Tag: airblast

Spraying Large Nut Trees – Part 1
Introduction
I’ve studied spray applications in a diversity of crops, both broad acre and specialty, but perhaps nothing is as challenging large tree nut canopies. Australia’s macadamia orchards can form >10 metre high, >4 metre deep canopy walls! So in writing this article I face the opposite situation I normally encounter when advising on airblast sprayer settings.
In my region, fruit orchard, cane, bush and vine crops are typically sprayed with airblast sprayers. Over the years, through breeding and crop management, these operations have densified. The idea is that smaller, uniform crops can be managed, protected and harvested more efficiently. The ratio of quality fruit to planted area goes up, and input costs go down.
However, our aging fleet of sprayers are overpowered relative to the target. This means much of what I do involves demonstrating to sprayer operators what sufficient coverage looks like, and then teaching how to restrain sprayer parameters to achieve this ideal coverage as efficiently as possible.
So, are there any commonalities?
Yes! The need to understand what “good coverage” looks like, and the parameters that affect it, is universal to any airblast operation. Assuming the operator already has product choice and pest staging well in hand, there are three major factors that influence the quality of the spray application: The sprayer settings, the geometry of the target and the environmental conditions.
In theory we can discuss each of these factors individually, but in practice they interact with one another. It is wrong to adjust one factor without considering the other two. This is also why you should be wary of anyone that tries to sell you a sprayer by demonstrating it in an empty lot on a calm day! Always calibrate a sprayer in the planting, in weather conditions you would normally spray in.
Air volume and direction
Air adjustments are perhaps the most impactful changes you can make to your operation. The air stream created by the sprayer not only conveys the spray solution to the target, but opens the canopy and exposes leaf surfaces to the spray. In order to achieve adequate coverage, the volume (and speed) of sprayer-generated air must be sufficient to span the distance from sprayer to target, and then displace the volume of air in the canopy while depositing the spray.
I admit to a bias when it comes to air shear systems. These sprayers utilize sprayer-generated air to atomize the spray liquid as well as convey it. As such, you cannot easily adjust the air without affecting spray quality (aka average droplet size or VMD). My preference is an arrangement where nozzle selection allows you to control spray quality independent of air settings. In any case, adjusting air settings requires the operator to “see” air.
In my region, I advise tying 25 cm lengths of flagging tape at the top, middle and bottom of the far side of the upwind tree. Then, drive past with the air on and the spray booms off. If the ribbons stand straight out, the sprayer is over-blowing and the operator can drop to a lower fan gear, reduce the tractor RPM’s (if using a positive displacement-style pump) or drive faster. If the ribbons don’t move, the opposite steps can be taken. If the ribbons still won’t move, the sprayer is under-powered, it’s too windy to spray, or the canopy is too large.
Learn more about these topics here .
Let’s explore that last point. In the case of a canopy as large as macadamia, it is unlikely a low-profile axial sprayer can produce sufficient air volume to displace all the air in the canopy – particularly at the top of the tree. In this case a more humble goal would be to move the leaves at the trunk, indicating that the sprayer is managing to drive the air to the centre. To monitor this, an observer wearing safety goggles would have to stand at the far side of the upwind trunk and (while being very careful of flying debris) watch for leaf movement.
This becomes increasingly difficult to monitor as the target gets fuller, higher, and farther away from the sprayer. Consider the macadamia trees in the following figure:

The observer will have difficulty seeing leaf movement at the top of either the taller or shorter tree, but we can safely assume there will be less movement as a function of height. Since our goal is uniform penetration throughout the canopy, we must somehow compensate for this differential. Consider the following figure which extrapolates the path between the sprayer air outlet and the tree:
In this figure we have divided each side of a low-profile axial sprayer into halves. The bottom half of the air outlet must produce enough air volume to displace area X. I realize I’m mixing area and volume, but bear with me. For the taller tree, the upper half of the outlet must produce enough air to displace 2.5 times the area versus the bottom half. Given that it is a single air outlet, this means inconsistent coverage.
Comparatively, the shorter tree requires a more uniform air distribution. While this improves matters, there are further challenges. Sprayer-generated air slows and disperses proportional to distance, requiring more air to compensate. Also, orchard wind speed increases with elevation, increasing the potential for interference and dispersion. So, the taller the tree, the harder it is to achieve uniform canopy penetration.
