in each of these burning zones, as set forth on
the figures 4 and 5. The thermocouple will at each point of time during the operation
of the tunnel furnace measure and transmit the actual temperature of the burning zone
the so-called 'IST' value, or value actually measured to the steering system of the
tunnel furnace.
[0056] The steering system will compare this actual or 'IST' value with the desired or 'SOLL'
value for that point in time for said burning zone and as a function of the difference
between both values, will steer the gas valves in the manner as set forth hereinafter.
[0057] Figure 5 shows how the temperature steering system steers the gas burners. As an
illustration, twelve bas burners are provided in the burner zone. In a typical configuration
these are positioned at both sides of a central gas supply line.
[0058] In the middle of the burning zone, the thermocouple is positioned that measures the
actual temperature in the burning zone and transmits same to the steering system according
to the invention.
[0059] The steering system of the tunnel furnace according to the invention is designated
by the block indicated 'ESA-technics', this is the provisional denomination of the
steering system to be implemented in the tunnel furnace.
[0060] The output of this steering system is the position to be held by the gas valve and
the air valve of the respective burning zone: the input parameters that are processed
by this temperature steering system are the desired initial and final values of the
temperatures in the respective burning zone, as well as the shift-time, this is the
residence time as determined for the tunnel furnace and for the respective load of
ceramic materials in the respective burning zone. (the shift-time is the time a trailer
loaded with to be baked materials resides in a given burning zone; this time is the
same for all of the burning zones, as the trailer passes stepwise and gradually through
the entire tunnel furnace.)
[0061] Usually on a trailer, various loads of to be baked materials are placed near to each
other, whereby openings are left between the various piles of materials in order to
enable the hot air and the baking heat to reach each pile in a more or less uniform
manner.
[0062] Usually two piles or rows of materials are loaded after each other in the moving
direction of the trailer, leaving sufficient space between both piles.
[0063] Usually the trailers pass beneath the burners, positioned on top thereof.
[0064] In some cases, burners are also placed sideways in the burning zones.
[0065] When a trailer passes through a burning zone, the trailers usually are placed such
that the flames of the burners positioned on top of or sideways of the trailers pass
through the middle of the piles of loads of ceramic materials on the trailers.
[0066] For the burners positioned on top of the burning zone, the flames are directed downwards;
for the burners positioned sideways of the burning zone, the flames are directed sideways.
[0067] According to such orientation of the flames, a quite uniform baking of all of the
ceramic materials loaded on the trailers is obtained.
[0068] During the transfer of a trailer to the next following burning zone, the burners
are not working. This procedure is followed so as to avoid that the top, resp. outer
pile of ceramic materials on the trailer is baked too hard.
[0069] For a tunnel furnace comprising for example seven burning zones and whereby the temperature
of the trailer loaded with ceramic materials and the ambient flue gases upon arrival
in the first burner zone amount to approximately 800°C, and assuming that the final
baking temperature is set at 1 000°C, in each burning zone a temperature increase
of approximately 30°C should be realized (1 000 - 800 = 200, /7 = approx. 30°C).
[0070] However, it is not a requirement that an identical temperature increase in each burning
zone should be realized; in some cases in the first zone(s) a greater temperature
increase is achieved, e.g. 40°C or more, whereby the required temperature increase
in the following zone(s) may be more moderate, e.g. around 30°C.
[0071] For the case that the residence time (or shift-time) of a trailer in a burning zone
amounts to approximately one hour, this implies that a temperature increase of 30°C
should be reached within one hour, so on average 1°C per 2 minutes of time. Hereupon,
the trailer can be transferred to the next following burning zone, whereby the same
process should be repeated.
[0072] For a tunnel furnace with nine burning zones, the table set forth hereinbelow illustrates
what the desired initial- and final temperatures could be:
Zone Nr. |
Initial-temperature |
End-temperature |
Delta temperature |
0 |
760 |
800 |
40 |
1 |
800 |
840 |
40 |
2 |
840 |
880 |
40 |
3 |
880 |
920 |
40 |
4 |
920 |
960 |
40 |
5 |
960 |
1000 |
40 |
6 |
1000 |
1025 |
25 |
7 |
1025 |
1050 |
25 |
8 |
1050 |
1050 |
0 |
9 |
1050 |
1050 |
0 |
[0073] The course of the temperature in each of the burning zones of this example is illustrated
in figure 3, upper curve.
[0074] The nine zones indicated above are also set forth on the curve shown in Figure 3.
