Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
W096l03533 219 ~~0~~5~~~~ ~ 1'CIYAU95I00458
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F-T .FCTRO-MAGNETIC PLUGGING MEANS FOR HOT DIP COATING POT
TECHNICAL FIELD
This invention relates to the pots used to hold baths of molten
:l
metal for use in the continuous hot dip coating of metal strip with liquid
metal coatings. It was developed for use in the continuous hot dip
galvanising of steel strip, wherein the coating metal is essentially zinc.
However, it will become apparent that it is applicable to any situation
wherein the substrate strip is metal and the coating is a liquid metal, for
example, hypereutectic aluminium-zinc alloys and other alloys.
More particularly the invention is directed to electro-magnetic
plugging means for preventing the leakage of bath liquid from the pot in
those instances in which the pot has an opening in it that is below the
surtace level of the liquid during normal operation..
BACKGROUND ART
In conventional continuous galvanising processes, steel strip, after
being cleaned and otherwise conditioned for the adherent acceptance of
the coating, is fed from above into a bath of molten zinc or zinc based
alloy. The strip passes around a so called "sink roll" submerged in the
- bath, then emerges from the bath, and passes between coating
thickness control devices, which return surplus liquid coating to the bath.
The coating is theri allowed or caused to solidify and the coated strip is
finally coiled for storage, further processing or sale.
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The sink roll, being submerged in the bath, operates in a hostile
environment and thus is a source~~:of trouble and unreliability unless
carefully maintained. Evela ;virhen adequately maintained, unavoidable
wear and tear requires its periodic replacement. Furthermore, dross is
sometimes dragged from the surface of the bath by the strip and may
become attached to the sink roll and rough alloy growths tend to form
on the roll's surface. That dross and those growths damage the strip
requiring frequent shut down of the line for removal and replacement of
the sink roll with a new or renovated roll. Thus it would be desirable to
eliminate the sink roll.
With that desirability in mind, it has been proposed to provide at
least one inlet opening in the pot, positioned below the normal operating
level of the bath liquid so that a strip to be coated may enter the pot,
either horizontally or from below, and depart, either through a similar exit
opening or through the mouth of the pot, without need for a change in
direction of the strip's pass line within the bath.
It is of course necessary to prevent the outflow of bath liquid
through the opening or openings and various electro-magnetic plugging
means have been proposed for that purpose.
For descriptive convenience the surface of the liquid metal that is
supported or otherwise restrained by forces generated by the electro-
magnetic plugging means rather than by a solid component of the pot is
referred to hereinafter as the "bare" surface of the liquid metal.
Prior proposed electro-magnetic plugging means have usually
fallen into either of two categories, namely those utilising either poly-
phase energising windings or multiple pole electro-magnets and
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W 0 96103533 21 g~~ p ~ ~ PCTlAU95I00458
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switching devices which provide moving magnetic fields passing through
the liquid or within the space into which the liquid might otherwise leak,
and those which are analogous to electric motors utilising either
permanent magnets or DC or single phase electro-magnets in
- 5 combination with a transverse electric current. All such electro-
magnetic plugging devices rely on the interaction of electric currents and
magnetic fields, either generated independently or induced one by the
other, and the currents are either DC .or power frequency and the fields
are likewise either steady or oscillating at power frequencies. In both
categories of plugging means, the magnetic field and/or the electric
current passes through the bath liquid adjacent to the opening to
generate restraining forces therein.
Prior proposed electro-magnetic plugging means of the kind
discussed above require relatively complex assemblies of components in
close association with the liquid metal, thus they all operate in a hot and
frequently crowded environment. This leads to design difficulties,
limitations on the size and shape of the openings that may be plugged
and low operating life expectancies.
Those prior proposed plugging means all suffer from operating
deficiencies as well. For example, those which use DC currents flowing
between electrodes in contact with the liquid metal, for example to
generate a lifting force in the liquid to prevent it falling through a strip
inlet opening in the floor of the pot, are inherently unstable. If an
adventitious localised downwards projection forms in the bare surface of
the liquid metal, then the current density in the projection becomes less
than the average current density in the bare surface skin as a whole.
