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About Ultrasonic Spray Nozzles
Nov 29, 2017

About Ultrasonic Spray Nozzles

As their name implies, ultrasonic nozzles employ high frequency sound waves, those beyond the range of human hearing. Disc-shaped ceramic piezoelectric transducers convert electrical energy into mechanical energy. The transducers receive electrical input in the form of a high frequency signal from a power generator and convert that into vibratory motion at the same frequency. Two titanium cylinders magnify the motion and increase the vibration amplitude at the atomizing surface.

Nozzles are configured such that excitation of the piezoelectric crystals creates a transverse standing wave along the length of the nozzle. The ultrasonic energy originating from the crystals located in the large diameter of the nozzle body undergoes a step transition and amplification as the standing wave as it traverses the length of the nozzle.

Since wavelength is dependent upon operating frequency, nozzle dimensions are governed by frequency. In general, high frequency nozzles are smaller, create smaller drops, and consequently have smaller maximum flow capacity than nozzles that operate at lower frequencies.

The nozzle body is fabricated from titanium because of its good acoustical properties, high tensile strength, and excellent corrosion resistance. Liquid introduced onto the atomizing surface through a large, non-clogging feed tube running the length of the nozzle absorbs some of the vibrate energy, setting up wave motion in the liquid on the surface. For the liquid to atomize, the vibrate amplitude of the atomizing surface must be carefully controlled. Below the so-called critical amplitude, the energy is insufficient to produce atomized drops. If the amplitude is excessively high, the liquid is literally ripped apart, and large “chunks” of fluid are ejected, a condition known as cavitation. Only within a narrow band of input power is the amplitude ideal for producing the nozzle’s characteristic fine, low velocity mist.

The fine control of input energy is what distinguishes ultrasonic atomizing nozzles from other ultrasonic devices such as welders, emulsifiers, and ultrasonic cleaners; these other devices rely on cavitation with input power of the order of hundreds to thousands of watts. For ultrasonic atomizing, power levels are generally under 15 watts. Power is controlled by adjusting the output level on the power supply.

Since the ultrasonic atomizing mechanism relies only on liquid being introduced onto the atomizing surface, the rate at which liquid is atomized depends solely on the rate at which it is delivered to the surface. Therefore, every ultrasonic nozzle has an inherently wide flow rate range. In theory, the “turn down” ratio (ratio of maximum to minimum flow rate possible) is infinite. In practice, this ratio is limited to approximately 5:1 as result of design constraints.

Ultra-low Flow Rate Capabilities

Since the ultrasonic atomizing process does not rely on pressure, the amount of liquid atomized by a nozzle per unit time is primarily controlled by the liquid delivery system used in conjunction with a nozzle.

Depending on the specific nozzle and the type of liquid delivery system employed (gear pump, syringe pump, pressurized reservoir, peristaltic pump, gravity feed, etc.), the technology is capable of providing an extraordinary variety of flow/spray possibilities.

Drop-size Range Selectivity

In general, the drops produced by ultrasonic atomizing have a relatively narrow size distribution. Median drop sizes range from 18-68 microns, depending on the operating frequency of the specific type of nozzle. As an example, for a nozzle with a median drop size diameter of approximately 40 microns, 99.9% of the drops will fall in the 5-200 micron diameter range.

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