CA1192391A - Coating thickness control nozzle - Google Patents

Abstract

COATING THICKNESS CONTROL NOZZLE

Abstract of the Disclosure An improved nozzle structure for directing a wide, thin jet stream of a gaseous fluid onto the surface of a running length of c flexible substrate emerging from a liquid coating bath to control the thickness of the coating liquid on the substrate employs a nozzle outlet construction producing laminar flow of the jet stream emerging from the nozzle structure by providing a relatively long, thin conduit extending between an internal plenum chamber in the nozzle structure and the outlet orifice which is defined by smooth, planar, substantially parallel opposed surfaces on the nozzle structure.
The internal configuration of the plenum chamber and the air inlet to the nozzle structure provide substantially uniform pressure throughout the length of the plenum chamber to thereby produce uniform flow from the nozzle across its full width.

Description

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BACKGROUND OF THE INVENTION
Field of the Inventlon This invention relates to hot dip coating apparatus and more particularly to an improved fluid nozzle assembly for directing a controlled, thin flat stream of gaseous fluid onto the surface of a running length of a substrate emerging from a liquid coating bath to wipe excess coating liquid from the surface and produce a controlled, uniform coating.
The nozzle assembly is particularly well adapted for hot dip metal coating of a running length of metal substrate wherein the nozzle assembly is located in a high-temperature environment.
Description of the Prior Art The use of a thin, wide jet or stream of fluid to wipe excess coating liquid from a moving substrate has long been known in the paper coating art and has more recently become widely used in the hot dip coating of metal substrates.
Since the nozzle structure of the present invention i.s par-ticularly well adapted for use in the hot dip coating of steel strip with zinc, ~in, aluminum, or an aluminum-zinc alloy, the invention will be described herein with reference to a hot dip galvanizing process, it being understood that the invention may be eq~ually useful in other coating opera-tions.
In the hot dip coating of steel strip with zinc in a continuous galvanizing operation, an excess of coat.ing metal _~_ D~

adheres to -the surface of the moving strip as it emerges from the bath. To remove the excess coating materi~l, a fluid such as steam, air, or a mixture of steam and air, is directed against the strip surface while the spelter is still in the liquid state. Normally two nozzles are employed, one on each side of the substrate, with the nozzles being adapted to direct the fluid in jet streams in substantially opposed directions across the full width of the strip so that forces applied to the strip by the two fluid jets tend to equalize one another. Such nozzles are generally referred to in the art as air knives, whether air or steam is employed as the control fluid. Also, the fluid jets of the prior art de-vices appear to have acted more in the nature of a pressure dam preventing excess coating material from being carried through than as a knife or blade which strips the liquid from the substrate.
While the use of air knives has largely replaced the older coating roll technique for controlling the coating weight in ho-t dip metal coating operations, the known air knives have not proven entirely satisfactory for various reasons. Difficulties ~ncountered include the inability to accurately control the coating thickness or to control the distribution of the coating material on the substrate surface, and an edge effect which can result both in an increased thickness of coating material adjacent the edges of the strip being coated and a tendency ~or the coating metal to adhere ~23~

to the side edge of the substrate to produce an uneven or rough edge on the final product. Edge effect, or edge build-up, has been a particularly difficult problem re-sulting not only in a waste of coating material but also in a defective or inferior product.
It is conventional practice for purchasers of coated metal such as hot dip galvanized steel strip to specify a minimum coating thickness, recognizing that the coating will not be precisely uniform. To meet the specifications, manufacturers of such product adjust the air knife position and air pressure to apply an excess of coating material sufficient to assure that even the most thinly coated por-tions will have the minimum coating thickness. This neces-sarily results in a substantial waste of expensive coating material.
In a hot dip galvanizing operation, the coated strip is conventionally led from the spelter bath in a generally vertical direction beyond the air knives for a distance sufficient to permit th coating metal to completely solidify.
Thereafter, the coated ~etal is wound into coils for handling and shipping. Excessive edye build-up presen~s problems in the coiling operation hy producing a condition known as spooliny. As the strip is wound into a coil, the thicker edge portions result in the ends of the coil haviny a diameter which gets progressively larger than the center. Conse-~239~

quently, there is a tendency for the strip to wind unevenlyon the reel. In extreme cases, stresses can elongate the edges, producing a wavy surface resulting in at least the edge portion being of inferior ~ .

