GB2466193A - Nozzle - Google Patents

Abstract

An electrostatic discharge nozzle has two concentrically arranged cylindrical portions (2, 3). At least one of the portions (2) has radial thickness that decreases along its axial length to a minimum value near the discharge end.

Description

Electrostatic Discharge Nozzle The invention is concerned with electrostatic discharge nozzles of the type used (for example) in the application of coatings to sheet metals in rolling mifis and the like.

The electrostatic application of coatings to sheet metal in a roping mi is we known.

Thus, the metal is afforded a degree of corrosion protection and, or lubrication which is useful during further processing of the material such as pressing, canning etc. In order to provide a reliable, predictable degree of protection, a homogeneous and even distribution (uniform thickness) of coating material (for example oil) is desireable.

The coating is typically deposited using an applicator (spray nozzle) having an annular spray gap. The present invention is concerned with improvements to such applicators.

In modern rolling mills, the high throughput of metal places heavy demands on the coating applicator systems. For example, Temper and Skin pass mills can typically pass metal with speeds in excess of l500mlminute with coating thicknesses of up to 3.5g!m2.

These demands frequently exceed the capacities of typical coating applicators. For example, a typical applicator might be able to deliver up tol 5cc of coating material per cm of applicator spray length but with a line speed of l000mImin, and a coating thickness of 2g/ m2, the applicator is required to deliver 22cc material per cm of strip.

This causes saturation' of the applicator edge leading to large or heavy droplets (coarse atomisation) of the material which in turn leads to uneven coating of the strip and/or application of a non-homogeneous layer of coating material.

One know approach to solving this problem is to increase the number of applicators arranged to deposit material on the strip but this is constrained by space limitations.

Another approach involves the appcation of an electrostatic charge to the applicator.

Such a charge facilitates atomisation of the oil or other coating material as is exits the applicator to form a fine mist. Again, however, there are practical limits on this solution.

It is known that for a given electric charge imparted to an object of given mass, the most intense electric field is realised where the object tapers to a point. For example, for a charge carrying object having a typical teardrop shape with one end being substantially hemispherical and the other tapering to a point, the intensity of the electric field is greatest around the point.

It is an object of the invention to provide a solution to the problems outlined above.

According to a first aspect of the invention such solution is provided by an applicator for the spray deposition of coatings having the features set out in claim 1 attached hereto.

The invention is described herein, by non-limiting example, with reference to the appended figures in which: figure 1 illustrates the distribution of electric charge in an object having a fine point; figure 2 illustrates a spray nozzle according to the prior art; figure 3 illustrates a typical spray pattern obtained using a nozzle according to the prior art; figure 4 illustrates a spray nozzle according to an embodiment of the invention; figure 5 illustrates a typical spray pattern obtained using a nozzle according to the invention and figure 6 i'lustrates a spray nozz'e according to an a'ternative embodiment of the invention.

The figures are schematic on'y and are not drawn to sca'e. Moreover, certain dimensions, gaps or ang'es may be exaggerated for i'lustrative purposes.

Referring to figure 1, it is known that where a given charge is imparted to an object having a shape which tapers to a point 1, the observed &ectric fie'd is most intense in the region around that point. Without being bound by theory, it is beUeve that this phenomenon arises because unba'anced repu'sion between charge carriers (e.g. e'ectrons) in the region of the point and charge carriers in the body of the object gives rise to a higher concentration of charge carriers around the point.

Referring to figure 2, an e'ectrostatic spray nozz'e according to the prior art inc'udes a so'id cylindrica' inner portion 2, arranged concentricay with a hoow cyindrica outer portion 3. The outer diameter of portion 2 and the inner diameter of portion 3 are s&ected to define an annu'ar gap 4. Coating materia' such as oii is directed to the annular gap via ducts (not shown) and exits as shown at 5.

Portions 2 and 3 are formed of eectricaUy conducting materia' such as metal and there is electricai continuity between the two, the potentiai being appiied between both and ground.

Referring to figure 3, an eectrica potential is app'ied to the prior art device giving rise to an improved spray pattern, whereby the materia' gathers at substantiay even'y spaced cusps 6 around the annu'ar gap and exits the device as a number of streams 7 of atomised materiaL Referring to figure 4, in a particular embodiment of the invention, the inner portion 2 of the applicator has a hollowed region at the end of the device where coating material exits, the hollowed region being shaped such that the inner portion exhibits a progressively decreasing radial thickness from a value (say R2) distal of the annular gap to a minimum value (Rmin) in the region of the annular gap.

By this arrangement, the invention provides a three dimensional (and in this case circular) analogue of the field concentrating point illustrated in figure 2 and the effect of an electrical potential of given value on the spray pattern is enhanced.

Referring to figure 5, the inventors have observed an improved spray pattern when employing an applicator according to the invention whereby the number of cusps 6 and streams 7 are increased, the streams are finer and the spray pattern is narrower. The narrower spray pattern allows for a greater number of applicators per unit area of sheet material, thus further assisting in meeting the coating material flow requirements.

Figure 6 illustrates one possible alternative shape for the inner portion 2. This is another of a large number of possibilities, the essential feature being that the radial thickness of at least one of the portions decreases along the axial length of the device toward the end where material exits. In other embodiments, this feature could be exhibited by the outer portion.

Claims (2)

1. C'aims 1. A nozz'e for appyng matera comprsng: an inner cyindrica portion arranged concentdcay with an outer cyindrica portion to define an annuar gap therebetween, characterized by at east one of the portions having a radia' thickness decreasing a'ong its axia' ength to a minimum va'ue proxima' to an end of the device where materia' exits.
2. 2. A nozz'e according to c'aim 1, where the inner portion has a decreasing radia' thickness.