How Ultrasonic Cleaning Process Work?
Ultrasonic cleaning has, however, been used to great advantage for extremely tenacious deposits, such as corrosion deposits on metals. In any case, cavitation forces can be controlled, thus given proper selection of critical parameters, ultrasonic can be used successfully in virtually any cleaning application that requires removal of small particulates.
Although the ultrasonic cleaning process has been used for over half a century, no reliable means of quantifying its cavitation activity has ever been developed. Indirect methods of measurements, such as erosion tests on metal surfaces, soil removal from weighted samples, acceleration of chemical reactions, thermodynamics studies, and white noise measurement, have been employed to a limited extent, but none of these methods has proved to be effective.
Thus, operators who seek to assess the performance of an ultrasonic cleaning system must rely almost exclusively on the evaluation of actual cleanliness levels achieved. Surface patterns produced on cavitating liquids can also be observed, as can the overall degree of agitation of the cleaning medium. Operators have also observed erosion patterns produced on aluminum foil erosion test,” as it is called, has come to be recognized as a fairly dependable, albeit subjective, means of demonstrating the existence of cavitation in ultrasonically agitated media. The measure has been used not only to provide an indication of the distribution of the sonic field throughout the bath, but also to locate the sites of the nodes and antinodes of the standing sonic waves. It can also generate fairly reliable side – by- side comparisons of different ultrasonic cleaning systems. In no way, however, can it be used to obtain quantitative measurements of cavitation activity.
The cavitation intensity in a sonic field is largely determined by three factors:
- The frequency and amplitude of the radiating wave.
- The colligative properties of the medium, including vapor pressure, surface tension, density, and viscosity.
- The rheological properties of the liquid, including static condition, turbulent flow, and laminar flow.
Frequency and Amplitude
The radiating-wave frequencies most commonly used in ultrasonic cleaning, 18-120 kHz, lie just above the audible frequency range. In any cleaning system, however, the harmonics of the fundamental frequency, together with vibrations originating at the tank walls and liquid surface, produce audible sound. Thus, an operating system that is fundamentally ultrasonic will nonetheless be audible, and low frequency (20-kHz) system will generally be noisier than higher-frequency (40- kHz) systems.
Moreover, ultrasonic intensity is an integral function of the frequency and amplitude of a radiating wave, therefore, a 20-kHz radiating wave will be approximately twice the intensity of a 40-kHz wave for any given average power output, and consequently the cavitation intensity resulting from a 20-kHz wave will be proportionately greater than that resulting from a 40-kHz wave.
The cavitation phenomenon will, of course, occur less frequently at 20 kHz, but this is not thought to have a significant bearing on cleaning effectiveness. However, the longer wavelengths of low frequency ultrasonic systems result in substantially different standing –wave patterns throughout the liquid medium.
The standing or stationary waves produced by ultrasonic in liquid media result from the simultaneous transmission of the surface-reflected wave motion and the wave motion originating at the transducer radiating surface. The fixed points of minimum amplitude are called nodes, and the points of maximum amplitude are called antinodes.
Obviously, the distance between the nodes and antinodes of the 20-kHz standing wave (2in.) will be approximately twice that of the 40-kHz wave. Because cavitation takes place primarily at the antinodes, the distance between cavitation sites will thus be larger with 20-kHz than with 40-kHz radiation, and the 20-kHz waves will also have larger dead zones (i.e., zones with little or no cavitation activity).
It is for this reason that cleaning resulting from 20-kHz radiation is likely to be less homogeneous and less consistent, even though this frequency produces more intense cavitation. Much of the inhomogeneity in ultrasonic fields can, however, be reduced or wholly eliminated through the use of sweep frequencies, or radiating waves with a multitude of different frequencies. By this means, several overlapping standing waves can be generated at the same time, thereby eliminating much of the dead zone.
The amplitude of the radiating wave is directly proportional to the electrical energy that is applied to the transducer. In order for cavitation to be produced in a liquid medium, the amplitude of the radiating wave must have a certain minimum value, which is usually rated in terms of electrical input power to the transducer. No cavitation can occur below this threshold value and the use of electrical power over and above the minimum level result not in more intense cavitation activity but rather in an increase in the overall quantity of cavitation bubbles. The minimum Power requirement for the production of cavitation varies greatly with the colligative properties and temperature of the liquid and with the nature and concentration of dissolved substances.
