US2764712A - Apparatus for electrostatically atomizing liquid - Google Patents

Description

' Sept. 25. 1956 J= w. JuvmAu.

APPARATUS FOR 'ELECTROSTATICALLY fi'romzmc LIQUID Filed May 31. 1951 INVEN TOR. JAMES W. JUVINALL United States Patent APPARATUS FOR ELECTROSTATICALLY ATOMIZING LIQUID James W. Juvinall,. Indianapolis, Ind., assignor to Ransburg- Electra-Coating Corp., Indianapolis, Ind., a corporation of Indiana Application May 31, 1951, Serial: No..229,247

9 Claims. (Cl. 31-7-3) .This invention relates to electrostatic atomizing methods and devices adapted to atomize liquids into finely divided particles and: project such particles in the that general type are broadly disclosed in the co-pending application of Edwin M.. Ransburg, Serial No. 143,994, filed February 13, 1950.

In order to obtain the most satisfactory results in atomizing liquid fromv the edge of a maintained liquid film, certain conditions must exist.. Among those conditions are a substantially uniform film-thickness at the edge of the film and a substantially uniform field-strength along such edge. In addition, the velocity with which the film approaches the site of atomization should be uniform along the edge and should not vary from time to time. A thin-edged film is preferable to a thick-edged film, and the velocity at which the film approaches the edge should therefore be relatively high. However, any accelerating effort applied-t0 the film to increase its velocity' of flow is preferably kept below a value which would cause the film to rupture.

It is therefore an object of this invention to produce an electrostatic atomizinghead of the rotating-typewhich will provide a thin, free-surfaced film of substantially uniform thickness and cause such film to flow at asatisfactory, stable rate toward its annular edge, where atomization takes place. A further object of the invention, is to produce a rotating head which will be compact, which can be simply and economically manufactured, and which will lend itself tothe maintenance of anadequate field strength adjacent its edge., Another object of the invention is to provide a simplified head-design. possessing standard characteristics which, can be; embodied in heads of different diameters and which will impart to those heads thev advantages previously mentioned' In. accordance with my invention, I provide-a generally cup-like atomizing head mounted for rotation about its axis, provided, at its center with an orifice through which it receives the liquid to be atomized, and having an internal surface which flares toward the periphery of the head, where the annularedge of the liquid film formed and distributed on such internal, surface by the: action of centrifugal force is presented v to the electrostatic field. The flare of the internal, film-supporting head surface adjacent the annular filmedge isdesirably comparatively small, preferably not exceeding, 45 in included. angle; although, depending on head-diameter and rotational speed, flare-angles (included). up to 90 or even greater may be used. In heads of relatively small. diameter, the flare may be constant from. adjacent the central orifice 2 to the periphery of the film-supporting surface; but in heads of relatively large diameter, the inner surface of the head may include an intermediate zone of much greater flare. The inner surface of the head blends smoothly with the walls of the central orifice, and any zones of different flare blend smoothly with each other in order that the liquid forming the film may flow from 'the orifice. to the periphery of the head without undergoing any abrupt change of direction which might cause ripples or eddies.

In the accompanying drawing, which illustrates the invention, Figs. 1, 2, and 3 are, axial sections through electrostatic atomizing heads of different diameter; and Fig. 4 is an elevation, illustrating atomizing apparatus embodying two heads of dilferentdiameter.

The head illustrated in Fig. 1 is one of relatively large diameter and embodies a central hub portion 10: and a cup-like body portion 11 concentric with such hub portion. The hub portion 10 has a central bore adapted to receive the. end of a rotatable shaft 12 to which the head is secured by means of one or more set screws 13. At the base or end of the bore in the hub 10, the head is provided with a central orifice 15 communicating with an axial passage 16 in the shaft 12.

With the head fixed ot the shaft 12 and with the shaft and head rotating at a sufficient speed, liquid supplied to the interior of the head through the passage 16 and orifice 15 will spread out over the inner surface of the head under the influence of centrifugal force and form a filmflowingtoward the peripheral margin of the inner head-surface. If an electrostatic field exists at the annular edge of the head, the edge of the flowing film will be broken up into fine, particles, and those particles will be projected from the head, such breaking up and projection occurring as the result of the: electrostatic forces of the field. To maintain the necessaryelectrostatic field at. the periphery of the head, I may employa highvoltage source 18 having one terminal grounded and its other terminal electrically connected to the shaft 12. In this manner, a high potential is applied tothe-liquid film on the inner surface ofthe head (and to-the head itself if it is of conducting material) and an electrostatic field will exist along the peripheral edge of the liquid film. Such field may be intensified and controlled in form, if desired, by disposing in opposed spaced relationship to the head and in line therewith an. electrode which is grounded or which is, at least maintained at a substantial potential difference relative to the liquid film. If the atomizer is used in the electrostatic application of liquid coatings, such oppositely disposed electrode may be an article-to-be-coated. Where an oppositely disposed electrode is used and a substantial: potential diiferenceis maintained between it and: the head, either or neither of the head and electrode may be grounded, and either may be at a positive potential relative to the other.