Thus the upwards electro-magnetic force on the projection is reduced,
and hydrostatic pressure causes it to grow. The more it grows the lower
x
WO 96103533 ~ ~ ~ _ PCTIAU95100458
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the restoring force becomes, until eventually the projection breaks away
from the bare surface as a droplet; 'of bath liquid. After breaking away
there is no current through ~f~e droplet, the restoring force on it drops to
zero, and the droplet falls through the opening. Indeed, in such plugging
means any disturbance of the bare surface usually leads to a continuous
rain of droplets from it, in that the break away of each droplet may
cause sufficient disturbance to initiate the formation of another.
Also, all such prior known proposals that operate at power
frequencies, produce flows of liquid within the bath adjacent the opening.
Thus the bath is turbulent at the very position where it contacts the strip
to be coated, this turbulence seriously degrades the surface quality of
the coating on the finished product.
Finally, it should be mentioned that the devices under discussion
have or produce powerful magnetic poles adjacent the opening, these
interact with ferrous substrate strips tending to attract the strip towards
the poles. Any adventitious deviation from the central pass line towards
one pole and away from another causes the attraction of the strip
towards the one pole to increase and that towards the other to
decrease, so producing a deviating force which increases with
increasing deviation. Thus the situation is clearly inherently unstable. In
the event, successful operation with steel strip requires expensive guide
rollers to be positioned closely adjacent to the opening, and an
objectionably high tension to be maintained in the strip to prevent
deviation, a tension which is not readily attained in practice.
In view of the foregoing and other deficiencies of what may be
termed zero or power frequency plugging devices, such devices have
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not found widespread acceptance or use within the metal coating
industry.
One other prior proposal has been suggested, namely the use of
a high frequency, oscillating, but spatially stationary electro-magnetic
5 field, positioned so as to exclude the bath liquid from the pot opening.
This proposal relies on the fact that such a field generates high
frequency eddy currents within the bath. Due to their high frequency the
eddy currents flow only in a thin surface layer of the liquid (the so called
and well known "skin effect"). The reaction between the surface
currents and the field is one of mutual repulsion, and at sufficiently high
frequencies the field is effectively excluded from penetrating the liquid.
In those circumstances the field behaves as a resilient cushion that. may
be distorted or compressed by the bare liquid surface but resists
penetration of the liquid into the space occupied by the field. The
resisting force is perpendicular to the direction of the flux lines of the
field and the bare surface of the liquid, and is proportional to the degree
of distortion or compression of the field. Thus, unlike lower frequency
interactions, the situation is inherently stable, in that an adventitious
projection of the liquid surface into the field space produces a localised
distortion of the field and an accompanying increased localised
resistance to further intrusion.
Furthermore, in the absence of electric currents or electric-fields
within the body of the bath liquid there is no turbulence induced in the
liquid by the operation of the plugging means.
Thus, high frequency plugging means overcome the major
deficiencies of zero or low frequency plugging means, but they are
subject to their own inherent limitations.
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In particular a high density magnetic field is required if sufficient
force is to be generated normal to the bare liquid surface to resist the
hydrostatic pressure at the bottom :of a liquid metal bath of a depth
sufficient to enable a reliable~:~continuous strip coating operation to
proceed. This in turn requires high energy generating coils and places a
premium on the use of pot and pot opening shapes and dimensions that
minimise the extent of the bare liquid area to be supported or otherwise
restrained. This at least requires the strip inlet opening to provide only
small clearances for the strip passing through it. This, in turn, requires
precautions to ensure that the strip does not deviate from the intended
pass line to any great extent.