Various prior art devices have been proposed for eli-minating or reducing edge build-up in a hot dip metal strip coating operation. These attempts have included the so-called bow-tie nozzle opening wherein the coating thickness control fluid is directed onto the surface of the strip through nozzle openings which are wider at their end near the strip edges than at the center. This arrangement is intended to deliberately produce a thicker coating in the central portion of the strip, or a thinner coating toward the edges. Even if edge build-up is not eliminated by this process, at least some of the adverse effects of spooling are avoided as a result of the thicker coating in the central portion of the strip. While spooling may at least be reduced by this expedient, it necessarily involves use of a substan-tial excess of coating metal.
Another known device for reducing the adverse effects of edge build-up is disclosed in U.SO Patent No. 4,128,668 which discloses a coating die having baffles which impart to air discharged through the nozzle a component of motion in a direction ~oward the edges of the strip being coated.

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U.S. Patent No. 3,~06,656 discloses a prior art air knife designed to eliminate edge effect in a hot dip yalva-nizing operation. In this device, the nozzle outlet is generally arcuate whereby air striking the central portion of the web will travel a greater distance in the atmosphere than air striking the edge portions.
The adverse effect of turbulence in the air stream as it leaves the nozzle has also been recognized and numerous patents have issued on arrangements intended to reduce this turbulence, one example being found in U.S. Patent No.
3,272,176. U.S. Patent Nos. 4,153,006 and 4l041,895 are examples of prior art coating nozzles incorporating means for equalizing the fluid pressure within the nozzle plenum chamber to produce a more uniform gas flow from the nozzle along its full width, while U.S. Patent No. 3,753,418 pro-poses the use of large headers in association with each nozzle to xeduce the rate of flow within the headers to a substantially quiet condition and ~hereby equalize pressure along the length of the-nozzle outlet.
U.S. Patent No. 3,459,537 utilizes a screen within the nozzle plenum chamber to equaliæe distribution of air within the nozzle structure in~ combination with off-setting the two nozzles along the lenyth of the strip by a predetermined amount~ to control e~ge effect.
From the above, it is apparent that, while the air 23~3~

knives or nozzle s-tructures of the foregoing and other prior art patents represent a substantial improvement over the previously employed coating roll apparatus in hot dip metal coating operations, they have not been entirely satisfactory either in controlling the thickness and distribution of the coating material or in avoiding edge build-up. It is, there-fore, the primary object of the present invention to pro-vide an improved nozzle structure which is capable of more accuratel~ and reliably controlling both coating thickness and coating distribution, and which substantially eliminates edge build~up.
Another ohject of the invention is to provide such an improved noz~le structure which is very stron6and dimensionally stable and which generally produces less noise and requires less air than known air knives.
In the preferred embodiment of the present invention, the nozzle structure includes a pair of elongated dies, or nozzle half-sections, of unitary construction. The two dies have interior die cavities which are substantially mirror imayes of one another, with a single air inlet in one of the dies communicating with the central portion of the die cavity. The dies cooperate, when assembled, to form a s-,nyle plenum chamber having a narrow outlet nozzle extending along its full lenyth. The improved nozzle design results in a laminar flow condition which produces the desired wiping action with a substantial reduction in total air flow and a 39~

consequent saving in energy. The reduced air flow also has the benefit of enabling a substantial reduction in volume, or cross-sectio~al area, of the nozzle plenum chamber without ~roducing objectionable pressure differentials in the chamber.
The improved nozzle construction has been made possible by the discovery that ~he outlet nozzle may be designed using standard pipe formula for calculating flow distances required for laminar flow, provided the outlet is considered as a plurality of separate channels or tubes aligned in side-by-side relation, but substituting in such standard formula the cross-sectional area of the entire nozzle outlet ~or the area of the individual tube section. While the reason for this is not known, it is believed that the thin outlet, which may be approximately 0~20 inches, and the use of contoured surfaces and smooth surface finishas in the plenum chamber and outlet portion of the nozzle may have made this modification of the formula possible. The very thin, rela-tively non-diverging ai.r stream resulting from the laminar flow condition at the nozzle outlet enables a reduction of flow to the extent tha~ the ratio of the cross sectional area of the plenum cha~ber to the total area of the outlet opening may be less th~n 4:1. In such a nozzle assembly used for wiping excess coating metal fronl steel strip in a hot dip galvanizing process, it has been found that the length, measured in inches, of the outlet nozzle portion from the plenum chamber to the outlet, may be within the range .3~ t of ~ to ~k~ times the cross-sectional area of the plenum chamber measured in square inches. This ratio is much less than expected and enables design of the very compact and highly efficient air knife of the present invention.
srief Description of the Drawi_ The foregoing and other features and advantages of the invention will become apparent from -the detailed des-cription contained hereinbelow, taken in conjunction with the drawings, in which:
lOFigure 1 is a fragmentary, schematic view, in eleva-tion, of a hot dip coating system embodying the improved coating nozzle of the present invention;
Figure 2 is a fragmentary side elevation view of the structure shown in Figure l, with certain parts broken away to better illustrate the invention;
Figure 3 is an exploded view~ in perspective, of a nozzle assembly embodying the present invention;
Figure 4 is a bottom-plan view of the upper half sec-tion of the nozzle struçture shown in Figure 3;
20Figure 5 is a top-plan view of the bottom half section of the structure shown -in Figure 3;
Figure 6 is an enlarged sectional view taken on line 6-6 of Figure 2;
Figure 7 is an enlarged fragmentary sectional view taken on line 7-7 of Figure 4;

