One of the pleasures of working in agricultural extension is when you’re able to help a grower solve a problem. This was one of those happy occasions. An orchardist recently purchased a Lipco multi-row recycling sprayer and he wanted help diagnosing his spray coverage.
We set water-sensitive paper along the trunk of his 12′, mature, high-density Royal Gala trees. The sprayer was driving at 5.0 km/h, operating at 11 bar using orange Albuz 80 degree air-induction flat fans. This resulted in about 350 L/ha, which was perhaps a little low-volume but not unreasonable because of the efficiency of shrouded sprayers.
The large shrouds behind each vertical boom encasing the tree row, blocking any cross-wind. For those sprayers with the air-assist option (preferred), a vertical plane of air entrains the spray and carries it into the centre of the tree canopy. Whatever spray blows through the tree hits the opposing shroud and is recycled back to the tank. All in all, how could you miss? (see Figure 1).
…we managed to.
Water sensitive papers were placed back-to-back facing each alley (in other words, facing the spray booms). Despite our best efforts, each pass resulted in inconsistent coverage. Papers were replaced in the same location and orientation for each pass and no settings were changed. Nevertheless, sometimes a paper got spray and sometimes it didn’t. What was going on?
That’s when I remembered a paper written by Quebec’s Dr. B. Panneton et al. about the direction of air streams relative to grape panels (download here). In essence, they noted that two air streams in direct opposition cancelled each other out (see Figure 2, configuration 1). They found the best coverage was achieved when the opposing air streams intersected at angles rather than meeting head-to-head (see Figure 2, configuration 2).
We decided to turn one of the boom/shroud/fan assemblies 15˚ by loosening the four bolts at the top of the gantry (see Figure 3). Now one side of the multi-row sprayer was adjusted to look like Figure 2, configuration 2. We left the other side in opposition as in Figure 2, configuration 1.
We replaced the water sensitive papers and ran another pass. The operator later told me he could see the leaves and branches rustling in the row where we made the adjustment, but not in the unadjusted row. The result on water-sensitive paper was dramatic (see Figure 4).
If adjusting one boom/shroud/fan assembly did that, what could we do with the other three? We didn’t like configuration 4 in Figure 2 because it might channel air into tree row and suck the spray out the back. We settled on configuration 3. This would separate the air streams and might encourage a vortex to form in the centre of the canopy (in theory). Additionally, it facilitated the sprayer turning and aligning with each tree row and minimized the chance of physically contacting the trees.
As a related departure, a few years after this experience I encountered another vertical boom sprayer with coverage issues. In this case the flat fan nozzles had swiveled in the nozzle caps, creating gaps. Quarter-turn caps are designed to align flat fans on a boom to prevent this problem, but mismatches between cap and nozzle can allow this to happen.
Since writing this article in 2013, I have been told that the Lipco instruction manual advises against air in direct opposition. It was a poorly translated and somewhat obscure sentence buried in the manual, but I concede that it was there. This lesson in air orientation is applicable to any multi-row, air blast sprayer. Just like the ending in the original Ghostbusters, you should cross the streams. You can even run them in parallel – just don’t leave them in direct opposition.