US3930075A

US3930075A - Method for controlling splashing resulting from the use of gas knives

Info

Publication number: US3930075A
Application number: US428468A
Authority: US
Inventors: Henry F Myers
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1973-12-26
Filing date: 1973-12-26
Publication date: 1975-12-30
Anticipated expiration: 1992-12-30

Abstract

In the use of gas-knives for the control of dipped coatings, a portion of the gas jet hits the bath surface causing undesirable splashing. The invention is directed to the use of a deflector system which is comprised of a substantially enclosed chamber surrounding the strip emerging from the coating bath. Gas pressure from the control jets rapidly builds within the chamber and acts as an air cushion to deflect the downward jet from the bath surface.

Description

I Unlted States Patent 1 [111 Myers Dec. 30, 1975 [54] METHOD FOR CONTROLLING SPLASHING 3,68l,l l8 8/l972 Ohama et 117/102 M RESULTING FROM THE USE OF GAS KNIVES Primary ExaminerCameron K. Weiffenbach [75] Inventor: Henry F. Myers, Monroeville, Pa Assistant Examiner-Ralph E. Vamdell [73] Assignee: United States Steel Corporation, Attorney Agem or F'rm Arthur Grelf Pittsburgh, Pa.

[22] Filed: Dec. 26, 1973 ABSTRACT [21] Appl. No.: 428,468

In the use of gas-knives for the control of dipped coatings, a portion of the gas jet hits the bath surface caus- [Szl CL 427,349 z g gg gj ing undesirable splashing. The invention is directed to [51] L ads!) 18 the use of a deflector system which is comprised of a {58] new g 7/102 L substantially enclosed chamber surrounding the strip g g H4 4C emerging from the coating bath. Gas pressure from R 3 6 43] 349 the control jets rapidly builds within the chamber and acts as an air cushion to deflect the downward jet [s6] Rekrences Cited from the bath surface.

UNITED STATES PATENTS 9 Claims, 5 Drawing Figures 3.589530 6/1971 Alexefi' 118/4 L/ 1 If: 4 1

U.S. Patent Dec. 30, 1975 Sheet 2 of5 3,930,075

FIG. 2

US. Patent D66. 30, 1975 Sheet 3 M5 3,930,075

US. Patent Dec. 30, 1975 Sheet 4 of5 3,930,075

FIG: 4

U.S. Patent Dec. 30, 1975 Sheet50f5 3,930,075

METHOD FOR CONTROLLING SPLASHING RESULTING FROM THE USE OF GAS KNIVES This invention relates to the use of gas knives for coating control, and more particularly to a method for the control of splashing during gas knive operation.

In the gas-knife control of hot dipped coatings on metal strip (the term strip is employed to generically define an elongated, flat rolled article such as sheet) severe splashing of the coating bath is encountered when the gas jets strike the surface of the coating bath. The problem is especially pronounced in high speed processes employing large orifice gas knives. In an attempt to minimize this splashing, the art has resorted to a number of expedients, all of which suffer from certain inherent deficiencies. One such practice operates the gas knife with an upward tilt with respect to the horizontal. Such upward tilting, while decreasing bath splash, causes increased splatter onto the gas knife system itself and results in drips at the strip edges being blown over a wide area of the coating operation. Another prior art practice, discussed more fully hereinafter, uses mechanical deflectors to baffle a portion of the gas jet striking the bath. However, because of the non-uniform nature of the velocity distribution, these mechanical deflectors must be placed very close to the planar surface of the strip to achieve an adequate amount of deflection. Such close placement cannot be employed in actual operating conditions, since the passage of the emerging strip becomes encumbered due to pass line variation and uneven strip shape. Finally, attempts have been made to employ gas knives a considerable distance from the bath surface. In general, this latter expedient has limitations because of the tendency of the molten coating to solidify and/or excessively to oxidize with long exposure prior to contact with the intense wiping region of the jet.

It is therefore a primary object of the instant invention to permit unencumbered passage of the emerging strip, while greatly reducing the degree of bath splashmg.

It is a further object to permit safe operation of high pressure wiping equipment, while maintaining the gas knife desirably close to the bath surface.

These and other objects and advantages of the invention will be more readily understood from the following specification and the appended claims when taken in conjunction with the drawings in which:

FIG. I is illustrative of the bath conditions existing under intense gas wiping.

