There are three general ways in which air is currently used as a part of some spraying systems. These include air-assisted sprayers, twin fluid systems and air-shear systems.

Air-assisted sprayers

Air-assisted spray systems use a standard spraying system and standard hydraulic nozzles to produce the spray droplets.

Normally the air is generated by one or more large fans, and the air is transported using an air bag which is delivered as a ‘curtain’ or ‘twin curtains’. The air system is used to constrain and transport the droplets to the target and is completely separate from the liquid delivery.

The air volumes are relatively high, while the air velocity is normally relatively low (often less than 70 to 80km/h), but is generally enough to suit spraying speeds less than 20km/h.

Strengths

• Using conventional nozzles means that these systems are generally easier to understand, and selecting the spray quality is the same as it would be for a standard hydraulic sprayer.

• Air assistance offers the potential for reduced downwind buffers (no- spray zones) due to the reduction in drift potential.

• Improved canopy penetration for fungicide applications and late season desiccation of large crops.

• Air assistance can be retrofitted to many spraying systems.

Possible limitations

• Where minimal ground cover or crop canopy is present it is possible to increase the drift potential if the spray quality, air speed and orientation of the air stream are not correctly set up.

• The width of the boom the system is able to be fitted to may be limited (e.g. often less than 36 metres), depending on the manufacturer’s current models and design.

Twin fluid systems

Twin fluid systems utilise air from a compressor injected into the spray liquid, which is delivered through hydraulic anvil-style nozzles.

These systems operate the nozzles similar to the way that standard air-induction nozzles work, but rather than using a Venturi to draw air into the nozzle, a compressor is used to inject air into the nozzle body. The volume of air or air pressure is set by the operator or adjusted by a controller. This allows the nozzle to produce either air- included or solid droplets.

By controlling the air pressure from the compressor the droplet size can be adjusted, so that a range of spray qualities can be obtained from a single nozzle and orifice combination.

Liquid flow is typically regulated by a standard automatic rate controller. However, there are two main differences in the systems that are available: those that require air pressure to be adjusted manually and those that can do this automatically.

Some twin fluid spray systems will automatically adjust the air flow in response to the liquid flow, while in others the air flow must be adjusted manually. The ratio of air to liquid is critical and has a large impact on the range of droplet sizes produced. In systems where the air flow is fixed, increasing liquid flow due to increases in speed can actually produce larger droplets.

Systems where the air pressure has to be adjusted manually (standard AirTech® or Optispray®)

These systems usually have the ability to hold a spray quality over a slightly wider range of speeds than a standard single line sprayer. Generally, the Optispray® has a wider range of operating speeds than the standard AirTech® system.

Systems that can adjust the air pressure in response to liquid flow.

These systems maintain the spray quality as spraying speed changes (examples include AirTech® fitted with a ‘Magic Box’ or the TeeJet® Airmatic).

Strengths

• Spray quality can be adjusted without changing nozzles, by adjusting the air pressure supplied to the nozzle.

• Some systems can adjust air pressure automatically to maintain droplet size as flow rate changes in response to changes in spraying speed.

• The system utilises only one set of nozzles, with a range of inserts or restrictors that can be changed for large changes in the target application volume (L/ha).

Possible limitations

• A single orifice size (inserted into nozzle body) may not cover all application volumes without adjusting spraying speeds.

• The increase in the range of spraying speeds is modest compared to some other options.

• The inserted orifice that controls the flow rate may need to be changed to maintain spray quality when switching application volume, or increasing the spraying speed.

• Nozzles are typically angled backwards, which may increase drift potential.

• Compressors may alter the weight and balance of the sprayer, and may require additional maintenance.

• Increasing liquid flow rates without increasing air flow tends to increase the size of the droplets, as well as the range of droplet sizes produced.

Air-shear systems

Air-shear systems use relatively high-speed air to impact on a stream of liquid to produce the droplets. The relationship between the air speed (indicated by air pressure in the air delivery system) and the liquid flow is critical for droplet size (spray quality) and the uniformity of the droplet spectrum.

Air-shear systems tend to produce a more even range of droplet sizes when set up to produce smaller droplets (typically fine to medium spray qualities). As the droplet size is increased, the range of droplet sizes produced also tends to increase.

Air-shear systems tend to have a relatively narrow range of operating parameters where the system can perform at its optimum. Operating outside of the manufacturers’ recommended spraying speeds and application volumes will generally result in reduced spray coverage or increased drift potential.

Summary of systems that use air

All spraying systems that utilise air require the operator to spend time ‘experimenting’ to find the optimum spraying speed, droplet size and air flow to match the canopy and the conditions.

Using water-sensitive paper (WSP) and image analysis tools such as the SnapCard App, is a valuable way to assess spray deposits on crop canopies or standing stubble. When evaluating the coverage obtained, operators should consider testing a range of travel speeds and air volumes or pressures at a range of application volumes. Often each system will have a series of ‘sweet spots’ where the all the parameters required line up to give great coverage.

Sprayers that use air can greatly improve penetration into large crop canopies. However, great care is required in fallow and low ground-cover situations, as there is potential for increased drift if the operator gets the airspeed wrong.