Spraying shorter nut trees with a low-profile axial sprayer is possible. The sprayer would require a large fan (≥1 m diameter), an aggressive fan blade pitch and a high fan speed. Air deflectors and air separation vanes would also be needed to segregate and focus the air. And travel speed would play a significant role.
Travel Speed
Travel speed should be considered as function of air penetration. A slower travel speed (~2 km/h) facilitates the displacement of stagnant canopy air with sprayer-generated air. Further, a slower travel speed reduces the wake effect that can suck finer droplets from the swath.
It may seem counter-intuitive, but slower speeds can result in greater productivity. There is no need to increase the volume sprayed per hectare, so additional refills are not an issue. Further, improving spray coverage at slower speeds may prevent the need for an additional “clean-up” application later on, saving time and reducing environmental impact. Time lost to slower travel speed can also be reclaimed with more efficient loading practices.
Learn more about travel speed here and productivity here.
Directed Sprays and Off-Target Deposition
When the height of the target tree exceeds alley width, or when branches overgrow alleys, many low-profile axial sprayers suffer from line-of-sight issues. Lower branches/leaves block the upper canopy and too many nozzles target the lower canopy. See the figure below.
One option is to direct spray vertically to ensure the swath reaches the top of the canopy. In this case it is hoped that droplets remain Coarse enough to fall from the swath and penetrate the canopy, or blow laterally with prevailing wind (left side of figure). This unadvisable strategy is unlikely to achieve consistent results and greatly increases the potential for drift.
Alternately, the top of the swath can be vectored directly at the top of the tree, but it must pass through canopy to reach it (right side of figure). This strategy increases the potential for drift, risks missing a portion of the upper canopy and is also unlikely to yield consistent results.
Ideally, we would use a sprayer design that brings the air (and nozzles) closer to the target. Hypothetically, there are several possible configurations, but in practice their success will be hampered by boom sway and roll (from sloped plantings or uneven alleys) and pressure drop restrictions (from boom height). Here are a few possibilities:
A. A vertical boom with a tapered inflatable bag to convey and redirect the air laterally (typically one-sided).
B. An axial sprayer topped with a ducted tower with vertical booms, terminating in either a second axial fan or one-sided cannon.
C. An axial sprayer with a vertical mast with a series of Sardi-style nozzle/fan assemblies distributed along the height.
Learn more about towers here.
In the following figure we see how two possible arrangements might work. On the left is a vertical boom with a tapered air assist system. This provides the shortest distance-to-target for each nozzle and in moving laterally, the air will more easily penetrate horizontal limbs. It also reduces the potential for drift.
On the right is a novel arrangement proposed by Dr. Ken Giles (UC Davis, California). A Sardi-style fan and nozzle assembly is elevated above the canopy from an axial sprayer. His intention was to create air and fluid interaction to generate turbulence that could improve uniformity and decrease drift. He proportioned 70% of the overall spray to the top fan, and the remaining 30% from the ground. Working in almond, he saw more even coverage distribution compared to a low-profile axial sprayer and noted it reduced off target drift. For a target as tall as macadamia, additional fans would likely be required.
In Part 2 we discuss Droplet size, Boom distribution, Spray coverage and diagnostics, California research and Canopy management.
April 20, 2018
Airblast Towers are Worth Considering
Are you considering shelling out for a tower extension for your airblast sprayer? Spray towers are an excellent investment, but they warrant special consideration. Towers move the air and nozzles closer to the target compared to the curved booms on a conventional airblast sprayer. When the distance-to-target is reduced, the odds of droplets reaching the target are improved. That means less pesticide drift and more deposit in the plant canopy.
Be Aware: Nozzles need a minimal distance from the target to create an optimal spray pattern, so do not get too close.
Many growers report savings when switching from conventional airblast to towers. The towers are more efficient at depositing the spray, so they have to reduce their typical sprayer volumes to prevent run-off. We worked with one apple grower that switched from a conventional sprayer to one with a tower. His lake-side orchard was plagued by wind, and his conventional sprayer had a relatively small fan diameter (~2 feet) that couldn’t compete. Traditionally, the grower used higher spray volumes to compensate. His new tower sprayer had a larger fan (~3 foot diameter) but perhaps equally import was that the tower reduced the distance-to-target. As a result, he was able to reduce his spray output by more than 200 L/ha while improving his overall coverage! That represented considerable cost savings and reduced environmental impact.
Towers may provide better coverage than conventional sprayers in orchards with horizontal scaffolding. The tower sprays between branches, penetrating more easily, while the conventional sprayer has to spray through them. Concept from K. Blagborne, British Columbia.
