US 3849195 A
Ultrasonic cleaning of a surface is carried out by applying a film of liquid to the surface and directing ultrasonic energy into the film to cause cavitation thereof and subsequently rinsing the surface. Apparatus for cleaning small articles includes nozzles for spraying water over the articles, a compressed air transducer for generating the ultrasonic waves, a spray rinse for removing loosened surface dirt and a conveyor for moving the articles sequentially past the initial spray, the transducer and the spray rinse.
Description (OCR text may contain errors)
United States Patent [191 Powell, Jr. et al.
[451 Nov. 19, 1974 ULTRASONIC CLEANING  Inventors: Harry C. Powell, Jr., Faber, Va.
22938; Donald W. Floyd, Rt. 2, Box 36, Afton, Va. 22920 221 Filed: Aug. 3, 1972 21 App1.No.:277,640
 US. Cl 134/1, 15/313, 15/3.2, 99/451, 134/6, 426/237, 426/238, 426/286  Int. Cl A23n 13/00, B08b 7/02  Field of Search 134/1, 184, 199; l5/3.1, 15/316, 3.17, 3.2, 3.21; 99/451, 1, 217,100
R; 198/22, 22 R, 229; 259/DIG. 44; 426/235,
 References Cited UNITED STATES PATENTS 2,213,489 5/1938 Durand 15/3.2 2,477,006 6/1943 Pierson 2,806,246 9/1957 Simjian 2,881,080 4/1959 Simjian 3,102,290 9/1963 Sannes 3,440,094 4/1969 Adam et al. 134/1 OTHER PUBLICATIONS B. Brown and J. Goodman, High Intensity Ultrasonics Industrial Applications, ILIFF Books Ltd., London, 1965, pp. 5960.
Primary Examiner-Morris O. Wolk Assistant Examiner-Barry I. Hollander Attorney, Agent, or FirmCushman, Darby & Cushman 57] ABSTRACT Ultrasonic cleaning of a surface is carried out by applying a film of liquid to the surface and directing ultrasonic energy into the film to cause cavitation thereof and subsequently rinsing the surface. Apparatus for cleaning small articles includes nozzles for spraying water over the articles, a compressed air transducer for generating the ultrasonic waves, a spray rinse for removing loosened surface dirt and a conveyor for moving the articles sequentially past the initial spray, the transducer and the spray rinse.
4 Claims, 4 Drawing Figures ('am zezss'eu was I "/51 tit/t ULTRASONIC CLEANING This invention relates to wet-cleaning of surfaces and in particular to cleaning processes and apparatus in which the cleaning action of a liquid is enhanced by the use of ultrasonic energy. The use of ultrasonic energy in wet-cleaning operations is known. One common system employs an immersion tank in which the object or objects to be cleaned are immersed in a liquid following which the mass of liquid in the tank is exposed to sonic or ultrasonic vibrations. The vibrations are usually generated by a transducer, suspended in the liquid, which converts electrical energy into mechanical vibrations in the neighborhood of 20 kilocycles/second. The vibrations impart kinetic energy to the liquid with the result that there is a rapid formation and collapse of millions of very small bubbles, referred to in the art as cavitation. The collapse of the bubbles on the surface of an object immersed in the body of liquid loosens and removes surface accumulations of dirt and other material.
It is also known in the ultrasonic cleaning art to supply ultrasonic vibrations to a pressurized stream of liquid admixed with a gas which is then impinged on the surface to be cleaned.
In the above-summarized cleaning techniques, large amounts of liquid, either the entire tank full or the entire stream, are supplied with ultrasonic vibrations, even though only a portion of the cavitated liquid actually contacts the surface to be cleaned. The result is that a significant portion of the power input to the liquid is not utilized for cleaning. The present invention overcomes this disadvantage by providing a filmcavitation technique in which only a film of liquid on the surface is supplied with ultrasonic energy. More specifically the technique comprises wetting the surface with a film of liquid and directing a beam of ultrasonic vibrations from a transducer through the surrounding atmosphere into the liquid film so as to cause cavitation of the latter. When small articles are being cleaned, the articles are caused to rotate while in the beam so that all sides of the articles will be exposed to a cavitating film. In order to concentrate the cavitating energy in the location where needed the transducers will usually be provided with a focusing means.
The technique has particular utility in the cleaning of soft articles such as fruit and certain other food products for several reasons. In general, soft materials tend to absorb ultrasonic energy, and when such materials are placed in an immersion tank type of ultrasonic cleaning system they may absorb so much of the power that not enough is left for producing cavitation of the liquid. A given system can generally be made operational by increasing the power input to the transducer, but the resulting power per unit volume of liquid is then substantially higher than is required for the cleaning of hard objects, such as glassware or watch parts. The film-cavitation technique of the present invention is not nearly as susceptible to these disadvantages, owing to the relatively small volume of liquid in the film.