A coating pot provided with a conceptually simple form of high
frequency electro-magnetic plugging means is disclosed in Japanese
patent No.04-099160 (Nippon Steel). In this instance the "pot" is a
hollow, rectangular prismatic cell of silicon carbide some 100 mm wide
with a slot some 20 mm wide in its floor. A steel sheet moves upwardly
through the slot and through a galvanising bath contained in the cell.
The lower part of the cell is surrounded by a solenoid coil that is
energised at 20 kHz and has a vertical centre plane coinciding with that
of the cell. The slot is set to one side of that centre plane. This is
described as causing the lower part of the bath liquid to be pushed
towards the centre plane of the cell clear of the slot but leaving the
upper part of the bath unaffected. A steel strip to be coated is shown
travelling upwardly through the slot and through the upper part of the
bath.
Bearing in mind that the 20 kHz field would be effectively
excluded from the bath and the strip, it is clear that the field of this
device would be asymmetric with regard to the strip in a critical zone
CA 02196056 2003-O1-23
7
immediately above the slot. Indeed because of the shielding effect of
the strip there would be very little if any field on the inside of the strip
to
push the liquid back from the inside of the strip immediately above the
slot. This prior proposal would have three major deficiencies, (i) the
above mentioned asymmetry would create substantial out of balance
lateral forces on the strip requiring special arrangements and
objectionably high tension in the strip to reliably maintain the strip out of
contact with the walls of the narrow slot, (ii) the deficient field adjacent
the inside of the strip would allow liquid to fall through the slot on the
inside of the strip, and (iii) the large, bare surtace area of liquid would
require a large volume, high density field with consequent high power
requirements for the generating coil. It might be thought that (i) and (ii)
would be overcome by centralising the position of the slot. Indeed that
may remove the out of balance force on the strip produced by the
plugging field and may prevent leakage when a strip is present in the
slot, but would stilt leave the strip free to move laterally from the pass
line. More importantly the naturally densest part of the field within the
bore of the coil is substantially vertical and so is not welt oriented to
provide a vertical restraining force. Thus, in the absence of a strip,
restraint would depend on the relatively lower density diverging field at
the top end of the solenoid. This would necessitate a very high power
coil producing an unnecessarily dense field overall if leakage is to be
efficiently prevented.
DISCLOSURE OF THE INVENTION
An object of an aspect of the present invention is to provide a hot
dip coating pot with high frequency electro-magnetic plugging means
that overcomes at least the above mentioned deficiency (I) of prior
proposed
2196056
W096103533 , PCTIAU95I00458
high frequency electro-magnetically plugged pots, and in preferred
embodiments alleviates deficiency (ii) thereof.
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The invention achieves th~tvbbject by providing plugging means of
the high frequency type which provide stabilising forces on the strip
tending to prevent it from deviating from its intended pass line through
the plugged opening, and which are more effective to prevent leakage of
the liquid metal in the absence of a strip than prior known plugging
means of that type.
The invention consists in a hot dip coating pot having a strip inlet
passage and electro-magnetic plugging means to prevent leakage of
bath liquid from the pot through that passage, wherein:
the plugging means comprise two magnetic field generators
disposed one on each side of the passage;
each generator projects an oscillating magnetic field into the
passage from at least two poles of opposite polarity that are adjacent
the passage and spaced apart in the through direction of the passage;
the said at least two poles of each generator are respectively in
substantial alignment with the corresponding poles of the other' in the
transverse direction of the passage;
the magnetic fields projected by the generators have flux patterns
which are substantially mirror images with reference to a plane of
reflection coinciding with a centre plane of the passage; and
both generators operate at a frequency of more than three
kiloHertz.
As a result of their mirror image flux patterns and mutual
registration, the projected fields provide identical repulsive forces on
opposite sides of a centrally positioned strip, if one be present in the
W O 96103533 219 6 Q ~ ~ s . ~, _ p~~A U95/OD458
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passage. If the strip deviates from the centre of the passage the
additional compression of the field on one side and the expansion of the
field on the other increases and decreases the repulsive forces
respectively to produce a restoring force tending to return the strip to the
centre position.
