US3436265A - Pressure gradient web cleaning method - Google Patents

Description

April 1969 T. A. GARDNER 3,436,265

PRESSURE GRADIENT WEB CLEANING METHOD Original Filed Aug. 19, 1963 Sheet of 2 April 1, 1969 'r. A. GARDNER 3,436,265

PRESSURE GRADIENT WEB CLEANING METHOD Original Filed Aug. 19, 1963 Sheet 4 of 2 My '7 U INVENTOR.

fidnrlxlazoamc United States Patent 3,436,265 PRESSURE GRADIENT WEB CLEANING METHOD Thomas A. Gardner, 513 Clark St, Neenah, Wis. 54956 Original application Aug. 19, 1963, Ser. No. 302,978, now Patent No. 3,239,863. Divided and this application Jan. 8, 1965, Ser. No. 424,246

Int. Cl. 1308b 3/02 U.S. Cl. 13437 3 Claims ABSTRACT OF THE DISCLOSURE The cleaning of a longitudinally moving web is accomplished by directing a transverse gas jet of at least 10,000 f.p.m. normally against the web and a second transverse gas jet at an acute angle to the movement of said web and toward said first-mentioned jet in the direction of web movement, and collecting from the vicinity of the web between the gas jets the gas from the secondmentioned jet and that component of the gas of the firstmentioned jet which travels counter to the direction of web movement. The first-mentioned gas jet originates at a distance from the web which is no more than 1.0 inch therefrom and the ratio of its cross sectional area at its origin to the distance of its point of origin from the web is within the range of 0.02 to 1 and 0.15 to 1. The angle of the second gas jet with respect to the web is within the range of 5 to degrees.

This invention relates to pressure gradient web cleaning method. This application is a division of my copending application Ser. No. 302,978, filed Aug. 19, 1963, now Patent No. 3,239,863.

Dust, lint and other free particles on the surfaces of webs of paper or foil or plastic have always presented a problem in numerous industries because of the practical diificulties in the way of removal of such particles by any apparatus or methods heretofore known.

For example, dust, lint, and other particles of dirt are generated in paper making processes. Lint and hair commonly originate from press and dryer felts. Cellulose flour and fiber originate from paper in the drying, calendering, and trimming or cuting operations in paper making. Much of the dust so formed adheres to the surface of the paper because of oily or moist surfaces on the particles, or electrostatic charge, or because the particles are wholly or partially immersed in the viscous boundary layer film of air on the surface. Usually a relatively large force is required to remove the particles.

Coating equipment presents another example. Most types of coating machines for applying clay coatings to paper depend on the action of rolls, rods, or steel blades to smooth out the wet coating on the paper surfaces. Dust and dirt are picked up by coating rolls or caught under rods and blades causing marking and streaking of otherwise smooth coating. Dust is often picked up by the coating applicator and circulated in the liquid coating system causing clogging of filters, loss of coating, and higher maintenance costs.

In the printing industry, dust is a problem in all types of printing. In the case of letterpress printing, dust clogs up type faces and hard dirt particles have even broken the printing plates. Offset press blankets, being moistened with water and ink, are particularly susceptible to dust that tends to build up on the blankets until the press is forced to shut down for washup. Intaglio printing is also affected by dust that blurs printing, clogs doctor blades, and transfers into the ink wells.

Neither vacuum devices nor brushes nor jets as here- 3,436,265 Patented Apr. 1, 1969 tofore known can effect dislodgement of the objectionable particles without using force sufliciently great to cause damage to the work or distribution of the objectionable particles within the ambient air around the apparatus. Employing suflicient vacuum to pick up the dust by any previously known procedures would displace the web upon which the work is being done and would in itself be a source of damage to the work.

The object of this invention is to provide an effective web cleaner and method, the preferred equipment being compact in size, simple to operate, economical to build and operate, safe to use on any web material, and capable of removing dust of finer particle size than has heretofore been possible.

The present invention accomplishes the objective of dust removal without damage to the work by disrupting the boundary layer of air on the surface of the work.

A variety of devices may be used to practice the method of the present invention. The preferred equipment is very simple, consisting of a plenum chamber from which two air slot nozzles open toward the work, the space between these nozzles being enclosed to constitute an exhaust chamber for the air supplied through the nozzles and dust displaced from the work.

The primary nozzle is arranged to jet the cleaning air upon the face of the work substantially at right angles to the work and sufficiently close thereto to produce an extremely narrow impingement Zone. Not only is the angle important but the orifice width in relation to its distance to the web is important as will hereinafter be shown.

