Ultrasonic “Double Boiler”

Maximum ultrasonic performance requires the most efficient transfer of ultrasonic vibrations from the ultrasonic transducers to the liquid in the process tank.  This is generally accomplished by applying ultrasonic transducers directly to the exterior surfaces of a tank containing the process liquid.  Transducer attachment techniques favor attachment to metals like stainless steel.  What if the chemistry to be used in the tank is not compatible with stainless steel?  One alternative that inevitably comes to mind is to use a metal tank with an internal coating that will be compatible with the chemistry.  Unfortunately, ultrasonic vibrations have been shown to quickly de-bond such coatings.

An option which has been proven practical in a number of applications is the use of a “double-boiler” approach where a secondary tank is positioned or floated in a primary tank made of stainless steel or another suitable metal.  Water is used to couple ultrasonic energy from the transducers mounted on the exterior of the primary tank to the walls of the secondary tank as shown below.

Setting up a tank in tank system is, admittedly, a bit cumbersome and maximizing ultrasonic penetration into the inner tank may require some experimentation and tweeking.  The following are a few tips that may be helpful – –

Glass is one material known to transmit ultrasonic energy well.  In cases where it is possible, pyrex beakers make excellent secondary tanks using one or more in a single primary tank.  In most cases, beakers are either suspended by their rims using a fixture or simply “floated” in the primary tank by filling each beaker to a level that will give it some stability.  Glass beakers are seamless and, although it may be my imagination, the rounded bottom configuration seems to focus sound waves in the beaker.  In no case should the beaker be set on the bottom of the primary tank as contact between the beaker and the tank bottom will cause erosion caused by surface cavitation.  The use of beakers as described here can also be employed to evaluate various cleaning chemicals without mixing up a batch large enough to fill a standard cleaning tank.  The clear beaker also facilitates viewing the cleaning in progress.

In larger applications, quartz or plastic insert tanks are usually used.  Designing such a system requires that several factors be considered including the tank material, the shape and thickness of the material, fabrication techniques, clearances and placement of the secondary tank in the primary tank as well as other considerations that are, unfortunately, not well defined.  For example, the laws of physics say that maximum transmission will be achieved if the thickness of the secondary tank bottom is equal to 1/2 of the wavelength of sound in the material.  Although this may be practical at higher ultrasonic and megasonic frequencies, it would indicate a bottom thickness of up to several inches at the more common ultrasonic frequencies below 100kHz – a design parameter that would be difficult to meet and has not been met (to my knowledge) in any existing equipment.  In general, it has been assumed that a thinner internal tank construction will result in better ultrasonic transmission.

It can not be argued that any obstruction to the transmission of ultrasonic energy will be of consequence.  Yet, as of this time, a “double boiler” construction is the only answer we have to cavitating liquids that are not compatible with the materials of construction of commercially available ultrasonic tanks.

–  FJF  –