Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR WIRE PATENTING BY RADIATION-
CONVECTION HEAT TRANSFER
Field of the Invention
The present invention is applied to wire patenting. It more specifically
relates to a method and a device for high-carbon wire patenting.
Background of the Invention
In wire production processes, the starting steel (in the form of a wire
rod) is drawn. The drawing operation gives the material metallographic and
mechanical properties that are not advisable for subsequent use thereof. For
this reason a patenting step is necessary, which again gives the wire the
suitable characteristics for either continuing the process or as an end
product.
Patenting is an isothermal transformation heat treatment consisting of
austenitization of the steel around 9002C (it can vary depending on the
carbon content) and rapid cooling to 5502C. The result is a fine perlite
structure (troostite) giving the wire high strength as well as good ductility.
Currently, most wire manufacturers use high-temperature fluidized bed or
open flame furnaces and lead baths in the rapid cooling step for patenting.
The use of lead in cooling means that it appears as a contaminant in
subsequent steps (cooling the wire in water, oxide cleaning with acids,
washing, even in the zinc bath in the case of being galvanized). This
classifies the waste as special, making it necessary for a waste management
company to treat and eliminate it. The high toxicity of lead thus makes it
necessary to search for alternatives.
Therefore, in the search for new patenting processes it must be taken
into account that they should be environmentally sustainable and energy
efficient, as well as not harmful for users.
Patent ES 2039708 T3 describes a wire patenting process using one
or several tubes filled with a gas, devoid of forced ventilation, modulating
the
heat exchanges throughout the cooling path of the wire and varying the
dimensions of the tubes, their length and in-line arrangement. The process
described in this document is a process for heat transfer based on natural
convection in a gas and the subsequent heat conduction through the wall of
the tube to the cooling fluid circulating through a coaxial annular channel.
This process presents the problems of having low energy efficiency, deficient
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heat modulation, complex adaptability to wires of different diameters, the
considerable length of the device for reaching the desired degree of cooling
of the wire, and the high cost of the installation. In particular, as can be
inferred from reading the description of the system, the heat transfer during
the cooling phase depends almost exclusively on the flow rate of the cooling
fluid and its log mean temperature. A minor log mean temperature difference
must result from the discussed process for heat transfer; accordingly, in
order
for the specific flow of heat through the wall of the tube in internal contact
with the gas to be large, the necessary flow rate of the cooling fluid must be
high; and it must be borne in mind that water is a scarce resource. On the
other hand, since the inert gas which fills each sector of tube is virtually
immobile, it will be progressively heated, accumulating heat, which is in
detriment to the efficacy of the process for the heat transfer from the wire
to
the cooling fluid.
Object of the Invention
These drawbacks and problems among others are solved by the
system and method for cooling wires of the invention. The invention proposes
a method for wire patenting comprising a cooling step, and where said
cooling step occurs by means of applying a turbulent fluid jet towards the
surface of the wire. The turbulent jet is preferably produced by at least one
flat jet nozzle situated such that the jet is perpendicular to the surface of
the
wire.
The method optionally comprises an in-line heating step for heating
the wire, before said cooling step, which is used to reach the austenitization
temperature of the wires circulating therein. It can further comprise a
drawing
step before entering the system for heating and a prior cleaning step,
whereby all the residues of lubricants from the previous drawing step are
eliminated. A system for heating by means of electromagnetic induction
currents individually wire-by-wire can be used in the heating step. The entire
transit of the wire in the process is preferably done in complete absence of
oxygen.
The invention also relates to a device for carrying out the methods
described above. Said device comprises a block of material having a very
high thermal capacity with a channel adapted for allowing the passage of a
wire to be cooled and at least one conduit for the circulation of a cooling
fluid,
and it further comprises at least one nozzle capable of injecting a turbulent
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fluid jet towards the wire to be cooled. The nozzles are preferably flat jet
nozzles and the device is axially symmetrical. It optionally comprises means
for modulating the intensity of the heat transfer from the wire. The number of
nozzles is also preferably predetermined depending on an assigned rate of
cooling and they are oriented according to radii perpendicular to the main
axis of the block.
