5.3 Nozzle Selection, Droplet Sizes & Spray Patterns

5.2 Nozzle Selection, Droplet Sizes & Spray Patterns

The nozzle is arguably the most important component of any sprayer system. Nozzles control the amount of fluid applied (flow rate), dictate the spray pattern, and determine the size of the individual spray droplets. Choosing the wrong nozzle can result in poor pest control, wasted chemical, crop injury, and severe environmental contamination from drift.

Common Nozzle Types and Spray Patterns

Different agricultural tasks require distinct spray patterns. Nozzle manufacturers engineer specific designs to meet these needs.

Extended Range Flat Fan

Flat fan nozzles produce a flat, oval-shaped spray pattern with tapered edges (the volume is heaviest in the center and lighter at the edges). They are the standard choice for broadcast herbicide applications on boom sprayers. Because the edges are tapered, adjacent nozzles must overlap significantly (usually 30% to 50%) to achieve a uniform distribution across the entire boom width. The "Extended Range" (XR) designation means the nozzle can operate effectively over a wider range of pressures (e.g., 15 to 60 psi) compared to standard flat fans.

Air Induction (AI) / Drift Reduction Nozzles

Drift reduction is a paramount concern in modern pesticide application. Air Induction (AI) nozzles, also known as venturi nozzles, are specifically designed to minimize drift. They feature a small hole in the side of the nozzle body that draws air in as the liquid passes through. The air mixes with the liquid, creating large, air-filled (coarse) droplets.

Because these droplets are large and heavy, they fall quickly and are far less susceptible to being blown off-target by the wind. When the air-filled droplets strike the plant surface, they shatter into many smaller droplets, providing excellent coverage without the drift risk associated with producing small droplets directly at the nozzle tip.

Hollow Cone vs. Full Cone Nozzles

Cone nozzles are generally used when thorough plant coverage and canopy penetration are required, commonly for insecticides and fungicides.

• Hollow Cone: These nozzles produce a spray pattern with the liquid concentrated in a ring on the outer edge of a conical shape, with little to no fluid in the center. They typically produce small droplets that swirl and swirl into dense foliage, making them ideal for airblast sprayers and directed canopy spraying.
• Full Cone: These nozzles produce a solid, circular spray pattern filled with fluid. They produce larger droplets than hollow cone nozzles and are useful when a high volume of liquid is needed over a specific area, such as for soil-directed herbicide applications or drenching.

Flood Nozzles

Flood nozzles, or flooding flat fan nozzles, produce a very wide-angle (up to 160 degrees) flat spray pattern. The liquid stream strikes a deflector surface, creating a wide swath of large droplets. Because of the wide angle, they can be mounted lower to the ground and spaced further apart on the boom. They operate at low pressures and are excellent for minimizing drift when applying pre-emergent herbicides or liquid fertilizers, though their pattern is less uniform than standard flat fans.

Droplet Size Classifications and VMD

Droplet size is the critical balance point between coverage and drift.

• Small droplets provide excellent coverage (more droplets per square inch) and stick well to plant surfaces, but they are lightweight and highly prone to drift.
• Large droplets fall rapidly, minimizing drift, but they may bounce off leaves or leave gaps in coverage.

To standardize droplet measurement, the industry uses the Volume Median Diameter (VMD), measured in microns (µm). A micron is 1/1,000th of a millimeter. If a nozzle has a VMD of 300 microns, it means that 50% of the total spray volume is made up of droplets larger than 300 microns, and 50% of the volume is made up of droplets smaller than 300 microns.

The ASABE (American Society of Agricultural and Biological Engineers) S572 standard categorizes droplet sizes using a color-coded system that applicator labels increasingly reference:

Applicators must read the pesticide label carefully; many labels now strictly mandate the use of nozzles that produce a specific droplet category (e.g., "Must be applied with a Coarse or coarser droplet size") to legally apply the product.

Nozzle Material and Wear Resistance

Nozzle orifices wear out over time. As the hole gets larger, the flow rate increases, leading to over-application, increased costs, and potential crop injury. The wear rate depends heavily on the abrasiveness of the spray mixture (wettable powders are highly abrasive) and the material the nozzle is made of.

The hierarchy of nozzle material wear resistance is generally: Ceramic > Hardened Stainless Steel > Stainless Steel > Polymer (Plastic) > Brass > Aluminum

• Ceramic: The most wear-resistant and most expensive. Ideal for highly abrasive formulations like wettable powders and dry flowables.
• Brass: Very inexpensive but wears out very quickly. Generally not recommended for modern agricultural applications, especially with abrasive products.
• Polymer: Offers an excellent balance of moderate cost and good wear resistance, making it very popular for general use.

Applicators must regularly calibrate their sprayers to detect nozzle wear. If a nozzle's flow rate exceeds the manufacturer's specification by 10%, the nozzle must be replaced.

Boom Configuration: Height, Spacing, and Overlap

When using flat fan nozzles for broadcast applications, the spray patterns of adjacent nozzles must overlap to ensure uniform coverage across the field. Standard flat fan nozzles are designed with tapered edges—meaning they apply less volume at the very edges of the pattern.

To compensate for these tapered edges, the boom must be configured so the patterns overlap by 30% to 50%. Achieving this specific overlap depends entirely on the relationship between two factors:

1. Nozzle Spacing: The physical distance between nozzles on the boom (commonly 20 or 30 inches).
2. Boom Height: The distance from the nozzle tip to the spray target (crop canopy or soil).

If the boom is set too high, the overlap will exceed 50%, resulting in heavy, concentrated streaks of pesticide. If the boom is too low, the patterns will not overlap adequately, resulting in untreated streaks (skips) between the nozzles. Operators must constantly monitor boom height, especially on uneven terrain, to maintain the critical 30-50% overlap necessary for an even, effective broadcast application.