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Publication numberUS3214101 A
Publication typeGrant
Publication dateOct 26, 1965
Filing dateMar 31, 1964
Priority dateMar 31, 1964
Publication numberUS 3214101 A, US 3214101A, US-A-3214101, US3214101 A, US3214101A
InventorsPerron Robert R
Original AssigneeLittle Inc A
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Apparatus for atomizing a liquid
US 3214101 A
Abstract  available in
Images(3)
Previous page
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Claims  available in
Description  (OCR text may contain errors)

Oct. 26, 1965 R. R. PERRON APPARATUS FOR ATOMIZING A LIQUID 3 Sheets-Sheet 1 Filed March 31. 1964 VENTOR. R tR. Perron Af orney Oct. 26, 1965 R. R. PERRON APPARATUS FOR ATOMIZING A LIQUID 5 Sheets-Sheet 2 Filed March 31, 1964 L Q R T WW w 1L R IL RU EC N w W B m M R 8 2 A O 6 6 7 N R T Q 3 T O E A q 7 u 5 o P% F A L 5 8 E 6 5 S 53 m 3 6 w I A l U I w I 1| A A E 7 7 W L Fig; 6

INVENTOR. Robert R. Perron SELF- RESONAN POWER OSCILLATOR Fig.5

Attorney United States Patent 3,214,101 APPARATUS FOR ATOMIZING A LIQUID Robert R. Perron, Beverly, Mass., assignor to Arthur D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts Filed Mar. 31, 1964,Ser. No. 356,323 11 Claims. (Cl. 239-102) This invention relates to atomizers and more particularly to atomizers suitable for use with burners designed to achieve complete and eflicient combustion of fuels.

Although the atomizer described hereinafter is suitable for atomizing a wide variety of liquids for many purposes, for convenience of presentation the apparatus will be described in terms of its use in atomizing fuel oil which is to be burned with air in a burner to supply thermal energy.

Fuel oil has long been known as an excellent source of heat, particularly for providing controlled heating for buildings such as homes and the like. In order to obtain the maximum thermal energy from fuel oil it is cutomary to generate large surface areas of the fuel oil through atomization. It is then subsequently necessary to provide proper mixing of the fuel oil and air to achieve proper combustion. A widely used type of oil burner for home heating, for example, burns fuel which has been atomized by forcing the fuel under pressure through a swirl nozzle embodying a small orifice. Often the fuel oil is pumped at a pressure of about 100 p.s.i. through the orifice to achieve the desired degree of atomization. This type of atomizer requires a high-pressure hydraulic system, which in turn means that considerable work must be done on the oil through the use of a high-pressure pump and an electric motor. Extended experience with the prior art atomizers described above has indicated that they normally require servicing about once a year to insure that the orifice remains open to permit the desired atomization. It would, therefore, be highly desirable to have available a-means for atomizing liquids and particularly fuel oil which was more reliable than the system now in use. It would also be desirable to have an improved atomizing device which could incorporate a low-pressure or gravity feed system which was safe, reliable, and achieved better atomization.

It is therefore a primary object of this invention to provide an atomizing device which is highly reliable in its operation and therefore which lends itself'in particular to home b-urner installations. It is another object to provide an atomizing device of the character described which is capable of self-regulation in that it is driven by a circuit which has the ability to keep in exact step with the natural frequency of the atomizer assembly. It is a further object of this invention to provide a device of the character described which achieves better atomization in that atomization can occur at a very low flow rate, and that the size of the atomized particles can be readily controlled by controlling the parameters of the atomizer. It is an additional object of this invention to provide an atomizer which does not require an orifice of such small size that it is subject to plugging. It is yet another object of this invention to provide such a device which does not require a motor for a high-pressure component, but which, on the contrary may use gravity feed or other convenient low-pressure means for delivery of the liquid to be atomized. It is yet another object to provide an atomizer which may be incorporated into a fuel burner, the entire apparatus being reliable, silent in operation, and safe to maintain. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the features of construction, combination of elements and the arrangement of parts which will be exemplified in the constructions hereinafter set forth and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings in which FIG. 1 illustrates the basic structure of the atomizer of this invention along with a suitable liquid feed means;

FIGS. 2 and 3 illustrate modifications in the method by which fuel oil is brought into operational contact with the atomizer;

FIG. 4 illustrates one embodiment of the atomizer showing the use of one type of clamping means;

FIG. 5 is an end view of the atomizer of FIG. 4;

FIG. 6 is another embodiment of the atomizer showing another type of clamping means;

FIG. 7 is a cross section of the transducer of FIG. 6 through the supporting means along line 77;

FIG. 8 presents a simplified equivalent circuit for the electromechanical transducer; and

FIG. 9 is a diagram of the power circuit for the atomizers of FIGS. 4 and 6.