FIG. 2 depicts the spreading and the velocity distribution of the downward wall stream.

FIG. 3 illustrates the deviation of the downward wall stream achieved by the instant method.

FIG. 4 is an isometric view of a preferred air-cushion apparatus for performing the method of this invention.

FIG. 5 is an additional isometric view of a preferred apparatus, showing the use of a supplemental edge guide.

The operation of a gas-knife in controlling the coating thickness of molten metal on a strip is illustrated in FIG. 1. For purposes of clarity, only one side of the strip is shown in this figure. However, it should be understood that, in general, the strip will emerge with liquid adhering to both planar surfaces (and the edges as well) and gas jets are therefore positioned on both sides of the emerging strip to control the thickness of said adherent liquid.

Referring to FIG. I, as the wiping jet J impacts on the planar surface of the strip 1 with its adherent liquid coating 2; the jet separates into upward and downward component streams. If the impact were perpendicular to the strip, the component streams would be equal. However, in the more general case where the jet is tilted downward, e.g., at angles of up to about 30 with respect to horizontal, the jet will split into unequal components with the downward component 1,, being the larger of the two, and acting as a wall stream that follows the strip down to the liquid bath 3. At the bath impact region, the wall stream forms a depression 4, which is opposed by the tendency of the moving strip to lift the portion of liquid adjacent to it. A large volume of relatively high velocity gas at this impact region, causes excessive splashing. It should be understood that a certain small amount of gas being blown across the surface of the bath would be desirable in that it would move floating dross away from the strip and thus aid in keeping the strip free of unwanted contaminants. However, the excessive bath splashing caused by the generally large amounts of gas results in undesirable losses of bath liquid and, more importantly, makes the immediate area hazardous to personnel, by reason of flying droplets of liquid. The latter effect is especially undesirable in the case wherein the liquid is a molten metal (i.e. Zn, Sn, Al or Pb) such as is commonly employed for coating steel strip.

Mechanical deflectors such as those shown in US. Pat. No. 3,681,118 have been employed in an attempt to overcome this problem. However, because of pass line variations and uneven strip shape, such deflectors must be set up to provide for the unencumbered passage of the strip, typically at a distance of about Va inch or greater from each side of the strip. Unfortunately, when the leading edge of the deflector is at such a distance from the strip surface, its effectiveness is greatly limited. The inherent limitations of these deflectors will be better understood by reference to FIG. 2, which depicts a typical velocity distribution of a downward directed wall stream I The velocity curve 9 will exhibit the general shape of one-half of a Gaussian distribution, except that at a position very near the strip surface the gas velocity rapidly decreases, due to viscous drag. The thickness of the stream at a point 10 immediately below impact of the jet with planar strip surface 11 is typically about 56 inch. The outer boundary 12 of the downward directed stream I approximates the region where the stream velocity falls to a value less than one-half of the maximum velocity Vmax at a given distance below the point of initial impact. This outer boundary will generally diverge at an angle of about 10 from the plane defined by the strip. The relative velocities of the stream at various distances from surface 11, are graphically depicted by curve 9 (superimposed on the figure). In a typical case where the leading edge of the deflector I3 is placed at a point 12 inches below the impact point and at a distance '95 inch from surface 11, the deflector would intercept about two-thirds of the area depicted. However, the unintercepted portion of the wall jet would nevertheless contain about 0.3 of the volumetric flow and in excess of 0.4 of the momentum of I (These values are based on the fact, as noted above, that the velocity distribution may be approximated by a normal probability curve. That portion very near the strip surface, where gas velocity drops to zero, has been neglected.)

The instant invention employs a device which is a combination of the above deflector with a restricted outlet chamber. The mechanism of its operation will better be understood by reference to FIG. 3. The wall stream I coming down the strip 21 with its adherent liquid coating 22, encounters the top portion of chamber 23, with its slotted opening 24. The portion of the stream outside the opening is deflected away from the bath in a manner similar to the conventional mechanical deflector. Initially, the portion of stream within opening 24, would proceed into chamber, where it would strike the bath therein and be deflected upward. As a result of the restricted outlet, the pressure at 25 within the chamber rapidly builds and attains an equilibrium state (i.e. wherein the amount of gas entering the chamber equals that leaving the chamber). The resultant back pressure creates an air cushion in the region of opening 24, sufficient to baffle the portion of the downcoming wall stream in that region. In this manner, the higher velocity portions of the gas stream which would normally escape the mechanical deflector (i.e. as in FIG. 2) are now substantially deflected by the air cushion at opening 24. The back pressure created within the chamber is self-regulating, in that it adjusts to variations in the wall stream flow. The chamber can be operated over a range of slot openings, thus enabling accommodation to variations in strip width and thickness.

