Since ultrasonic spray nozzles deliver such a soft, low-velocity
spray, the spray envelope produced may not be suitable for a
particular application without further shaping. In many applications,
such as coating blood collection tubes, dispensing fragrances
onto non-woven fabrics, and introducing chemicals into reaction
chambers, the soft spray is perfectly suited "as is."
However, for other applications, such as coating wide substrates,
focusing the pattern into a very narrow, well-defined line, or
producing a pattern with precise outlines, auxiliary means for
spray shaping must be employed.
To produce wide sprays from a single nozzle, the nozzle is mounted within a specially designed air-handler that uses low-velocity air to both shear the spray to the desired width, and propel it in the desired direction in a uniform wedge-shaped pattern. Uniform spray patterns up to 20 inches wide can be produced with this type of equipment. One type of air-handling device used to accomplish this is shown below.
 |
The jet block is an acetal plastic part, approximately 4" wide, that serves the dual purpose of holding the nozzle and providing two controllable jets of air that are directed toward the spray as it emerges from the nozzle.
The jets shear the spray, moving the plume outward in a symmetric fashion. The velocity of the jet air, which is controllable, determines the width of the ultimate pattern. Obviously, the higher the velocity, the wider the pattern.
Notice that the jet directed air is not perpendicular to the stream as it emerges from the nozzle, but rather is angled such that spray pattern has forward momentum. The reason for this is so that the pattern can reach the next spray shaping device, referred to as a an air guide. This device is essentially an adaptation of an air knife, providing a fan-shaped sheet of air in a plane perpendicular to the plane of the impinging spray pattern. The velocity of this airsteam is generally held fixed. The drops of liquid are readily entrained by the air sheet, and are propelled toward the target substrate in a uniform mist. Since the air guide produces a fan-shaped pattern, the resultant spray pattern is wider than that of the impinging spray produced by the jet block.
In this way, uniform patterns of spray can be generated, spanning a width range of 2 - 20". This air-handling system is insensitive to orientation. The spray can be directed upwards, downwards, or horizontally with equally good results.
The technique used to generate narrow-line patterns, or other types of patterns with well-defined edges consists of surrounding the nozzle's atomizing surface with a housing that injects a stream of low-pressure air into, or immediately around the spray, parallel to the flow direction of the spray. The diagram below depicts the general method used.
Using auxiliary air in order to shape the spray does increase drop velocity above that produced by the ultrasonic atomization process. However, the spray is still very gentle compared with pressure sprays.
Click here for more information on ordering the new book, Ultrasonic
Atomization, Theory and Application.