The annular edge 20 of the head marks the outer terminus of a frustoconical'delivery zone 21 having a relativelysmall flare or taper. Where the head is of; such diameter that it would be ofundue axial length if the delivery zone continued inwardly at substantially uniform flare or taper until it met the-walls of'the orifice 15, the interior of the head may be-formedto provide: a feeding zone 22 having a flare or taper materially greater. than that. of the delivery zone 21; Where the interior surface of, the head comprises a: delivery zone, 21' and a feeding zone 22 of materially different taper, those two zones are interconnected by an outer connecting zone 23 which is tangent to the zones 21 and 20 and which, in axial section, has a substantial: radiusof curvature A. so. that the liquidflowingfrorn the-feeding Zone to the delivery vZQne will not bev subjected; to any abrupt; changes of direction.

At the base of the head, a second, or inner connecting zone 24 is provided to connect the feeding zone 22 with the walls of the orifice 15. The inner connecting zone 24 is tangent to the orifice wall and to the feeding zone 22 and desirably has in axial section a substantial radius of curvature B.

The head illustrated in Fig. 1 has proportions found suitable for a head of inches diameter and 3 /2 inches depth measured along the axis from the orifice to the plane of the edge 20. The delivery zone 21 has an included angle of 30, the feeding zone 22 has an include-cl angle of 150, the radius A is 1 /2 inches and the radius B /3 inch. Rotated at about 900 R. P. M. and maintained at an electrical potential of 100,000 volts relative to ground or to an opposed electrode spaced 8 to 12 inches in front of it, such a head is capable of reducing to a fine state of atomization about ten fluid ounces of liquid per minute. At some sacrifice in respect to the fineness of the atomization produced, liquid may be atomized at several times that rate. Where an opposed, oppositely charged electrode is employed, the fine liquid particles created by the electrostatic atomization which occurs at the periphery will form a well defined spray and will be attracted to the oppositely charged electrode.

The head described possesses the flow-creating and flow-directing characteristics necessary to satisfactory atomization. Because of the substantial radius B, the liquid emerging from the orifice 15 is not carried out of contact with the inner surface of the head as a result of its axial momentum; but instead, such liquid, by virtue of the centrifugal force acting upon it, tends to hug the inner surface of the head at the inner connecting zone 24. As the liquid flows outwardly over the inner connecting zone 24 and the feeding zone 22 the centrifugal force acting on it increases and the velocity of liquid flow toward the edge accordingly increases. In Fig. 1 I have indicated force triangles resolving into components parallel and perpendicular to the adjacent surface the centrifugal forces acting on liquid particles and 26 located respectively at an intermediate point of the feeding zone 22 and at a point adjacent the outer edge of the delivery zone 21. It will be clear from those triangles that although the centrifugal force acting on the particle in the delivery zone is greater than that acting on the particle in the feeding zone the component of centrifugal force parallel with the surface is materially greater for the particle 25 in the feeding zone than it is for the particle 26 in the delivery zone. Thus, the liquid flowing over the feeding zone accelerates rapidly while that flowing over the delivery zone tends to accelerate less rapidly. When frictional forces are taken into account, there may be substantially no acceleration of liquid flowing over the delivery zone. The relatively large component of centrifugal force normal to the surface of liquid in the delivery zone promotes uniform film thickness throughout the circumferential extent of such zone and thus promotes uniform atomization at the edge 20.

It will be obvious that it is possible to vary the diameter of the head within wide limits, maintaining the same head-length, the same radii A and B, and the same tapers or flares of the delivery zone 21 and the feeding zone 22. With those constants maintained, an increase in head diameter would result in an increase in the width of the feeding zone 22 and a decrease in the width of the delivery zone 21, while a decrease in head diameter would entail a reduction in the width of the feeding zone 22 and an increase in the width of the delivery zone 21. It is generally desirable that the head diameter be not increased to the point where the delivery zone 21 disappears. It is also generally desirable that the included angle of the delivery zone be relatively small, and preferably not greater than 45.

