Ultrasonics and Turbulation Don’t Mix!

Previous blogs have explained the benefits of relative motion between the part being cleaned or rinsed and the liquid in contact with the part.  Relative motion can be accomplished by moving either the part or the liquid relative to the other.  Ultrasonic agitation has also been identified as beneficial to cleaning and rinsing for somewhat the same reason but on a “micro” scale.  Not surprisingly, combining ultrasonic agitation and the larger effect provided by part agitation or liquid turbulation may provide results superior to either those seen when either is used independently.

Now things get interesting!  Both turbulation (under water spray) and agitation of the part in the liquid provide relative motion between the part and the liquid.  One might assume that, since both provide the same effect, they could be used interchangeably.  The fact, however, is that turbulation is detrimental to the ultrasonic effect.  Let me explain.

Efficient transmission of sound at any frequency is, at least, somewhat dependant on the continuity of the sound conducting media be it gas, liquid or solid.  A good example of the effect of dis-continuity on the transmission of sound waves is two people talking over some distance on a calm vs. on a windy day.

Illustration showing the effect of wind on sound transmission
In the top illustration, the sound waves are not distorted by wind with the result that the listener can easily hear the speaker. In the lower illustration, distortion of the sound waves by wind make it difficult or impossible for the listener to hear the speaker.

On a calm day, sound waves travel effectively between the speaker and the listener.  On a windy day, however, the sound waves become distorted by the external effects of the wind with the result that it is difficult or impossible for the listener to hear the speaker.  In a more personal example, on a calm day I can easily hear church bells located about a mile from where I live.  On a windy day, I hear nothing.  I’m sure we can all think of examples of this effect.

Turbulation of liquid produces much the same effect on ultrasonic transmission as wind does on the transmission of audible sounds through air. The effectiveness of ultrasonic waves is significantly reduced once they are distorted by turbulation of the liquid.  For this reason, part agitation is often superior to liquid turbulation when it comes to maximizing a cleaning or rinsing effect.  The part moving through the liquid at reasonable speed, although it creates essentially the same relative motion between the part and the surrounding liquid, does not cause the liquid distortion seen in turbulated systems using turbulation to create the same relative part to liquid motion.  Although it is often a better choice, part agitation may be more difficult or costly than liquid turbulation using eductor nozzles or other devices providing a similar effect.

As an alternative to part agitation, turbulation and ultrasonics can be used sequentially to provide excellent results.  There is no real downside to providing ultrasonic agitation continuously in a system with both turbulation and ultrasonics, but the turbulation should be discontinued periodically for sufficient time to allow the ultrasonic waves to become established in the liquid and then resumed to complete the cleaning or rinsing cycle.


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