About one-half of the air supplied to this nozzzle escapes along the surface of the web in the direction of web travel and is not picked up in the exhaust chamber. However, this air is clean and therefore does not distribute dust into the surrounding atmosphere. The other half of the air from the primary nozzle establishes a pressure gradient which results in disruption of the boundary layer on the surface of the work and therefore frees dust and other foreign matter and carries it 011..

Before the work reaches the primary nozzle, it passes beneath a nozzle which creates a jet of air directed angularly toward the surface of the web and toward the pri mary nozzle. This nozzle is herein designated as a secondary nozzle because, even though it is the first nozzle beneath which the work passes, its function is secondary to that of the nozzle which disrupts the boundary layer. The jet from the secondary nozzle may dislodge a relatively small percentage of relatively free particles but its primary function is to seal the chamber against loss of dust-laden air from the primary nozzle and to contribute to the establishment of a steep pressure gradient at or immediately adjacent the primary nozzle. Substantially all of the air from the secondary nozzle is intercepted by the exhaust chamber. The distance between nozzles is not critical but is limited by practical considerations.

Air velocities of 10,000 f.p.m. to 40,000 f.p.m. are contemplated but a relatively low volume of air is used and the arrangement is such that no damage is done to the work even if the web is a very light film.

In the drawings:

FIG. 1 is a view of apparatus for the practice of the invention as it appears in front elevation, portions being broken away.

FIG. 2 is a diagram of a circulatory air system in which such a device as that shown in FIG. 1 is preferably incorporated.

FIG. 3 is a view taken in cross section on the line 33 of FIG. 1.

FIG. 4 is a fragmentary detail View showing an alternate secondary nozzle construction.

FIG. 5 is a diagram showing the air flow along the surface of the work, the nozzles and the work being fragmentarily illustrated.

FIG. 6 is a diagram greatly enlarged as compared with FIG. 5 and showing relative tensions of the boundary layer on the surface of the work as aflected by the primary jet.

FIG. 7 is a fragmentary diagrammatic view taken in cross section like FIG. 3 and showing a modified embodiment of the invention.

FIG. 8 is a fragmentary diagrammatic view like FIG. 3 showing a further modified embodiment of the invention.

FIG. 9 is a fragmentary diagrammatic view like FIG. 3 showing another modified embodiment of the invention.

FIG. 10 is a fragmentary diagrammatic view similar to FIG. 3 and showing another modified embodiment of the invention.

As best shown in FIGS. 1 and 3, preferred equipment for the practice of the invention comprises a housing 6 which contains a plenum chamber 8 supplied with air through duct 10 for discharge through the primary nozzle 12 and the secondary nozzle 14.

The orifice 16 of the primary nozzle 12 is conveniently made by holding an outer wall 20 and a partition wall 22 in proper converging relationship defined b spacers 18. In practice, these spacers are used every two and one-half inches and are offset upwardly from the orifice 16 as clearly appears in FIG. 3. The nozzle opening 16 preferably extends the full width of the work without any substantial interruption. The walls 20 and 22 desirably converge at approximately 10 degrees or more toward the orifice and the spacers 18 are remote from the orifice 16 by an amount at least equal to one spacer diameter. After passing the spacers, the air stream flowing to the nozzle outlet tends to become rectified so that the jet is virtually uninterrupted across the entire width of the work. The nozzle 12 is designed to project a primary jet of air against the surface of the work in a transverse plane substantially normal thereto.

The nozzle orifice width in relation to its distance from the work is important. An excessively small ratio is wasteful of energy and loses effectiveness whereas an excessively large ratio is also wasteful of energy and requires quantities of air so great as to be impractical. The preferred ratio is 0.10 to 1. Ratios in a range between one extreme of 0.02 to 1, and another extreme of 0.15 to 1 are operative. The objective is to produce maximum impingement velocity with the least expenditure of energy and without excessive flow of air. Increase of distance from the web requires a proportionate increase in nozzle orifice with resulting increase in air requirements if the best operation is to be maintained. A small impingement area is essential to the desired effect for reasons hereinafter explained.

The nozzle cannot be so close to the work that it might be contacted by the moving web, with resultant damage to the work. Neither can the orifice be so small that it will be difficult and costly to make. In practice a spacing of the nozzle from the work at about 0.3 inch is optimum and spacings in a range between 0.2 inch and 1.5 inches are within practical limits.