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Figure 8 is an enlarged fragmentary sectional view taken on line 8-8 of Figure 4;
Figure 9 is a graphical representa-tion of the zinc coating thickness, measured at ~-inch intervals across the width oE a random sample of a steel strip haviny unequal coating weights applied. to its opposed sides by a hot dip galvanizing operation using the nozzle construction o~ the present invention to control coating weight; and Figures 10, 11 and 12 are graphic representations similar to Figure 9 and illustrating zinc coating thickness on random samples of commercial hot dip galvanized steel strip produced by three different manufacturers.
Description of the Preferxed Embodiment ~eferring initially to Figures 1 and 2 of the drawings, a running length of steel strip 10 is shown passing through a galvanizing pot 12 containing a molten zinc, or spelter bath 14, in a hot dip galvanizing operation. The path of travel of the strip 10 is established by a sequence of guide rolls around which the strip is led. The guide rolls in-clude a sin~ roll 16 wi~hin the spelter bath and an idlerroll 18 positioned far enough above the bath so that the molten spelter has solidified by the time the strip 10 reaches this elevation. A motor-driven coiler 20 draws the strip throuyh the apparatus, and a stabilizing roll 22 within the spelter bath near i~s surface presents the strip in a 3~L

planar, stable form to a coating thickness control apparatus indicated generally by the reference numeral 24. Strip 10 is thus guided through ~he coating thickness control apparatus in fixed spaced relation relative to the opposed coating nozzle assemblies 26, 28.
A liquid coating thickness control system including nozzles of the general type employed in the present inven-tion, and the operation thereof, are described in detail in U.S~ Paten~ No. 3,499,418. Briefly, however, strip ].0 passes 10 upward from the spelter bath and carries on its surfaces a layer, or coating, of molten zinc. However, due to various factors, substantial excess coating metal is carried out of the bath by the strip~ and the thickness of the coating on the two sides of the strip is controlled by the wiping or 15 damming action of pressurized gaseous fluid directed onto the surface of -the strip by the nozzles 26, 28. This wiping of excessive coating metal back into the spelter bath is affected in accordance with the basic principles taught in U.S. Patent NoO 3,499,4180 Coating control nozzles 26, 28 form a part of the coating thickness control assembly which includes a nozzle support frame structure 30 in the form of a horizontal plat-form 32 having mounted thereon a pair of downwardly depending legs 34, 36 for supporting the nozzle assemblies 26, 28, X 1]. -~9Z3~

A pair of adjustable brackets 38, 40 mounted on legs 34, 36, respectively, support the nozzle 26 and a similar pair of adjustable brackets support nozzle 28. Since the nozzles are preferably identical, only the nozzle assembly 26 will be described in detail, it being understood that the des-cri tiOIl applies to both nozzle assemblies.
Referring now to Figures 3 - 8, it is seen that nozzle 26 comprises an elongated body assembly made up of upper and lower die members 42, 44, respectively, normally firmly retained in rigidly assembled relation by a plurality of bolts ~not shown) extending through openings 46 along the back edge and ends of the die members. An elon~ated shim member 48 is positioned between the back and the end portions of the die members 42, 44 to retain them in fixed spaced relation relative to one another. As will be understood, the thicknesses of shim 48 may be utili~ed to accurately control the spacing of the die members and-the dimension of the no2zle outlet, as will be described more fully herein-below.
~0 Upper and lower die members 42, 44, are each ~ormed from a single, elongated piece of metal and, except as des-cribed hereinbelow, are of substantially identical configura tion. Thus, die 42 has a substantially flat surface 50 which is essentially the mirror image of the corresponding flat surface ~2 on die member 44, with the surfaces 50 and 52 being retained in fixed spaced relation by the shim 480 ~L9Z3~

