Process and installation for heating a metallic strip,
notably for an annealing
The invention relates to a process and an installation for heating a
metallic strip to a set temperature needing the presence of a reducing
combustion atmosphere (lack of fuel in comparison with stoechiometric
quantity).
The invention is described as for its application to heating of a metallic
strip in the form of a strap transmitted to an annealing installation, notably for
making galvanized steel.
Various types of annealing installations for straps are already known.
Fig. 1 is a schematic view of a first example of a known installation for
preheating a strap to a set temperature appropriate to an annealing installation.
It comprises an oven delimiting a chamber 102 for preheating the strap 100 and
a chamber 104 for heating the strap in order that, at the installation exit, the
strap is at a set temperature, for example of 750°C.
In the preheating chamber 102, comprised between the oven entry 108
and a place 110 where post-combustion air from a duct 112 is injected, hot
gases with no reducing power, so that the atmosphere in the oven, which could
flow out through the entry 108, is not toxic (i.e. it contains no CO), flow from the
heating part 104 to the proximity of the entry 108, in a direction opposite that of
the strap 100. The heat exchanges between a surface and a current parallel to
this surface are not very efficient, because of the presence of laminar layers.
Consequently, the chamber 102 must have a great length.
In the combustion chamber 104, practically comprised between the air
injection place 110 and the oven exit 114, burners 116 receive a fuel
transmitted by a feed circuit 118 connected to each burner 116, and combustion
air by a circuit 120 also connected to each burner. The air in the circuit 120 has
been preferably preheated in a heat exchanger 122 in which also circulate the
hot gases evacuated by a duct 124 from the proximity of the oven entry end
108.
In the heating chamber 104, the burners 116 are open fire burners
working in a reducing combustion atmosphere, i.e. in the presence of CO. In the
heating chamber 104, the strap temperature increases from a temperature
around 350 to 400''C, near the air injection place 110, to a set temperature for
example of 750°C at the exit 114.
The just-described installation has some drawbacks.
First, because of the simple circulation of the hot gases in the direction
opposite that of the strap in the preheating chamber 102, the heat exchange
between the gases and the strip is not efficient, so that the chamber 102 has a
very great length and the oven takes up thus much room.
Then, the whole system using open fire burners for forming the reducing
combustion atmosphere with the air preheated by the exchanger 122 has a
relatively low efficiency, in the order of 50%, so that the fuel consumption is
high.
At last, the heat losses are important, essentially due to the evacuation of
the gases, still at a high temperature, into the atmosphere, because the heat
exchanger 122 which collects the heat of the gases evacuated by the duct 124
has a moderate efficiency.
In the document JP2002-294 347, it is also described another example of
a known installation for preheating a strap, before annealing it, which does not
have the drawback of taking up very much room at the floor level.
More precisely, this installation comprises a preheating chamber 202 with
a small length, because it comprises a device 210 for preheating the strap by
projecting hot gases in a direction perpendicular to the strap. This device
comprises chambers whose wall, facing the strap, comprises a plurality of
orifices projecting as many jets of hot gases. Such a device 210, sometimes
called "plenum", enables to get an efficient heat exchange over a small length.
This preheating chamber 202 is connected to a preheating chamber 204
in which the strap follows a zigzag path between radiant tubes. The preheating
chamber and the heating chamber have the same protective atmosphere. The
combustion gases evacuated by the radiant tubes, which are separated from
the protective atmosphere in the chamber 204, are extracted by ducts 205 to a
heat exchanger 26, with the help of a blower 207. In the heat exchanger 206, a
protective atmosphere circulates under the control of a blower 208. The
circulation circuit comprises two parallel connected ducts, one duct 209 feeding
the device 210 for projecting hot gases onto the strap and another duct 211
comprising a control valve 212 which opens more or less for modulating the
quantity of gas transmitted to the device 210 for projecting hot gases.
Thus, the set temperature sensor 218, which measures the temperature
of the strap at the installation exit, enables to control the valve 212 in order that
it regulates the quantity of gas able to circulate through the duct 209, and thus
the preheating power of the projection device 210, in order that the set
temperature, measured by the sensor 218, does not vary.
The installation, represented in Fig. 2, does not have the drawback of
taking up much room, but it has other drawbacks.
First, the efficiency of the radiant burners used is not high.
Then, as the heat of the combustion gases is only partially collected by
the heat exchanger 206, the installation has an energetic efficiency which is
relatively reduced and does not exceed a value in the order of 50%.