According to a preferred embodiment of the invention, in the first zone, wherein the
temperature is below the gas ignition temperature, the steering or control system
of the tunnel furnace according to the invention, is not applied. According to a further
preferred embodiment, the system also is not applied in the two last zones.
[0075] In the seven zones of the so-called 'savings-zone', the system is applied.
[0076] In Figure 3, the upper curve indicates the course of the temperature according to
the state of the art.
[0077] In each burning zone, according to the temperature steering systems known in the
state of the art, the temperature is steered from the initial value for said burning
zone until the final value for said burning zone in a way such that a quite strong
increase of temperature is achieved in the initial stage or phase. As soon as the
desired or set temperature is then reached, the temperature is kept constant, which
is shown by the horizontal part of the curve for each burning zone.
[0078] In the above example, the first zone is numbered 0. The initial temperature in said
zone is situated below the gas ignition temperature; as a result, this zone is not
comprised within the so-called savings-zone of the steering system of the present
invention.
[0079] Further, the last two zones are numbered 8 and 9, the sinter-zones; in these zones
the temperature of the ceramic materials is kept constant and no further temperature
increase is performed.
[0080] According to the steering system of the tunnel furnace according to the invention,
the temperature in each of the burning zones of the 'savings zone' will evolve according
to a course or a curve that is clearly situated below the abovementioned temperature
curve according to the state of the art. As set forth supra, the 'savings zone' is
the zone starting as from the gas ignition temperature until reaching the maximum
temperature in the tunnel furnace, the latter being the temperature upon which the
sintering process starts. In this case either a linear curve is followed, like the
curve indicated in figure 3, or a curve that globally is situated below such linear
curve.
[0081] The term 'linear curve' for a burning zone should be understood as the curve that
for the given burning zone shows the course of the temperature as a function of time
and whereby the temperature increases steadily, uniformly or linearly in the given
burning zone from the initial temperature value until the final or end value for the
given burning zone, as a function of the residence time of the to be baked ceramic
materials in this burning zone. This residence time is referred to supra as the 'shift
time'.
[0082] The characteristic feature of the invention resides in the fact that the temperature
in a burning zone is steered or controlled by the temperature steering system of the
tunnel furnace of the invention in such a way that for a given burning zone the actually
followed temperature course is situated equal to or below this linear or regular curve
for the said burning zone.
[0083] The temperature curve obtained in such a way may for example be a parabolic curve,
the initial value whereof corresponds to the initial temperature in a given burning
zone and the final or end value whereof corresponds to the set or desired temperature
in the given burning zone. The characteristic feature of such a curve is that the
temperature increase in the burning zone at the initial stage of phase of residence
of the ceramic materials in the burning zone is less pronounced and that up near the
end of this residence time, the temperature increases in a more steep manner, for
instance according to the abovementioned parabolic curve, until the set or desired
final temperature for the given burning zone has been reached.
[0084] Figures 7 and 8 show an example of such a parabolic curve. In these figures the ordinate
indicates the temperature and the abscissa indicates the time for a given burning
zone.
[0085] Figure 7 shows how the temperature for a given burning zone is steered and as a result
hereof varies as a function of time, more in particular as a function of the residence
time of the to be baked ceramic materials.
[0086] All in total four 'cycles' are shown in figure 7, whereby each cycle is characterized
by an increasing curve, followed by a (strongly) decreasing curve.
[0087] The increasing part of the curve shows the increasing temperature in the given burning
zone, as soon as a load of ceramic materials has arrived in the given burning zone.
In the example shown in figure 7, the temperature in the burning zone is increased
from approximately 780°C to approximately 800°C, this is the desired or set value
for the temperature in this burning zone.
[0088] More in general terms, the temperature in the burning zone should be increased from
an initial value situated in the range varying from 750°C to 850°C, more preferably
from 790°C to 835 °C to a final value situated in the range from 950°C up to 1150°C,
more preferably from 975°C up to 1100°C, still more preferably from 1000 °C up to
1075 °C.
[0089] The figure clearly shows how the course of the increasing temperature globally is
situated under the linear curve of the initial set value of 780°C up until the final
or end set value of 800°C. As soon as the final temperature for the given burning
zone has been reached, in this example 800°C, the load of ceramic materials is transferred
to the next burning zone and a new load of ceramic materials is loaded in the given
burning zone.
[0090] In this period, the temperature in the given burning zone decreases from the final
value of 800°C down to the initial temperature, in this particular case, 780°C. During
this period, the gas burners are off. As soon as the new load of ceramic materials
has arrived in the burning zone, the gas burners are activated again according to
the steering system of the invention and the temperature will again increase up until
the set value for the given burning zone.
