Category: Drift

Articles about managing spray drift and inversions with horizontal boom sprayers

What is Delta T and why is it important for spraying?
Click here to listen to Audio Article
Humidity is important in spraying. With the average tank of pesticide being 90 to 99.5% water, evaporation plays an important role in both droplet size and active ingredient concentration. Low humidity causes droplets to evaporate faster, potentially increasing drift and reducing uptake. But relative humidity (RH) isn’t the best way to measure this effect because the same RH at two different temperatures results in two different water evaporation rates.
Instead, we present Delta T, also known as “wet bulb depression”. Delta T is an atmospheric moisture parameter whose use in spraying has made its way to North America from Australian operations. It is defined as the dry bulb temperature minus the wet bulb temperature, and provides a better indication of water evaporation rate than RH. Higher Delta T means faster water evaporation.
The recommendations from Australia are to avoid spraying when the Delta T is either too high or too low, with a range of two to eight being described as ideal.
Figure 1: Delta T chart used in Australia (Source: Australian Gov’t Dept of Meteorology)
Delta T is being reported on an increasing number of weather stations, and it’s time we took a closer look at what it means.
Measuring Relative Humidity
In the early days of weather reporting, relative humidity was calculated from psychrometric charts. All one needed was a hygrometer, usually a sling psychrometer. A sling psychrometer is two identical thermometers side by side whose bulbs could be slung in a circle, exposing them to moving air. One bulb was covered in a cotton wick moistened with distilled water, the other was left exposed and dry.
Figure 2: Sling psychrometer (Source: ScienceStruck.com)
As the bulbs met moving air, water evaporated from the cotton wick and that reduced the temperature of that thermometer. The dryer the air, the greater the evaporation rate and therefore the greater the temperature drop. The dry thermometer was unaffected by this movement.
On measuring the wet and dry bulb temperature, one consulted a psychrometric chart. This chart converted the two temperatures to total water content in the air, compared it to total water-holding capacity, and expressed it as Relative Humidity. Psychrometric charts are useful for many other air parameters such as dew point, vapour pressure, or enthalpy. (Pause briefly to give thanks that we don’t need to know what enthalpy is.)
Figure 3: Psychrometric Chart (Source: Carrier Corporation)
Turns out that RH is a poor measure of water evaporation rate. An RH of 24% at 20 C has exactly the same evaporation rate as an RH of 44% at 35 C. That’s why Delta T is the preferred measurement: it’s linearly related to evaporation.
Note: Modern electronic weather stations don’t need two thermometers to measure air moisture content, and use polymers whose capacitance or resistance changes with atmospheric moisture. Add an internal look-up table, and we have all the information we need.
Pros and Cons of Water Evaporation
It’s important to note that our Australian colleagues caution against spraying when water evaporation rate is both too high and too low.
Too High:
- Water evaporates rapidly, reducing droplet size and pre-disposing the smaller droplets to drift;
- Deposited droplets dry quickly, reducing pesticide uptake which is more effective from a wet deposit.
Too Low:
- Water doesn’t evaporate, maintaining the smaller droplets in a liquid state. These small droplets are already drift prone, but are now more potent because of more effective uptake. Overnight conditions that are inverted are usually humid, adding to harm potential from the inversion.
Delta T in North America
The addition of this parameter to our spraying weather lexicon has been useful. But it’s important to understand the context in which it was developed to properly judge its suitability.
Aussies started talking about Delta T because the use of finer sprays under the hot dry conditions found during their summer sprays resulted in significant evaporative losses, significantly greater drift potential, and potential reduction of product performance. The guidelines to avoid spraying when Delta T exceeds eight or ten originate there.
A few changes have happened since these guidelines were developed. Over the past ten to 20 years, we’ve observed greater use of low-drift sprays, with the coarser sprays’ larger droplets resisting fast evaporation. In the past five to ten years, water volumes have increased due to our heavier reliance on fungicides, desiccants, and contact modes of action. Both of these developments have helped reduce the impact of a dry atmosphere. We simply can’t say if a Delta T of 10 is too high with these new application methods.