To project a magnetic field, any magnetic field generator
necessarily has at least two spaced apart magnetic poles. The poles
are necessarily of opposite polarity at any instant and the projected field
extends from one to the other along part of an endless flux path. Those
poles may be real (solid bodies from which the field emanates) or virtual
(a spatial location from which the field emanates). Therefore, to project
mirror image fields into the passage in accordance with the invention
each generator must have at least two poles of opposite polarity closely
adjacent the passage, spaced apart in the through direction of the
passage, and in alignment with the corresponding poles of the other
generator, as aforesaid.
However in preferPed embodiments of the invention there is a
further limitation, namely that the polarities of the respectively aligned
poles be such as to ensure that when no strip is present in the passage,
the projected fields combine, and the combined field extends
transversely of the passage from each of the poles of one generator to
the corresponding pole of the other generator.
By using two high frequency generators as aforesaid such a
change in field pattern is made possible. This change provides the
major benefits of the invention. In one pattern the combined field
extends transversely of the passage and is ideally positioned to plug the
passage when it is open for its full width due to the absence of the strip.
CA 02196056 2003-02-26
in the other pattern the fields extending along the passage on each side of
the
strip not only respectively plug the narrow passage spaces an each side of
the strip but also react with the strip to maintain ~t central of the inlet
passage
as a whole, thus enabling a narrow inlet passage to be used along with low
5 tension in the strip.
The invention also extends to a continuous, hot dip, strip galvanising
line or like metal coating apparatus, wherein the coating pot ~s a pot
according
to the invention.
In conventional coating Imes the pot is necessarily large enough to
house the sink roll and allow it to be partly or fully submerged in the liquid
coating metal. An advantage of the present invention is that the pot may be
made very much Smaller than has been possible hitherto. Thus the term "pot"
as used hereinafter includes small but elongated trough-like containers
somewhat different in shape and size from the normal concept of a
conventiional prior art pot, although fulfilling the same function as before.
Therefore, in accordance with the present invention there is provided:
'?o A hot dip coating apparatus compnsmg_
a pat for cantaming a bath of molten metal,
duct means defining an elongate strip wlet passage having a
longitudinal direction extending downwardly from an opening in a floor surface
of the pot, the passage and the opening being dimensioned so that a metal
z5 strip to be coated moves upwardly through the passage and the opening into
the bath without contacting the apparatus or the duct means, and
electro-magnetic plugging means to prevent leakage of batri fluid from
the pot through the passage, wherein:
the plugging means comprise two magnetic field generators disposed
30 ofle on each side of the duct means;
each generator projects an oscillating magnetic field into the passage
from at least two poles of opposite polarity that are adjacent the passage and
spaced apart in said tongitudW al directson;
CA 02196056 2003-O1-23
10a
said at least two poles of each generator are respectively directed
towards and in substantial alignment with the corresponding poles of the other
generator in transverse directions substantially perpendicular to said
longitudinal direction to form two pairs of mutually aligned poles;
the magnetic fields projected by the generators extend along flux paths
which are substantially mirror images about a longitudinal center plane of the
passage;
both generators operate at a frequency of more than three kiloHertz;
1o and
the poles of each mutually aligned pair of poles are of opposite
instantaneous polarity, whereby;
in use in the absence of an intervening strip to be coated, the projected
magnetic fields extend across the passage between the poles of each said
mutually aligned pair of poles and,
in use in the presence of an intervening strip to be coated, the
projected magnetic fields extend longitudinally of the passage between the at
least two poles of each generator without crossing said intervening strip.
BRIEF DESCRIPTION OF THE DRAWINGS.
By way of example, several embodiments of the above described
invention are described in more detail hereinafter with reference to the
accompanying drawings.
Figure 1 is a diagrammatic sectional view of a bottom portion of a
coating pot according to the invention showing a magnetic field pattern
established in the absence of a strip to be coated.
