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. See page 211.
Rotary atomizer with direct drive
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:
(15)
Where:
Dvs = Sauters mean diameter in ft (add 15-20% to get volume mean diameter)
K1 = Constant depending on the atomizer (0.37-0.40)
r = Radius of the wheel in ft
Mp = Mass flow per total wetted periphery (lbs/min. x ft)
P = Liquid density, lbs/ft3
N = Atomizer speed, rpm
μ1 = Viscosity, lbs/ft. x min.
Ơ = Surface tension, lbs/min2
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.