Looking at it another way, if Delta T values are very high, increasing water volume and droplet size will mitigate that to some degree, as the Aussies state in their extension materials (linked earlier).
Formulation
Pesticide formulation can also play a role in evaporation. Once the water is gone, oily formulations may still have good uptake because the oily active ingredient stays dissolved in the oily solvents. This is both good and bad, helping on-target efficacy but also increasing the risk of more potent drift. Solutions, on the other hand, are more likely to leave their actives stranded on leaves as crystals once the water is gone.
Bottom Line
Delta T is definitely useful information when spraying. It will typically rise and fall with air temperature as the day proceeds, and it is wise to consider suspending operations when values are critical. Take note of the Delta T when spraying the same product throughout these hot days and learn from the experience. Remember, the atmosphere affects not just sprays but also plants and insects, and due to this complexity we may not be able to attribute success or failure to just one measurement.
June 19, 2018
“Bee” Responsible with Pesticide Sprays
Horticultural crops cannot be produced commercially without the use of pesticides to manage the impacts of insects and pathogens. Growers recognize the importance of pollinators and in some cases, rely on bees for pollination. Growers are practicing due-diligence to try to minimize the effects of necessary pest management activities on bees. There’s a fine balance between managing pests effectively and economically and minimizing the effects of pesticides on pollinators. Impact on pollinators is a major consideration for the registration of pesticides.
Not all pesticides are toxic to honeybees.
Not a honeybee, but a great photo of a pollinator on a spray boom near some nozzles. Too good not to use.
Growers use IPM practices which means that they are spraying only when necessary (monitoring for pest levels) rather than following a calendar-based program. Because each droplet of spray that does not land on the target (the crop) is wasted money, growers are more conscious of drift and are using technology to reduce off-target drift.
The Ontario Bees Act states “No person shall spray or dust fruit trees during the period within which the trees are in bloom with a mixture containing any poisonous substance injurious to bees unless almost all the blossoms have fallen from the trees.” While some crops, like grapes and peaches, do not rely on insects for pollination, bees may still visit their flowers and they are still present in vineyards and orchards before and after bloom, foraging for nectar and pollen on flowering plants in row middles and surrounding vegetation areas. We have been promoting row middle management with flowering plants to encourage the presence of beneficial insects. Honey bees are also attracted to these plants. For this reason, it’s important to recognize that sprays applied to manage pests may have adverse effects on honey bees as well.
One of the most important things to do is to maintain communication between growers/custom operators and beekeepers. While it’s common sense to not allow insecticides to drift directly onto bee hives, bees will usually forage up to 3 km from a hive and when food sources are scarce, they are known to fly as far as 12 km (8 miles) (Download reference).
BeeConnected is an app connecting registered beekeepers with registered farmers and spray contractors, enabling anonymous communication on the location of hives and crop protection product activities. The app is available free of charge through a web browser, the Apple App Store and Google Play.
Here are a few others things you can do:
Read the pesticide label:
Carefully follow listed precautions with regard to bee safety. In some cases a product may not be used while bees are actively foraging.
Product selection:
Pesticides (insecticides and fungicides) are not all equally toxic to honey bees. It is also important to be familiar with the relative toxicity of pest control products to bees. In Publication 360, Fruit Crop Protection Guide, the relative honeybee toxicity is now listed in the fungicide and insecticide activity tables of each chapter. The impact of products that are moderately toxic to bees can be can be minimized if dosage, timing and method of application are correct. Highly toxic products may cause severe losses if used when bees are present at treatment time or within a few days thereafter.
Choose the least hazardous insecticide formulation. Emulsifiable formulations normally have a shorter residual toxicity to bees than wettable powders and flowables which, in addition to having residual characteristics can be more easily picked up from the flowering plant while bees are gathering pollen.