While there are many benefits associated with towers, they are not suitable for all situations:
- Towers must be taller than the highest target (e.g. treetop)
- Towers should be used on level ground. Towers will roll on the vertical axis (i.e. tip left and right) on uneven ground, potentially missing or over-shooting targets
- Towers must be able to clear netting, trellises, or an overhanging canopy.
The perils of towers on uneven ground. For towers to be effective, the tower must be at least as tall as the target. When the target is only slightly higher than the tower, some sprayer operators install an additional nozzle body on the top deflector plate to extend the reach.
A home-grown airblast sprayer with tower. PVC ducts, sheets of plastic, a squirrel cage blower and grower ingenuity. While it looks suspect, and difficult to clean, it reputedly works very well in highbush blueberries.
Occasionally, we have discovered areas along tower outlets where there is reduced air flow. You can usually feel these “dead zones” with your hand (beware flying debris), but it’s better to observe short ribbons attached to the nozzle bodies as described in our articles about adjusting air direction and speed/volume. In low fan gear, watch to see if any ribbons flag or appear slack from a lack of air, you can “borrow” air by re-positioning neighbouring deflectors. If that’s not possible, try replacing the conventional nozzles in the dead zone 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.
In the end, if a tower will fit in our operation, we suggest it’s a worthwhile investment that will make coverage more consistent, reduce off-target drift and possibly reduce the volume of spray needed per hectare.
Towers come in many shapes and sizes. Orchards aren’t the only good fit for towers; grapes, bushes and canes can also benefit from small towers.
April 12, 2018
The Agitation over Agitation
Sprayers101 recently received a couple of seemingly unrelated questions about airblast sprayers:
What are the advantages and disadvantages of mechanical versus hydraulic agitation? Why would someone want a stainless tank versus the cheaper poly or fiberglass options?
Recognizing that each manufacturer has their own reasons for the features and materials used in their sprayers, we posed these questions to Mr. Kim Blagborne (formerly of Slimline Manufacturing). The following article was written from Kim’s response, and it turns out these two questions are very much related. Kim writes:
This is a great debate among customers and manufacturers, and it’s difficult to stay neutral. Let’s consider the following:
Hydraulic Agitation
The flow required for hydraulic agitation requires about 30% of the pumps total capacity. This is very important because many sprayers cannot achieve, or maintain, this minimum requirement whilst spraying. This may be why it’s rare for a sales person to demonstrate agitation while the sprayer is spraying; quite often, the agitation slows or even stops. And, of course, because everyone gets wet.
Let’s say an airblast sprayer has a pump with a manufacturer-listed capacity of 26 gallons per minute (gpm) (Click to download the spec sheet for the pump). The figure in that output chart is determined on a bench at 540 rpm and at 50 psi. However, when an operator uses that pump in the field, they run it at ~150 psi, and that brings the pump capacity down a bit to 25.5 gpm.
Now we build in the line pressure drop associated with the sprayer’s plumbing. Effectively, another 8-10% of the pump’s output is lost to plumbing (a figure easily measured by collecting the total output capacity of the pump). Let’s say we are now down to a practical capacity of 23 gpm.
If the operator’s crops are on 14 foot rows, it would be reasonable to spray 200 gpa at a travel speed of 3 mph at 150 psi. With both booms spraying that’s a required flow of 16.8 gpm.
Remember, our hypothetical 26 gpm pump can only provide 23 gpm in the field. When we subtract the 16.8 gpm required for spraying, we’re left with 6.3 gpm excess capacity for agitation. But, we said we needed 30% of the pump’s 26 gpm capacity, and that comes out to 7.8 gpm. We’re short by 1.5 gpm, or stated differently, we’re about 20% short of what we need.
Why don’t we see that deficit? Because the flow to the booms is prioritized, and therefore the sprayer output matches the calibration, so everything seems OK. But no one sees the reduced return flow through the regulator, and certainly no one peeks into the tank while spraying to see that the hydraulic agitation is greatly reduced.
And so, while everything looked great during loading, the spray mix (especially SC and WDG formulations) may not stay suspended correctly during spraying. In extreme cases, that could lead to burning a crop (high concentration) at the start of a spray job, and reduced efficacy (low concentration) at the end. We’re quick to blame the chemical, but no one ever thinks to question hydraulic agitation.
Let’s consider it from another angle: TeeJet suggests a model number 62905c-5 jet agitator for a sprayer with a 250 US gallon tank. To correctly agitate the contents of this tank, we will need 30 psi and 7.6 gpm (see the chart below).
