This invention relates to rotary atomisers of the kind which include a rotary dispersion member in the form of a hollow truncated cone having a central region with a peripheral frusto-conical wall inclined outwardly of the central region, the inner surface of the wall being formed with grooves, each of which is symmetrical and extends radially outwardly from the central region to an innermost free edge of the wall. Such a rotary dispersion member will hereinafter be referred to as being of the kind specified. An outermost edge of the wall is usually formed with radially outwardly projecting teeth, each having an apex and a root, the teeth being disposed at the same angular spacing as the grooves and aligned along a radius of the dispersion member, so that an apex of each tooth is in alignment with a base of a corresponding groove.
Rotary atomisers have been used successfully for some years as a way of atomising liquids where it is necessary to control the range of spray droplet sizes produced for greater accuracy in liquid application and/or to reduce the volumes of spray liquid required. Such applications are the atomisation of pesticides (including herbicides) in agriculture, horticulture, and amenity situations, the atomisation of water for humidification/dust suppression and the accurate application of special additives in various applications such as paper making.
Rotary atomisers consist typically of a rotating dispersion member which maybe of a simple flat disc form. The rotary member can be driven by an suitable means such as an air or hydraulic motor, electric motor (A.C. or D.C.), either directly or indirectly via gears or belts and pulleys, a propeller in an air stream, or a turbine. The speed of the rotating member can be in the range 0-100,00 r.p.m. The liquid to be atomised is then fed through a flow calibration orifice onto the rotating disc. The liquid travels to the edge of the disc and centrifuged from it as discrete droplets (either directly or from the break-up of liquid ligaments formed at the disc edge).
One problem with these basic simple forms is that if the edge of the disc is plain, the surface tension of the liquid is sufficient to prevent the liquid leaving the edge in anything other than the random break up of a sheet of liquid if not at equilibrium of liquid flow rate and disc speed. The addition of tooth-like pointed serrations acting as zero issuing point as described in a paper by Bals. `Design of rotary atomisers` (Proc. 4th Int. Agric. Aviat. Congr., Kingston 1969) overcomes this problem. A second problem is one of feeding the spray liquid onto the disc. Ideally the feed should be at the centre of the disc where the angular velocity is at its lowest. As in practice this is normally where the drive shaft is, the feed is normally at some radius from the centre, usually resulting in poor spreading and distribution of the liquid to the disc edge. This problem has been overcome on later practical rotary atomiser designs by pointing the liquid feed towards the centre of the disc and turning the edge of the disc up to make a `frusto-conical` form. In order to ensure that liquid is fed to each tooth as regularly as possible, channels or grooves are added to the inside of the frusto-conical form. The number of teeth is normally the same as the number of grooves and can vary from 2 to 720 or whatever is practical for the disc size which can be from say a few millimetres to 1,000 mm. The design of this type of disc (frusto-conical with teeth and grooves) is described in U.K. Patent No. 1515511.
Whilst these discs made significant advances in controlling rotary atomiser droplet production, observation showed that on early designs when the liquid reached the end of the groove, because it had to turn a corner and cross a plain section of disc to reach the tooth tip, it could be disturbed by air movement caused by rotational speed and hence the stream of liquid was sometimes split and when it rejoined it as `corkscrewing` with subsequent deterioration of droplet spectrum. This phenomenon is illustrated in FIGS. 1 and 2 which show a groove 10, angle .alpha. through which liquid has to turn, liquid path 11, tooth 12 and liquid corkscrews 13 giving droplets 14.
This is overcome as described in U.K. Patent No. 2026904, which shows the interlocking of teeth and grooves and hence the shortening of the path from the end of groove to the tooth tip. In another invention as described in U.K. Patent No. 2004204 an attempt was made to ensure liquid contact from groove to tooth by means of an asymmetric tooth form. This however, suffers from the problem that it is a uni-directional rotary atomiser.
In recent years in the area where rotary atomisers are used there has been a move towards higher feed rates onto discs. This has resulted in another problem as illustrated in FIG. 3, which shows a groove 10, a liquid path 15 therein, and a body 16 of the atomiser. As can be seen, only part 17 of the stream of liquid remains in contact with the disc at the point where it leaves the groove and turns through the angle to reach the tooth tip. Another part 18 of the liquid stream splits off and emerges directly from the groove possibly resulting in contamination of the atomiser body and consequent dripping of spray liquid.