In brief, the atomizing apparatus of this invention comprises a transducer, means for driving the transducer, a power supply means, and liquid delivery means. The transducer in turn comprises in combination an atomizing surface, means for oscillating the atomizing surface in a direction normal to the plane of the atomizing surface and at a supersonic rate. The power supply circuit associated with the transducer includes means for achieving a feedback voltage, and a voltage amplifier the output of which exhibits a phase relationship, as compared to the output of the power amplifier which is part of the transducer driving means, such that the complete system is caused to oscillate at a frequency determined by the acoustical resonance of the transducer and at an optimum frequency to achieve liquid atomization. As Will become apparent, modifications of these various components of the apparatus are possible and are to be construed as falling within the scope of this invention.

Although the supersonic oscillation of a surface to achieve atomization is known it is believed that the atomizer of this invention is unique in its incorporation of a voltage feedback component which achieves a self-regulating of the driving means such that the oscillating amplifier is in acoustical resonant frequency with the transducer and the driving means always drives the transducer at its natural frequency, whether or not that frequency is varied by changes in transducer temperature or by virtue of any other external influence.

The transducer portion of the atomizer of this invention is illustrated in FIG. 1. All of the figures and their accompanying descriptions will be directed toward the atomization of fuel oil as an example. However the method and apparatus are not limited to atomizing liquid fuels, but may be used with any other liquid equally well. FIG. 1 is presented to illustrate the basic structure of the transducer portion of the atomizer of this invention.' It

is seen to comprise a stepped horn formed of a large segment 10 and extension or small segment 11. A stepped horn of such a configuration is known to achieve a displacement amplification which is directly related to the ratio of the area of the large segment face to the area of the small segment face.

The small segment 11 affords an atomizing surface 12 at its terminal end. Fuel droplets 13 are developed on this atomizing surface 12. In the apparatus of FIG. 1 the liquid to be atomized is fed through a fuel conduit 14 which in this case passes through the stepped horn and terminates in an orifice 15 in the atomizing surface 12. The horn is driven by a suitable driving means which includes two piezoelectric crystals with a common electrode between them. The crystals are formed of a natural piezoelectric material such as quartz or a polarized ceramic such as a lead zirconate titanate polycrystalline material or a barium titanate polycrystalline material. The term crystal is used hereinafter to refer to both the natural and synthetic materials. In the structure of FIG. 1 the driving means comprises two crystals 17 and 18, between which is an electrode 19 which makes physical contact with the crystal faces.

An opposing mass stub 20 is mechanically aflixed to the transducer and is designed such that the nodal point occurs at the plane of the electrode. The stub must be so shaped and of such a length that an integral multiple of one-quarter wavelengths of the resonant frequency of the transducer occurs between the end of the stub and the nodal plane at the electrode. The choice of the number of one-quarter wavelengths will depend on whether the driving crystal is located at a nodal or antinodal point in the transducer. If it is located at a node point, the stub should be an odd multiple of quarter wavelengths. If at an antinode, the stub should be a multiple of half wavelengths. In the modification of FIG. 4, the op posing-mass stub is seen to take the form of an opposing stepped horn and the acoustical lengths between the tip of the horn and the nodal plane of the electrode is wavelengths. This is a preferred length for the stub.

In FIG. 1 the entire atomizing apparatus is assembled and held together by a mechanical clamping means which in FIG. 1 is shown as a screw 21. Other mechanical clamping means are illustrated in FIGS. 4-6. Finally, there is provided an electrical lead 22. The electrical contacts and circuits will be described and illustrated in detail in connection with the description of FIGS. 4-8.