[0091] The above is illustrated by the second increasing temperature curve of this figure
7. Once the final temperature again has been reached, the loads of ceramic materials
again are shifted through the tunnel furnace and the temperature in the given burning
zone again decreases, illustrated by the second decreasing line of the curve shown
in figure 7. These temperature courses or evolutions throughout a burning zone are
repeated as long as the passing of the ceramic materials through the tunnel furnace
continues.
[0092] Figure 7 in total shows such four cycles. The temperature course in each cycle follows
the course of a parabolic curve.
[0093] Figure 8 shows an alternate temperature course for such parabolic curve, for a single
cycle.
[0094] The temperature increases from about 780°C till about 800°C as shown in this figure.
Compared to the temperature course as shown in figure 7, the temperature increase
and decrease in this case is slower.
[0095] The course of the temperature during the heating stage in the given burning zones
also in this case follows a parabolic curve.
[0096] According to the way the burners are steered following the control systems known
in the state of the art, the burners are fully operational, this means they operate
at full or maximum power upon arrival of the trailers in the burning zone until the
set final temperature for the given burning zone has been reached. As soon as such
temperature has been measured by the thermocouple, the burners are off. Only when
thereafter the temperature decreases with a set value of e.g. 1°C, the burners are
temporarily again activated until the set value has been reached.
[0097] Given the capacity of the burners, in most of the tunnel furnaces and for the vast
majority of ceramic loads, such set value for the temperature is quickly reached,
e.g. after 5 minutes of a set shift time of e.g. 60 minutes. This implies that during
55 minutes the to be baked ceramic materials on the trailer in the given burning zone
are stabilized at that temperature.
[0098] Figure 6 shows a gas burner as it is commonly installed in a tunnel furnace.
[0099] It comprises a supply line for the gas and a separate supply line for air. Each burner
installed in a tunnel furnace is equipped with variable valves for gas and air. Upon
installation of the temperature steering system as described in the present specification
in a tunnel furnace, these valves are not changed; they keep their fixed positions
upon commissioning of the tunnel furnace. Only the pulse time of the burners is being
steered by the steering system as described in this specification.
[0100] In the steering system of the tunnel furnace according to the invention, the gas
burners are not continuously operational, but intermittently.
[0101] In a steering system according to the state of the art, the gas burners are being
operated at full power when the difference between the desired temperature and the
temperature measured by the thermocouple is sufficiently high.
[0102] When the delta between both temperature values is smaller, a state-of-the-art steering
system will not have the gas burners work at full power, but at a lower rate. In such
a case however, the length of the flame varies. This in turn has a negative impact
on the quality of the baked ceramic materials; in particular it gives rise to uneven
or not-uniformly baked materials.
[0103] In such a case indeed, the materials that are situated on top of the pile of materials
loaded on the trailers will be baked longer or more intensively compared to the materials
that are situated lower in the pile of loaded materials on the trailer. These 'upper'
materials indeed are in closer contact with the flames of the burner compared to the
'lower' materials. To compensate for these drawbacks, in a number of cases a higher
supply of air is used, giving rise to sufficient turbulence and as a result hereof
a more uniform heating of the materials residing in the burning zone.
[0104] In the temperature steering system of the tunnel furnace according to the invention,
use is being made of pulse-wise steering. In such a steering system, the burners are
regularly put on and off again (pulse-wise burning).
[0105] The big advantage thereof compared to the state of the art is that - always when
the burners are on or operational - the length of the flame is constant, whereas at
the same time a sufficient degree of turbulence is achieved in the burning zone. This
in turn gives rise to an increased quality level of the baked ceramic materials. The
burners are steered according to a small air surplus or excess resulting in a more
even or uniform baking of the ceramic materials. According to a further preferred
embodiment, the burners may be operated alternately or in turn, again given rise to
an enhance quality of the final baked products.
[0106] Also a decrease of the load of ceramic materials on the trailers may yield an increase
of quality of the final baked products.
[0107] So as to achieve a sufficient degree of turbulence in the burning zones, in the state
of the art, an extra air supply could be applied. This however yielded a degree of
air excess which was too high, often more than a factor 2. This means that the amount
of air is twice the supply of gas. The ratio of air over gas is steered on the basis
of the temperature of the furnace. The air excess level can then be controlled at
a lower level. But given the fact that there are multiple variable factors that influence
the rate of burning, this does not have an ideal effect on the gas consumption, given
the fact that one needs to adjust manually the settings for burning and pause. It
is impossible to perform manually the necessary corrections so as to reach the ideal
burning curve, given the fact that there are not many variables that have an influence,
such as the volume of the loads of ceramic materials, losses throughout the process,
ambient air temperature, ...