Another advantage of the film-cavitation technique with respect to cleaning soft materials is that it permits the use of relatively low frequencies (e.g., 3 to I kilocycles/second) which are less costly to generate than the higher frequencies (e.g., above 100 kilocycles/second. The absorption of ultrasonic energy by soft materials decreases as the frequency of vibration is increased, but increasing the frequency is not always economical because the amount of power required for initial cavitation is higher for the higher frequencies.
This consideration is not of great significance in the film-cavitation technique because of the small volume of liquid in the film.
In one preferred embodiment of the present invention a compressed air transducer is employed to produce ultrasonic vibrations in the range of about 3 to about 30 kilocycles/second in air. Higher frequencies may be employed if the atmosphere through which the vibrations are transmitted is, for example, helium. This type of mechanical transducer is capable of efficiently propagating ultrasonic vibrations through gases and of producing a beam which can be focused rather easily. Electrical transducers of the electro-mechanical, magnetorestrictive and piezo-electric type may also be used, although for many applications these devices are inefficient.
The invention will be further understood from the following more detailed description of an illustrative embodiment taken with the drawing in which:
FIG. 1 is a schematic longitudinal sectional view of an apple-cleaning device embodying the principles of the present invention;
FIG. 2 is a schematic transverse sectional view of the apparatus of FIG. 1;
FIG. 3 is a fragmentary view of the transducer of FIG. 1; and
FIG. 4 is an end view of the transducer.
For purposes of illustration the invention is hereinafter described in terms of an apparatus for cleaning apples but it will be understood that the principles of the film-cavitation technique are applicable to the cleaning of any surface, including the surface of other soft objects and of hard objects and thesurface of large stationary surfaces either flat or curved.
In FIG. 1 there is illustrated an apple-cleaning ma chine which includes a tank 10 and conveyor and brushing means in the form of two parallel rotating brushes 12 and 14 disposed above the bottom of the tank 10 and extending longitudinally of the tank 10. The brush-type conveyor may be of a conventional construction of the type which acts on objects, for example apples 16, placed in their nip to move the objects along the length of the brushes in the direction of the arrows while simultaneously and continuously brushing the surface of the objects and causing the objects to rotate so that all portions of the surfaces are brushed. To this end one of the brushes 12 or 14 should have its bristles arranged in a spiral pattern, and both brushes l4 and 12 should be driven, as indicated schematically by a motor M. Conveniently the shafts of the brushes may be joumaled in bearings 18 secured to the end walls of the tank 10.
Above the brushes l4 and 12 is disposed a liquid spray system for applying a film of wash liquid to the apples l6 and for subsequently rinsing the apples 16. The system may take any convenient form such as a pair of horizontal manifold pipes 20 on which nozzles 22 and 24 are provided. The nozzles 22 are located above the upstream ends of the brushes l2 and 14, and the rinsing nozzles 24 are located above the downstream ends of the brushes l2 and 14. Wash liquid is supplied to the manifold pipes 20 in any convenient manner either from a source (not shown) or from the lower portion of the tank by means of a pump 26.
lntermediate the nozzles 22 and 24, and preferably closer to the upstream nozzles 22 is a transducer 28 for generating ultrasonic vibrations and beaming them through the surrounding air to the film of wash liquid on the apples 16 passing beneath the transducer 28. In this illustrated embodiment the transducer 28 is a mechanical transducer of the I-Iartmann type, operated from a source of compressed air and utilizing the same principle as a steam or air whistle or organ pipe. This type of transducer offers the advantages of simplicity, ease of manufacture, high power output that is easily focused and simply compressed air for the power supply. While it may suffer from a lack of positive frequency control, this is not critical in most cleaning systems.
As shown in FIG. 3 the transducer 28 includes two main parts, an air jet member 30 and a resonator 32. The jet member 30 is constructed of a hollow cylinder having a conical end portion 34 which terminates in a small orifice 36, and a cylindrical end connected to a source of compressed air. The cylindrical end is fixed within an outer sleeve 37, as by means of set screws (not shown). The bore of the jet 30 tapers toward the orifice 36 as seen at 38.