Unfortunately, there is no simple way for an operator to measure the agitation pressure or the flow, so it goes unchecked. The only way to determine if the flow demand is satisfied is to apply the generic rule of 30% of pump capacity and make an estimate. That’s pretty loose math since we’ve already established that the listed capacity may not reflect reality.
Still another angle: Many operators now employ the Gear Up, Throttle Down (GUTD) approach to match their sprayer air settings to the crop canopy. However, when we reduce PTO input speed we also reduce pump capacity. Remember our piston diaphragm pump with the 26 gpm capacity at 540 rpm? We still need 16.8 gpm to spray, but reducing the rpm’s by 100, per GUTD, drops our pump output to only 23.16 gpm.
23.16 minus 16.8 equals 6.36, and we needed 7.8 gpm to maintain sufficient hydraulic agitation. Oops.
Mechanical Agitation and Tank Material
There are definite advantages to mechanical agitation. It is not affected by the PTO speed because it is already excessive at 540 rpm. This means there is no pump capacity issue and it allows the operator to take advantage of GUTD.
There are also a few disadvantages. Unlike a hydraulic system, mechanical agitation requires maintenance, such as regular (daily?) greasing. The packing where the the system inserts into the spray tank also requires occasional inspection and adjustment to prevent leaks.
And of course there’s sticker shock. Many European manufacturers offer hydraulic agitation because it is ~$500.00 CAD less expensive. Further, mechanical agitation creates vibrational stress on tanks walls, which fiberglass or plastic tanks can’t handle for long. The solution is stainless tanks, which is a more expensive material. Further, stainless cannot be moulded around pumps and rotating parts, so more steel is required, adding to expense and weight.
In my opinion, there is sufficient benefit to stainless to easily recover the investment. Beyond permitting mechanical agitation, there’s durability. We have stainless tanks built in 1948 that are still operating today, and we’ve never found a plastic or fiberglass tank that can claim that. There’s also sprayer sanitation. It has long been know that stainless cleans more easily and more reliably that plastic or fiberglass, especially as the tanks begin to age.
Closing
The decision to buy a sprayer with hydraulic agitation or mechanical agitation lies, ultimately, with the consumer. But be sure to look past the price tag, and under the hood. Ensure that you have sufficient agitation to properly suspend your tank mix, and give you the flexibility to Gear Up and Throttle Down to improve your spray coverage and efficacy.
October 10, 2017
Should Backpack Sprayers be Used to Test Airblast Products? – Part 2
In Part One of this article, we showed that approximately 40% of minor use label expansions and registrant submissions rely on data from hand booms and guns. We also showed that a hydraulic backpack or knapsack will not give the same coverage as an airblast sprayer, and we concluded by suggesting that small plot researchers use spray equipment that reflects grower practices.
Unfortunately, practical logistics prevent most researchers from using a full-size airblast sprayer. They may not have access to such a sprayer, and if they do, it takes considerable time to mix and clean between treatments. Further, treatments are often only a single row, or even a single plant. It takes too much pesticide, too much time, and too much plot space to justify using a full-sized airblast sprayer, even if the relevance of the results are questionable.
Would another method of application better emulate an airblast application but retain the convenience of a hand boom or gun?
The motorized backpack mistblower
Using the same methods used to compare airblast to hand boom spray coverage in the previous article, we compared airblast sprayer coverage to that of a motorized backpack mistblower in grape, raspberry and peach (July, 2013). Once again, coverage was analyzed as overall percent coverage (see first graph) and droplet density (average droplets per square centimeter – see second graph).
Comparison of average % coverage in peach, raspberry and grape using a mistblower and air blast sprayer emitting he same volume
Comparison of droplets per square centimetre in peach, raspberry and grape using a mistblower and air blast sprayer emitting the same volume
Results and Observations
The mistblower met, or in the case of droplet density, exceeded the coverage obtained using an air blast sprayer in most crops. The results led to a few observations:
- The significantly-higher droplet density is a function of the Finer spray quality produced by the mistblower (see water sensitive papers below). This may still represent a confound between small plot work and large scale airblast applications.
- Drift between proximal treatments may be an issue given how far the mist was blown. This should be considered when planning plots.
- While not shown here, spray coverage was more consistent throughout each canopy, of each crop, when using the mistblower. This is likely because the operator was able to aim the output as they swept the spray over the canopy, thereby ensuring all surfaces were hit from multiple angles.