The secondary nozzle 14 directs a secondary jet at an acute angle to the work as will clearly appear from FIG. 3. The outer wall 26 may constitute a flange on the side wall of housing 6. The inner wall 28 may constitute a flange on the partition 30. The spacers 32 are similar to those shown at 18 in the primary nozzle and they are similarly remote from the nozzle orifice 34. By way of example, and not by way of limitation, this nozzle is shown to be set at an angle of about 20 degrees to the surface of the work and the length of the inner wall 28 of the nozzle is about one and one-fourth inches in practice.

The purpose of the angled nozzle is primarily to block the momentum of flow from nozzle 16 along the surface of the work in what may be characterized in an upstream direction (to the right as viewed in FIG. 3). But for this, air from the primary jet would escape from the exhaust chamber upstream along the web. Also, the cleaning effect of the primary jet would be greatly reduced.

Both the size of Orifice 34 and the angle of impingement are critical. If the angle exceeds degrees, substantial amounts of air will blow out into the ambient atmosphere, carrying much dust with it. If the angle is too small, it will similarly permit flow beneath the secondary jet. This will discharge dust into the ambient atmosphere. In normal practice the angles of nozzle 14 to the work would not exceed about 25 degrees and the minimum practicable angle is about 5 degrees.

The orifice size is important because the momentum of the jet should substantially balance the momentum of flow along the surface from the primary nozzle so that neither is capable of overcoming the other to cause air to blow out of the exhaust chamber 36 as defined by partitions 22, and 38. The range of appropriate sizes for the orifice 34 is from fifty to one hundred percent of the size of the orifice 16 which produces the main jet. A ratio of sixty-seven percent is considered optimum.

An alternate arrangement giving somewhat comparable results is shown in FIG. 4 in which the flange 260 is like flange 26 and the partition 300 simply terminates in properly spaced relation to the flange 260 leaving an orifice at 340. Tack welds (not shown) on two and onehalf inch centers will hold the parts in proper relationship. If the flange 260 is at 22 degrees to the web, the resulting jet will be directed at approximately the preferred angle of 20 degrees, as in the structure shown in FIG. 3.

The exhaust chamber 36 has one or more ducts 40 opening from it. It is not necessary that the air be recirculated but this may be a very desirable procedure, particularly if temperature or moisture or other favorable components should advantageously be preserved. In the preferred system shown in FIG. 2, the discharge ducts 40 are connected by lines 42 with a line 44 which opens into the filter plenum 46. Makeup air is likewise admitted to the plenum 46 through an opening 48 controlled by damper 50.

All such air passes through a deep mat air filter diagrammatically shown at 52 en route to a high pressure blower or other fan 54 having an outlet line 56 leading to the supply duct 10.

A relatively high degree of vacuum in the exhaust chamber 36 in lieu of the secondary jet would overcome any tendency for air and dust to be discharged into the ambient atmosphere but it is impractical to use any substantial degree of vacuum because the resulting pressure differential on the web will cause much difficulty in the handling of the web. In the instant device only very slight vacuum is required to prevent loss of air at the ends of the housing 6. The damper 50 is preferably adjusted to produce only such vacuum as will cause a slight inflow at the ends, an ideal arrangement being one in which the pressures are substantially balanced so that there is neither inflow nor outflow.

The spacing between the primary nozzle 12 and the secondary nozzle 14 is not at all critical. The nozzles should not be so far apart that the energy of flow is materially dissipated before the flow of the respective jets meet each other.

The work is here representd by a web 60 which may be assumed to be moving from right to left as viewed in FIG. 3. While the blower 54 is characterized as a high pressure fan or blower, the velocity of air movement to the respective nozzles is relatively low. High velocity at the jets is achieved by restrictions at the nozzles, 10,000 f.p.m. to 40,000 f.p.m. being contemplated. Only about three and one-half pounds of air pressure in the plenum 8 is required to produce a jet velocity of 40,000 f.p.m.

Since the pressure in the exhaust chamber 36 is very close to ambient pressure, the flow of the main jet from nozzle 12 will divide nearly equally on the surface of the work 60, half flowing to the left (FIG. 3) into the ambient atmosphere and half to the right into the exhaust plenum 36. Since the web is clean after it passes under the center line of the primary jet, the air which blows into the room is clean air and substantially free of dust. The part of the primary jet flow which enters the exhaust chamber 34 will carry substantially all dust from the web. Substantially all flow from the angled secondary jet produced by nozzle 14 Will enter the exhaust chamber 36, along with any of the relatively loose foreign matter picked up by this jet.