Three factors, however, must be considered for effective operation of the chamber:

a '1. The normal distance d between the planar surface of the strip and the oppositely facing long edge of the slot, i.e. the lip. This distance should be less than four times (and preferably less than, as shown in FIG. 3) the effective thickness of wall stream passing therebetween. The effective thickness of the stream is defined by the boundary (i.e. l2 in FIG. 2) where the stream velocity falls to less than one-half the maximum velocity. Thus, if openings more than four times the thickness of the wall stream are employed, the stream after striking the bath would be deflected along the horizontal surface thereof, upward along the walls of the chamber and then exit on the furthest edge of the slot opening, with little or no interaction with the downcoming wall stream. This kind of operation, although tending to shield and trap a portion of the splash inside the chamber, would have little effect in reducing its magnitude and could further cause it to disrupt the liquid coating adhering to the strip. At slot openings within the prescribed range, the stream interference and back pressure generation in the chamber is effective to markedly decrease the bath splash. It should be noted, however, that even with relatively narrow slot openings, some splash may be noticed. This results from the fact that the stall pressure of the wall stream varies with distance from the strip surface, while the air-cushion pressure along the normal distance d will tend to be substantially uniform due to the uniform pressure inside the chamber. Thus some penetration of the highest velocity portion of the wall stream (see FIG. 2) may be encountered.

2. The leakages out other possible exits of the chamber must not be excessive. That is, the chamber outlets must be restricted, sufficiently to create the requisite back pressure. However, there is no requirement that the chamber be air tight (at regions other than the slotted opening) so long as the losses are only a minor fraction of the incoming wall jet. This factor permits 4 the utilization of a controlled exhause of gas near the bath surface to carry away accumulated top dross and thus maintain the cleanliness of the region at which the strip exits from the bath surface.

3. The closest approach of the lip or slot at the top portion of chamber to the wiping jet. This lip portion, i.e., the topmost portion of the chamber must be placed an effective distance below the wiping jet (i.e. the region of intense wiping) so that it will not interfere with the jet to the extent that it will (a) disrupt effective wiping of the coating or (b) not intercept a sufficient amount of the downcoming stream to create the requisite back-pressure.

A device for accomplishing the above objectives is shown in FIG. 4. This device is particularly preferred since its features enable (a) adjustment to vary the total width of the slot opening and thus vary d, the normal distance between the strip surface and the long edge of the slot, (b) centering about the strip pass line (c) adjustment of the clearance between the bath surface and the bottom portion of the chamber sides to provide for the gas cleanout feature mentioned above, and (d) the easy removal and installation of the chamber. Referring to FIG. 4, the chamber 33 is composed of two similar half-units 34 and 35 and their

respective supports

36 and 37. It should be noted that while the strip is depicted as emerging vertically, the instant method is applicable to any coating process in which the strip emerges from the bath in a generally upward direction, i.e. with respect to vertical. The half-units are constructed with provision for meshing when offset by their wall thickness. The supports and half-units are sufficiently rigid to resist the opening and lifting forces on the units resulting from the buildup of back-pressure in the chamber. The ends of the supports (not shown) are connected to a suitable mechanism for (i) adjusting the dimensions of the total width of the slot opening y (ii) centering the opening about the strip pass line, and (iii) raising or lowering the chamber to provide for gas escape, e.g. at region 38. In practice, the total slot opening y, should be sufficiently wide to permit the unencumbered passage of the emerging strip. Thus, y may vary, for example, from about #4. to 3 inches. It should be noted that with y equal to inches, that d will necessarily be less than 16 inches, an approach to the strip closer than is permissible with the conventional mechanical deflector depicted in FIG. 2. This closer permissible approach results from the ability of the instant deflector system to be placed a greater distance from the wiping jet (i.e. closer to the bath surface) and nevertheless achieve desired interception of the downcoming stream. With such lower placement to the bath surface, the pass-line stability is improved and the unencumbered distance d may be decreased in comparison with a deflector placed at a greater distance from the bath surface. The bottom portion of the chamber sides must be approximal to the surface of the liquid bath, sufficiently so to provide the requisite restricted outlet. Thus, the term approximal includes, for example, those situations in which all such bottom portions are immersed beneath the surface of the bath, as well as those situations in which one or more of those portions is raised (e.g. about 1 to 2 inches) to provide a gas escape and dross cleanout region, e.g., that of region 38 shown in the figure. While the chamber top portion, 39, which runs in a direction substantially parrallel to the width of the strip, could take various shapes (e.g. flat or convex), it is preferable that it be 5 concave (as illustrated) to the downcoming wall stream striking thereupon. This concave feature deflects the wall stream from the immediate area of the chamber, while decreasing turbulence compared to that which would result from using a flat top portion.

