Ultrasonic Nozzle Flow Rate Ranges
Sono-Tek’s ultrasonic nozzles cover a wide range of flow rates, from a few microliters/min to as much as 6 gallons/hr.
The flow rate range of a specific nozzle is governed by three factors:
- Orifice size
- Atomizing surface area
- Liquid characteristics
The last factor, liquid characteristics, is covered in the section The Compatibility of Ultrasonic Atomization with Various Liquids. The more difficult a liquid is to atomize, the lower will be its maximum flow rate for a given nozzle.
Orifice size plays a principal role in determining both maximum and minimum flow rates. The maximum flow rate is related to the velocity of the liquid stream as it emerges onto the atomizing surface. The atomization process relies on the liquid stream spreading out onto this surface and creating capillary waves. At low stream velocity, surface forces are sufficiently strong to “attract” the liquid, and cause it to cling to the surface. As the velocity of the stream increases, a critical velocity is reached where the surface forces are overcome by the kinetic energy of the stream, causing the stream to become totally detached from the surface. (The act of pouring water from a pitcher is a good analog from our everyday experience.)
As a result of our observations over the years, the critical velocity is on the order of 13 in./sec. As an example, for a nozzle with an orifice diameter of 0.100 in., this translates into a maximum flow rate of about 1.7 gph (ml/sec).
In theory, there is no lower flow rate limit for any orifice size since the process is independent of pressure. However, in practical terms, lower limits do exist. As the flow is reduced, a point is reached where the velocity becomes so low that the liquid emerges onto the atomizing surface in a non-uniform circumferential manner, causing the atomization pattern to become distorted. In some applications, where stable spray patterns are unimportant (e.g. some chemical reaction chambers), this distortion may be tolerable. In other applications, where the integrity of the pattern is vital (e.g. surface coatings), the low-velocity stream distortions are unacceptable. As a practical matter in such cases, the minimum velocity of the stream from an orifice of a given size is about 20% that of the maximum velocity. For our example above, where the maximum flow rate is 1.7 gph, the minimum flow rate is approximately 0.35 gph.
The amount of atomizing surface area available is the final factor influencing the maximum flow rate available from a given nozzle. This aspect of ultrasonic nozzle theory is somewhat involved so that we will not go into details here. If you are interested in the details, further information is available.
An atomizing surface of a given size obviously has a limitation as to how much liquid it can support and still create the film that is required to create capillary waves. If the quantity “dumped” onto the surface becomes too great, it overwhelms the capability of the surface to sustain the liquid film.
Studies performed at Sono-Tek over a decade ago show that the maximum sustainable flow rate not only depends on the amount of real estate available, but also on the a nozzle’s operating frequency. Lower frequency nozzles can support greater flow rates than higher frequency nozzles having the same atomizing surface area. Taking into account both the surface area and operating frequency, we arrived at a partially experimental/partially theoretical relationship for maximum flow rate as follows:
The experimentally determined constant, k is primarily dependent on surface area, and is modified by the frequency relationship, which is theoretical. A is the atomizing surface area. The magnitude of k is on the order of 28,500. The chart below presents these results for the frequency range of nozzles produced by Sono-Tek. The vertical axis, specific flow rate, r, is simply the maximum flow rate, F max, divided by area, A.
In summary, there are three factors that can determine maximum flow rate for a given nozzle. However, in every instance, only one of these factors will set the limit. If we are dealing with a hard-to-atomize material,for example, it is likely that the maximum flow rate will not depend on orifice size nor available surface area, but solely upon the atomizability of the liquid. Similarly, if we have a nozzle with an orifice whose capacity exceeds that of the available atomizing surface area, the surface area becomes the limiting factor. This interplay among the limiting factors is important in specifying a nozzle for a given application.