Although it is preferred to introduce the fuel onto the atomizing surface 12 through a fuel conduit within the horn as shown in FIG. 1, it may be supplied to the atomizing surface by other means and techniques as illustrated in FIGS. 2 and 3. In FIG. 2 a liquid feed line 24 is located within the interior of the horn which has at its terminal end internal of the horn an enlarged flared space 25 which in turn is in communication with a porous body 26 through which passes the liquid to be atomized. The porous body 26 is held in position by extensions 27 of the small segment 11 of the horn, and the liquid is made available at the porous atomizing surface 28. In the modification in FIG. 3 the liquid to be atomized is supplied through an external conduit 30 and directed onto a solid imperforate atomizing surface 31 which is the terminal end of the small segment 11.

FIGS. 4-6 illustrate preferred embodiments of the atomizer structure of this invention. The embodiments of FIGS. 4 and 6 illustrate two forms of opposing stub masses and two different clamping means. In these FIGS. 4-6 electrical circuit and associated power means are indicated generally as a self-resonant oscillator. The circuit which makes up the power supply means of this invention will be described in detail with reference to FIGS. 7 and 8.

Turning now to FIGS. 4 and 5 it will be seen that the stepped horn formed of components 10 and 11 is directly opposed by a mass which in this embodiment is an identical stepped horn, the components of which are indicated by primed numbers. Because of the stresses set up in the horn, a generous fillet 33 is supplied at the point where the small segment of the horn 11 joins the large segment 10.

The two crystals 35 and 36 used in the embodiment of the atomizer of FIG. 4 are placed between the two horns and share a common electrode 37 which is typically a thin sheet of brass placed between them. The entire atomizer assembly is held clamped together by means of the collars 40 and 41, and the long joining bolts 42. The atomizer is joined to the main fuel line through a fixed line 46, which is positioned in collar 41, screw connector 47, adjustable line 48 and conduit connector 49. The

electrode 37 is connected to the power supply means through lead 51 and bolt 52 which passes through collar 41. Bolt 52 is affixed by means of nuts 53 and it passes through an insulating bushing 55 in collar 41, and is electrically insulated from the collar by means of insulating washers 54. FIG. 5 illustrates how ground connection 56 and the electrode, through bolt 52, are connected by lead lines 57 and 58 to the power-supply means 59.

The use of two crystals in the driving means has the advantage of permitting the outside electrodes of both crystals to be at ground potential which means that there are no hot surfaces associated with the atomizer since the electrode can be and is isolated from the surroundings.

FIG. 6 illustrates another means of clamping the horn and the use of an opposed mass stub 61 which is not a horn. Both the atomizing horn and the stub 61 are equipped with flanges 62 and 63, respectively. The two crystals 35 and 36 are positioned between these flanges and separated by the electrode 37. The entire assembly is clamped together by means of bolts 65 and 67, which are inserted through suitable holes around the periphery of the flanges, and held in their clamping positions by means of screws 66 and 68. Any suitable number of such bolts may be used.

FIGS. 6 and 7 also illustrate a preferred way of sup porting the transducer. As noted previously, the A wavelength stub dimension is preferred. One reason for this is that it permits the supporting of the transducer (such as shown in FIGS. 4 and 6) at a convenient nodal point. At this point in the stub mass 61 (FIG. 6) three holes are drilled. These are adapted to receive pins 101 which terminate in screw ends 102. These in turn are adapted to be screwed into a support ring 103. The pins 101 are of a size which permits them to move radially within the holes 100. By placing the supporting pins at a nodal point there is no longitudinal motion of the transducer to consider. However, due to the fact that the oscillation of the transducer effects tension or compression in the stub, there is radial motion at this nodal point and to clamp the stub firmly in a support would increase losses in the transducer. The supporting means illustrated in FIGS. 6 and 7 avoids the incurring of such losses by the use of the pins 101 which are free to move in a radial direction, thus putting no restraints on the radial motion of the transducer at this nodal point.

It has been known for some time that liquids can be atomized from a surface oscillating at ultrasonic frequency. However, effective atomization demands higher excursion than can be conveniently produced at the face of the piezoelectric materials. This in turn demands some form of mechanical amplification which is accomplished in the atomizer of this invention by clamping the crystal to a solid bar of metal which is of such size and shape as to form a resonant system with the crystal. This is the horn which is then necked down to increase the amplitude at the small end, the increase being the inverse ratio of the areas of the ends of the large segment 10 and the small segment 11. In this manner it is possible to attain amplification ratios as high as 20.

As pointed out in conjunction with the description of the basic structure of FIG. 1 there is also supplied in the assembly an opposing mass stub which permits placement of the center of the electrode between the crystals at a nodal point. The stub is wavelengths acoustical length from the end to the nodal plane at the electrode and serves to terminate the transducer at an antinodal point.

