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

86 Sally A. Bound
ling the volume of liquid output, distributing the droplets in specified patterns and
providing direction for the droplets (Banks et al., 1990).
3.1. Hydraulic pressure
The use of liquid pressure to produce a spray is one of the oldest and simplest means
of atomisation. The conventional hydraulic nozzle is a device with a small hole.
When liquid is forced through the orifice under pressure, hydraulic energy spreads
it into a thin sheet which becomes unstable, disintegrating into spray droplets.
This disintegration process is largely uncontrolled, resulting in a wide range of
droplet sizes. There are a range of nozzle types which can be broadly categorised
as plain jet, fan, impact, and conical spray (see Alcorn (1993) for a further description
of each type). For a given nozzle, a minimum liquid pressure is required to
break up the liquid sheet and fully develop the pattern of spray for which the
nozzle was designed.
Advantages of hydraulic pressure are that the components are simple, cheap to
manufacture and easy to maintain and use. However, the different ways in which
the sheet breaks up leads to a wide droplet spectrum, causing problems of drift
from the smaller droplets and chemical wastage from the larger droplets. Other
disadvantages include rapid wear of components, particularly nozzle tips, and the
need to select appropriate nozzle types for a particular job. Care needs to be taken
to operate nozzles at the correct pressure and replace worn nozzles to maintain
spraying efficiency.
3.2. Centrifugal energy
Centrifugal energy atomisers consist of a spinning disc or a rotating cage in which
centrifugal force is used to accelerate liquid and produce droplets. This method of
droplet formation is known as Controlled Droplet Application (CDA) and produces
droplets whose size can be controlled within very close limits.
In the spinning disc, liquid is fed near the centre of a rotating surface so that
centrifugal force spreads the liquid to the periphery where droplets are formed.
The droplet spectrum produced depends on liquid properties such as density, surface
tension and viscosity, as well as the disc design, diameter, speed, feed position,
presence of grooves and teeth on the disc, surface properties of the disc and the
presence of multiple discs (Banks et al., 1990). At low flow rates, individual droplets
of near-uniform size are formed directly at the disc edge. As the flow rate increases
liquid leaves the disc in long continuous threads, or ligaments, which break up
into droplets. If the flow rate is increased further the disc edge becomes flooded
and the liquid leaves the disc as a sheet. In this case droplet formation is similar
to that of hydraulic nozzles and a wide range of droplet sizes is produced.
In rotating cage atomisers, a cylindrical cage that may be porous, perforated,
vaned or slotted replaces the spinning disc. The size of the droplets produced is
largely dependent on the rotational speed of the cage, however the type of mesh,
cage diameter and type of formulation being applied also impact on the droplet
spectrum.