If, while maintaining the same respective values for the radii A and B and the flares of the delivery and feeding zones 21 and 22, the head diameter is progressively reduced, a point is reached where the feeding zone 22, as a well defined surface of substantially constant flare, disappears, and the connecting zones 23 and 24 become directly tangent to each other. Such a head is illustrated in Fig. 2. That head, which has the proportions of one four inches in diameter, has the same axial length as the head of Fig. 1, its delivery zone 31 has the same flare as the delivery zone 21 of the head shown in Fig. 1, and the radii of the outer and inner connecting zones 33 and 34 are the same as the corresponding radii in Fig. l; but because of the reduction in head diameter, the feeding zone 22 has disappeared and the two connecting zones are directly tangent to each other. The head of Fig. 2 possesses the same property as the head of Fig. 1 in that the component of centrifugal force parallel to the surface has its greatest value at a point located inwardly of the head from the rim or edge.

In Fig. 3, which illustrates the proportions suitable for a head of 2 inches in diameter, the outer connecting zone has disappeared; and the delivery zone 41, which has the same flare as the delivery zone 21 of Fig. 1, joins the wall of the orifice through a narrow connecting zone 44 having an axial section of the same radius as have the connecting zones 24 and 34 in Figs. 1 and 2. In this head, the flow-creating component of centrifugal force has its maximum value at the periphery of the head.

The rim of the head, whatever its diameter, desirably has a relatively sharp edge. If the delivery zone 21 flares at an included angle of 30 and if the exterior of the head is made cylindrical adjacent to the edge, the included angle between the inner and outer surfaces of the head-rim will be 15, an angle which has been found very satisfactory in practice. The edge need not be of knifelike sharpness, and is desirably rounded off on a radius of a few thousandths of an inch to lessen the likelihood of injury to personnel and damage to the head.

All the heads shown in the drawing possess the capability of converting the relatively thick stream flowing through the central orifice into an annularly extended thin film the leading edge of which is presented to the action of an electrostatic field for atomization. The film edge is of substantially uniform thickness throughout its circumferential extent and flows to the edge at a stable rate. The relatively slight taper or flare of the delivery zone, where the film is thin, promotes uniform film-thickness by respectively reducing and increasing the components of centrifugal force parallel and normal to the film surface. The form of the head provides a concentration of field strength at the peripheral edge, where atomization is to occur.

In the larger diameter heads of Figs. 1 and 2, the increased flare-angle which the inner surface of the head possesses inwardly of the delivery zone reduces the axial extent of the head and thus conserves space. In addition, it possesses an advantage in promoting smooth, uniform flow of liquid to the periphery of the head. In general, the rate of liquid-delivery through the central orifice will be proportionate to the circumference of the head. If the flare of the inner head surface adjacent the orifice and beyond were the same in large heads as in small heads, the inner portions of the liquid film would be materially thicker in large heads than in small ones. Since the liquid of the film, in flowing generally outwardly, traverses zone-portions of ever-increasing linear velocity, and since increased velocity can be imparted to the liquid only by virtue of friction at the inner headsurface, thick films are to be avoided. That is, if the film is unduly thick, undersirable turbulenec may exist within it as a result of differences of the velocities, both tangential and in the feeding direction, possessed respectively by liquid at the film surface and liquid in contact with the inner head-surface. The increased flare possessed by the inner portions of larger heads embodying my invention results in a relatively quick spreading and thinning of the film; and hence the film is rapidly reduced to a thickness such that differences in velocity between its surface and inner portions will not be great enough to cause objectionable turbulence. At the same time, because the preferred form of even the larger-diameter heads possess no portions normal to the head-axis, the inner head-surface always provides a measure of support for the liquid, reduces the feeding component of centrifugal force, and tends to prevent rupture of the film under the influence of such force.

Heads of the type above described are especially advantageous when used in apparatus where a plurality of the heads are arranged to produce atomization in a common plane or where heads of different diameters are to be used alternatively and it is desired to maintain a fixed axial distance between the rim of the head and a fixed point on the shaft which supports it. To illustrate this, I have shown in Fig. 4 an apparatus embodying two heads 50 and 51 respectively corresponding in proportions to the heads shown in Figs. 1 and 2. The heads 50 and 51 are secured to shafts 52 and 53 rotatably supported from a common support 54. The shafts have their head-supporting ends located in a common plane, both heads have the same axial dimension, and the rims of the heads are therefore located in a common plane. The heads can be interchanged on the shafts 52 and 53, and their peripheral edges will still lie in the same common plane. Atomizing apparatus embodying two or more rotating heads carried by a common support, as indicated in Fig. 4, is frequently used in electrostatic application of liquid coating materials where the desired distribution of coating material over the surface being coated is facilitated by the use of heads of difierent diameter.