Moreover, the whole installation, comprising both chambers and the gas
circuit for feeding the chambers for projecting gases onto the strap, contains a
protective atmosphere which can be toxic and then does not fulfill the "hygienic"
requirements for the combustion, or which is inert and expensive.
At last, as the strap follows a sinuous path inside the preheating
chamber, the installation is onerous and prone to failures.
Preheating units with "regenerative" burners are otherwise known,
notably from the document JP-2001/304 539. The term "regenerative" indicates
that, in a first phase, some heat extracted from combustion gases is
accumulated, and, in a second phase, the accumulated heat is returned to the
combustion air. Such units usually comprise two burners 301a, 301b which are
mounted in tandem, the one working in a combustion mode in which some
combustion air in a duct 316 circulates through a regenerative mass 306 before
participating to a combustion, and the other working in a heat recuperation
mode in which the combustion gases from the first burner circulate through its
regenerative mass 306 and heat it. After a certain length of time, for example a
fraction of a minute, both burners exchange their ways of working. In the above-
cited document, each burner 301a, 301b also comprises an air feeding device
308 which, in the regeneration mode, introduces post-combustion air which
mixes with the combustion gases before these circulate through the
regenerative mass 306. A regenerative mass can be formed of a ceramic, for
example in the form of balls.
Thanks to the heat recuperation by the combustion air in the regenerative
mass 306 and to the absorption of the post-combustion heat of still reducing
gases with the post-combustion air introduced by the device 308, the efficiency
of the heating unit is Increased in a very important manner in comparison with
that of simple open fire burners working in a reducing combustion atmosphere
and used in an installation of the type described in reference to Fig. 1.
We do not currently know any use of such heating units with regenerative
burners in continuous heating installations, for example of the type used for
feeding an annealing installation for straps.
The invention implements a combination of characteristics of the
installations represented in Figs. 1 and 2 and of the regenerative burners
represented in Fig. 3, while reducing heat losses due to the use of heat
exchangers. The gases that are evacuated from the regenerative burners are
projected onto the strap in the preheating zone, participate to a noticeable
increase in the total thermal efficiency of the installation and enable to control
the temperature. Besides, a protective atmosphere which can be toxic exists
only inside the heating chamber, and the installation thus works in good
"hygienic" conditions for the combustion.
More precisely, the invention relates to a process for heating a metallic
strip to a set temperature needing the presence of a reducing combustion
atmosphere, of the type comprising a preheating of the strip to an intermediary
temperature needing not the presence of a protective atmosphere, by projecting
hot gases towards at least one side of the strip, and a heating of the strip in a
reducing combustion atmosphere from the intermediary temperature to the set
temperature, with the control of the set temperature by varying the projection of
hot gases for the preheating; according to the invention, the preheating in a
reducing combustion atmosphere is carried out by means of regenerative open
fire burners in a first phase which comprises, in at least one first burner, a
combustion with the help of air that has absorbed heat from a first thermal
absorption mass, and, in at least one different burner, a heat absorption
regeneration of the combustion gases from the first burner by at least one
different thermal absorption mass, and in a second phase in which the functions
of at least the first burner and at least the different burner on the one hand and
the functions of the first thermal absorption mass and of the other thermal
absorption mass on the other hand are reversed, the heat absorption
regeneration of the combustion gases is carried out in an operation which
comprises, before the passage of the combustion gases through the
regeneration mass, the mixing of these combustion gases to an additional
quantity of post-combustion air, the preheating by projection of hot gases
comprises the use of at least a part of the gases evacuated from the heating by
regenerative burners, and the control of the set temperature comprises the
adjustment of the quantity of hot gases for the projection preheating.
Preferably, the adjustment of the quantity of hot gases used for the
preheating comprises the adjustment of the proportions of the hot gases
transmitted to the projection step on the one hand and to the hot gas evacuation
step on the other hand.
Preferably, the quantity of air introduced for the mixing to the combustion
gases is sufficient for the evacuated gases to have no reducing power.
Preferably, the mixing of the combustion gases to the post-combustion
air introduced before the passage of the combustion gases through the
regenerative mass is adjusted according to the result of a measure of the
reducing power of the combustion gases.
Preferably, the process comprises the implement of a gas circulation
directly from the heating step to the preheating step and the injection of post-
combustion air between both these steps.
In an example, the Intermediary temperature is in the order of 400°C, the
set temperature is a temperature for annealing the metallic strip, the metal of
the metallic strip is steel, and the metallic strip is a strap.