WO 96/03533 219 f 0 5 6 PCTIAD95J00458
ii
Figure 2 is a view similar to figure 1 showing the subject matter of
that figure and the magnetic field pattern established when a strip is
present.
Figure 3 is a perspective view of a pair of electromagnetic field
generating coils, being components of the pot of figure i .
Figure 4 is a view similar to figure 3 of an alternative pair of coils.
Figure 5 is a diagrammatic sectional view of a bottom portion of a
coating pot according to another embodiment of the invention showing a
magnetic field pattern established in the absence of a strip to be coated.
i 0 Figure 6 is a view similar to figure 5 showing the subject matter of
that figure and the magnetic field pattern established when a strip is
present.
Figure 7 is a diagrammatic sectional view of a yoke and strip,
being components appearing in figures 5 and 6 drawn to a larger scale,
showing dimension indicia as referred to elsewhere in the description.
Figure 8 is a view similar to figure 6 oif another embodiment of
the invention.
BEST MODE OF CARRYING OUT THE INVENTION
As shown in figures 1 and 2, a hot dip coating pot in a continuous
strip coating line contains a bath 1 of a molten metallic coating material,
for example zinc or an aluminium-zinc alloy. The pot has a floor 2 with
a downwardly directed duct 3 of generally rectangular cross-section
VI'O 96103533 2 ~ 9 ~ Q 5 6 PCTIAU95l00458
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defining a strip inlet passage 4 providing clearance for the entry into the
pot of a metal strip 5 that is to be coated.
The strip 5 is guided by rolls (not shown) to enter the pot from
below and travel upwardly through the bath 1. Prior to reaching the
passage 4 the strip 5 may be cleaned and otherwise conditioned in
conventional manner to receive the coating. Thus a steel strip, for
example, would normally be pre-conditioned and fed in a conventional
manner from a heating furnace having a controlled reducing
atmosphere, through a hood (not shown) likewise containing a reducing
or at least inert atmosphere 6, into the passage 4. Having emerged
from the bath 1, the strip 5 would be treated, also in a completely
conventional manner, to become finished product. Therefore, apart from
the provision of rolls to bring the strip to the pot from below and the
shape of the mentioned hood, the line equipment downstream and
upstream of the coating pot may be conventional in all respects.
The pot may be made from a ceramic or other refractory material,
for example a titanium stabilised alumina, silicon carbide or boron
nitride.
Two high frequency magnetic field generators comprising coils 7
are respectively disposed on opposite sides of the duct 3. Those coils
may, for example, be optionally one or other of the coils illustrated in
figures 3 and 4. They are shown in section in figures 1 and 2, the
section being taken on line X-X appearing in figures 3 and 4.
In each instance, each coil 7 is a single turn comprising an upper
coil side conductor 8, a lower coil side conductor 9 and a coil end
conductor 10. The coils are connected together by interconnecting
a. if F 1.
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WO 96f03533 ~ ~~ 9 6 0 ~ 6 PCTIAU95/00458
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a
conductors 11 and are fed by supply conductors 12 extending to
terminals 13. In the figure 3 arrangement the interconnecting
conductors 11 are such that the two coils are in series, whereas in the
figure 4 arrangement the coils are in parallel.
In both instances the coils preferably extend rigidly as self
supporting cantilevers from the terminals 13. To that end the coils may
be fabricated from copper tube, preferably 'tube of non-circular cross
section such that each of the coil side conductors 8 and 9 presents a
broad flat face towards the adjacent surfaces of the pot and duct. For
example, the coils may be fabricated from hollow rectangular section
(HRS). The joints between conductors in the coils are preferably brazed
with bronze alloy.
The terminals 13 may be lengths of copper tube adapted to be
clamped by pipe clamp formations in or on rigid supply bus-bars
extending to a high frequency power supply transformer. The terminals
may be internally threaded at their lower ends, as indicated at 14 in
figure 3 where a part of the terminal has been cut away, to receive
coolant supply hoses (not shown) whereby coolant may be circulated
through the coils 7 while they are in operation.
