Production Practices and Quality Assessment of Food Crops. Vol. 1

With centrifugal energy systems droplet sizes with optimum target collection efficiency
can be produced (Harden, 1992), meaning that less volume of total liquid
is needed as large drops are not produced. Drift can also be minimised, as there
are no droplets at the small end of the spectrum. However, CDA equipment requires
constant careful maintenance for optimal efficiency, and the system needs to be
understood to be used effectively as flow rate, liquid viscosity and rotational speed
are critical for droplet formation.
3.3. Airshear
The airshear principle involves the shattering of a liquid stream with a fast moving
column of air. The liquid is sheared into small droplets, with droplet size controlled
by the liquid/air ratio. Smaller droplets are produced with reduced liquid flow
and higher air velocities. In order to obtain maximum liquid shatter and correct
droplet formation, it is necessary to generate air speeds of 65–90 m/sec at the nozzle
(Alcorn, 1993). The energy of the motor is used primarily to drive the fan producing
the air flow and the ducting is designed to provide maximum air speed at the point
of droplet formation. Advantages of this system are the ability to use reduced carrier
volumes and the air movement created by the equipment moves droplets to the target
and provides energy for impaction/capture. The liquid/air flow ratio must be correctly
adjusted and maintained if a narrow droplet spectrum is to be produced.
Airshear can also be used for secondary break up of larger droplets produced
initially by hydraulic nozzles (e.g. orchard air-blast sprayers). The orientation of
the hydraulic nozzles in relation to the air stream affects the degree of secondary
airshear and therefore the droplet spectrum (Harden, 1992).
3.4. Kinetic
Droplets can be formed in either low-energy vibrators or ultrasonic atomisers. The
former tends to produce relatively large droplets and are not widely used in
pesticide application except where large volumes of pesticide per unit area and
minimum drift are required. Liquid is gravity fed through a series of orifices which
are vibrated to break up the liquid stream into droplets (Banks et al., 1990).
Ultrasonic atomisers use electric and magnetic transducers and can produce uniform
droplets as small as 50 micron (µm) in diameter.
3.5. Thermal
Droplets can be produced by vaporising the pesticide in a stream of hot gas which
is then cooled by rapid expansion. Condensation produces a dense fog of very
small droplets. Such droplets are very susceptible to drift. This system is not suited
to the orchard situation.
3.6. Electrodynamic methods
Spray Technology in Perennial Tree Crops 87
No mechanical energy is required for droplet formation in this system. The liquid
stream is fed between two charged plates which subject it to an intensely diver-