Spray timing:
Whenever possible, apply products with toxicity to bees in late evening, night or early morning while bees are not foraging (generally between 8 p.m. and 8 a.m.). Evening applications are less hazardous to bees than early morning applications. Warm days and nights can extend the foraging period; therefore applications may be necessary later in the evening or earlier in the morning under unusually warm conditions. Do not apply insecticides when cool temperatures are expected after treatment. Residues will remain toxic to bees for a much longer time under cool conditions. Do not apply insecticides that are toxic to bees on crops in bloom, including crops containing weeds or cover crops in bloom. Avoid treating during hot evenings if beehives are very close to the target field and honey bees are clustered on the outside of the hives.
Remove alternate pollen sources:
Where feasible, eliminate weeds or flowers in row middles by mowing at least 2 days before a pesticide with toxicity to bees is to be applied.
Minimize off-target drift:
Drift of spray applications can cause significant bee poisoning problems, particularly when drift reaches colonies or adjacent flowering weeds. In general, sprays should not be applied if wind speed exceeds 10 mph and favors drift towards colonies. Give careful attention to position of bee colonies relative to wind speed and direction. Ensure that there are no colonies directly in the orchard at the time of spray. Select drift-reducing spray nozzle technology, whenever possible. Since fine droplets tend to drift farther, apply spray at lower pressures or choose low-drift nozzles that reduce drift by producing a medium to coarse droplet size.
Calibrate spray equipment often. Air-blast sprayers can produce finer droplets with greater drift potential. When using an air-blast sprayer, consider redirecting or turning off nozzles, or use technologies that reduce drift (for example, towers, multirow, tunnel and target-sensing sprayers). Shut off sprayer when making turns at field ends or gardens, near large puddles, ponds and other sources of water that may be used by pollinators and other wildlife.
There is a precaution to nighttime spraying: you must be aware of inversions. When you spray during an inversion, the larger drops fall quickly (per normal), but smaller lighter droplets fall very slowly (a few centimetres per second). They do not disperse. Instead, they move with the air they were released into, evaporating very slowly, over great distances. These small particles, as well as vapours from volatilizing products, are capable of moving for kilometers and are therefore subject to drift.
The only sure way to know if you are in an inversion is to take two air temperature readings: the first about 10 cm from the ground, and the second about three metres off the ground. If the surface air temperature is cooler, you are in an inversion. The magnitude of the difference indicates how strong the inversion is. Accurate measurements are difficult to manage with conventional thermometers (Although the new Spot-On Inversion Detector makes it possible). It is generally easier for sprayer operators to watch for the following cues:
- Large temperature swings between daytime and the previous night.
- Calm (e.g. less than 3 km/h wind) and clear conditions when the sun is low.
- Intense high pressure systems (usually associated with clear skies) and low humidity where you intend to spray.
- Dew or frost indicating cooler air near the ground (fog may be too late).
- Smoke or dust hanging in the air or moving laterally.
- Odours travelling large distances and seeming more intense.
- Daytime cumulus clouds collapse toward the evening.
- Overnight cloud cover is 25% or less.
If you suspect a strong inversion, don’t spray. Postpone the application if possible.
Reducing pesticide injury to honey bees requires communication and cooperation between beekeepers and growers and applicators. It is important that beekeepers understand cropping practices and pest management practices used by farmers in the vicinity of their apiaries. Likewise, pesticide applicators should be sensitive to locations of apiaries, obtain a basic understanding of honey bee behavior, and learn which materials and application practices are the most hazardous to bees.
Furthermore a number of native pollinators such as bumblebees, leaf cutter bees, sweat bees and squash bees are also important pollinators in some crops and they too require consideration. While it is unlikely that all poisonings can be avoided, a balance must be struck between the effective use of insecticides, the preservation of pollinators and the rights of all — the beekeeper, farmer and applicator.
