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Soft, Low-velocity Spray, Elimination of Overspray, Material
Savings, and Precise Control One feature that distinguishes pressureless, ultrasonic atomizing nozzles from most other spray nozzles is a soft, low-velocity spray, typically on the order of 6-14 cm per second. Other common atomization techniques, which use pressure in order to generate a spray, generally produce drops with velocities well over 100 times that generated by ultrasonic atomization. This velocity differential means that pressure sprays generate on the order of 10,000 as much kinetic energy as do ultrasonically atomized sprays. This striking contrast in spray energy has important, practical implications. |
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In coating applications, the unpressurized, low-velocity spray
significantly reduces the amount of overspray since the drops
tend to settle on the substrate, rather than bouncing off it.
This translates into substantial material savings and reduction
in emissions into the environment. The spray can be controlled and shaped precisely by entraining the slow-moving spray in an ancillary air stream. Spray patterns from as small as 1,5 mm wide to as much as 30-60 cm wide can be generated using specialized types of spray-shaping equipment. Ultra-low Flow Rate Capabilities Since the ultrasonic atomization 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. |
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| The flow rate range for the entire family of Sono-Tek ultrasonic nozzles is from as low as a few microliters per second to up to about 24 liters per hour. | ||
| 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. | ||
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Drop-size Range Selectivity In general, the drops produced by ultrasonic atomization 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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Ultrasonic Atomization Simply stated, when a liquid film is placed on a smooth surface that is set into vibrating motion such that the direction of vibration is perpendicular to the surface, the liquid absorbs some of the vibrational energy, which is transformed into standing waves. These waves, known as capillary waves, form a rectangular grid pattern in the liquid on the surface with regularly alternating crests and troughs extending in both directions as shown in the photomicrograph below. |
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| When the amplitude of the underlying vibration is increased, the amplitude of the waves increases correspondingly; that is, the crest become taller and troughs deeper. A critical amplitude is ultimately reached at which the height of the capillary waves exceeds that required to maintain their stability. The result is that the waves collapse and tiny drops of liquid are ejected from the tops of the degenerating waves normal to the atomizing surface. A useful analogy that helps visualize this process comes from our everyday experience. Ocean waves coming into shore go through a transition from stability on the open water to instability as they approach shore. The instability is evident as the waves form foamy breakers. | ||
| The reason for instability in this type of wave is that as it approaches shore, the bottom of the wave contacts the ocean floor and is slowed down by frictional forces. The wave top, on the other hand, continues to move ahead unimpeded. The net result is that the wave topples over. In this process of breaking up, a spray of tiny drops is ejected from the wave surface. Although the mechanisms governing the creation of a spray from capillary and ocean waves differ, the results are similar. | ||
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Ultrasonic Spray Nozzles 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. The nozzle is designed (as shown below) such that a nodal plane is located between the crystals. For ultrasonic energy to be effective for atomization, the atomizing surface (nozzle tip) must be located at an anti-node, where the vibration amplitude is greatest. To accomplish this the nozzle's length must be a multiple of a half-wavelength. 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. |
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| Liquid introduced onto the atomizing surface through a large, non-clogging feed tube running the length of the nozzle absorbs some of the vibrational energy, setting up wave motion in the liquid on the surface. For the liquid to atomize, the vibrational 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. | ||
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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 atomization, power levels are generally under 15 watts. Power is controlled by adjusting the output level on the power supply. Since the atomization 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. |
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| Different applications - different spray nozzles | ||
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