Flat surfaces 50 and 52 ex-tend in opposed, parallel relation at the front of the nozzle assembly to define the elongated, thi.n outlet conduit or slot 54 terminating in an outlet opening 55.
Die member 42 has an elongated recess defined by con-toured surface 56, and a similar recess defined by contoured surface 58 is formed in die member 4~. Surfaces 56 and 58 cooperate to define an elongated concavity or plenum chamber 60 when die members ~2 and 44 are assembled together. Con-cave surfaces 56, 58 are substantially identical and pre-ferably are tapered from back to front progressively from the central to the end portion.s of the die members so that the cross-sectional area of the plenum chamber is substan-tially greater in the cen*ral portion than at its ends.
Also, the surfaces 56, 58 are machined to provide a smooth surface to reduce friction.
Die member 42 has a control fluid inlet opening 62 formed in its top and extending into the plenum chamber 60.
As best seen in Figure 4, inlet 62 is in the form of a rela-tiv~ly long, narrow opening defined by downwardly and out-wardly inclined, generaily arcuate end walls 63, 64 joined by generally parallel flat sidewalls 65, 66. The relatively long, narrow, diverging nature of the inlet 62 tends to im-part a component of mot.ion longitudinally of the nozzle assembly to air admitted into the plenum chamber 60. To this end, air supplied to the nozzle through a h.igh pressure ~ ~39~L

conduit 68 is initially discharged into a diffusion box 70 of generally isoceles trapezoidal configuration and having a bottom open end overlying and generally corlforminy to the top of air inlet 62. Diffusion box 70 is rigidly secured, as by welding, to the top surface of die 42. Thus, air delivered from conduit.6~ expands in the diffusion box 70, reducing its velocity and giving it a component of motion longitudinally of the nozzle before reaching the inlet o~ening 62. As illustrated in Figure 2, the inclined wall portions of diffusion box 7Q may extend at a substantially 45 degree angle relative to the vertical, while walls 63, 64 of inlet 62 are more diverging, and may extend at about a 60 degree angle relative to the vertical. All edges are rounded to minimize turbulence as air enters the plenum 60 through the inlet opening 62.
As best seen in FIG. 6, the plenum cham~er ~0 is defined by smooth, arcuate walls contoured to produce minimum resistance to fluid flow in~o the entrance or throat 72 of elongated outlet slot 54. The length L of the outlet slot 54, from the throat 72 to the outlet 55 at the forward edges of walls 50, 52 is sufficient to pxoduce laminar air flow at the nozzle outlet 55, wlthin the pressure ranges and gas flow velocities employed by the apparatus. This results in the air being discharged in a relatively thin, flat stream at the outlet and avoids the tendency of air to spread into a rela-39~

tively wide, fan-like patte.rn of the prior art devices illus~
trated, for example, in the Hunter et al. patent discussed above. There will, of course, be some diversion or diffusion of air from the stream along the side edges due to frictional contact of the s-tream with ambient air; howe~er, since the outlet of the nozzle is normally retained within the range of about 1/2 to 1 1/2 inches from the moving metal strip, this diffusion of the jet stream into ambient air is negligible.
The result is a jet stream which impacts with the moving coated strip in a very narrow band and which is extremely efficient and effective in controlling the removal of excess coating fluid from the moving strip.
By maintaining a substantially uniform pressure within the plenum chamber 60, throughout its full length, the jet is highly uniform across of the full width of the strip. The smooth finish and rounded contours of the internal surfaces of the plenum chamber, in combination with the longitudinally diverging configuration of the diffusion box 70 and inlet 62, enables the use of a plenum chamber of substantially reduced volume while at the same time maintaining the uniform pressure within the chamber throughout its lengthO This substantially uniform pressure along the length of the plenum chamber, in combination with the uniform outlet slot, produces the uniform jet.
An added and somewhat unexpected advantage of utilizing a thin, non-diverging air jet of uniform volume and velocity Z3~

