Pressure Changes Spray Angles

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About Jason Deveau (Spray_Guy)

Dr. Jason Deveau (@spray_guy) has been the OMAFRA Application Technology Specialist since '08. He researches and teaches methods to improve the safe, effective and efficient application of agricultural sprays in specialty crops, field crops and controlled environments. He is the co-administrator of Sprayers101, co-author of the Airblast101 Textbook, a slow cyclist and a slower runner.

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When we consult a nozzle catalogue we are interested in the flow and droplet sizes produced at a given pressure. Perhaps we should also consider the effect of pressure on spray angle. We have several articles discussing the collective impact of spray overlap, nozzle spacing and boom height on coverage uniformity (Check here and here for example). However, we don’t really address the fact that fan angle is not a constant. This may be more relevant with the growing adoption of spot sprayers.

To illustrate the potential for fan angle variation, we assembled a collection of red, flat fan nozzles (‘04s) from several manufacturers. We plugged each nozzle into a spray pattern table, set the regulator at a given pressure, and photographed the spray angle and flow distribution. This process was repeated for each nozzle at seven different pressures within the manufacturer’s approved range of 20-80 psi. After digitizing the photos, we measured the spray angle using a digital protractor.

We anticipated a concomitant increase in spray angle as the pressure increased. This is not news. Anyone who has operated a sprayer has seen the spray pattern open up as the boom fills and pressurizes. Bear in mind this was only performed once (i.e. n=1), so while it illustrates trends it shouldn’t be mistaken for a rigorous scientific comparison. Further, this demonstrates a static situation and not a dynamic one where travel speed, wind conditions and the vortices from the sprayer it self will influence matters.

We saw similar trends with nozzles other than 110˚ fans, but let’s focus on 110˚s due to their popularity.

Fan angles for five common 110 degree AI flat fans over their manufacturer-recommended pressure range
Fan angles for five common 110 degree AI flat fans over their manufacturer-recommended pressure range

The spray angle for 110˚ nozzles ranged from 75˚ at 20 psi to approximately 143˚ at 80 psi. One nozzle failed to reach 110˚ at any pressure. Conversely, there was another that was over 110˚ at nearly all pressures. Ideally, spray nozzles should be operated around the middle of their manufacturer-recommended operating range. Three of the nozzles tested came close to 110˚ at that median pressure, but only the TeeJet AIC110-04 measured 110˚ at the middle of its recommended range (~50 psi).

Using that nozzle as an example, let’s look at the pressure, spray angle and subsequent distribution of flow along the swath at three different pressures. At 20 psi, the spray angle was 85˚. The yellow balls are floats that reflect flow as a series of cross sections of the swath. We see that aside from the tapered edges (which illustrate the need for 100% overlap between neighbouring nozzles) the distribution was fairly even. One of the priorities in nozzle design is to ensure a low coefficient of variability over the operating pressure range. In other words, the length of the swath may change, but the spray quality and uniformity in that swath is still within spec. At 50 psi the nozzle produced the expected 110˚ fan, and the spray distribution remained even. At 80 psi, the angle spread out to 125˚, spanning a greater distance, but it started to produce a less-even distribution.

Photographs of spray angle and distribution for the TeeJet AIC110-04 at the extreme low, middle and highest pressures of its recommended pressure range.
Photographs of spray angle and distribution for the TeeJet AIC110-04 at the extreme low, middle and highest pressures of its recommended pressure range.

When fan angle changes with pressure, it can have significant implications. Nozzle spacing on a boom varies from sprayer to sprayer. Generally 50 cm (20 inch) centres are the standard in North America, but we’ve seen 15″ and even 10″. Nozzle spacing and boom height collectively determine the degree of spray overlap. Excessive overlap isn’t a problem, although additional nozzles do do mean added expense, cleaning time and potential for plugging. Conversely, gaps in the pattern could lead to sub-lethal applications or flat-out misses. For example, in this soybean demo plot (below) we sprayed a contact herbicide at low pressure to collapse the spray pattern. You can see the alternating stripes of hits and misses that resulted from an incomplete overlap of spray.

Soybean demo plot sprayed with a contact herbicide using 110 degree air induction flat fans at 20 psi. The collapsed spray pattern did not overlap sufficiently to burn the entire crop down, leaving a striped pattern and demonstrating the poor coverage.
Soybean demo plot sprayed with a contact herbicide using 110˚ air induction flat fans at 20 psi. The collapsed spray pattern did not overlap sufficiently to burn the entire crop down, leaving a striped pattern and demonstrating the poor coverage.

Nozzle manufacturers generally recommend a 100% spray overlap for flat fans. This creates sufficient overlap when the boom sways low to the ground. It also increases the degree of droplet size homogeneity under the boom as coarser and fewer droplets are generally found at the “horns” or edges of the pattern compared to the centre. In order to ensure this degree of overlap, sprayer operators should observe and consider changes in fan angle over their typical pressure range. Otherwise, the cost of poor deposit uniformity under the boom could be high.

  • Operate nozzles around the middle of the manufacturer-recommended pressure range. However, just because a nozzle is rated over a range of pressures does not mean the angle is constant.
  • Lower pressures are a greater concern than higher pressures. 30 psi is the absolute lowest pressure for operating a 110˚ air induction flat fan; the ideal operating range for these nozzles is 50-70 psi.
  • If nozzles are not maintaining the recommended 100% overlap at your preferred pressure range, then consider switching nozzle rates, and adjusting pressure and boom height.

This work was performed by Victoria Radaukas, 2015 OMAFRA application technology summer student, who also co-wrote this article.