The resonator 32 is a solid conical member arranged coaxially with the jet member 30 and provided at its apex end with a small cylindrical cavity 40 which faces the orifice 36. The opposite end of the resonator 32 terminates in a circular flange 42. The jet member 30 and the resonator 32 are fixed in position by means of three tie rods 44 having threaded ends which extend through unthreaded holes in the flange 42 and in the sleeve 37. Nuts 46 and 48 engage opposite surfaces of the flange 42 and sleeve 37, respectively, to fix the latter members in place and to allow for adjustment of their longitudinal positions during tuning of the system for greatest power output.
This type of transducer generates vibrations in the pattern of a thin circular disc, with its center at the orifice 36. In order to concentrate the energy in the form of a beam 49 at the surface of the apples 16 passing beneath the transducer, a reflector in the fomi of a parabolic metal strip 50 is disposed around the orifice 36. The strip 50 is set at a small angle to the plane of the orifice 36, as seen in FIG. 3, because the disc-shaped energy pattern radiating from the orifice 36 is in a plane which lies at an angle of 1015 to the plane of the orifice 36. The strip is fixed in position in any suitable manner, as by having its rear convex surface welded or brazed to two of the tie rods 44.
In operation of the apparatus the brushes l2 and 14 are rotatably driven by the motor M, and the apples 16 are fed sequentially to the upper surfaces of the brushes at the inlet end of the tank 10. An inlet chute 52 may be provided for feeding the apples 16 to the nip of the brushes. Rotation of the brushes revolves each apple 16 rapidly and at the same time moves it to the left toward the outlet end of the tank 10 where an outlet chute 54 may be provided. Prior to and after passing beneath the transducer 28 each apple 16 is sprayed with wash liquicl, for example water, which drains downwardly through the brushes into the tank 10. The liquid applied by the nozzles 22 drains away before the respective revolving apple 16 enters the energy beam 49 from the transducer so that the beam 49 acts only on the film of liquid remaining on the apple 16. The resulting cavitation of the film, which requires relatively low power owing to the small liquid volume of the film. loosens surface accumulations of dirt and natural waxes on the apple 16, and these accumulations are subsequently flushed into the tank 10 by liquid being sprayed from the nozzles 24.
While not illustrated in the drawing the cleaning technique may include several nozzles 24 and transducers 28, or more than one pass of the apples l6 beneath the transducer 28, so as to expose the apples 16 to a sequence of cavitation and rinsing operations.
Referring to the operation of the transducer 28 it has already been explained broadly that the latter produces a disc-shaped beam 49 of ultrasonic vibrations in the surrounding air which is focused downwardly onto the apples 16. Specifically, the air escapes from the orifice 36 at supersonic velocity and sets up regions of instability in the air stream. The effect of the resonator cavity 40, which is disposed in the air stream at a region of instability is to excite the air stream into oscillation at a frequency that is dependent upon the dimensions of the resonator cavity 40. In the illustrated embodiment supersonic air velocity through an orifice 36 of 0.1162 inches in diameter was obtained with air pressure of about 300 psi, using about 5 cubic feet of air per minute. The resonator cavity 40 was of the same diameter, and under these operating conditions the device generated about 55 acoustic watts at about 20 kilocycles per second. The dimensions of the apparatus are of course not critical, and as explained previously, frequencies of between 3 and 30 kilocycles per second in air are suitable.
The invention has been illustrated in simplified form and it will be understood that various modifications may be made to the illustrated process and apparatus without departing from the scope of the invention.
What is claimed is:
1. Apparatus for ultrasonically cleaning the surfaces of articles comprising: a transducer for generating a beam of ultrasonic vibrations in the surrounding atmosphere; conveyor means for transporting an article transversely through the beam and for simultaneously revolving and brushing the article, said means including two closely spaced parallel brushes rotatably driven in the same direction; liquid application means for wetting the surface of the article carried by the conveyor means with a film of liquid before the article enters the beam; and rinse means for rinsing the article after it has been exposed to the beam.
2. Apparatus as in claim 1 wherein said transducer is a mechanical device which generates the beam of vibrations from a stream of compressed gas.
3. Apparatus as in claim 1 wherein said liquid application means and said rinse means include nozzles for spraying liquid on to the revolving article as it is being conveyed by said brushes.
4. A method for removing surface dirt from articles of fruit comprising: wetting the surfaces of the fruit with a film of liquid; conveying the wetted fruit along a path while simultaneously revolving and brushing all surfaces of the fruit; generating a beam of ultrasonic vibrations and directing the beam through the surrounding atmosphere into the film of liquid while the articles of fruit are being conveyed, revolved and brushed so as to cause cavitation of the liquid film and thereby loosen material on the surface of the articles by the cavitation and the brushing; and rinsing the surfaces of the articles with liquid to remove loosened materials while the articles are being conveyed, revolved and brushed.