The horns, bolts, screws and the opposing mass stubs are preferably constructed of aluminum and the two segments of the horn are joined as previously noted with a generous fillet to prevent stress cracking. Aluminum is preferred since its stiffness to mass ratio is high, it has low internal damping, and is economical to fabricate using the same metal throughout, the thermal expansion of the material need not be considered in the design.

. Theclamping means used to assemble the atomizer (for example the collars, bolts, and screws of FIG. 4 or the flanges, bolts, and screws of FIG. 6) must be capable of bearing relatively heavy stress. For proper acoustic coupling it is necessary to have the ceramic transducer (crystal) and the aluminum horn surfaces flat to within about 0.0005 inch and to use clamping pressures of the order of 5,000 to 6,000 pounds per square inch. The clamping pressures must be maintained during and after the temperature cycling experienced by the atomizer in service. Final-1y, the clamp pressure should be applied at-a point of minimum motion, i.e., a nodal point. The clamping means shown in FIGS. 4 and 6 adequately meet these specifications.

Certain operational relationships are desirable to provide fuel droplets of optimum size and surface areas for efiicient combustion. The parameters which are adjustable to provide these relationships are transducer frequency, the extent of mechanical amplification which is determinedby the relative diameters of the large and small segments of the horn, and rate of fuel feed. The frequencies necessary to provide a sufliciently large number of small dropletsin the atomization process must be in the ultrasonic range and preferably in excess of 50 kc. For a fuel oil burner, droplets in a size range from 20 to 100 microns are acceptable. At a frequency of 55 kc., 50% of the mass of liquid atomized is in the range of 45 microns. Higher frequencies produce smaller drops but begin to impose impractical design limitations on the apparatus.

Maximum mechanical amplification which is achieved through decreasing the diameter of the small segment of the horn, is desirable. However, it is limited by the extent to which the small segment can be reduced and still retain sufficient stiffness to prevent any appreciable transverse motion or whipping of this segment. Once the oscillation frequency and mechanical amplification of any transducer are established the fuel feed rate can readily be determined. It should not be so great as to cause any appreciable quantity of the fuel to be forced beyond the atomizing surface before it can be atomized to the desired droplet size.

To make the atomizer of this invention reliable and eflicient it is necessary to provide a combination of driving means and power-supply means which are so coupled and coordinated as to assure that the horn is continuously and constantly operating at its natural frequency even though that frequency be changed through changes in horn dimensions brought about by thermal expansions and contractions. This is accomplished through the use of the phase shift oscillator circuit of FIG. 8 which in combination with a phase shift circuit, provides the phase shift required to maintain oscillation at the preferred operating frequency of the transducer.

Before describing the circuit of FIG. 9 in detail it will be helpful to point out the fundamental characteristics of the transducer circuit with reference to FIG. 8. An electromechanical transducer such as the one used for atomizing has an equivalent circuit as shown in that figure. In this simplified circuit C is the capacitance of the ceramic piezoelectric element, R is the resistance of the ceramic element, R is the motional resistance associated with internal mechanical losses, L is an equivalent inductance associated with the mass of the system, and C is an equivalent capacitance associated with the stiffness of the system.

This circuit will have two resonant frequencies, one associated with the series resonant circuit, comprised of L, C, and R and the other with the parallel circuit which consists of L, C, R and C As an example, a suitable transducer for this atomizer has a series resonant frequency of 55 kc., and a parallel resonant frequency of 55.7 kc. However, with the type of oscillator circuit employed to drive the transducer, the transducer and oscillator combination operates best at a frequency of 55.5 kc. The

6 preferred operating point is achieved by adjustment of the inductor L in the phase shift circuit, When properly adjusted, the phase of the amplifier input voltage is 180 degrees from that of the output voltage and the tube draws a minimum plate current.

In the circuit of FIG. 9 power from a line voltage is controlled by switch and the closing of the switch is indicated by indication light 71. The AC. line vlotage is introduced into a power transformer 72 and a full wave rectifier 74. A smoothing capacitor 75 insures the delivery [of smoothed DC. power into the pentode 78 by way of power line 76. Inductance 80 isolates the high frequency currents from the DC. power supply to prevent high frequency current from feeding back into the DC. power supply. Resistor 82 is a voltage drop resistor for the screen grid of pentode 78, while capacitors 84 and 87 are by-pass capacitors designed to by-pass the high frequency currents to ground so they will not feed back into the DC. powre supply. Capacitor is an output coupling capacitor. The high frequency currents are supplied to the crystals through this coupling capacitor 85 which has a low impedance to the high frequencies but is like an open circuit to the DC. plate voltage. Resistor 88 is a grid resistor designed to supply bias to the pentode 78.

