4
GENERAL INFORMATION
The ultrasonic power supply converts 50/60 Hz line voltage to high frequency electrical
energy at 20 kHz or 40 kHz (20,000 or 40,000 cycles per seconds), depending on the
model. This high frequency electrical energy is transmitted to the piezoelectric transducer
within the converter, where it is changed to mechanical vibrations. The ultrasonic
vibrations are intensified by the probe and focused at the tip where the atomization takes
place. The liquid travels through the probe, and spreads out as a thin film on the
atomizing surface. The oscillating tip disintegrates the liquid into micro-droplets, and
ejects them to form a gentle, low velocity spray.
Unlike conventional atomizing nozzles that rely on pressure and high-velocity motion to
shear a fluid into small drops, the ultrasonic atomizer uses only low ultrasonic vibrational
energy for atomization. The liquid can be dispensed to the atomizing probe (nozzle) by
either gravity or a small low-pressure metering pump, and atomize continuously or
intermittently. The rate at which the liquid is atomized depends, within limits, solely on
the volume that is being delivered onto the atomizing surface, and the frequency. The
amount of material atomized can be as little as 2 µl/sec. Because the droplets typically
drift downward at low velocity under the influence of gravity, the probe should be
mounted with tip facing downward, and air disturbances kept to a minimum. A wide
variety of coatings, chemicals, lubricants, and particulate suspensions can readily be
atomized. However, factors such as viscosity, miscibility, and solid content deserve
consideration. For optimum atomization, the viscosity should be under 500 cps and the
solid concentration kept below 30%. Because the atomization process depends on setting
a liquid film into motion, typically the higher the viscosity – the lower the flow rate, and
the more difficult the application. The atomization of liquids containing long-chained
polymer molecules is problematic, even in diluted form, due to the highly cohesive nature
of the material. In many cases, mixtures with particulates can be atomized, because the
solids are simply carried along in the drops. The low transport velocity of the liquid
through the probe permits even abrasive slurries to be processed with negligible erosion
of the passageway. Compared with conventional pressurized nozzles, the feed channel
running through the probe and exit orifice are relatively large, and practically
uncloggable. Drop size is primarily a function of frequency, and the higher the frequency,
the smaller the drop diameter. The median drop size at 40 kHz is 50 microns.
Atomizing probes are one-half wavelength long tools that act as mechanical transformers
to increase the amplitude of vibration generated by the converter. They are fabricated
from high grade titanium alloy Ti-6Al-4V because of its high tensile strength, good
acoustical properties at ultrasonic frequencies, high resistance to corrosion, low toxicity,
and excellent resistance to cavitation erosion.
