How do spray droplets behave, or misbehave?

Before you can decide which airblast nozzles to use, you should understand how spray droplets behave. It is the nozzle’s function to meter out the spray solution as droplets, making it more likely to cover a target evenly and accurately. There are a staggering variety of nozzles available, so selecting the right nozzle for the job requires an understanding of how droplets behave (or misbehave). Once the spray leaves the nozzle, the operator has no more control over the application, so it’s important to account for as many contributing factors as possible.

Spray quality

The name, symbol and range of droplet sizes used to describe the median droplet diameter produced by nozzles.

The name, symbol and range of droplet sizes used to describe the median droplet diameter produced by nozzles.

Spray quality is an umbrella term that describes the range of droplet sizes, the number of droplets, and the spray shape emitted by a nozzle. Some also include the rate (e.g. L/min. or gal./min.) in that definition. Nozzles create a variety of spray qualities depending on their design, the operating pressure and the viscosity of what’s being sprayed.

A single, hydraulic nozzle produces a range of droplet sizes, spanning from finer to coarser diameters and measured in microns (µm). North American and European ISO standards use symbols to describe the average (or more accurately, median) droplet size emitted by a nozzle for a given pressure. In North America, these ratings go from “Extremely Fine – XF” to “Ultra Coarse – UC” (There is less range in the original European system). Nozzle manufacturers include this information in their catalogues so an operator knows what to expect from a nozzle. Where the nozzle follows convention, the colour of the actual nozzle itself represents the nozzle rate (e.g. L/min. or gal./min.). For example a “red” flat fan means the nozzle emits 0.4 gal./min of water at 70 °F and 40 psi. Red flat fans are sometimes referred to as “Oh-Four’s” for short.

Unfortunately, droplet size is sometimes associated with a colour. Do not confuse this with the colour of the actual nozzle itself, which represents the nozzle rate (e.g. L/min. or gal./min.)

Unfortunately, droplet size is sometimes associated with a colour. Do not confuse this with the colour of the actual nozzle itself, which represents the nozzle rate (e.g. L/min. or gal./min.)

Finer droplets have a low “settling velocity”, which means they take a long time to fall out of the air. Coarser droplets have a high settling velocity, which means they fall out of the air more quickly. Imagine you are outside, facing into in a light wind and holding a golf ball and a Ping-Pong ball in one hand. The Ping-Pong ball represents a fine droplet, and it has much less mass than the heavier golf ball, which represents the coarse droplet.

Now, toss them into the wind.

The golf ball will follow a simple trajectory, arcing up into the air and falling again predominantly because friction and gravity. The Ping-Pong ball behaves more like a soap bubble – wind, thermals, humidity and many other factors will change where it goes because it is too light to resist them. It may even land behind you, blown by the prevailing wind.

The one to eight rule: Every time the median diameter of spray is doubled, there are eight times fewer droplets. Conversely, every time the median diameter of spray is halved, there are eight times more.

The one to eight rule: Every time the median diameter of spray is doubled, there are eight times fewer droplets. Conversely, every time the median diameter of spray is halved, there are eight times more.

A few years ago I gave a workshop in a nursery. The operator was spraying whips, which are young, tall trees with very few lateral branches; essentially a forest of leafless sticks. He was using a cannon sprayer to spray 30 rows of whips (15 from each side) and he didn’t like the look of the swath blowing through the trees. He decided it was unnecessary to use the air, and instead would rely solely on pressure to propel the droplets. We set up water-sensitive papers throughout the whips and had him spray from both sides with the fan off. The coverage was erratic: Drenched nearest the sprayer followed by an unpredictable array of good, bad and ugly throughout the rest of the trees. We replaced the papers, and asked him to spray again with the fan on, but only from one side (just to prove a point). When we recovered the papers this time, coverage was greatly improved in consistency and in overall quality. In fact, it was night and day compared to spraying with no air, and that was only when spraying from one side! The fate of finer droplets is therefore more difficult to control. This is why sprayer air direction and speed is so critical – the air generated by the airblast sprayer carries the spray to the target before releasing it to deposit in the canopy. Why, then, would anyone elect to spray finer droplets when coarser droplets behave more predictably?

Once again, a metaphor: Imagine the volume a nozzle emits as a cake. No matter how many slices you cut the cake into, you still have the same amount of cake. The finer the slices, the more people can have a slice, albeit not very much. Similarly, the finer the median droplet size emitted by a nozzle, the more droplets are available to provide coverage. In fact, every time the median droplet size doubles, there are 8x fewer droplets created! Now you might be thinking the finer the spray, the better. But, remember that fine droplets are very difficult to control; even though there are more of them, they misbehave and rarely go where you want them to.

These water-sensitive papers were sprayed under controlled conditions and they demonstrate the role droplet size plays in coverage. As the droplets get finer, there are more of them, increasing coverage. However, this is really only hypothetical as many drift off target before impinging. As the droplets get coarser, there are less of them, and coverage may be compromised. To compensate for this, higher volumes are used. Credit – Dr. T. Wolf, Saskatchewan.

These water-sensitive papers were sprayed under controlled conditions and they demonstrate the role droplet size plays in coverage. As the droplets get finer, there are more of them, increasing coverage. However, this is really only hypothetical as many drift off target before impinging. As the droplets get coarser, there are less of them, and coverage may be compromised. To compensate for this, higher volumes are used. Credit – Dr. T. Wolf, Saskatchewan.

Take a breath – It gets better!

What if the target is the underside of a leaf, or an area deep inside in a dense canopy? If coarser droplets behave like golf balls, they move in a single direction unless some other force deflects them. They shatter, they ricochet, and they run off targets. Finer droplets waft and change direction rather easily, and will follow turbulent air behind and under targets if there is sufficient air to carry them there. Finer droplets are also less likely to accumulate to the point that they begin to run off the target.

The net result of all this is that the sprayer operator must find an appropriate balance in the range of droplet sizes. Small enough that there are a lot of drops that can move with sprayer air to deposit all over the canopy, but large enough that they can be directed into the canopy and not drift away if they miss.

Hey, if it was easy, anyone could do it!

How droplets behave (2013).