From the foregoing it will be clear that the instantaneous current
direction in the two upper coil side conductors 8 is always the same.
Likewise the instantaneous direction in the lower coil side conductors 9
is always the same and always in the opposite direction to that in the
upper conductors.
Thus the coils are of the same polarity, and each coil 7 may be
regarded as having three virtual poles that are spaced apart in the
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VVO 96103533 PCTIAU95100458
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longitudinal direction of the passage 4, namely the region 15
immediately above the upper coil side;8, the region 16 immediately
below the lower coil side 9, and 3h~e"region 17 at the centre of the coil. ,
The fields from virtual poles 15 and 16 will always be in a common
direction (say, at a specific instant, towards the passage 4) and field
from virtual pole 17 wili be in the opposite direction (at that instant, away
from the passage 4), as indicated by arrows on the flux lines shown in
figures 1 and 2. Virtual pole 17 may be regarded either as a single pole
having twice the strength of each of virtual poles 15 and 16 or as two
closely adjacent poles which are the respective counterparts of poles 15
and 16.
In the absence of a strip 5,. the close proximity and mutual
registration of the corresponding poles of the two generators, which with
the simple air cored identical coils of the present embodiment equates
with the axial alignment of the coils, ensures that the electromagnetic
fields generated by the coils links both coils. Also, for the same coil
current, the transverse component of the field would have a value equal
to twice that produced by either coil acting alone.
More importantly, in the absence of a strip, the registration of
poles 15 ensures that a portion of the field extends from one to the
other transversely of the passage to provide a barrier to the descent of
liquid from the bath through the passage. This may be seen in figure 1,
wherein, although the relevant flux lines are seen to be bowed
downwardly where they cross the passage in response to the liquid
pressure, the repulsive force on the bare surface of the liquid is still
primarily upwards.
WO 96103533 PCTlAU95/00458
;: ..
In general terms, since the repulsive forces between a high
frequency field and the liquid are effective at the interface of the field
and the liquid, it is only the field of the poles 15 that is directly
effective
as a plug, and thus it is only those poles and the pole 17 counterpart of
5 each of them that are necessarily in register in the longitudinal direction
of the passage. Thus, in this and other embodiments having simple
coils as the field generators, the coils are disposed so that at least their
upper coil side conductors are in register with each other in the direction
of strip travel, that is to say, assuming the strip travels vertically through
10 the duct the upper coil sides lie in the same horizontal plane.
Specifically, the lower coil sides could be more remote from the
passage, for example they might lie in the same horizontal plane as the
upper sides if desired. In other embodiments the arrangement may be
further varied to maximise the field at the lower position of poles 17 and
15 to use the field from these poles for levitation of the liquid.
This complementary polarity and the mutual registration of at least
the upper coil sides are significant features of the present embodiment
of the invention, in that, in the absence of a strip, it maximises the
horizontal component of the generated flux intersecting the duct
immediately below the floor 2 of the pot. This in turn maximises the
levitating force acting on the bare surface of the liquid metal 1 at the
mouth of the duct.
The coils 7 are energised from a preferably constant magnitude,
alternating voltage source at a frequency of at least 3 kHz and
preferably in excess of 7 kHz.
If the strip 5 is present the field from each generator coil is
substantially restricted to that part of the passage and strip on its side of
R'O 96103533 ~ PCTIAU95100458
16
the centre plane of the strip. The generated fields then adopt the mirror
image patterns shown in figure 2, wherein the field from each pole 15
enters the passage more or less transversely and then turns to extend
f. ~'~ ".4
longitudinally of the passage and:,'stnp for a distance before turning
again to depart more or less transversely from the passage to pole 17.
The fields projected by poles 15 and 17 then serve to plug the
passages on each side of the strip. In this regard it should be noted
that the field penetrates the strip, albeit to a very small depth, so that
the strip and field together provide for the complete containment for the
liquid.