across the width of the strip is the substantially complete elimination of edye build-up. Indeed, while the prior art has generally taught that the so-called bow tie nozzle openiny, or other means for directing an increased air flow toward the edge portions of the strip is necessary, the nozzle of the present invention may actually produce a slightly reduced coating thickness along the edge ~y use of a uniform thin flat laminar flow jet. At the same time, if it is desired to provide a greater coating thickness in the central portion of the strip to facilitate coiling, a shim of slightly greater thickness at ends of the nozzle may be used. For example, the shim 48 may have a thickness of 0.020 inches throughout a major portion of the length, and have a thickness of 0.022 inches near the ends.
Although it has long been recognized that it is desirable to avoid excessive turbulence at the nozzle outlet of an air knife, size limitations of the prior art devices have generally been such that laminar flow at the nozzle outlet has not been possible, and devices such as that shown in U.S. patent 3,408,983 can not produce laminar flow as asserted. It is a well recog-nized principle of fluid mechanics that, for the gas pressuresand velocities required in an air knife, laminar flow can only be produced when the gas flows through a conduit having a uni-form section over a predetermined minimum length. Empirical formulae are available, for example from Marks' Mechanical Engineering Handbook, for calculating the length of a conduit required to produce laminar ~low~ However, it has generally ~;239~

been considered that the length of the conduit leading to the ou-tlet orifice in an air knife, calculated from such standard formulae, would be so great as to make the resultiny design imprac-tical.
Applicant has discovered that the long thin configura-tion of the outlet slot of an air knife results in a flow condition which does not conform to standard pipe flow condi-tions and that laminar flow can~ indeed, be produced in a nozzle structure which is actually of smaller overall size than previously considered necessary. This size reduction has been made possible by applicant's discovery that the outlet nozzle may be designed, using standard pipe for~.ulae for calcu-lating flow distances required for laminar flow, provided the outlet slot is considered as a plurality of separate channels or tubes aligned in side-by-side relation across the full width of the nozzle rather than as a single conduit, but substituting in such standard formulae the cross-sectional area of the entire nozzle outlet for the area of such imaginary lndividual tubes.
While it is not fully understood why this modificaiton of such standard formulae is possible, it is believed to be -the result of the geometry of the-outlet slot, which may be up to 80 inches in length and have a thickness, or width, in the range of about O.OlS to about 0.025 inches~ in combination with the use of very smooth or polished surfaces on the interior oE the nozzle out-let slot.

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The very thin, relatively non-diverging air jet resulting from laminar flow at the nozzle outlet enables a substantial reduction in total air Elow while producing the desired wiping actionO The reduced air flow not only resu]ts in a saving o~
energy, but also enables a substantial reduction in the total volume or cross-sectional area of the nozzle plenum chamber while maintaining a very uniform air jet.
The ratio of the maximum cross-sectional area of the plenum, i.e., the area in the central portion of the plenum to the area of the outlet orifice may be less than 4 to 1 and preferable is no more than about 5 to 1. This is in contrast to the prior art nozzles such as U.S. patent 3,~53,578 wherein the ratio of the cross-sectional area of the plenum, for a single inlet nozzle, to the area of the outlet is stated to be at least 8 to 1 and preferably 1~ to 1~ It is noted, also, that this prior patent is primarily concerned with a low pressure, high volume nozzle utilizing air pressures down to about 0.4 psig and necessarily requiring a relatively large outlet opening whereas ~he high-speed nozzle of the present invention is intented f~r use with air pressures within the range of about 20 to about 60 psig.
The very thin, relatively non-diverging air jet resulting from laminar flow at the nozzle outlet may be proauced in a nozzle structure in which the uniform slot leading to the outlet has a length, measured from the throat 72 to the outlet 55, Which is substantially less thanwould be expected fxom conventional nozzle formulae calcu]ations. For example, in such a nozzle assembly used for hot-dip galvanizing of steel strip, or in a similar process used for coating steel strip with an alloy of zinc and aluminum, it has been found that satisfactory results are produced when the throat length, measured in inches, is within the range of about 0.28 to 0O4 times, and preferably about 0.28 to 0,4 times the cross-sectional area of the plenum chamber measured in square inches.
10 This ratio is much less than expected; particularly in view of the very substantially reduced cross-sectional area of the plenum chamber of the present invention, and enables the use of a very compact and highly efficient air knife.
as indicated above, a nozzle assembly constructed in 15 accordance with the present invention has been employed to coat steel strip on a high-speed commercial galvanizing line under normal mill run conditions. The nozzle had an outlet opening measuring 81 inches in length i.e, the dimension transfers to the strip and 0.020 inches in width i.e. the 20 dimension longitudinal of the strip. The throat length of the slot from the plenum to the outlet orifice was 2.5" Long radius turns were employed to provide a smooth contour to the internal surface of the plenumO The plenum chamber had a cross-sectional area, at its center, of approximately 7.25 25 square inches and approximately 4.25 square inches near the ends~ This nozzle assembly was installed on a 72 inch galvan~
izing line, i.e., a galvanizing line for coating steel strip up to 72 inches wide~

