BUCHI Ultrasonic Package - Manual (Page 17)

BÜCHI Labortechnik AG Description of function | 4
Ultrasonic Controller 17/46
4 Description of function
4.1 Purpose and design
The Ultrasonic Nozzle generates wet droplets with a mean size of 50–200 μm
after feed injection into the Mini Spray Dryer B­290. The droplet size depends
on the feed properties and process conditions such as solvent type, viscosity,
surface tension, type of feed, atomization power and feed rate.
4.2 Function
4.2.1 Ultrasonic Atomizing Nozzle
An ultrasonic atomizing nozzle is a device that vibrates at frequencies beyond
those of human hearing; that is, in excess of 20 kHz. The atomized spray pro­
duced results from the breakup of unstable capillary waves developed in the liq­
uid introduced onto the rapidly vibrating atomizing surface of the nozzle. Ultra­
sonic atomization is solely a surface phenomenon. The amount of liquid atom­
ized depends exclusively on the rate at which liquid is introduced onto the sur­
face.
Flow rate
Ultrasonic nozzles have infinite variability with respect to flow rate. Although
practical considerations related to the nozzle design limit this variability (typi­
cally the achievable ratio of maximum to minimum flow rates is at least 5:1), the
ability to precisely adjust flow rates by adjusting the rate at which liquid is deliv­
ered to the nozzle is often useful.
Spray velocity
The spray velocity is typically low with 18–36 cm per second as compared to
10–20 m/second for standard pressure atomizing nozzles. This approximately
100 fold reduction in spray velocity is equivalent to a 10’000 times reduction in
kinetic energy.
The reduced velocity and kinetic energy enables improved floating of the
droplets in the drying gas. This reduces the risk that droplets collide with the
spray chamber especially at the bottom before complete dryness.