FIG. 5 depicts a further embodiment of the invention which is particularly desirable for situations where the strip is appreciably narrower than the width of the wiping jet. In such a case, there will exist a region where the two opposing wiping jets will intersect in space. When this occurs, resultant jets are formed beyond each edge of the strip, whose directions and magnitudes are vector sums of the downward and upward components of the intersecting jets. if the opposing wiping jets are at the same angle with the strip and are in perfect balance, the downward resultant streams will proceed in a direction generally parallel to the plane of strip and toward the bath. The upward resultants play no part in bath surface disruption and are therefore of no concern. The downward stream will then impact the region of the deflector opening beyond the strip edges and will contribute to maintaining near uniform orifice pressure within the chamber (provided the length of the slot is less than the width of the source jets). This self-aiming arrangement is, however, an unstable situation; and mechanical means for stabilization are generally advisable. As shown in FIG. 5, supplemental edge jet guide 40 is suspended to hang near the edge of strip 41 in a generally parallel position to the strip with a small separation 42 between the two edges. This separation should be as small as is practical. Splash and droplets from the strip edges will restrict this closeness of approach, but such splash can be partially offset by directing some of the wall jet flow inward, to counteract the outward divergence caused by the presence of the strip edge. This inward deflection can be achieved by vanes or raised areas 45 on the surface of the jet guide to direct some of the stream toward the gap 42. This small gap can be maintained by mechanical guides from the strip edges or by non-contact sensors and positioning drivers (not shown). For stability reasons the point of suspension, or the equivalent axis of rotation 43 of the suspension system should be above the region of impingement 44 of the wiping jets. The bottom edge of edge jet guide can be left free to position itself if the strip rises perfectly vertically out of the bath and if the two streams have equal momentum. However this is not the usual situation, and some control over the position of the bottom edge of this guide is necessary to keep it parallel with the plane of the strip and aimed toward the center of the slot opening. The use of the edge jet guide 40, provides an additional benefit in that it reduces the noise level resulting from the turbulence produced by the intersection of the wiping jets.

I claim:

1. in the method for the continuous hot-dip coating of a metal strip, wherein the strip emerges from a liquid bath in a generally upward direction and the thickness of the liquid layer adhering to said emerging strip is controlled by gas jets impinging against the planar surfaces of said strip at an angle of about to 30 below the horizontal, said impingement causing each of said 6 jets to divide into component streams, which components include wall streams flowing downward on both planar strip surfaces, said downward wall streams causing undesirable turbulence of the liquid bath within a region proximate the metal strip;

the improvement for minimizing said liquid bath turbulence which comprises; employing a baffle chamber, the sides of which are substantially enclosed to form a restricted outlet for said downward wall streams, the bottom portions of said sides being approximal to the surface of the liquid bath, whereby the surface of the liquid bath serves as the bottom enclosure for said chamber to substantially restrict the egress of said wall streams; the topmost portion of said chamber lying an effective distance below the point at which said gas jet impinges upon the planar strip surfaces, said topmost portion having a generally rectangular slot opening, the long edges of which are substantially parallel to the planar faces of said strip, said slot opening being of a dimension at least sufficient to permit the unen cumbered passage of said strip therethrough, but in which the normal distance between a planar surface of said strip and the oppositely facing long edge of said slot is no greater than about four times the thickness of the wall stream passing therebetween, whereby the resultant restricted wall streams within the chamber form a back pressure therein, sufficient to provide an air cushion to baffle the downcoming wall streams.