The invention also refers to an installation for heating a metallic strip to a
set temperature needing the presence of a reducing combustion atmosphere,
which comprises a preheating chamber provided with a projection device for
projecting hot gases towards the strip, a heating chamber provided with an
open fire heating unit for heating in a reducing combustion atmosphere, the
heating unit comprising at least two regenerative burners working in tandem, at
least one of the burners working in a combustion mode in which the combustion
air circulates through a regenerative mass before participating to a combustion,
and at least another burner working in a heat recuperation mode in which the
combustion gases from at least the first burner circulate through the
regenerative mass of the other burner and heat it, the at least two burners
exchanging theirs working modes, each burner comprising an air feeding device
working in a regeneration mode, in order that this air mixes up to the
combustion gases in the reducing state before they circulate through the
regenerative mass, a duct for evacuating the gases from the heating unit, an
adjustable three-way valve with an entry connected to the duct for evacuating
the gases from the heating unit, an exit connected to the gas projection unit of
the preheating chamber, and an exit connected to a duct for evacuating the
gases outside the installation, and a control device comprising a set
temperature sensor for the metallic strip and an adjustment component for the
three-way valve, in order that this valve controls the quantity of hot gases
transmitted to the projection device.
Preferably, the air feeding device working in a regeneration mode
introduces an essentially constant quantity of air, which is at least sufficient for
the evacuated gases to have always no reducing power.
Preferably, a second control device, which comprises a sensor for
measuring the reducing power of the gases and an adjustment component for
adjusting the introduced quantity of air according to the signal from the sensor
for measuring the reducing power, controls the quantity of air introduced by the
air feeding device.
Preferably, the preheating and heating chambers are adjacent and
collinear.
Preferably, the installation further comprises a device for introducing
post-combustion air between the preheating and heating chambers, the quantity
of air introduced by this device being sufficient for the preheating chamber to
contain gases with no reducing power.
In comparison with the installation represented in Fig. 1, the advantage of
the invention is that it enables the realization of an installation with a reduced
length, thanks to the considerable reduction of the preheating zone.
In comparison with the installation represented in Fig. 1, the advantage of
the invention is that it is simpler, insofar as the strap follows a linear path.
In comparison with all the described installations, the invention enables
to considerably reduce energetic losses and to very significantly increase the
energetic efficiency, essentially obtained by using units with regenerative
burners and thanks to the absence of any heat.
Thus, if an installation, such as represented in Fig. 1, has a total
energetic efficiency in the order of 50%, the efficiency of a similar installation of
the type represented in Fig. 4 will greatly exceed 60%.
All these advantages are obtained by combining the use of a rectilinear
path, the use of a projection preheating device, the use of open fire burners of
regenerative type, and the use of a set temperature control by the preheating
device. More precisely, the regenerative burners are implemented in order to
obtain a temperature of the evacuated combustion gases which is much higher
than in the state of the art. Indeed, in all the known applications, the
regenerative burners, whether they work in an oxidizing atmosphere or in a
reducing atmosphere, produce combustion gases at temperatures in the order
of 150°C. It is partly for these reasons that all the known regenerative burners,
including that of the type described in the document JP-2001/304 539, are
implemented with an evacuation of the combustion gases to the atmosphere.
Within the scope of the invention, the burners work on the contrary so as to
obtain a hot gas temperature which enables their direct use by projection into
the preheating chamber, advantageously in the order of 400°C.
Other characteristics and advantages of the invention will be better
understood from the reading of the following description of an embodiment; in
the figures, Fig. 4 is a schematic representation of an installation according to
the invention, the Figs. 1 to 3 having already been described.
The installation represented in Fig. 4 comprises a number of parts similar
to those in Fig. 1.
Thus, a strap 1 enters an oven 6 which comprises a first chamber 2
working in an oxidizing environment, or at least in an environment with no
reducing power, and a second chamber 4 with a reducing combustion
atmosphere. The strap entry 8 enables only the passage of the atmosphere in
the first chamber 2, i.e. an atmosphere which is not toxic and which contains no
CO, thanks to the introduction of post-combustion air through an entry 10, with
the help of a duct 12. The strap leaves the oven through an exit 14 at a set
temperature, after passing the burners 16. These burners are supplied with fuel
by a circuit 18 and with air by a circuit 20. The gases, which are evacuated in
the end near the entry 8, leave the installation through a duct 24 which is
connected, for example, to a smokestack.
The other characteristics of the installation represented in Fig. 4 are
different from those in Fig. 1.
First, the preheating chamber 2 comprises a device 11 for projecting hot
gases for the preheating, which device is supplied with hot gases by the heating
chamber 4, as described afterwards.
