In rotary atomizers the liquid is continuously accelerated to the wheel
edge by centrifugal forces, produced by the rotation of the wheel. The liquid
is distributed cen-trally and then extends over the wheel surface in a thin
sheet, discharged at high speed at the periphery of the wheel. The degree of
atomization depends upon peripheral speed, properties of the liquid, and feed
rate.
The wheel should be designed, so that it will bring the liquid up
to the peripheral speed prior to the disengagement. Very often the wheels are
therefore with vanes of different design to prevent liquid slippage over the
internal surface in the wheel. The vanes also concentrate the liquid at the
disc edge, producing there a liquid film analogous to the one considered in
pressure nozzles. The wheel will act as a fan and air is sucked into the
concentrate due to the rotation. Different wheel designs and properties decide
how much air is incorporated in the atomized droplets.
In spite
of intensive investigations into the mechanism of atomization from rotating
atomizer wheels, the prediction of spray characteristics still remains
uncertain. The effect of individual variables has been established over a
limited range and there is only a few dealing with high capacity, high speed
industrial atomizers. However, the relation between droplet size and various
products and operation characteristics is as follows:
LIQUID FEED RATE
Droplet size varies directly with feed rate at constant wheel speed, and
will increase with increased feed rate (power of 0.2)
PERIPHERAL SPEED
The peripheral speed is depending on the diameter of the wheel and the wheel
speed and is calculated as follows:
Vp = ∏×D×N / 1000×60 (14)
Where:
Vp = Peripheral speed (m/sec)
D = Diameter of the wheel
(mm)
N = Speed of the wheel (r.p.m.)
The peripheral speed is widely
accepted as the main variable for adjustment of a specified droplet size.
However, it has been shown that droplet size does not necessarily remain
constant, if equal peripheral speeds are produced in wheel designs of various
diameter and speed combinations, and there is a tendency that bigger wheels
produce bigger particles all other things being equal. However, in the choice
of wheel diameter one should rather look at the reliability of the atomizer, as
the differences in spray characteristics are negligible. Further, smaller
wheels are easier to handle when cleaned.
VISCOSITY OF THE LIQUID
Droplet size varies directly with the viscosity (power of 0.2) and bigger
particles are therefore obtained when the viscosity in the feed becomes higher.
In order to ensure an optimal atomization, the viscosity is therefore normally
kept as low as possible, often by heating the concentrate prior to the
atomization. Regarding droplet size distribution this becomes broader with
increased viscosity - an effect sometimes used when powder bulk density is to
be increased.
The prediction of the mean droplet diameter can be
summarized in the following equation which was evaluated for peripheral speeds
not over 90 m/sec. However, experimental results from tests with peripheral
speeds up to 150-160 m/sec. have indicated that there is a close agreement
between the results obtained using the formula from above mentioned tests:
Where:
D
vs = Sauters mean diameter in ft (add
15-20% to get volume mean diameter)
K
1 = Constant depending on
the atomizer (0.37-0.40)
r = Radius of the wheel in ft
M
p =
Mass flow per total wetted periphery (lbs/min. x ft)
P = Liquid density,
lbs/ft
3 N = Atomizer speed, rpm
μ
1 = Viscosity,
lbs/ft. x min.
Ơ = Surface tension, lbs/min
2 n = Number of
vanes
H = Height of vanes, ft.
Above mentioned formulas for
predicting the mean diameter should naturally only be used as a guide and are
only stated to give the readers an idea of the relation between the mean
diameter and the various technical and technological parameters.