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In operation of the galvanizing line utilizing the nozzles just described, air was used as the coating ~luid.
Both zinc and a zinc-aluminum alloy were used as the coating material with excellent results both from the standpoint o~
coating thickness and edge effect. Successful coating has been accomplished at speeds up to 360 feet per minute.
Tests conducted using these nozzles have demonstrated that they are extremely versatile in operation. Coating thickness can be controlled very accurately, and adjustements may be made as necessary to maintain such control. The nozzles may discharge the jet in a stream perpendicular to the moving strip or they may be adjusted to direct the jets downward at an angle of up to about 10 to achieve slight adjustments in coating thickness. Preferably the spacing of the nozzles both relative to the strip and to the coating may be adjusted and the air pressure may be varied to control coating thickness.
I~ desired~ unequal coating thicknesses may be applied to the two surfaces o~ the strip without producing edge build-up, as illustrated in FIG. 9. Air was used in the nozzles in place of steam required by the nozzles previously employed on the same coating line, and it was learned that the air pressure required was substantially less than the steam pressure previously employed.
FIG. 9 represents the zinc coating thickness, at four inch intervals across the width of a 56 inch steel strip coated on the commercial hot-dip galvanizing line, using the nozzle -20~

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assembly of the present invention. The nozzles on opposed sides of the strip were intentionally positioned to produce an unequal coating thickness on the two sides of the strip.
A random sample of the strip coated was tesked and the coating thickness, measured in ounces of zinc per square food of surface area of the base steel,.on the thick-coated side is represented by curve 7~. The corresponding coating thickness on the thin coated side is represented by curve 76. Surprisingly, the contour of the thicker coating correlates very closely with the contour which i5 normally attempted to be achieved using nozzles of the so-call~d bow tie design wherein less coating material i5 applied to the side edges to avoid the adverse effect of edge build-up known as spooling. No special effort was made, however, to achieve this result by intentionally directing excess air to the side edges. A surprisingly flat, or uniform coating was achieved on the side having the thinner coating, and the uniform coating extended substantially throughout the full width of the strip.
~ FIGS. lO, ll and 12 are curves similar to FIG. 9~
graphically illustratin~ the coating thickness of three separate commercially produced steel stripsO The samples were selected at random from material supplied to a customer by three diferent manufacturers. The three strips had previously been produced with a thin coating on one surface, and the coating metal on the thin side had been stripped from the steel base metal prior to shipment to the customer.

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Extensive testing of steel strip coated on the line just described have shown that a substantially more uniform coating is achieved by use of the air knife of the present invention.
The more uniform coating has enabled operation at relatively high speeds with the maximum or control coating thickness closer to minimum limits, with a substantial reduction in the total coating metal used. Savings of 2.5 to 3 percent are readily achieved under mill production conditions. On a single 48 inch hot-dip galvanizing line, the savings may be 10 within the range of about 7,000 to 8,000 tons of coating metal per year.
Tests have also shown that the nozzle assembly of the present invention operates satisfactorily under mill production conditions utilizing shop air instead of steam, both in a high-speed, hot-dip galvanizing operation and for hot-dip coating of steel strip with a zinc-aluminum alloy. Further, the reduction in gas flow, and the lower pressure required, substantially reduces the noise generated by the air knife.
Where a greater coa~ing thickness is desired on the center portion of a strip to facilitate coiling, a shim 48 which is slightly thicker at its end portions can be employed to produce a thinner coating at the edges. Since only a slight increase in nozzle opening is required, the geometry of the outlet is not materially af~ected and laminar flow is not interrupted.
While we have disclosed and described a preferred embodi-ment of our invention, we wish it understood that we do not intend to be restricted solely thereto, but rather that we -22~

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intend to include all embodiments thereof which would be apparent to one skilled in the art and which come wi.thin the spirit and scope of our invention.
We claim:

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a coating apparatus for applying a coating liquid to a substrate in strip form by passing the substrate through a bath of the coating liquid, withdrawing the substrate upwardly from the bath with an excess of coating liquid adhering to the surfaces thereof, then removing the excess coating liquid by passing the substrate between and in closely spaced relation to a pair of opposed fluid nozzles and discharging a jet of gaseous fluid from the nozzles against the surfaces of the moving substrate, the improvement wherein each such nozzle comprises, a pair of elongated die members adapted to be assembled together and supported with their length dimension disposed transversely of and generally parallel to the length of the substrate as the substrate exits the coating bath, the assembled die members cooperating to define an elongated nozzle body having an outlet opening extending at least the full width of the substrate, a recess formed in each said die member, said recesses being in substantially opposed relation and coopera-ting to define a plenum chamber extending substantially the full length of and in spaced relation to said outlet.
said plenum chamber having a cross-sectional area, measured in a vertical plane perpendicular to said outlet opening, which is no greater than about five times the area of said outlet opening, a single fluid inlet formed in one of said die members for admitting pressure fluid into said plenum, a smooth substantially planar surface on each said die member extending from said plenum to said outlet, said sub-stantially planar surfaces being disposed in spaced, substan-tially parallel relation to one another and cooperating to define a thin outlet slot of uniform thickness extending between said plenum and said outlet, spacing means between said die members retaining said flat surfaces in said spaced relationship, the length of said outlet slot being sufficient to produce laminar flow of gaseous fluid flowing therethrough from the plenum before the fluid reaches the outlet, and the ratio of the length of said substantially planar surfaces from the plenum to the outlet opening, measured in inches, to the maximum area of the plenum chamber, measured in square inches, being within the range of about 2.5 to 1 to about 3.5 to 1.

2. In the coating apparatus according to claim 1, the further improvement wherein said fluid inlet is in the form of an elongated generally slot-like opening having its maximum dimension generally parallel to the length of the plenum chamber, the length of said elongated opening being substantially greater at the plenum chamber than at the external surface of the die member whereby a component of motion longitudinally of the nozzle is imparted to at least a portion of the air entering the plenum chamber.

3. In the coating apparatus according to claim 2, the further improvement comprising a diffusion chamber mounted on the outer surface of said one die member and overlying said fluid inlet and projecting upwardly therefrom, said diffusion chamber conforming generally to the configuration of said fluid inlet at said die member and tapering upwardly there-from to an inlet portion having a substantially smaller cross-sectional area.

4. In the coating apparatus according to claim 1, the further improvement wherein said plenum chamber has a maximum cross-sectional area, measured in a generally vertical plane, at its center and a substantially smaller cross-sectional area adjacent its end portions, and wherein the plenum chamber is defined by smooth contoured surfaces to minimize fluid friction loss and thereby reduce turbulence within the plenum and promote laminar flow.

5. The coating apparatus according to claim 1, wherein the maximum cross-sectional area of said plenum chamber is within the range of about 4 to about 5 times the area of said outlet opening.

6. In the coating apparatus according to claim 5, the further improvement wherein the width of the nozzle outlet opening is within the range of about 0.015 to about 0.025 inches and the length of said thin slot from the plenum chamber to the outlet opening is about 2,5 inches.

7. In the coating apparatus according to claim 6, the further improvement wherein said spacing means comprises shim means having a thickness corresponding to the nozzle opening.

8. In the coating apparatus according to claim 1, the further improvement wherein the width of the nozzle outlet opening is within the range of about 0.015 to about 0.025 inches and the length of said thin slot from the plenum chamber to the outlet opening is about 2.5 inches.

9. In the coating apparatus according to claim 8, the further improvement wherein said fluid inlet is in the form of an elongated generally slot-like opening having its maxi-mum dimension generally parallel to the length of the plenum chamber, the length of said elongated opening being substan-tially greater at the plenum chamber than at the external surface of the die member whereby a component of motion longitudinally of the nozzle is imparted to at least a portion of the air entering the plenum chamber.

10. In the coating apparatus according to claim 9, the further improvement comprising a diffusion chamber mounted on the outer surface of said one die member and overlying said fluid inlet and projecting upwardly therefrom, said diffusion chamber conforming generally to the configuration of said fluid inlet at said die member and tapering upwardly there-from to an inlet portion having a substantially smaller cross-sectional area.