How and why air induction nozzles cut drift – Summer ’19 edition
It’s eight years since the results of the last independent tests on droplet sizes from air induction nozzles were published. With so many new developments on sale, we turn to one of the original researchers to find out how and why they cut drift.
Less than 10% of current spray applications are made using flat fan nozzles. Now, with increasing numbers of air induction nozzles (AI) on the market, it’s time to re-assess nozzle choice and to make informed decisions on what to choose and why.
Clare Butler Ellis, research manager at the Silsoe Applications Unit, says the way nozzle choice influences application performance is complex. “We know a lot about some aspects, but much less about others. Droplet size and velocity, the structure of the spray cloud and individual droplets will affect the quantity of pesticide that reaches the target, the amount that potentially contaminates the environment and the way the product behaves once it’s deposited,” she says.
Images from the Silsoe Applications Unit’s wind tunnel clearly show the difference in drift from the standard nozzle in Pic 1, compared with the 90% drift reduction air induction nozzle, Pic 2.
In the video below, Teejet’s laser clearly shows the drift from a standard 110° flat fan, which is still hanging around when the AI XR is turned on. It’s not only possible to see the large size of the droplets, but also that there is virtually no drift from TTI 90% drift reduction nozzles at the end.
How do air induction nozzles cut drift?
AI nozzles produce larger droplets that are moving slower out of the orifice than a flat fan. While they don’t eliminate all the tiny drops from the spray fan, the volume of liquid in drops that are prone to drift is reduced.
So why don’t we just use a conventional coarse nozzle? Coarse droplets from a flat fan nozzle are unlikely to work as well as current products, because they have too much energy (large size and high velocity) to be easily retained on foliage. Also, flat fan nozzles that give coarse sprays also need high water volumes – 240 litres/ha is quite normal.
Larger AI droplets not only travel slower, but they also contain air bubbles, which act like shock absorbers, which prevent them bouncing off the target.
“They cushion the impact and the droplets sit on the surface,” explains Clare. “But, they do not burst on impact – we don’t think there is any evidence to show that happening.”
AI Droplet sizes differ
Droplet sizes from different nozzles for the same output vary enormously. Tests carried out at Silsoe, published in 2010, show some of the droplets produced by different AI nozzles were twice as large as others – with droplets made by a range of 02 nozzles varying from less than 250 microns up to over 400 microns.
At the time, Dr Paul Miller, who carried out the research, said: “All the evidence shows the differing droplet sizes from AI nozzles do have a significant influence on control. The smaller the target, the more sensitive the amount deposited is to droplet size. Similarly, bigger droplets improve drift reduction,” he explained.
Droplet size comparisons for nozzles can be found on the AHDB nozzle selection chart. Unfortunately this doesn’t include many of the recent developments and is in serious need of updating.
Low pressures for LERAPs
Many AI nozzles carry LERAP 3 star ratings and this is another big reason behind their rise in popularity. However, it’s very important to note, to achieve the LERAP rating some of the nozzles must work at very low pressures – 1 bar to 2 bar is quite normal, explains Clare.
“This is due to the correlation between pressure and drift. Usually as the pressure increases, so does the drift,” she adds. “In practice, operators simply only need to slow down to achieve the correct setting for the LERAPS, because the autorate controller will to drop the pressure automatically.”
It’s important to point out that any drift reduction benefits provided by AI nozzles can all be lost if the boom is just 20cm above the optimum height.