Each open fire burner 16 is associated to a regenerative mass 26, similar
to the regenerative mass 306 described in reference to Fig. 3. A post-
combustion air entry 28 is similar to the entry 308 described in reference to
Fig. 3. Two burners 16 and 16', arranged one opposite the other, work in
tandem, in the manner described in reference to Fig. 3.
In a first embodiment, the quantity of post-combustion air introduced
through the entry 28 placed before the regenerative mass 26 can be sufficient
for the gases evacuated by the burner to have always no reducing power. In
that case, the burners work with an excess of post-combustion air.
In a variant, a reducing power sensor (not represented) is incorporated in
the burners, before or after the regenerative mass, in order to control the
quantity of post-combustion air introduced in each burner. In this manner, the
thermal efficiency is optimized and the energy consumption of the burners is
reduced to the minimum.
Despite the post-combustion reaction which emanates an additional
quantity of heat, notably by combustion of CO in the reducing combustion
atmosphere, the gases evacuated by the regenerative masses 26 are at a
temperature which is sufficiently low for the transmission of a combustion gas
evacuation duct 31 by a blower 30 to a three-way control valve 32 and to the
hot gas projection device 11, at a temperature which needs not the presence of
a reducing combustion atmosphere for the strap. The presence of a blower,
which is in all the cases necessary to extract the combustion gases, further
enables to take advantage of the dynamic pressure acquired by these gases to
increase the thermal efficiency of the projection preheating. This also
contributes to the increase in the efficiency of the system according to the
invention.
It is also represented a temperature sensor 34 for determining the set
temperature of the strap 1 at the exit of the oven 6. A control circuit (non
represented), for the control according to the temperature determined by the
sensor 34, controls an operation device 36 which adjusts the three-way valve
32. In this manner, the quantity of gases evacuated from the burners, which is
used for the preheating in the device 11, can be regulated at a high reaction
speed, necessary because of the high speed of the strap in the oven.
The heating chamber 4 in the installation represented in Fig. 4 has a
length similar to that of the heating chamber 104 in the installation represented
in Fig. 1. On the contrary, the preheating chamber 2 in the installation
represented in Fig. 4 is much shorter than the chamber 102 in the Installation
represented in Fig. 1.
The installation represented in Fig. 4 enables a linear passage of the
strap 1 which is simply locally supported by some rolls inside the oven, contrary
to the complex installation for the zigzag circulation of the strap in the
installation represented in Fig. 2. Thanks to the presence of two distinct
atmospheres, the one which can be oxidizing and is not reducing in the
combustion sense, in the preheating chamber, and the other which is reducing
in the combustion sense, in the heating chamber, the combustion gases can be
used directly for the preheating, contrary to the installation represented in Fig. 2
which needs a heat exchanger between the combustion gases and the gases
used for preheating the strap.
Another advantage of the invention lies in the fact that, thanks to the
preheating to a high temperature and to the increase in the thermal efficiency of
the regenerative masses in the burners, which can reach 80%, the evacuated
smokes can contain reduced quantities of nitrogen oxides NOx, for example
inferior to 200 mg/m^ in normal conditions for 3% of oxygen in the smokes,
when this system for preheating the combustion air is associated to a high-
performance technique as regards pollutant emissions (for example flameless
oxidation). Besides, thanks to the increase in the thermal efficiency of these
high-performance burners, it is possible to heat the strap more quickly and thus
to get an increase in productivity, with a reduction of fuel consumption and of
nitrogen oxide emission.
Though the invention has been described as for its application to the
heating of a metallic strip, this heating can be considered as a real thermal
treatment, and not as a simple operation prior to a thermal treatment, such as
an annealing.
The invention has thus important advantages, not only from the
profitability and cost point of view, but also from the point of view of fossil
resources preservation and of environment protection, thanks to the reduction
of consumed fuel and emissions and to the increase in security.
