Process Parameters

The effective application of an ultrasound cleaning process needs to take various parameters into consideration. While time, temperature and chemistry are just as important as they are with other washing technologies, there are other factors that have to be taken into consideration in order to maximise the effectiveness of the process. Those variables that regard the intensity of the ultrasound cavitation in the liquid are particularly important.
 

Cavitation maximization

Maximising the cavitation of the washing liquid is obviously key to the success of the ultrasound cleaning process. Many variables affect the cavitation intensity.

Temperature is the single most important parameter to be considered in relation to cavitation intensity maximisation. This is because many of the properties of the liquids that affect cavitation intensity are related to temperature. Changes in temperature result in changes in viscosity, the solubility of the gas in the liquid, the percentage diffusion of the gases dissolved in the liquid and the vapour pressure, each influencing the cavitation intensity. In pure water, the cavitation effect is maximised at approximately 160°F.

The viscosity of the liquid needs to be minimised for maximum cavitation effect. Viscous liquids are so and do not react quickly enough to form cavitation bubbles and a violent implosion. The viscosity of most liquids falls as the temperature increases.

For most effective forms of cavitation, the cleaning liquid must contain the minimum possible quantity of dissolved gas. The gas dissolved in the liquid is released during the cavitation bubble growth phase and obstructs its violent implosion, necessary in order to obtain the desired ultrasound effect. The amount of gas dissolved in the liquid falls as the liquid temperature increases.

The percentage diffusion of gases dissolved in the liquid increases at high temperatures. This means that, at the highest temperatures, liquid releases dissolved gases more readily than at low temperatures, helping to minimise the amount of gas dissolved in the liquid.

A moderate increase in the liquid temperature takes it closer to its vapour pressure, meaning that vaporous cavitation is easier to achieve. Vaporous cavitation, in which cavitation bubbles are filled by cavitation liquid vapour, is the most real form of cavitation. However, when close to boiling temperature, the cavitation intensity falls when the liquid begins to boil in the cavitation spots.

The cavitation intensity is directly related to ultrasound power at the power levels generally used in ultrasound washing systems. When the power increases substantially over the cavitation threshold, the cavitation intensity levels off and can only be increased further through the use of focusing techniques.

The cavitation intensity is inversely related to the ultrasound frequency. When the ultrasound frequency rises, the cavitation intensity falls due to the smaller size of the cavitation bubbles and their resulting less violent implosion. The reduction of the cavitation effect at higher frequencies may be overcome by increasing the ultrasound power.