As the moving web 60 carries particles of dust into the cleaner, the particles are first subjected to the high velocity jet produced by nozzle 14. The more adherent particles will not be dislodged by such a jet but as the work progresses toward the primary nozzle 12 the particles will encounter progressively increasing velocity and progressively smaller boundary layer thickness until, at the edge of the impingement zone directly beneath the main jet, the boundary layer is almost eliminated and the full impingement velocity of the main jet is flowing parallel to the work at an infinitesimally small distance from the Work. This is shown diagrammatically in FIGS. 5 and 6.

The primary jet 120' issuing from primary nozzle 12 is directed normally with high velocity onto the surface of the work. At the work the jet is divided into two approximately equal streams 68 and 70, flowing at high velocity in opposite directions away from the impingement zone 62 along the surface of the work. The flow continuously expands from the primary nozzle to the work and from the impingement zone to the point of exhaust 72, and progressively loses velocity as the cross section of flow increases.

All real fluids have viscosity which causes fluid particles to cling to surfaces and to each other. Thus when a fluid flows along a surface boundary, the particles in contact with the surface cling to it, and the particles once removed from the surface cling to the particles in contact, and so on. As a result a region of reduced velocity is created adjacent to the surface. This region is called the boundary layer film. Velocities within the film relative to the surface vary from zero at the surface to full stream velocity at the full depth of the film as shown in the local velocity contour on the righthand side of FIG. 6.

The trick in increasing heat transfer or in picking up dust is to find a way to make this sticky viscous film thin so that full stream velocities are brought as near to the surface as possible. Then any particle that happens to be bigger than the thin boundary layer is exposed to high velocity drag trying to lift it along.

In the impingement zone, the surface area equal to the cross section of flow approaching the surface from an impinging jet, there is a pressure created on the surface that is a maximum at the stagnation point 62, and falls to atmospheric at the edge of the zone. I have found that the rate of change of that pressure affects the boundary layer thickness in the zone. Thus it is desirable to obtain as high a pressure gradient as possible. The width of the impingement zone is not the width 74 of the nozzle as shown in FIG. 6, but the width of the impinging flow, which is greater because of jet expansion. As is true of all flow from a nozzle, the jet expands out at an angle of about 7 degrees. Consequently, depending on the spacing of the jet from the work, the resulting impingement zone and the thinned boundary layer areas 64, 66 will be wider than the nozzle.

The thinness of the boundary layer 64, 66 in the impingement zone is important to the success of this web cleaner. Within the impingement zone and starting from the stagnation point of the jet, both the pressure gradient and velocity increase at a rate approximately linearly related to distance from the stagnation point. An approximation of the boundary layer thickness drawn from the Navier- Stokes equations (chapter 9, page 14, Handbook of Fluid Dynamics, 1st ed. (1961)) using the above conditions shows that the thickness is constant in that region and its size depends on the pressure gradient, and the impingement velocity. It is thus clear that the size of the impingement area must be kept as small as possible for any given impingement velocity in order to increase the pressure gradient in the impingement area. It can thus be shown that the boundary layer under an optimum nozzle orifice A" away from the surface develops a thickness 50% thinner than an optimum nozzle orifice l" away and requires only 25% as much air energy in doing so. Beyond a practical maximum distance of 1" the jet is so far diffused as to be relatively ineffective. However, it may be as much as 1 /2" away if air velocity is adequately high, but this is not usually practicable.

The diagram of the boundary layer under the main jet shows schematically the development of the boundary layer. This is greatly magnified for illustration purposes. The actual maximum thickness shown would be in the range of 0.020". The diagram shows clearly the effects occurring in the impingement zone and beyond. The boundary flow shown and existing in fact has a 10W Reynolds number and is therefore laminar. Flow beyond the impingement zone thus corresponds quite closely to the conditions of flat plate flow predicated by Pohlhausen, and as a result the boundary layer thickness increases inversely as the square root of the distance traveled. At some distance, generally in excess of one inch from the impingement zone, the flow eventually becomes turbulent. It is clear, however, that any dust particle moving with the work toward the stagnation point would have great ditficulty in passing through the falling thickness of the boundary layer and finally through the impingement zone.

Velocity of impingement is also important, but only in relation to the type and size of the dust particles and nature of the web surface. For example, large lightweight particles in the range of 100 microns in size (millionths of a meter) on a smooth surface free of electrostatic charge would easily be picked up by using a velocity of 10,000 f.p.m. at the nozzles. On the other hand, fine particles in the range of 10 microns in size on the same surface would require as much as 25,000 f.p.m. nozzle velocity. Whatever the particle size and type, however, the present device is capable of doing the cleaning job many times more efficiently than previous devices and is further capable of cleaning finer particles from the surface of a web than was heretofore possible.