May 8, 2018
Question of the Week: Fine Sprays for Fungicides?
The following question arrived from one of our prairie clients last week:
“A retailer is promoting the use of hollow cone nozzles to be used on field sprayers (20” spacing) to apply fungicides which he claims out-perform any regular and twin fan tips. Claims:
- create an extra fine droplet for maximum coverage on the canopy
- use less water, less time spent filling
- apply at 3.5 gpa
- add vegetable oil to reduce drift
“So his direction to a specific customer was to use the TEEJET CONEJET TXA8001VK nozzle at 80 psi – travelling at 10 – 12 mph to achieve a 3.4 gpa application rate with a ‘very fine’ droplet size.
“What are your thoughts?”
Here’s how I answered (edited for clarity):
That recommendation sounds familiar – it originates from a consultant with experience in South America, where this idea is promoted to improve (aerial) spray productivity.
I fundamentally disagree with his approach. Adopting and promoting it is not only illegal (contravenes every modern label’s water volume and spray quality requirements), it also puts a generation’s worth of stewardship efforts on drift management at risk.
To be balanced, let’s explore the attractiveness of this approach. Finer sprays do provide superior coverage and save water. Every child knows this. Finer sprays also go places in the canopy where the coarser sprays can’t, for example very dense lentil canopies.
Over the years, we’ve explored the performance of fine fungicide sprays in canola, pulses, and cereals in research trials with the U of S and AAFC. To our surprise, droplet size played only a small role in fungicide performance. Water volume was much more important. Droplet size management with pressure through a low-drift nozzle was enough to get the best disease control.
The main drawbacks of very fine sprays are:
1. The fine droplets evaporate to dryness very quickly, in seconds. As they shrink, their drift potential is increased even more, and once dry, the remaining particles work much less well. The proponent corrects for this by adding an oily adjuvant as an evaporation retardant. With oil, the fines remain liquid much longer. Although many products become more effective this way, they also become more phytotoxic and less safe for the applicator and bystander. Completely off label, completely risky for crop safety, unknown effects on MRLs, extremely unsafe for the environment and humans. Remember when people dissolved 2,4-D ester in diesel, back in the 40s and 50s and sprayed it with their brass 6501 tips? That’s what this is.
2. Cone nozzles are designed for airblast sprayers and do not produce good pattern overlaps for boom sprayers. The proponent of this method actually recommends that the boom be raised to overcome the bad patterns and to (believe it or not) simulate aerial application. If this were done, the spray would be re-distributed by air-currents and come down wherever the wind blows it. Probably far away. The concept of on-target, uniform application, the practice that makes product use acceptable, and the thing we try to achieve with flat fans at a low boom height, is completely lost.
3. Producers will not have the support of pesticide manufacturers should a performance issue arise. Even worse, if regulators find out about this off-label practice, significant fines (fines for fines, get it?) can be charged under the Pest Control Products Act.
4. Airborne spray drift with an air-induced spray like the AirMix, GuardianAir, AIXR and the like, applying 10 gpa, is about 1% of the applied amount, measured at 5 m downwind of the downwind edge of the swath in a 20 km/h wind. We’ve never measured hollow cone drift from a boom sprayer, but when we used a flat fan at 5 gpa, drift increased to about 8% of applied. I’d guess a high pressure hollow cone would easily double or triple that. Illegal and irresponsible.
5. Travel and boom turbulence is a part of faster travel speeds. This would affect the finer droplets much more than the coarser ones, as we can imagine. It’s similar to drift. With a low-drift spray, the proportion of the total spray volume that is “fine”, say less than 150 microns, is about 5%. For a very fine hollow cone, it might by 50 to 75%. So a much greater proportion of the sprayed dosage would be susceptible to uncontrollable movement. This could be good, when turbulence redirects the spray to places that are unreachable by larger droplets. Or it could be bad, as turbulence pushes droplets away from an important target, creating a miss. On balance, bad. Very bad.
