Nozzles for Coating Applications
Engineering Consideration - Uniformity of spray and droplet size
Generally costing is achieved by using flat fan or full cone spray patterns. Flat fans will be used to coat a target that is moved on a conveyor with full cone nozzles being used to coat static targets. There are a number of different designs of nozzle that can achieve each pattern and some produce more even coverage than others.
Uniformity of spray
All major designs of flat fan nozzles produce a uniform density of spray with the exception of the very ends of the spray, where less fluid is delivered but with knowledge of this adjustments can be made to overlap the sprays to give very even distribution. A simple example is the tapering of spray at the edges of a flat fan pattern.
Nozzles such as the TF spirals produce concentric rings of spray to make up a full cone pattern. This property is very useful in certain applications, such as fire fighting, but is a problem for coating applications as the spray is not uniform. If a full cone pattern is required for the coating application (perhaps if the target ca not be moved through a spray bar for some reason) then axial whirl designs of nozzle will produce a more even full cone pattern.
Droplet size
Some coating applications will necessitate a relatively fine spray. As a general rule of thumb the finer the spray is atomised the more uniform the spray will be and so, assuming an even coating is required, finely atomised sprays are generally desired for coating applications. This is particularly important for very light coatings.
As can be seen from the diagram to the right the same volume of fluid will form a far more evenly distributed coating if it is more finely atomised. The smaller droplets will also cover a much larger area.
Perhaps just as importantly as mean droplet size is uniformity of the sizes of droplets in the spray. All spray nozzles produce a variety of different droplet sizes but some produce a more narrow range than other. The relative span is a measure of how wide a spectrum of droplet sizes is produced by a spray nozzle. For coating applications nozzles with a low relative span are desirable as they will produce far more evenly distributed sprays.
Summary
Air atomising nozzles have low relative spans, very small droplets and low flow rates so are ideal for delicate coating applications.
Flat fan direct pressure nozzles have average relative spans and product fairly coarse droplets. This means they are acceptable for faily heavy coatings but for refined light coatings the large droplet sizes may be problematic.
Axial whirl nozzles produce even full cone patterns with medium droplet size this makes them acceptable for moderate to heavy coating applications but the larger droplet sizes may cause problems for very delicate coatings.