As a result of the device and method of the invention, the current
processes for wire patenting, which use sulfuric/hydrochloric acid in their
cleaning systems and lead in their cooling baths, and consume a large
amount of energy, are replaced.
The number of nozzles, their geometric dimensions, length, width of
the outlet groove, cone angle, etc., as well as the relationship between them,
and their orientation with respect to the normal to the surface of the wire
can
vary according to if there are needs of the process for convection heat
transfer from the hot wire.
Brief Description of the Drawings
For the purpose of aiding to better understand the features of the
invention according to a preferred practical embodiment thereof, the following
description is accompanied by a set of drawings in which the following has
been depicted by way of illustration:
Figure 1 is a general scheme of the system for cooling patented wire
object of the patent application.
Figure 2 shows a cross-section view and a longitudinal view of one of
the possible configurations of nozzles, gas conduits and cooling fluid
conduits which respond to the fluid dynamic and heat transfer requirements
described above.
Figure 3 shows an alternative example of the invention, but it
maintains the same operating principle.
Figure 4 is a graph showing how the non-uniformity of the flow over
the object translates into a non-uniform distribution of temperature and of
the
heat transfer over its surface.
Detailed Description of the Invention
The patenting process comprises preferably a drawing step for
drawing the wire, a cleaning step for removing possible residues of lubricant
used in the previous step and an in-line heating step for heating the wire to
the austenitization temperature. After heating, cooling occurs without the
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need for lead baths.
As a complement to the arguments set forth above, the information
about the physical phenomena on which the system for cooling the wire by
heat transfer from it by the combined processes of radiation-convection and
conduction can be extended.
To extract heat from the wire without contact with a cold solid surface,
from a practical industrial point of view only the processes of radiation and
convection heat transfer can be considered.
Given a wire surface at a high temperature, the intensity of the heat
emission by radiation depends on its temperature and on the temperature of
the receiving surface in relation to the wire, both to the fourth power, on
the
composite emissivity and on the view factor, besides the value of the Stefan-
Boltzmann constant. Accordingly, in the case at hand, the major variable is
the temperature of the receiving surface.
Assuming a heat capacity of the material of the solid surface, its
temperature will depend on the efficiency with which it is cooled. Said
cooling
can be achieved simply by heat conduction through the solid material
towards the surfaces in contact with the cooling fluid, by the combination of
said conduction with a process for the forced convection produced by
blowing said surface with a gas that is at a lower temperature.
It is obvious that the cooling capacity of the process for heat transfer
by the association of heat conduction and forced convection is considerably
greater than the cooling capacity by heat conduction alone.
The capacity of forced convection heat transfer is characterized by its
Nusselt number. Of all the techniques for applying forced convection for heat
transfer in industrial processes, the one that has been proven most efficient
is the use of fluid jets, whether the fluid is a gas, a liquid or a gas-liquid
mist,
having a high turbulence intensity, which is achieved by means of nozzles,
mainly those referred to as flat jet nozzles. The flat jet nozzle, the
longitudinal
groove of which coincides with the direction of the axis of the cylindrical
body
on which the gas jet is projected, is the optimal configuration for the
following
reasons:
1. The ratio of the distance of the nozzle discharge section to the surface
that receives the jet with respect to the width of its groove is constant
throughout the entire area of action.
2. The core of the flow, i.e. the width of the jet in which the speed of the
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ejected fluid is maximum, is constant throughout the entire area of
action.
3. The hydraulic diameter of the nozzle discharge section involved in the
definition of the Reynolds number is small compared to that which
corresponds to other geometric configurations with an identical area of
the outlet opening.
As a result of the previous characteristic, the regimen of the current in
the jet is two-dimensional, the turbulence intensity is very high and its
distribution is spatially uniform. This results in a high capacity for heat
and
momentum transfer in the surface which the jet strikes.
