Piezoelectric Transducer
What is the difference between an ultrasonic nebulizer and ultrasonic nozzle?
Ultrasonic nebulizers and ultrasonic nozzles both utilize piezoelectric transducers to generate atomized particles. Both systems apply voltage to piezoelectric transducers which will vibrate at high frequency in an up and down motion. Ultrasonic nebulizers utilize the surface of the piezoelectric disk as the surface of atomization, whereas an ultrasonic nozzle will use the piezoelectric transducer to vibrate a metal, such as titanium, at a resonate frequency. The vibration of this titanium is a similar concept to a vibrating tuning fork. This same up and down motion of the piezoelectric transducer is transferred to the ultrasonic spray nozzle tip, which will also vibrate in the same up and down direction. The surface of the nozzle is from which atomization occurs in contrast to a nebulizer which directly uses the surface of the piezoelectric disk. I have a great video showing this principal.
Both systems are called ultrasonic since they operate at the ultrasonic range which begins at 20 kilohertz or 20,000 hertz up to several gigahertz.
Ultrasonic Nebulizer
Ultrasonic nebulizers used industrially to create atomized sprays, typically operate at a frequency of 1-2 MHz. The power source is often in the 12-15 watts range. Frequency determines droplet size. A 2.4 MHz system for example will generate an average particle size of 1-2 microns. Compare this to a 1.65 MHz system and the average particle size may be in the 5-7 micron range.
Since the liquid is making direct contact with the piezoelectric transducer it can accumulate solids which will effect its performance, making many industrial nebulizers a maintenance challenge. Ultrasonic nebulizers are popular in spray pyrolysis applications.
Ultrasonic Nozzles
Ultrasonic nozzles operate at a much lower frequency to ultrasonic nebulizers and have physical limitations to their operational range. Think of an organ pipe. The larger the pipe the lower the sound, thus the lower the frequency. In contrast, smaller organ pipes make higher sounds, but they also vibrate more vigorously than larger ones. This is important concept in ultrasonic nozzle construction.
For several decades the operational range for ultrasonic spray nozzles was stuck between 25 and 120 KHz. Last several years, new material science and manufacturing practice have pushed this range a little further, but not by much. The main barrier is as nozzles are built at higher frequency they become very small, but packed with even greater forces, that generate higher stress and heat on the system. Why try to build a higher frequency nozzle? Droplet size is the answer. As frequency increases, the ultrasonic nozzle generates smaller droplets. Droplet size is important not only in many thin film applications, but where the spray is being transformed, such as spray drying applications, spray pyrolysis, etc. I mentioned that nebulizers are often used in spray pyrolysis and you would think smaller droplet size is better. Many studies have been done on the subject. See spray pyrolysis articles. Ultrasonic nozzles in some studies have shown that the resulting droplet and/or particle is smaller and more uniform when compared to nebulizers. This is a good point to make, many customer are so focused on droplet size they forget about how evaporation plays into their system set up. See my article on this topic. It is important to match the correct technology and frequency to the application. Don’t hesitate to reach out to us on this topic.