I claim as my invention:

1. In electrostatic atomizing apparatus, a rotatable atomizing head having a concentric cavity flaring to a peripheral edge, means for supplying liquid to the apex of said cavity, means for rotating the head to cause liquid so supplied to be formed under the influence of centrifugal force into a free-surfaced film covering the surface of the cavity and flowing toward said edge, and means including a high-voltage source electrically connected to said film for maintaining an atomizing electrostatic field adjacent said edge, the wall of said cavity at an annular delivery zone adjacent said edge flaring at an included angle no greater than 45 2. The invention set forth in claim 1 with the addition that inwardly from said delivery zone the cavity-wall possesses a flaring feeding zone having an included angle of flare materially greater than that of the delivery zone.

3. The invention set forth in claim 1 with the addition that said delivery zone constitutes the inner surface of a rim portion on said head, said rim portion being of decreasing thickness toward said edge with an included angle between the inner and outer surfaces of said head of not more than 15 and with said edge rounded on a radius of not more than a few thousanths of an inch.

4. The invention set forth in claim 2 with the addition that said delivery zone constitutes the inner surface of a rim portion on said head, said rim portion being of decreasing thickness toward said edge with an included angle between the inner and outer surfaces of said head of not more than 15 and with said edge rounded off on a radius of not more than a few thousanths of an inch.

5. In electrostatic atomizing apparatus, a rotatable atomizing head having a concentric cavity flaring to a peripheral edge from an axial orifice in the head, means for causing liquid to flow through said orifice into the head-cavity, means for rotating the head to cause liquid so supplied to be formed under the influence of centrifugal force into a free-surfaced film covering the entire surface of the cavity and flowing toward said edge, and means including a high-voltage source electrically connected to said film for maintaining an atomizing electrostatic field adjacent said edge, the edge of said orifice being rounded on a substantial radius to provide a smooth connecting zone tangent to the walls of the cavity and orifice.

6. The invention set forth in claim 1 with the addition of a rotatable shaft to one end of which said head is secured, said shaft having an axial, liquid-supply passage communicating with said cavity at the apex thereof.

7. The invention set forth in claim -1 with the addition that the means for supplying liquid to the apex of said cavity comprises an axial orifice whose entranceway into said cavity is smoothly rounded on a substantial radius to provide a smooth connecting zone between said orifice and cavity.

8. In electrostatic atomizing apparatus, a pair of parallel, rotatable shafts having ends located in a common plane, atomizing heads respectively secured to such shaftends, said heads having cavities respectively concentric with their associated shafts and flaring to peripheral edges of different diameters and located in a common plane, means for supplying liquid to each of said cavities, means for rotating said shafts and heads to cause the liquid supplied to each cavity to be formed under the influence of centrifugal force into a free-surfaced film covering the surface of the cavity and flowing to the peripheral edge thereof, and means including a high-voltage source electrically connected to said film for maintaining .an atomizing electrostatic field adjacent the peripheral edges of the cavities, the walls of the cavities having adjacent such edges delivery zones flaring at substantially the same included angle.

9. In electrostatic atomizing apparatus, a rotatable atomizing head having a concentric cavity flaring to a peripheral edge, a rotatable shaft to one end of which said head is secured, said shaft having an axial liquidsupply passage communicating with said cavity at the apex thereof, means for supplying liquid to said passage (for flow therethrough to said cavity, means for rotating the head to cause liquid so supplied to be formed under the influence of centrifugal force into a film on the surface of the cavity and flowing toward said edge, and means including a high-voltage source electrically connected to said film for maintaining an atomizing electrostatic field adjacent said edge, the flare of the film-engaging Wall of said cavity being substantially less in a first zone adjacent said edge than in a second zone nearer the cavityapex, said film-engaging cavity wall having a smoothly radiused connecting zone between said first and second zones and tangent to each thereof.

References Cited in the file of this patent UNITED STATES PATENTS 656,774 Moore Aug. 28, 1900 1,832,096 Chalfee Nov. 17, 1931 1,861,475 Hopkins June 7, 1932 1,958,406 Darrah May 15, 1934 2,049,940 Barthel Aug. 4, 1936 2,302,289 Bramston-Cook Nov. 17, 1942 2,357,355 Penney Sept. 5, 1944 2,369,216 Crisp Feb. 13, 1945 2,466,906 Miller Apr. 12, 1949 2,526,178 Weber Oct. 17, 1950 OTHER REFERENCES R-ansburg Coating Processes, Ransburg Company. Received in Patent Oflice October 30, 1947.

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