These types of recommendations are concocted by people who want to tell a unique story that is popular with some. Their approach differentiates them from the rest of the crowd, an old and effective marketing trick. But these proponents do not have the best interests of the industry in mind.
Our individual and collective agricultural practices must be respectful of others. Of safety. Of the law. Of the environment. We have lots of opportunities to make shortcuts…nobody’s watching most of the time. But that doesn’t make it right. It’s certainly not in ag’s long-term interest.
When considering our agricultural practices, imagine describing them to a young non-farming person. Can you justify your actions? Do your practices make you proud? If not, you have work to do.
Here’s a task: If your boom sprayer has nozzles that produce very fine sprays, take them off and throw them in the garbage. Might sound radical, but it’s the right thing to do.
July 5, 2017
Deciding on the Right Way to Spray
“What is the right way to apply this pesticide?” It’s one of the classic questions. Applicators know that spray method determines the efficacy of the application as well as its environmental impact. And it has to use time and water resources efficiently to make sense.
To answer the question properly, we need to take things one step at a time.
1. Canopy: To start, we need to look at the canopy that our application will go into. If it’s an early season spray into a seedling crop, then the canopy won’t be much of a barrier. Lower water volumes can be possible. Droplet size will only depend on the target type and the pesticide mode of action.
Small weeds require more smaller droplets to secure effective targetting
If it’s a later application into the bottom of a maturing canopy, the foliage may intercept the spray before it reaches the target area. More water will likely be needed, and droplet size may become more critical for getting the spray to its destination. Dense canopies are a real challenge and lower-canopy deposition usually benefits from finer sprays because the small droplets can turn corners better.
Dense canopies are very difficult for a spray to penetrate. Higher water volumes and smaller droplets are the key tools that help.
2. Water Volume: Regardless of canopy, the range of application possibilities will depend on the water volume and spray quality combination. It’s math: assuming some constant amount of coverage on each leaf, more layers of foliage will require more water. Using less water volume will make it necessary to use finer sprays to keep droplet numbers constant. More water will allow coarser sprays. This decision has implications for drift, and by extension, affects the number of hours we can spray in a day. More drift tolerance means better application timing and overall productivity.
The tradeoffs between water volumes and droplet sizes are seen in this figure. Once a certain threshold of coverage has been reached, a further increase in coverage may not provide any additional control.
3. Target Type and Droplet Behaviour: Whatever spray we use, the target plant or insect needs to intercept, collect, and retain the spray droplet. This is where the fun begins. Target leaves may be vertical or horizontal, large or small. Their waxy surface may be easy-to-wet or difficult-to-wet. The general rules of thumb are that larger, more horizontal and easy-to-wet surfaces are better suited for coarser sprays – these are intercepted more efficiently and stick readily. That is a reason why most broadleaf weeds and crops are very compatible with low-drift sprays.
Large targets (left) are most efficient at intercepting larger droplets (provided droplet bounce is not a problem) because smaller droplets may evade capture. Smaller targets are usually missed by larger droplets but are very capable of capturing smaller droplets.
On the other hand, smaller, vertically oriented and difficult-to-wet plants require finer sprays for effective targetting. Larger drops tend to miss these targets or bounce off them. Most grassy, and some broadleaf weeds (especially at early growth stages) fall into this category.
4. Mode of Action: There are nearly 30 modes of action on the herbicide world, and another ten modes for insecticides and fifteen for fungicides. The effect of droplet size and water volume on their uptake and translocation varies, and it’s probably not correct to generalize too much. There is one notable product, glyphosate. For this product, research has consistently shown that large droplets and more concentrated mixtures provide better uptake. But we’ve also seen problems when this is over-done, causing localized toxicity and limiting translocation.
