As a leading ultrasonic manufacturer, we are often asked to manufacture large, industrial ultrasonic cleaning systems producing a particular ultrasonic power density based on laboratory trials. In fact, it may not be appropriate to base the watts per gallon requirement for a large industrial application on those found effective on a laboratory scale. A paper at the following link explains, generally, why this is so. The continuing blog below the link provides a little more depth.
Considerations for Ultrasonic Power Density vs. Tank Volume
The effectiveness of an ultrasonic cleaning system really doesn't depend totally on how many watts of power are delivered per unit of liquid volume. The key to cleaning success is how those watts are utilized. The real goal to assure effective ultrasonic cleaning is to make sure that there are a sufficient number of cavitation events occurring in the tank and that those cavitation events result in energetic implosions. To repeat what I've said in previous blogs, cavitation without implosion does very little to enhance the cleaning process.
Let's analyze, briefly, the life of a sound wave. Once ultrasonic sound waves are introduced into a liquid, all of the energy in the sound waves has to go someplace or we are breaking one of the laws of physics - "conservation of energy." In an ultrasonic cleaning tank, the ultrasonic energy ends up as either heat or sound radiated into the air. In fact, nearly ALL of the energy goes into heat as very little energy is consumed in sound radiated into air. The following link provides an interesting (and somewhat humorous) demonstration of how little energy is actually contained in sound radiated into air.
Although the transformation of sound energy into heat energy is a result of cavitation and implosion, a lot of other heat-producing mechanisms also take place due to the vibrations of the sound waves. Among these are heat losses in the ultrasonic transducers, heat losses due to internal friction of the liquid and heat losses caused by vibrations of the tank (and all of the things connected to it). These vibrations all result in heating due to internal friction. In fact, the ONLY vibration to heat transformation that benefits cleaning is that accounted for by cavitation and implosion. So, the key to effective use of ultrasonic energy is to maximize that portion of the ultimate conversion of sound energy into heat that is provided by the production and implosion of cavitation bubbles.
How does this relate to the reduction in the watts per gallon requirement for effective ultrasonic cleaning? As the tank size is increased, proportionately more sound energy results in the production and collapse of cavitation bubbles. Since cavitation and implosion are somewhat self-limiting (once the liquid is saturated with cavitation bubbles the existing bubbles prevent the growth of further bubbles by becoming absorbers of sound themselves), the addition of more sound energy will not result in additional cavitation but, rather, will be converted to heat by other means. The energy has to go somewhere, but it does not necessarily result in added cavitation and implosion. The key measure is not watts per gallon but, rather, how those watts are utilized that makes the difference.
Unfortunately, many potentially good applications for ultrasonic energy have been passed by for economic reasons because of the assumption that a particular watts per gallon density was required. In fact, they may have been practical had there been a better understanding of the concepts discussed above.
- FJF -