The magnitude of the energising voltage needed to plug the
passage in any instance depends on the physical parameters of the
installation, (for example, passage width, strip width, number of turns in
energising coil etc.) and on the liquid pressure. The last mentioned
depends on the density of the coating material and the depth of the
bath.
The frequency of the power supply is chosen to produce an
optimum balance between conflicting effects. At higher frequencies the
so called skin depth of the coil conductors, that is to say the thickness of
the surface layer to which the current is largely confined, is reduced and
the coil resistance becomes higher. This leads to higher resistive
losses. On the other hand, the repulsive force on a conducting body, in
this instance the coating liquid, rises as the eddy currents in it become
more nearly confined to its surface, that is as the frequency rises. The
cross over point between attraction and repulsion of the steel strip
occurs at a frequency within the range of from 3 to 7 kHz. Thus
frequencies in excess of that up to about 100 kHz are preferable.
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VS'O 96/03533 ~j 'i"'# ' ' ' '
PCTIAU95I00458
i7
In the present embodiment~mall of the fields from the respective
poles, not only that of poles 15 and 17 are projected into the passage
and are mirror images of each other, thus the fields projected into the
passage by poles 17 and 16 also contribute to the restoring forces
- 5 preventing the strip pass line from deviating from the desired centre
position.
The coils 7 may be effectively in air or other non-magnetisable
medium as illustrated. Alternatively the generator coils may be partly
externally enclosed in C-sectioned magnetisable shells. Such shells
increase the magnetic flux for a given energising current which is
advantageous, but also increase the inductance of the coil. which
requires a higher energising voltage and is disadvantageous. Thus, if
shells are present, a design balance has to be struck to optimise the
efficiency of the plugging means.
Figures 5 to 7 illustrate another preferred embodiment of the
invention, wherein each of the field generators is in the nature of an
electromagnet comprising an energising coil wound upon a ferro-
magnetic, preferably G shaped, core.
In this instance, the pot may have a floor 20 that is thick enough
to enable two, high frequency magnetic field generators 21 to be housed
within elongated recesses formed in the confronting faces of the floor 20
that define the pot's strip inlet passage 22.
Each of the generators 21 comprises an energising coil 23
encircling the web of a C- or G-sectioned ferro- or ferri-magnetic
core 24 and, preferably, copper or other non-ferrous electrically
conductive shields 25 and 26. Those shields constrain the high
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18
frequency magnetic field;;so~ that virtually all of the fields generated by
the coils 23 emerge from the elongate end faces 27 and 28 of the
cores 24. Those faces 27 and 28 are therefore the poles of the field
generators 21.
Each of the coils 7 is energised from a preferably common,
preferably constant magnitude, alternating voltage source at a frequency
in excess of 7 kHz. They are connected to the source so as to produce
the preferred polarity, such that when one pole 27 is a north pole the
other is a south pole and vice versa.
Under those circumstances a flux pattern as shown in figure 5 is
produced if and when no strip is present in the passage, whereby the
field extending between poles 27 plugs the passage 22, and the field
extending between poles 28 plays no direct part in the operation of the
pot, other than to complete the flux path.
When a strip 29 is present the flux pattern assumes that shown in
figure 6, and it will now be clear from the description of the figure 1
embodiment that this not only plugs the passage but also centralises the
strip in the passage 22.
Each side of the passage 22 is lined with a non-metallic refractory
or other heat resistant, insulatory face plate 30, that provides a barrier
between the molten metal of the bath 31 and the upper parts of the
shields 25 and cores 24.
Various aspects of the design of the components of this
embodiment are discussed below.
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WO 96103533 219 6 Q 5 6 pCT~AU95100458
19
Each core 24 is made of a low loss material having a high
permeability and high saturation magnetisation. For example, high
density ferrites, magnetic metallic glass or- iron powder may be used.