With many products, we’ve sometimes seen better performance with finer sprays due to improved coverage, yet at other times less performance due to rapid evaporation. On the whole, it’s probably still fair to say that contact modes of action require finer sprays and higher water volumes, even if there is the occasional exception. And systemic products can typically handle coarser sprays. We’ve always been surprised just how coarse we seem to be able to push the system before any loss of efficacy.
What does it all mean? In spraying, we need to accommodate a lot of diversity. The average application is broad-spectrum, targeting large and small broadleaf and grassy plants. Many sprays are tank mixes of several modes of action. It’s impossible to prescribe a specific spray for each situation. We need a little bit of everything. And the spray should not be drift-prone. It’s easy to see that we need to aim for the middle to accommodate everything.
The traditional flat fan nozzle, either in its conventional or low-drift form, generates a wide range of droplet sizes that can range from 5 µm to about 2000 µm. If we need fine droplets, they’re there. If we need larger droplets, they’re also there. The proportion of the total spray volume in each specific size fraction depends on the nozzle choice and size, the spray pressure, and the adjuvant mix in the tank. Overall, the system is very robust, and although it requires some tweaking, a well chosen average spray can achieve most tasks well enough.
A typical spray quality chart shows the expected spray quality for a range of nozzle sizes and pressures. Spray quality measurements follow standards set by the ISO and ASABE, these change from time to time and therefore charts tend to become outdated.
Our research has repeatedly shown that a Coarse spray is a good starting point that does most things well. It is acceptable to move into a Very Coarse or coarser category provided water volumes are also raised, and provided the target types and modes of action are suited for this change.
It is rarely necessary to spray finer than Coarse, and when this is done, we recommend against spraying finer than a Medium spray. There is simply no advantage from product performance, and drift risk becomes unacceptable.
Tweaking the System. In order to maximize the performance of your spray, and the efficiency of your overall spray program, here is some advice:
1. Know the spray quality of your nozzles, and their response to spray pressure. Manufacturers publish this information in their catalogues and on-line. Make this your homework assignment.
2. Use the coarsest spray that you can afford to. This will make the application safer, it will widen the weather window, and it will simply let you get more done in a day or a season. Coarse sprays work.
3. Use spray pressure and water volume to fine tune the application for a specific purpose. If using a contact product, you can keep the same nozzle you used for a systemic product. Apply more water or use more spray pressure to generate more droplets.
4. Do not skimp on water. Higher water volumes tend to make an application more uniform, robust, and crop-safe. Spray coverage improves. Canopy penetration improves. Coarser sprays are possible. The only exception to this rule is glyphosate, which works better in lower water volumes. But with higher glyphosate rates and more tank mixing, even that exception is disappearing.
5. Learn as much as you can about how your pesticides work and where they need to be in your canopy. Apply your knowledge to select optimal water volumes and spray qualities.
6. Be wary of people who advise very low water volumes in conjunction with fine sprays. They want to appeal to your need for efficiency, but do so at the cost of consistency and environmental stewardship. Plus these types of applications are illegal for many of our products.
June 7, 2017
How Do Hydraulic Low-Drift Nozzles Work?
Low drift nozzles have become the standard way to apply pesticides from a boom sprayer. In order to use them properly, we need to understand how they are designed and how they are intended to work.
Sprayer nozzles have three functions on a sprayer.
1. Metering flow
2. Atomizing liquid
3. Distributing liquid uniformly
Accurate metering of the flow is done through precise machining or molding of the nozzle.
Atomization of a liquid occurs by imposing some sort of force on the liquid that causes it to break up from a stream or a sheet into droplets of the desired spray quality.
Distribution is done by generating a pattern that, in combination with adjacent nozzles, produces similar dosages in appropriate droplet sizes and densities, along the target area.
All three of these functions are confirmed by the nozzle manufacturer, but the properties are likely to change with wear.
