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 a little light but not unreasonable because of how the Lipco is supposed to work.
The Lipco has large shrouds behind each vertical boom that block cross-wind by encasing the tree row. A vertical plane of air captures 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.
The 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. We were replacing the papers in exactly the same place, in the same orientation, each time, and yet sometimes a paper got spray and sometimes it didn’t. What was going on?
That’s when I remembered a paper written by 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 degrees by loosening the four bolts at the top of the gantry (see Figure 3). Now one side of the multirow sprayer was adjusted to look like Figure 2, configuration 2. We left the other side in opposition as in Figure 2, configuration 1.
We reset the water sensitive papers and ran another pass. The operator told me after that 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.
This lesson should be 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.