How fluid circulation affects ultrasonic cleaning
The effectiveness of an ultrasonic cleaning system is greatly reduced when the cleaning fluid is in motion, or circulating. This is the primary reason why filtration systems which are installed on an ultrasonic cleaning tank are typically operated intermittently, or with significantly reduced flow rates. Therefore, fluid circulation in ultrasonic cleaners is something that is generally avoided.
When ultrasonic cleaning fluid is in motion, cavitation is difficult to achieve since cavities are created when sound waves separate fluid molecules momentarily, followed by an immediate collapse of the cavity which products a microscopic jet of liquid which strikes the surface of the parts to remove contamination. When fluids are in motion, the ultrasonic energy is never working on the exact same location in the fluid at any time, which prevents ultrasonic cavitation from occurring. In addition, the turbulence can buffer the jet of liquid which reduces its striking force on the surfaces of parts being cleaned.
How much circulation can take place before ultrasonic cleaning is affected?
The amount of circulation that can exist before ultrasonic cleaning action is diminished, and how fast the system can recover from fluid circulation once the circulation source has stopped, will depend upon the size and shape of the tank being considered, the amount of liquid flowing, and the location of the liquid flow. The amount of time it takes before ultrasonic cleaning action is restored is called the Circulation Recovery Period. Small ultrasonic tanks tend to have greater watt densities than larger tanks, and circulating fluids do not remain in motion for long after the circulation source has stopped. However, large tanks over 100 gallons in size may require several minutes before circulation has stopped to a degree where full ultrasonic power is restored. Not only do fluids remain in motion longer in larger tanks, but watt densities are also less in larger systems, which exacerbates the problem. Also, square or round tanks will require more time for ultrasonic recovery after the circulation source has been stopped since these tanks require more time to reduce fluid circulation. There are no right-angle walls to reduce the turbulence.
The LOCATION where the liquid flows also plays a role in how much the process is affected. Liquids flowing across the surface of a bath such as encountered when including spargers which push floating oils off of the surface of the liquid will not drastically affect the process whereas liquid turbulence directly above the transducers will greatly affect the process.
How is degassing affected by turbulence in an ultrasonic tank?
The state of ultrasonic degassing will also play a role in the circulation recovery period as well. When fluids are not fully degassed, the recovery period is extended since the dissolved gasses are circulating as well, thereby creating a moving wall of entrapped air which further reduces the effectiveness of the system. However, there is a positive side to this property. By circulating the fluid during the degassing period, the rate of degassing is increased. Fluids can be degassed much faster if fluids are in motion or lightly circulating since the moving air entrapped within the fluid is allowed to join with other smaller air bubbles which can float to the surface. More information on ultrasonic degassing can be found in the Technical Info drop down menu above under “Degassing of Fluids.”
Does turbulence in a deionized water rinse tank affect ultrasonic cleaning?
Fluids which are circulating in deionized water rinse tanks are subject to complete elimination of ultrasonic activity. Deionized water is very difficult to cavitate for any ultrasonic cleaning system since the surface tension of the liquid is very high, making molecular separation of water difficult to achieve. Circulating deionized water is highly detrimental to the ultrasonic cleaning action in these tanks.