WE CLAIM:
1. Process for heating a metallic strip to a set temperature needing
the presence of a reducing combustion atmosphere, of the type comprising:
a preheating of the strip to an intermediary temperature needing not the
presence of a protective atmosphere, by projecting hot gases towards at least
one side of the strip, and
a heating of the strip in a reducing combustion atmosphere from the
intermediary temperature to the set temperature,
with the control of the set temperature by varying the projection of hot
gases for the preheating;
characterized in that
the heating in a reducing combustion atmosphere is carried out by
means of regenerative open fire burners in a first phase which comprises, in at
least one first burner (16, 16'), a combustion with the help of air that has
absorbed heat from a first thermal absorption mass (26), and, in at least one
other burner (16, 16'), a heat absorption regeneration of the combustion gases
from the first burner by at least one different thermal absorption mass (26), and
in a second phase in which the functions of at least the first burner (16, 16') and
of at least the different burner on the one hand and the functions of the first
thermal absorption mass (26) and of the different thermal absorption mass (26)
on the other hand are reversed,
the heat absorption regeneration of the combustion gases is carried out
in an operation which comprises, before the passage of the combustion gases
through the regenerative mass (26), the mixing of these combustion gases to an
additional quantity of post-combustion air,
the preheating by projection of hot gases comprises the use of at least a
part of the gases evacuated from the heating by regenerative burners, and
the control of the set temperature comprises the adjustment of the
quantity of hot gases for the projection preheating.
2. Process according to claim 1, characterized in that the adjustment
of the proportions of the hot gases transmitted to the projection step on the one
hand and to the hot gas evacuation step on the other hand.
3. Process according to anyone of the claims 1 and 2, characterized
in that the quantity of air introduced for the mixing to the combustion gases is
sufficient for the evacuated gases to have no reducing power.
4. Process according to anyone of the claims 1 and 2, characterized
in that the mixing of the combustion gases to the post-combustion air introduced
before the passage of the combustion gases through the regenerative mass is
adjusted according to the result of a measure of the reducing power of the
combustion gases.
5. Process according to anyone of the preceding claims,
characterized in that it comprises the implement of a gas circulation directly
from the heating step to the preheating step and the injection of post-
combustion air between both these steps.
6. Installation for heating a metallic strip to a set temperature needing
the presence of a reducing combustion atmosphere, characterized in that it
comprises
a preheating chamber (2) provided with a projection device (11) for
projecting hot gases towards the strip (1),
a heating chamber (4) provided with an open fire heating unit for heating
in a reducing combustion atmosphere, the heating unit comprising at least two
regenerative burners (16, 16') working in tandem, at least one of the burners
(16, 16') working in a combustion mode in which combustion air circulates
through a regenerative mass (26) before participating to a combustion, and at
least another burner (16, 16') working in a heat recuperation mode in which the
combustion gases from at least the first burner (16, 16') circulate through the
regenerative mass (26) of the other burner (16, 164) and heat it, the at least two
burners (16, 16') exchanging theirs working modes, each burner (16, 16')
comprising an air feeding device working in a regeneration mode, in order that
this air mixes up to the combustion gases in the reducing state before they
circulate through the regenerative mass (26),
a duct (31) for evacuating the gases from the heating unit,
an adjustable three-way valve (32) with an entry connected to the duct
(31) for evacuating the gases from the heating unit, an exit connected to the gas
projection unit (11) of the preheating chamber, and an exit connected to a duct
(24) for evacuating the gases outside the installation, and
a control device comprising a set temperature sensor (34) for the metallic
strip and an adjustment component (36) for the three-way valve (32), in order
that this valve controls the quantity of hot gases transmitted to the projection
device (11).
7. Installation according to claim 6, characterized in that the air
feeding device working in a regeneration mode introduces an essentially
constant quantity of air, which is at least sufficient for the evacuated gases to
have always no reducing power.
8. Installation according to claim 6, characterized in that it comprises
a second control device which comprises a sensor for measuring the reducing
power of the gases and an adjustment component for adjusting the introduced
quantity of air according to the signal from the sensor for measuring the
reducing power, and which is intended to control the quantity of air introduced
by the air feeding device.
9. Installation according to anyone of the claims 6 to 8, characterized
in that the preheating and heating chambers (2, 4) are adjacent and collinear.
10. Installation according to anyone of the claims 6 to 9, characterized
in that it further comprises a device (10) for introducing post-combustion air
between the preheating and heating chambers (2, 4), the quantity of air
introduced by this device being sufficient for the preheating chamber (2) to
contain gases with no reducing power.

The invention pertains to the heating of a metal strip, and
relates to equipment for heating a metal strip that comprises
a pre-heating housing (2) provided with a device (11) for
projecting hot gases towards the strip (1), a heating housing
(4) with regenerative burners (16, 16 ), a duct (31) for dischar
ging the gases from the heating housing, a three-way adjustaable
valve (32) and an adjustment device including a sensor (34) for
detecting the setpoint temperature of the metal strip and a
member (36) for adjusting the three-way vaive (32) so that it
can adjust the amount of hot gases fed to the projection
device (11). The invention can be used in equipment for heating
nnarrow strips before annealing the same.