Importance of Liquid Velocity
Ultrasonic spray nozzles unlike conventional air driven nozzles, do not rely on the force of air to break apart a liquid stream for atomization. Instead, atomization is created by an entirely different process (see How Ultrasonic Nozzles Work). In the case of conventional air driven nozzles, flow rate or liquid velocity is largely driven by how much pressure (p.s.i.) is used in the system, i.e. the higher the air pressure, the higher the flow rate. Ultrasonic nozzles on the other hand are not necessarily dependent on air pressure for dictating flow rate. This obvious advantage of not needing air to create atomization is the resulting plume has little to no kinetic energy. Ultrasonic nozzles can be said to be airless systems under certain conditions. Air shaping is typically used for shaping the atomized plume to give it force and direction. Air is used in this case as a secondary assist. True airless systems are confined to applications that utilize the plume such as in chemical vapor deposition, spray pyrolysis, spray drying, vacuum applications, etc. Typically applications where a chamber is involved and a plume is created.
So if flow rate is not a function of air, what drives liquid velocity in the case of an ultrasonic nozzle? The answer is the pumping mechanism and the orifice or hole in which the liquid is dispensed to the surface of an ultrasonic nozzle for atomization. Let’s look at the importance of the orifice first.
When a liquid is pumped through an ultrasonic nozzle, it is critical that the flow rate not exceed the critical velocity of the nozzle design. The critical velocity is where the exiting liquid is unable to wet out properly onto the nozzle tip. In an extreme example, if the pump causes the nozzle in layman terms, to “pee-out,” the liquid velocity is far in excess of critical velocity. This condition will exist even with the nozzle powered off. More common, is a condition in which the critical velocity is only moderately exceeded. Resulting atomization is incomplete, where some liquid wets out and properly atomizes while other material is not atomized creating large droplets. For this reason, published flow rates per a given orifice size are provided that allow one to operate an ultrasonic nozzle within a safe margin of exceeding critical velocity.
At the other extreme of critical velocity are liquid velocity or flow rates so low, that a liquid does not properly wet out concentrically on the tip. Imagine water exiting a garden hose at high pressure verses a trickle of water flowing in a large sewer drain. A garden hose, the exiting water touches all parts of the nozzle opening, verses a sewer drain will have water only touching the opening at the lowest point. For proper atomization, a minimum amount of flow is required for the liquid to wet out properly 360 degrees at the point of exit.
Ultrasonic nozzles can have a variety of different orfice sizes for a give frequency and tip design. It is important to understand the application requirements, thus required flow rate range. Microspray can help you determine the appropriate frequency nozzle, tip design and orfice.
Ultrasonic nozzles can be used with a variety of liquid delivery systems, such as syringe pumps, gear pumps, peristaltic pumps, pressure tanks and gravity fed reservoirs. Irrespective of the system that is used, as long as the liquid is delivered in a stead flow within the operational range of the nozzle any of these systems will work. Pulsation however should be avoided since a pulse can result in the liquid falling outside the operational range even if momentarily. This is especially pronounced with low flow applications such as stent coatings. It is not uncommon for a customer wanting to push a nozzle below its designed range which will result in less than ideal atomization and/or pulsation. Many customers find that using ultrasonic technology for the first time they use a lot less material due to the high transfer efficiencies of low velocity spray, thus less material needs to be dispensed.