Atomization
Atomization forces could be air-shear (used in some aircraft, airblast, or twin-fluid nozzles), centrifugal energy (used in rotary atomizers), electrical energy (used in some electrostatic sprayers), or hydraulic pressure (used in the most common nozzles, the flat fan or hollow-cone tips).
Typically, the higher the applied energy, the greater the break-up of the spray. More air-shear resulting from faster aircraft or fan speeds, faster rotation of a cage, or more hydraulic pressure all have similar effects: they create finer sprays.
Most nozzles produce polydisperse sprays, comprised of a large number of different droplet sizes. For hydraulic flat fan nozzles, droplets ranging from 5 to 2000 µm can be produced. The exact distribution of the volume in these droplet sizes depends on the nozzle design, the spray liquid, and the pressure. Here are three examples, representing approximately Medium, Coarse, and Extremely Coarse sprays.
Droplet size distribution by number and volume from a Medium spray. Note the majority of the droplets are small, but the majority of the volume (dose) is in somewhat larger droplets.
Droplet size distribution by number and volume from a Coarse spray. Like in the Medium spray, the majority of the droplets are small although there is fewer of them. The majority of the volume is in intermediate sized droplets.
Droplet size distribution by number and volume from a Very Coarse spray. While the majority of the droplets are small as in the finer sprays, their overall number is sharply reduced from the finer sprays. The volume is now in the largest droplet sizes.
Let’s focus on hydraulic nozzles, by far the most common in agriculture.
Spray Pressure
Spray pressure is a useful tool for controlling droplet size from any hydraulic nozzle. Need a finer spray? Add pressure. It is also the basis for the age-old recommendation that lower pressures are a good tool for reducing drift.
We impose practical limits on the upper and lower range of recommended pressures based on several other factors, chief among them the spray pattern.
Spray patterns of a certain width, or angle, are required for proper pattern overlap. The convention is to space hydraulic nozzles at 15 or 20 inch intervals along a boom, and operate them at about 20” above the target. Boom height values will depend on the fan angle of the nozzle and the degree of overlap required. For low-drift flat fan tips, a minimum 100% overlap is best. With 100% overlap, the spray pattern width at target height is twice the nozzle spacing. With this approach, at any point under the boom, the target receives droplets from the closest two nozzle patterns.
Pattern angles are published by manufacturers, but in practice, angles often differ from those values and can vary with spray formulation. Importantly, they tend to become narrower at lower pressures. The exact pressure at which this happens depends on the tip design, but experience shows that pressures below 20 psi for conventional nozzles, and 30 to 40 psi for low-drift nozzles, result in poor (too narrow) patterns. Narrow patterns reduce overlap, resulting in poor distribution.
TeeJet AI11003 at 20 psi
TeeJet AI 11003 at 80 psi
We might also limit pressures at the upper end, based on drift potential. Most conventional flat fan nozzles, for example, drift excessively at pressures above 60 psi or so, hence that limit.
Low Drift Nozzles
Low drift nozzles were quickly adopted by applicators due to their ability to reduce drift and thereby widen the window of safe spray application. They work by using a two-stage design (often called “pre-orifice”) to reduce the internal operating pressure of the tip. The pre-orifice, the original liquid inlet, is round and sized for the nominal flow of the tip. The exit orifice is eliptical in shape and has a larger flow capacity than the pre-orifice, by about 1.2-fold to 2.5-fold. The larger exit creates an internal pressure drop, so the pattern formation produces larger droplets as though the operating pressure had been reduced. Most modern low-drift tips also introduce air into the nozzle via a built-in venturi. This further suppresses the formation of driftable droplets and introduces air into the interior of the nozzle, adding some pressure back to the system.. The Albuz AVI nozzle schematic below explains the venturi design.
Cross-section of the Albuz AVI venturi nozzle.
The tapered channel inside the nozzle is a venturi, which draws air into the nozzle via integrated ports. When low-drift nozzles are operated beside conventional nozzles at the same pressure, low-drift nozzles produce much fewer driftable fines, and also more larger droplets.
