Air Induction Nozzles for Airblast

Air induction (aka Venturi) nozzles create coarser droplets that are less prone to spray drift compared to the finer droplets produced by conventional hollow cone nozzles. Spray settles more quickly, producing a briefer and less obvious plume over the operation. This is good when there are bystanders but it makes shoulder-check assessments more difficult for the sprayer operator.

Note the difference in the visibility of the spray swath when spraying coarser (left) versus finer (right) droplets.
Note the difference in the visibility of the spray swath when spraying coarser (left) versus finer (right) droplets.

Beyond my own experience, researchers in Quebec performed a literature review of more than 30 peer-reviewed, international studies and concluded that applications from air induction hollow cones are as efficacious as conventional hollow cones. Translated from French, they conclude: “What is important to note is that anti-drift nozzles could be used in the same operating conditions than conventional nozzles, with no change in airflow and without change of application rate.”

There are, however, some conditions where they may not be a good fit:

  • When the canopy is very dense (e.g. un-pruned trees or grapes with a closed canopy) Coarse droplets don’t penetrate as well. Canopy management is a must.
  • When the canopy is very wide, Coarse droplets lose momentum and fall out of the air more quickly than finer droplets, and that can affect penetration depth. This is typically a concern in orchards, or operations where multiple rows are sprayed with one pass.
  • If the operator is already using a low volume (e.g. L/ha or gal./ac), there may not be enough droplets to provide sufficient coverage – especially when disease pressure is high.
Air induction nozzles on a Turbomist. On the left are conventional flat fans. On the right is the same rate using air induction flat fans. When this image was captured, the sprayer operator apologized to me for forgetting to turn on the right-hand boom – he could not see the coarse spray from the cab!
Air induction nozzles on a Turbomist. On the left are conventional flat fans. On the right is the same rate using air induction flat fans. When this image was captured, the sprayer operator apologised to me for forgetting to turn on the right-hand boom – he could not see the Coarse spray from the cab!

These are extreme situations, and failures are rarely reported as a result of using air induction nozzles. When used with sufficient volume and on a reasonable canopy, spray distribution can be excellent. Always confirm coverage with water-sensitive paper when you make a change to your sprayer.

Be aware: A red hollow cone air induction tip does not have the same flow rate as a conventional red hollow cone nozzle – they are not (yet) standardized in North America. Always consult nozzle tables.

Here are a few emerging uses for air induction nozzles on airblast sprayers in Ontario, Canada:

  • Use air induction nozzles in the top two positions on each side of the sprayer, and conventional moulded hollow cones in the remaining positions. Given that the top nozzles are often the biggest contributors to off-target movement of spray, the larger droplets produced by the air induction nozzles tend to fall out of the air rather than drift away. Further, the ballistic movement of coarser droplets tends to keep them moving in a straight line after leaving the nozzle, helping to direct them to the tops of trees.
  • Try replacing the conventional nozzles in any tower “dead zones” with air induction nozzles; coverage should improve in that zone because pressure propels coarser droplets further than finer droplets. We’ve seen significant improvements using this technique in high density orchards.

The spray quality produced by an air induction nozzle is affected the by air speed and the orientation of the nozzles relative to the air outlet. Coverage patterns on water-sensitive paper indicate that nozzles located outside but aimed into the air stream (e.g. Turbomist) result in smaller droplets then nozzles located inside and aimed parallel with the air stream (e.g. Durand-Wayland).

This is likely due to air-shear, which is known to “shred” spray as the angle between the nozzle and the entraining air stream becomes more acute. The effect may be more pronounced due to the delicate nature of with the air-filled droplets created by air induction nozzles; It is unlikely they would remain intact given the shear forces generated by the airblast sprayer.

Reaching the tops of 16-20 foot hop plants on 7 by 7 foot spacing can be a tall order This Rears sprayer is doing a good job of it, but would air induction in the top positions help? Photo Credit – D. Groenendale, Washington.
Reaching the tops of 16-20 ft high hops on 7×7 ft spacing can be a tall order. This Rears sprayer is doing a good job of it. Would air induction in the top positions help or hinder coverage? Photo Credit – D. Groenendale, Washington.
The more acute the angle between air and spray, the more air-shear. This results in higher numbers of finer droplets.
The more acute the angle between air and spray, the more air-shear is generated. This results in higher numbers of finer droplets.
Left: water-sensitive paper sprayed with an air induction nozzle located within the air stream. Right: water-sensitive paper sprayed with an air induction nozzle outside the air stream. This comparison may be exaggerated because the left paper was from a higher volume spray and less-dense canopy than the right. However, the droplets captured by the papers are always finer when nozzles are positioned outside the entraining air.
Left: water-sensitive paper sprayed with an air induction nozzle located within the air stream. Right: water-sensitive paper sprayed with an air induction nozzle outside the air stream. This comparison may be exaggerated because the left paper was from a higher volume spray and less-dense canopy than the right. However, the droplets captured by the papers are always finer when nozzles are positioned outside the entraining air.

A Real World Example

It was spring when we calibrated an older FMC with a 24 inch diameter fan in low gear. We were spraying mature high-density Royal Gala at tight cluster. The operator was travelling at 5.5 km/h spraying ~400 L/ha. This sprayer operator runs his sprayer at the lowest operating pressure I’ve ever encountered on an airblast sprayer: 60 psi. It was 10:00, the wind was a light 2 km/h, relative humidity was ~55% and the temperature was ~21 °C: beautiful conditions for spraying.

Two water-sensitive papers were clipped back-to-back, facing the alleys at the top of an 11’ tree. The nozzle in the top position was a TeeJet TXVK 18 (0.24 gal/min. @ 60 psi). The coverage was good, but given the light wind and the fact that it was early spring and the paper was at the top of the tree, it didn’t make me confident. Such small droplets could easily be blown off-course as the wind picked up.

We decided to install an air-induction hollow cone in that position. The TeeJet AITX 8002 emitted the same rate (0.247 gal./min/ @ 60 psi), but sprayed fewer and larger droplets. When the papers were recovered, we decided to go up one more nozzle size to increase the droplet count. We used the TeeJet AITX 80025 (0.3 gal./min. @ 60 psi). Later, we digitized the water-sensitive papers using the DepositScan software developed by Dr. Heping Zhu (USDA ARS in Ohio). The results are presented in the adjacent picture: the resultant coverage was exactly as we suspected.

The tree-top coverage obtained using three different nozzles in the top position during a spring calibration. Note that percent coverage and droplet density generally increase with nozzle rate, but moving to an air induction nozzle requires higher rates to compensate for reduced droplet counts.
The tree-top coverage obtained using three different nozzles in the top position during a spring calibration. Note that percent coverage and droplet density generally increase with nozzle rate, but moving to an air induction nozzle requires higher rates to compensate for reduced droplet counts.

One side is nozzled with conventional nozzles (fine/medium droplet size). The other with air induction flat fans at the same rate (coarse droplet size). Can you guess which is which? (2009)

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