The two-dimensional characteristic of the outlet groove of the nozzle
and its longitudinal orientation facilitate the evacuation of the jet once it
strikes the surface of the solid with which it exchanges heat, directing it
towards the surfaces of the enveloping wall, cooling them.
High Nusselt number values are thereby achieved, these numbers
being given in the case of flat nozzles by
Pr P
Nu c. Re'.. r n .
Pro
wherein c is a numeric constant dependent on the geometric configuration of
the nozzle - contour surface, Re is the Reynolds number, Pr is the Prandtl
number and m, n and p are numeric coefficients which depend on the shape
and dimensions of the nozzle, as well as the orientation of the jet with
respect
to the normal to the surface which the fluid strikes, and very dependent on
the ratio between the distance from the nozzle discharge section to the
surface receiving the jet and the hydraulic diameter of the latter.
In the system for cooling wire object of the invention, it is precisely this
process of forced convection heat transfer by means of flat nozzles having a
highly turbulent flow that contributes to a large extent to the
intensification of
the heat transfer, because not only does it activate the direct cooling of the
wire but also of the entire surface receiving the radiant flux emitted by the
wire and reduces part of the heat conducted by the solid mass towards the
cooling fluid, whereby the length of conduit necessary for cooling the wire
and the required consumption of cooling fluid- water- is considerably
reduced. The claimed system for cooling has the novelty of using a highly
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efficient forced convection circuit which incorporates flat nozzles generating
very turbulent gas jets, the flow rate and temperature of which can be
regulated at will. These jets and the gas reflux resulting after striking the
surface of the wire assure that not only is a very high Nusselt number
obtained in the heat exchange with the wire, but also the control of the
temperature of the enveloping tube which in turn controls the radiant flux of
heat from the wire and, in summary, the reduction of the flow rate of the
cooling fluid necessary, as well as the reduction of the length of the
installation.
The device for heat treatment of the invention is a device for heat
transfer by the suitable combination of radiation, convection and conduction,
preferably being axially symmetrical, for example cylindrical. It is made up
of
a continuous channel or a channel formed by several consecutive sectors
having different dimensions aligned according to one and the same axis,
provided with several radially oriented flat nozzles through which a gas, or a
mixture of gases, a finely sprayed liquid or a mist is ejected in a highly
turbulent regimen at a temperature that can be externally regulated.
The device is formed by a block of material (Figure 1) the thermal
capacity of which is very high, in which there are several conduits 5 for
feeding fluid to the nozzles 1, whether for the subsequent extraction from the
chamber or for the circulation of cooling fluid for the purpose of controlling
the
temperature of the material of the block and, accordingly, for regulating the
radiation-convection heat transfer of the solid which is displaced at an speed
which can be regulated through the inside of the block through a channel 9
(Figure 2).
It furthermore has means for modulating the intensity of the heat
transfer from the solid in motion through the external control of the
temperature of the gas, of the cooling fluid, and of their respective flow
rates.
According to the elements detailed in Figure 1, the operation is as follows:
The flat nozzle 1 described in Figure 1 is used to eject a turbulent gas
jet towards the wire traversing the tube. Once the gas has struck the surface
of the wire, it is oriented towards a chamber 2, which is used to recirculate
said gas. In the system, the gas is introduced in the chamber by means of
the impulsion of a gas blower 3 that has variable-speed and is under
regulated pressure and flow rate. Said gas is introduced at a temperature
controlled by means of the system 4 for controlling the programmed
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temperature of the gas. The system is cooled by means of the cooling
conduits 5 (of the recirculated gas, of the tube for receiving the radiation
emitted by the wire and the solid structural parts of the system). The cooling
fluid impelled by a circulating pump 6 for circulating the fluid of the
cooling
circuit, with variable speed regulation for controlling the flow rate,
circulates
through said cooling conduits. Said system for cooling includes a
programmed regulation of the temperature 7 of the cooling fluid.