But while the two-stage design is useful for managing drift, it also conceals the actual operating pressure of the exit orifice in these tips. The exit orifice is important – it is the part of the nozzle that does the atomizing and that forms the pattern.
Let’s illustrate the pressure inside a low-drift tip by operating an air-induced low-drift nozzle at 60 psi. This nozzle has a pre-orifice size of 03 (0.3 US gpm at 40 psi, blue) and an exit orifice size of 06 (0.6 US gpm at 40 psi, grey). The operator sees 60 psi on the gauge. What is the exit orifice pressure?
The exit tip has twice the flow-rate of the pre-orifice, and therefore operates at one quarter the pressure, or 15 psi. Recall the square-root relationship between flow rate and pressure.
The relationship between spray pressure and flow rate. Doubling the flow rate requires a quadrupling of pressure
That’s not the whole story. The internal venturi is drawing additional air into the nozzle chamber, and depending on the operating pressure, this could be from 5 to 15 psi. The amount added depends on the specific nozzle, its flow rate, and its pressure. Let’s add 10 psi in this case. The exit tip is actually at 25 psi.
Now let’s assume the pressure gauge reads 40 psi, and that the venturi generates 5 psi additional pressure. The actual exit orifice pressure is now only 15 psi. This is at the lower limit at which a spray is atomized, and at which a good pattern can form.
Our general recommendation with venturi-style low-drift tips has been to avoid pressures below 30 or 40 psi for that reason. We’re trying to prevent the spray becoming too coarse for adequate coverage, and also to prevent the spray pattern from collapsing.
The upside of this design is that the same principle allows for much higher-pressure operation without creating excessive drift. These types of nozzle can, in fact, be operated at 70 to 90 psi without becoming very drift-prone because the pressure at which the spray liquid is atomized is likely only 30 or 40 psi (the actual exit pressure and drift potential will depend on the nozzle and the formulation).
Speed Range
A low-drift nozzle with a pressure operating range from 30 to 90 psi (i.e., 3-fold) would have a flow rate range of 1.73 (i.e., the square root of 3 due to the square root relationship of flow rate and spray pressure). This means that the fastest travel speed (at 90 psi) would be 1.73 times the slowest travel speed (at 30 psi).
A conventional nozzle operating between 20 and 60 psi would have the same travel speed range. So why don’t we just do that? The main reason is that the two-stage design lowers the overall amount of drift substantially, something a conventional nozzle can’t achieve even at very low pressures.
A second reason is that even at high pressures, a two-stage design will likely drift less than an conventional nozzle. This is still the case if the conventional nozzle is operating at low pressures. Any spray quality chart comparing spray qualities of conventional and low-drift tips will demonstrate that.
Pulse Width Modulation
PWM uses a solenoid to intermittently shut off nozzle flow, between 10 and 100 times per second (Hz) depending on the manufacturer. This has implications for nozzle design because the nozzle must not leak liquid during the brief off-cycle. If it does, the small amount of liquid leaving the nozzle will not only not atomize properly, it will also cause a pressure drop within the nozzle which must be replenished with the next on-pulse. This will mean the on-pulse will operate at a lower initial pressure, affecting pattern development and atomization. For this reason, venturi-style low-drift nozzles have not been recommended with PWM. The venturi provides an alternate exit for air or liquid, compromising nozzle performance.
And yet, some venturi style nozzles do, in fact, produce acceptable patterns with PWM according to the nozzle manufacturers. This goes to show that nozzle design can continue to evolve to provide the best in drift reduction technology with PWM. Design for PWM suitability should be at the top of nozzle manufacturers’ agendas.
Nozzle design continues to evolve. But in the foreseeable future, spray pressure will continue to control pattern width and droplet size. That’s why understanding the pressure limits of any specific nozzle type, and maintaining pressure within those limits, is so important in any spray operation.
April 4, 2017