- While we always try to be brand-neutral, it should be noted that we’ve used multiple Solo mistblowers over the years, and all of them required significant maintenance (no matter how they were cleaned and stored). It was very difficult to find brand parts and repair expertise in Ontario. The Stihl brand currently has far more dealers, and more accessible parts, and has not caused us any difficulties (yet).
- Always use the highest grade gasoline in two-stroke engines to avoid ethanol gumming up the carburetors!
- Always calibrate mistblowers by volume because raising and lowering the boom will affect the flow rate.
Conclusion
Hand booms, and likely hand guns, are not appropriate for testing agrichemical products intended for use with an airblast sprayer. Data derived from these methods should be questioned. An airblast sprayer is the best choice for any such research, but a mistblower is a viable alternative. Transparent, standardized operating protocols for testing products intended for use in airblast sprayers should be required.
Try this calculator to determine what droplet sizes are produced by a backpack mistblower. It will help dial-in your operational settings.
Thanks to Vaughan Agricultural Research Services Ltd. for their assistance in the research performed for this article, and to Mark Ledebuhr for sharing the data used to develop spray quality app.
March 27, 2017
Should Backpack Sprayers be Used to Test Airblast Products? – Part 1
Peer-reviewed journal publications claim there is a significant difference in spray coverage and deposition patterns when an agrichemical product is applied using an airblast sprayer versus a hydraulic hand boom. An airblast sprayer creates Fine droplets that shear in entraining air and are carried into a plant canopy. Properly calibrated, the air opens the canopy to expose all target surfaces to the spray. By comparison, a hand boom relies on pressure to propel fine droplets into a canopy, and while there is some air-entrainment surrounding the spray, it cannot travel as far or displace as much canopy. As a result, most of it impacts on the outer surfaces of the canopy.
Knowing this, it is surprising that so many products intended for use with airblast sprayers are applied by researchers and consultants using hand booms or the high-pressure arborist-style handgun (see ‘Survey of Submissions’).
Survey of Submissions
This graph represents a random selection of 150 minor use label expansion studies and registrant submissions from Canada and the USA spanning 1990 to 2011. It shows the application method by crop.
In 2012, we performed some research with the following goals:
- To demonstrate the difference between spray deposition and coverage when using a hand boom versus an airblast sprayer.
- To create a sound basis for questioning and potentially improving how agrichemical products for orchard, bush, and vine are tested in Canada.
Using water-sensitive paper to diagnose spray coverage, airblast sprayer application was compared to hand boom application in highbush blueberry, apple and grape.
Target locations in highbush blueberry.
Target locations in apple.
Target locations in grape panel.
Sprayers were calibrated to emit the same volume per planted area via hollow-cone nozzles. Volumes selected were based on typical application volumes for Pristine or Captan (commonly sprayed in Canada). While there is no standardized protocol (and there should be) we followed typical practices of 500L/ha for grapes, blueberry and apples until plant growth warrants higher carrier volumes. At that point, many researchers go up to 1,000 L/ha. Coverage was quantified by collecting and digitally scanning water-sensitive papers to calculate overall percent coverage (see graph) and droplet density (average droplets per square centimeter – see graph).
Overall percent coverage
Droplet density
Conclusion
In all cases, airblast applications deposit > %50 more spray than a hand boom. In the case of grape, you’ll note there are three bars. This is because spraying 1,000 L/ha with the airblast sprayer drenched the targets (it was late in the season and the canopy was sparse), making it impossible to discern droplet density. When we reduced the output to 375 L/ha, we were able to register droplet density, which was still significantly higher than that produced by the hand boom at 1,000 L/ha. This raises significant questions about the validity of efficacy and residue studies performed with hand booms when growers apply the same products using airblast sprayers.
When this data was shared at extension conferences, it was sometimes noted that many researchers choose to spray the target until it is drenched, ensuring the dose administered to the crop reflects what was intended. This does not, however, invalidate the fact that a growers spray equipment and practices are significantly different, and the dose and spray distribution they achieve will not reflect the original research.
The recommendation is that researchers use the same equipment to test products as the growers use to apply them. But, recognizing the difficulties associated with performing small plot experiments with full-sized airblast sprayers, an alternative is needed. That topic will be addressed in part two of this article.
Horticultural Crops Ontario, the grower co-operators and former OMAFRA summer student Carly Decker are gratefully acknowledged for making this research possible.
March 22, 2017