Ultrasonic Systems Operating Considerations
The capillary wave mechanism that governs ultrasonic atomization is discussed on the Nozzle Technology page. There, it states that the atomization process is confined to a relatively narrow input power range. Below the critical power level, there is insufficient energy to cause atomization. The power range in which atomization proceeds normally is generally confined to a narrow region, approximately 1-2 watts above the critical power level. At power levels above this range, the liquid is literally “ripped apart” by the excess energy provided, causing large chunks of material to be expelled, rather than the characteristic soft spray of fine drops. This condition is known as cavitation.
The exact magnitude of power required depends on several factors:
- Nozzle Type
- Liquid Characteristics (e.g. viscosity, solids content)
- Flow Rate
Nozzle types in ultrasonic systems, because of their specific geometry and other factors, will generally have a different critical power level for the same liquid. For example, the critical power level of a 48 kHz nozzle, designed with a conical atomizing surface to deliver a wide spray pattern at substantial flow rates, will generally be in the neighborhood of 3.5-4 watts of input power when atomizing water. Another nozzle, operating at the same frequency, but designed for microflow operation (a very small atomizing surface), may require only 2 watts to atomize water.
The type of liquid being atomized strongly influences the minimum power level. More viscous liquids or liquids with high solids content generally increase the minimum power requirement. For example, the 48 kHz nozzle with a conical atomizing surface mentioned in the last paragraph, might require at least 8 watts of input power if the liquid being atomized were a 20% solids-content, isopropanol based material. See The Compatibility of Ultrasonic Atomization with Various Liquids for further information on how the nature of a liquid determines whether or not a material is a good candidate for ultrasonic atomization.
The flow rate also plays a role in determining minimum power level. For a given ultrasonic system, the higher the flow rate, the higher will be the power required, since the nozzle is working harder at higher flow rates. See Flow Rate Ranges and Liquid Delivery Issues for further information on how flow rates bear on a nozzle’s capabilities to atomize.
The piezoelectric transducers that comprise the active elements of ultrasonic nozzles are limited as to maximum operating temperature. The limit is characterized by the Curie point, defined as the temperature at which the piezoelectric property of a material vanishes, as a result of the loss of its permanent polarization. For the lead zirconate-titanate transducers used in ultrasonic nozzles, the Curie point is just over 300 degrees C.
However, this does not mean that the transducers can be operated at temperatures anywhere near this limit, because the degradation in piezoelectric performance degrades gradually, not suddenly, with increasing operating temperature. A practical upper limit is approximately 150 degrees C. There is no lower temperature limit.
Therefore, the nozzles incorporating these transducers are likewise limited as to operating temperature, both in terms of the environment in which they can be placed and the temperature of the liquid running through them. Methods have been developed for air or gas cooling so that it is possible to operate nozzles at elevated temperatures under certain circumstances. Another factor that must be included in the thermal equation is that the nozzles themselves generate some heat. It is possible for a nozzle operating at a high power and at a 100% duty cycle to experience a 30 degree C temperature rise. Although this represents an extreme case, this factor should be remembered in assessing what, if any, cooling is required.
Consult Sono-Tek if you have an application that involves elevated temperatures. A solution using one of our precision ultrasonic systems is usually available.