Table 1: Changes in travel speed range for two nozzles
U. S. gal/acre L/ha
One of the most challenging aspects of new sprayer technology is adjusting to the higher travel speeds that these machines are capable of. Marketing for these sprayers often cites their increased productivity as a key justification for the often enormous cost of such units. For example: “More acres per hour results in more profits!” But in practice, fast travel speeds raise challenges, some of which are difficult to manage. They are:
Maintaining volumes at fast speeds requires the use of nozzles with larger flow rates. This produces coarser quality sprays and complicates nozzle selection.
MORE HORIZONTAL TRAJECTORY FOR LARGER DROPLETS
Upon leaving the nozzle, all droplets initially move forward with the boom. As they move away from the nozzle, only the larger ones can maintain that trajectory. The smaller ones lose their inertia and can be easily displaced by wind.
MORE TURBULENCE, CREATING DRIFT
At higher speeds the boom, spray pattern, and tractor units can generate significant aerodynamic effects that can reduce spray deposition. Air moving past the edges of spray patterns creates turbulent vortices that pull fine droplets from their flight path. These swirling vortices of fine droplets hang behind the sprayer and are vulnerable to drift.
MORE VARIATION IN SPRAY PRESSURE
Sprayers with automatic rate controllers link pressure to travel speed in a square root relationship. That means for every time travel speed is doubled, pressure has to increase four times to maintain a constant application volume. At higher average travel speeds, necessary variations in speed can result in dramatic changes in spray pressure that either exceed the nozzle’s capabilities (at low speeds) or the pump’s capacity (at high speeds.)
psi 34 61 96 kPa 235 420 660
34 235 16.0 25.8 61 420 20.0 32.2 96 660
*Nominal flow rates stamped on nozzles are given in U. S. gallons per minute of water at 40 psi (275 kPa) spray pressure.
Assume, for example, that a given nozzle has a 60 psi (415 kPa) operating range. It develops an acceptable pattern at 30 psi (205 kPa) and can be operated to 90 psi (620 kPa). This pressure range allows for a 25 per cent travel speed variation from the average (See Table 1.) At a speed of eight mph (13 km/h), the sprayer can slow down to six mph (10 km/h) and speed up to 10 mph (16 km/h) without exceeding the nozzle pressure range. However, the picture changes at an average speed of 16 mph (26 km/h). Now, the minimum speed becomes 12 mph (19 km/h), hardly slow enough for some rough terrain or tight turns. Slowing down further means that pressures will drop below those optimal for these nozzles and could result in reductions in herbicide performance due to poor spray patterns.
The opposite problem occurs with units capable of more than 20 mph (32 km/h). Accelerating to those speeds from 16 mph will require pressures that may be beyond the pump capacity, or can result in fine, drift-prone sprays for some nozzles.
GREATER DAMAGE IN WHEEL TRACKS, MORE DUST
Faster speeds mean faster wheel revolution. For a sprayer wheel, this can mean not only greater soil disturbance in the track, but can also cause significant displacement of air behind it. Both factors are aggravated by the weight of larger sprayers, which increase the odds that weed control will be reduced in the wheel tracks. Adding higher flow nozzles, preferably well back of the wheel, can partially overcome this problem.
At higher speeds sprayers can also kick up more dust. This can be a contributing factor to reduced herbicide performance, especially for water-soluble herbicides that can bind strongly to soil particles. (Glyphosate and diquat are the best-known examples.)
Faster speeds give operators less time to respond to boom movements resulting from uneven terrain. Many find it necessary to raise the boom to prevent contact with the ground. If a boom is too high it will increase drift potential, decrease canopy penetration, and lower the effectiveness of any angled nozzles. An automatic boom height controller would help in this situation.
Travel speed is used to justify higher productivity, which in turn justifies higher capital costs. However, boom width may be a more sensible means to increase productivity with fewer disadvantages. Changing from a 90-foot to a 120-foot boom increases capacity by 33 per cent at a reasonable cost. Wider booms weigh more, require higher pump capacity, and may need to be operated at a higher height but these problems can be managed with existing technologies like automatic boom height controls.
TO READ THE FULL ARTICLE…
This is one section of a larger article called “Best Management Practices for Herbicide Application Technology.” You can read the whole article at the Saskatchewan Soil Conservation Association’s online journal called Prairie Soils and Crops. To subscribe, visit the website at www.prairiesoilsandcrops.ca.
Tom Wolf is a research scientist with Agriculture and Agri-Food Canada’s Saskatoon Research Centre.