Various modifications are shown in the drawings. In FIG. 7, the primary nozzle 12 and plenum 8 are essentially as shown in FIG. 3. However, the secondary jet has been completely eliminated and in lieu thereof the housing 6 has its wall curved to be substantially parallel at 82 to the web 60, which is here shown to be moving from right to left as in FIG. 3. The movement of the web 60 beneath the closely parallel wall 82 tends to carry ambient air into the exhaust chamber 36 thus serving, to a degree, as a substitute for the secondary nozzle in blocking escape of dust-laden efiluent from the primary nozzle. While the device is not as effective as that above described, it does enable the primary nozzle 12 to achieve some of the advantages of the invention to a substantial degree.

In the embodiment shown in FIG. 8, the primary nozzle 12 remains unchanged and the wall 84 extends directly toward the web '60 and is provided with a terminal brush 86 lightly brushing the web. To the extent that such a brush tends to loosen adherent particles of dust and also to block the escape from the exhaust plenum 36 of air from nozzle 12, the device is reasonably effective.

In FIG. 9 the construction is very closely similar to that of FIG. 8 except that a rotary brush is used at 90, the wall 92 having its lower terminal margin 94 curved around the cylindrical rotary brush toward the web which repre-- sents the work. The rotary brush would tend to block the escape of air even if it were actuated with a peripheral speed corresponding to the linear travel of the web but it preferably is operated in the direction indicated by the arrow 96 and at a speed such that the peripheral speed of the brush materially exceeds the rate of linear web travel.

The embodiment shown in FIG. uses the same nozzle arrangement shown in FIG. 3 but gives normally unnecessary electrostatic assistance for dislodging dust. As the web approaches the cleaner, it passes a wiping plate 98 which imparts an electrostatic charge to particles on the surface of the moving web. In the area between the nozzles 12 and 14, and beneath the web, there is a field plate 100 which has a similar charge. Spaced slightly over the web within the exhaust plenum 36 is a transversely extending electrode 102 that is oppositely charged. The wiring is schematically illustrated.

It is desired to indicate by the various modifications disclosed that the method invention is not limited to the use of any particular physical structure. At the same time, it is desired to emphasize that the various alternatives shown are not by any means intended to exhaust the possibilities for modification. The essential feature of the method is a primary jet having the characteristics above described and preferably used in the specified relation to the moving web and with a secondary jet and with the step of confining and removing the dust-laden air resulting from dislodgement of dust from the boundary layer on the work.

I claim:

1. A method of cleaning dust and other particle material from a longitudinally moving web, said method comprising the steps of directing a transversely elongated gas jet of at least 10,000 f.p.m. against the web in a direction substantially normal to one face thereof so as to interrupt the gaseous boundary layer adjacent to the web, discharging that component of the jet which moves downstream with the web after impingement thereon, directing onto the web and from a position upstream therefrom a second transverse gas jet in a direction at an acute angle to the movement of said web and toward said first-mentioned jet in the direction of web movement, and collecting from the vicinity of the web between the gas jets the gas from the second-mentioned jet and that component of the gas of the first-mentioned jet which travels counter to the direction of web movement, the first mentioned gas jet originating at a distance from the web which is no more than 1.0 inch therefrom and the ratio of its cross sectional area at its origin to the distance of its point of origin from the web being within the range of 0.02 to 1 and 0.15 to 1, and the angle of the second gas jet with respect to the web being within a range of 5 to 25 degrees.

2. A method according to claim 1 in which the first jet originates at about A" from the web.

3. A method according to claim 1 in which the first and second jets are sufiiciently close in the direction of web travel so that each retains substantial energy when the flows of the respective jets meet, the momentum of flow being substantially balanced.

References Cited UNITED STATES PATENTS 1,196,437 8/1916 Doyle 15-306.1 1,658,485 2/1928 Hayward 15307 2,022,593 11/1935 Fuykers 15-345 XR 2,139,628 12/1938 Terry l5316 XR 2,358,334 9/1944 Knowlton 15--309 XR 2,482,781 9/ 1949 Knowlton et al. 15303 XR 2,515,223 7/1950 Hollick 15306.1 XR 2,648,089 8/1953 Mayer 15306.1 2,920,987 1/1960 Landry et a1. 151.5 XR 2,956,301 10/1960 Bruno 15306.1 3,045,273 7/1962 Bruno 15306.1

FOREIGN PATENTS 732,972 7/ 1955 Great Britain.

MORRIS O. WOLK, Primary Examiner.

I. ZATARGA, Assistant Examiner.

US. Cl. X.R. 134-1, 15, 21

Images