Being of high permeability the core 24 concentrates the magnetic field in
the air gap G adjacent the upper pole face 27, preventing excessive
"wastage" of the field outside the gap. The yoke preferably has a G
shape so that the window occupied by the coil23 may be made
arbitrarily large, independent of the pole separation S, whilst maintaining
a reasonably compact cross-sectional shape for the coil.
A large window for the coil allows larger conductors to be used for
a given number of turns, this permits lower current densities which, in
turn, gives lower resistive power loss in the coil 23. On the other hand
increasing the coil size by increasing its conductor size increases the
leakage field inside the coil, so that a higher proportion of the field
generated does not pass through the upper pole face 27. A balance
between these two competing effects has to be reached when
determining the size and shape of the core 24.
If the pole separation S is reduced the reluctance of the magnetic
path is reduced, so that, for a given number of ampere turns in the
coil23, the total field passed is increased. However, if the pole
separation S becomes too small the field may become concentrated in a
small region near the poles, and may not penetrate at full strength
across the air gap G to the strip 29. In this instance the region of
weaker field near the strip may not form an effective plug and so may
allow liquid coating metal to escape. As a general rule, the pole
separation S should be about, and preferably not less than, three times
the air gap G. Thus the spacing between the poles in the through
direction of the passage should be within the range of from two to ten
W O 96103533 219 6 ~l 5 6, PCTIAU95100458
times the width of the air gap between the strip being coated and the
side of the passage. , 4
c
The shields 25 and 26 are made from high conductivity material,
for example, copper, aluminium or silver. If the energising coil is very
5 large, additional shielding may be placed between the conductors of the
coil to reduce internal flux leakage. Such an embodiment is illustrated
by figure 8 wherein such additional shields are shown at 32 and 33.
Eddy currents will be induced in the shields. This will cause heating in
the shields and of course they are in a hot environment. Thus forced
10 cooling of the shields may be required, for example by passing a cooling
liquid through tubes brazed or other wise joined to the shields in a
thermally conductive manner. The figure 8 embodiment includes such
tubes referenced 34 in that figure.
In preferred embodiments the number of conductor turns in each
15 coil 23 is small, for example no more than ten, preferably from one to
four, depending on the frequency of operation, the geometry of the
components and the characteristics of the power supply. As is well
known, a multi-filament conductor of the same cross-sectional area as
a single filament conductor is more efficient than the single filament
20 conductor at high frequencies. That is to say, other things being equal a
multi-filament conductor has a lower power loss than a single filament
conductor. This is because high frequency current is largely restricted to
the surface skin of the conductor and the multi-filament conductor has a
greater surface to cross-sectional area ratio than the single filament
conductor. Thus in preferred embodiments each turn of the coil 23
comprises a plurality of tubular conductors in parallel. Those conductors
are preferably cooled by means of cooling fluid pumped through their
bores.
WO 96/03533 ~' ~ PGTIAU95100458
21
For the reasons elaborated in relation to the first described
embodiment the field frequency is preferably within the range of from 7
kHz to 100 kHz although even higher frequencies are quite feasible.
- By way of example, the details of an appropriate design for a
galvanising pot furnished with plugging means according to the last
described embodiment of the invention and holding a zinc or aluminium-
zinc bath with a nominal depth of 0.5 metres are indicated below:
PARAMETER VALUE
Air gap (G) l0mm
Number of turns in coil 1
Number of tubular conductors in 126
parallel in each turn
Outside diameter of each tubular 12.5 mm
conductor
Frequency of excitation 20 kHz
Length of upper pole face (P) 30 mm
Pole separation (S) 80 mm
Window height (H) 150 mm
Yoke depth (D) 150 mm
Flux density in air gap* 0.414 T (peak)
Energising current* 33,000 A (rms)
Applied voltage* 2161 V (rms)
Power loss in coil* 20 kW
Plugging pressure (ideal)* 33.1 kPa
WO 96/03533 2 ~ g 6 p S 6 PCTlAU95100458
22
Head of liquid zinc (ideal)* 488 mm
*Calculated values.