[0108] The tunnel furnace according to the invention does not make use of a pulse and pause
timing sequence that can be manually set. These appear to react too slowly upon changes
in the oven to effectively save on gas consumption. Contrary thereto, the furnace
and method according to the invention calculates at set time intervals, situated between
10 and 60 seconds, e.g. every 10, 15, 20, 25, 30, 35, 40, 45 50 55 or 60 seconds,
the required pulse so as to obtain the desired temperature course in a burning zone.
In this way, the furnace and the method according to the invention will quickly and
accurately detect differences between the temperature as measured by the measurement
means (e.g. a thermocouple) (this is the 'IST' value) and the desired or set value
for such temperature in the given burning zone (this is the 'SOLL' value). On the
basis of such temperature difference, the furnace and method will then react so as
to clear away such difference by steering the pulse and pause time of the burners
of the given burning zone. This steering method allows to reach the pre-set or desired
temperature for a given burning zone only at the point in time when the pre-set residence
time for a load of ceramic materials in the given burning zone is reached. The longer
such temperature is reached, the less gas will be consumed.
[0109] The effective reduction or saving in gas consumption in the furnace according to
the invention will depend on the particular dimensions and operational process of
the furnace.
[0110] The quality improvements that are obtained by the pulse-wise operation of the gas
burners are remarkable. The different way of achieving the end or final temperatures
in the burning zones even give rise to substantive improvements in quality of the
baked ceramic materials.
[0111] Other external factors (such as ambient air, losses, defects, air supply systems,...)
that may have an influence on the actual gas consumption, are also taken into account
in the steering system of the tunnel furnace according to the invention.
[0112] In the tunnel furnace according to the invention, the steering system and method
as described supra is only implemented in the burning zones that have an influence
on the quality and the color of the ceramic materials to be baked. This implies that
the steering system and method is preferentially not applied in such zones that operate
below the gas ignition temperature and not in the sintering zones that maintain the
temperature such that the baked ceramic materials are in conformity with the applicable
standards.
[0113] With respect to the practical implementation of the steering system and method in
a tunnel furnace: so as to reach the above set goals, the inventors have installed
the steering system and method in a steering box of a tunnel furnace. In such box
the program as written and designed by the inventors (named 'ESA-technics') was implemented
on a Siemens Simatic manager S7-1200. The inventors obtained from the owner of the
tunnel furnace the measured values of the temperature in the burning zones, so that
remotely the baking curves could be steered and monitored.
Practical mode of implementation:
[0114] The steering system and method according to the invention as applied in a tunnel
furnace in a real productive baking process; its results were compared to the operation
of a traditional or state of the art steering system.
[0115] This yielded the following results:
- 1) Operation according to the state of the art:
energy consumption of the furnace: 1 797 MWh per month for 450 loaded trailers in
that month;
- 2) Operation according to the steering system and method according to the invention:
natural gas consumption amounted to 1 509 MWh per month for the same load and cycles.
[0116] As is illustrated by the above example, the application of the steering system and
method in the tunnel furnace according to the invention gave rise to a substantive
saving in energy.
[0117] Such saving in energy in turn leads to a corresponding decrease in the output of
CO
2. The invention consequently also contributes to the reduction of CO
2 output as imposed upon industry.
[0118] Surprisingly, apart from the reduction in gas or energy consumption brought about
by the implementation of the present invention, also the fallout ratio of disapproved
products on the basis of the steering of the temperature according to the system and
method of the invention could be substantially decreased.
[0119] This is an unexpected but quite important effect obtained by the implementation of
the present temperature steering system and method according to the invention. Evidently,
apart from the gas consumption savings, this unexpected effect of the invention yields
an important added value for the operators of tunnel furnaces in the baking process
of ceramic materials, provided the temperature is steered in accordance with the system
and method of the present invention.
[0120] In the claims as set forth hereinafter, the word "comprising" does not exclude other
elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.
The mere fact that certain measures are recited in mutually different dependent claims
does not indicate that a combination of these measures cannot be used to advantage.
[0121] In the claims, the claimed methods are not limited to the order of any steps recited
unless so stated thereat.