The modulation of the intensity of heat transfer, given a rate of
passage of the wire through the device for cooling, is achieved by regulating
the temperature of the gas ejected by the flat nozzles over the wire by means
of the system 4, regulating the mass flow of gas or varying the operating
speed of the gas compressor, or acting on both.
In addition to the previous basic modulation action, it is also possible
to vary the flow rate and temperature of the cooling liquid, the equipment for
controlling the temperature 7 of the cooling liquid, and the flow rate of the
liquid impelled by the pump 6.
The system is designed such that means such as mixing chambers,
mist chambers, sprayers, etc., can be incorporated so that the fluid of the
jets
projected through the flat nozzles is a mixture of gases, a mist, a sprayed
liquid or a chemical vapor serving either for the heat transfer effects or for
reactive-chemical effects on the surface of the solid in motion, for example:
descaling of metal surfaces by acid, Cr-Ni passivation of steel surfaces by
means of nitric acid mist, bonding reactions in the interface of composites,
etc.
The number of nozzles necessary is a function of the rate of cooling of
the wire assigned to the convection process. Once this rate has been fixed,
the Nusselt number value is determined and, from the latter, the Reynolds
number is calculated. The Reynolds number is expressed as Re = V dk/v
where dh is the hydraulic diameter of the nozzle discharge section, V is the
velocity of the fluid therein, and V is the kinematic viscosity of the fluid.
The Reynolds number is a dimensionless parameter of the relative
measurement of inertia forces with respect to the viscous forces in a fluid
current. The value of the Nusselt number, which in turn defines the heat
transfer coefficient, depends on the value of this parameter.
Once the Reynolds number is known, a fluid dynamic optimization
process is developed in which the nozzle length, the width of the nozzle
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discharge section and the separation between them are interactively
involved, the number thus being determined. The optimization process
includes comparing the analytical results obtained by applying the available
empirical correlations.
The orientation of the nozzles in the most important applications is
defined by the direction of the jet ejected, usually according to the line
that is
normal to the surface it strikes, in the case at hand, the surface of the
wire.
Nevertheless, other orientations can be applied in the search for a greater
surface of contact of the jet with the surface of the wire, there being a
compromise between said orientation and the uniformity of the field of
temperatures in the surface being struck.
Figure 4 shows how the non-uniformity of the flow over the object
translates into a non-uniform distribution of temperature and of heat transfer
over its surface.
The mass flow of gas and its temperature are externally regulated
according to the scheme of the system shown in Figure 1. The mass flow is
regulated by varying the speed of the drive motor of the blower according to
a routine which is determined by the characteristic curve of the blower
installed. The signal necessary for applying the regulation routine comes
from one or two pressure sensors installed in the gas circuit. The
temperature of the gas is regulated by means of an external heat exchange
the flow of cooling fluid of which is established by means of a routine the
signal of which is from the thermocouples installed in the gas circuit. The
regulation can be on-off, proportional or proportional-integral, according to
the desired precision for the value of the temperature of the gas at the
nozzle
discharge.
On-off control is understood as all-nothing, e.g., a reference
temperature is fixed in the N2 circuit, when the thermocouple for measuring
the temperature at the outlet of the blower detects a temperature difference
with respect to the reference temperature, a signal is produced whereby
acting on the external heat exchanger by completely closing or opening the
valve for the passage of water through the exchanger (a step regulation).
The differential regulation is implemented using the temperature
difference read in the N2 current, before the heat exchanger and after the
blower and, according to the proportional band of the regulator, the valve for
the passage of water through the exchanger is opened or closed
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proportionally.
In integral control, the measurement of the temperature difference and
that of the flow rate impelled by the blower are combined to integrate them by
means of a routine which determines either the regulation of the flow rate of
the blower, the temperature difference upon passing through the external
exchanger, or both in order to reach a maximum energy efficient operating
state.