2. The method of claim 1, wherein said normal distance between the strip surface and said long slot edge is equal to or less than the thickness of the wall stream passing therebetween.

3. The method of claim 1, wherein the bottom portion of said sides is immersed beneath the surface of said liquid bath.

4. The method of claim 1, wherein the bottom portion of at least one of said sides is immersed beneath the surface of said liquid bath.

5. The method of claim 4, wherein said normal distance between the strip surface and said long slot edge is equal to or less than the thickness of the wall stream passing therebetween.

6. The method of claim 5, wherein at least 10 percent of the volume fraction of each of the wall streams does not enter said chamber and is diverted away from the bath, at said region proximate the metal strip, by the top portion of said chamber.

7 The method of claim 6, wherein the top portion of said chamber which is substantially parallel to the planar surfaces of said strip tapers downwardly and outwardly from said planar surfaces.

8. The method of claim 7, wherein said tapering top portion is concave to the fraction of the wall stream striking thereupon.

9. The method of claim 8, wherein said metal strip is steel and said liquid bath is molten metal containing a major amount of a metal selected from the group consisting of Zn, Sn, Al, or Pb.

Claims

1. IN THE METHOD FOR THE CONTINUOUS HOT-DIP COATING OF A METAL STRIP, WHEREIN THE STRIP EMERGES FROM A LIQUID BATH IN A GENERALLY UPWARD DIRECTION AND THE THICKNESS OF THE LIQUID LAYER ADHERING TO SAID EMERGING STRIP IS CONTROLLED BY GAS JETS IMOINGING AGAINST THE PLANAR SURFACES OF SAID STRIP AT AN ANGLE OF ABOUT 0* TO 30* BELOW THE HORIZONTAL, SAID IMPINGEMENT CAUSING EACH OF SAID JETS TO DIVIDE INTO COMPONENT STREAMS, WHICH COMPONENTS INCLUDE WALL STREAMS FLOWING DOWNWARD ON BOTH PLANAR STRIP SURFACES, SAID DOWNWARD WALL STREAMS CAUSING UNDERISRABLE TURBULENCE OF THE LIQUID BATH WITHIN A REGION PORXIMATE THE METAL STRIP; THE IMPROVEMENT FOR MINIMIZING SAID LIQUID BATH TURBULENCE WHICH COMPRISES; EMPLOYING A BAFFLE CHAMBER, THE SIDES OF WHICH ARE SUBSTANTIALLY ENCLOSED TO FORM A RESTRICTED OUTLET FOR SAID DOWNWARD WALL STREAMS, THE BOTTOM PORTIONS OF SAID SIDES BEING APPROXIMAL TO THE SURFACE OF THE LIQUID BATH, WHEREBY THE SURFACE OF THE LIQUID BATH SERVES AS THE BOTTOM ENCLOSURE FOR SAID CHAMBER TO SUBSTANTIALLY RESTRICT THE EGRESS OF SAID WALL STREAMS; THE TOPMOST PORTION OF SAID CHAMBER LYING AN EFFECTIVE DISTANCE BELOW THE POINT AT WHICH SAID GAS JET IMPINGES UPON THE PLANAR STRIP SURFACES, SAID TOPMOST PORTION HAVING A GENERALLY RECTANGULAR SLOT OPENING, THE LONG EDGES OF WHICH ARE SUBSTANTIALLY PARALLEL TO THE PLANAR FACES OF SAID STRIP, SAID SLOT OPENING BEING OF A DIMENSION AT LEAST SUFFICIENT TO PERMIT THE UNENCUMBERED PASSAGE OF SAID STRIP THERETHROUGH, BUT IN WHICH THE NORMAL DISTANCE BETWEEN A PLANAR SURFACE OF SAID STRIP AND THE OPPOSITELY FACING LONG EDGE OF SAID SLOT IS NO GREATER THAN ABOUT FOUR TIMES THE THICKNESS OF THE WALL STREAM PASSING THEREBETWEEN, WHEREBY THE RESULTANT RESTRICTED WALL STREAMS WITHIN THE CHAMBER FORM A BACK PRESSURE THEREIN, SUFFICIENT TO PROVIDE AN AIR CUSHION TO BAFFLE THE DOWNCOMING WALL STREAMS.

7. The method of claim 6, wherein the top portion of said chamber which is substantially parallel to the planar surfaces of said strip tapers downwardly and outwardly from said planar surfaces.