A jack 90 is provided for plugging a milliammeter into the circuit for initial circuit adjustment. Plug-in connectors 91 are used to connect the atomizer to the power supply.

The phase shift network is indicated by the dotted line and numeral 92. It is seen to be comprised of resistor' 94, inductor 95 and capacitors 96 and 97.

It is now possible to explain the operation of the circuit making up the power-supply means. The four compo nents which are required for any system to oscillate are an amplifier, means for limiting the oscillation amplitude, a feedback circuit, and a frequency controller. As will be seen, amplification is provided by pentode 78 as the amplifiers; oscillation amplitude is limited to an equilibrium value by circuit non-linearities and a grid bias resistor 88; feedback is provided by the phase shift network; and frequency control is a result of the transduceramplifier circuitry combination.

Two conditions must be met for oscillations to be sustained. The first of these conditions is that there be a phase shift close to degrees between the output voltage applied to the transducer and the grid input voltage applied to the input of the amplifier. The second condition is that a self-regulating amplification factor is required such that equilibrium is attained. This occurs when the amplification of the amplifier is equal to one.

T he'first requirement, a 180-degree phase shift between the output and the input of the amplifier, is achieved by the phase shift network which is comprised of resistor 94, inductance 95, and capacitors 96 and 97. Inductance 95 is adjusted so that the required 180-degree phase shift occurs at the frequency at which the transducer prefers to oscillate.

The second requirement of sustained stable oscillation is provided by the grid bias resistor 88. Initially, the pentode 78 amplification is high enough to initiate oscillations. As oscillations build up and grid current flows, the bias increases. This increase in grid bias reduces the amplification of the tube and equilibrium is reached when the amplification of the tube is equal to one. At this point the power output is just able to sustain the amplitude of oscillations required to produce the output power, and there is no surplus to cause a further increase in amplitude.

The equivalent circuit for the mechanical resonance of the transducer can be considered as a tuned circuit containing an inductor and capacitor. This tuned circuit is the frequency controller for the frequency of the oscillator. The power dissipated in the transducer due to power factor loss in the ceramic dielectric and internal damping of the horn metal causes the transducer to warm up with operational time. Expansion of the metal of the horn with increasing temperature causes the transducer length to increase slightly. Although this increase in length causes a decrease in the resonant frequency of the transducer, the oscillator, nevertheless, remains locked in on this frequency and remains in step with the transducer. Thus the required self-regulation is achieved.

The following may be cite-d as an example of the operation of the atomizer of this invention. This example is meant to be illustrative and not limiting. An assembly, such as shown in FIG. 6, was constructed using two lead zirconate titanate polycrystalline ceramics as the driving means. The crystals were separated by a brass foil 0.005 inch thick as an electrode and the horn and opposed mass stub were aluminum, 1.0 inch in diameter and having 1.4 inches diameter flanges. The diameter of the small segment of the horn was 0.285 inch, the fuel conduit within the horn was 0.046 inch in diameter. The clamping means was likewise aluminum and exerted about 3800 pounds on the assembly.

A power supply circuit as shown in FIG. 8 was used and line voltage of 115 A.C. volts was employed. The transducer (crystals, horn, and mass stub) had a series resonant frequency of 55 kc. and a parallel resonant frequency of 55.7 kc. (see FIG. 8). An optimum frequency of 55.5 kc. was determined for normal operating conditions. When No. 2 fuel oil was fed to the atomizing surface 12 of the small segment of the horn, at a rate of onethalf gallon per minute, it was atomized to droplets which were distributed in size ranging between about 20 and 100 microns. Most of these droplets were less than 45 microns in diameter. Previous experience indicates that this degree of atomization is adequate for very etficient combustion in an oil burner.

The above description of the atomizer of this invention, as well as the illustrative example given, shows that there is provided an eflicient and reliable means for atomizing a liquid, particularly fuel oil. Moreover, the atomizer is so designed and constructed to provide such performance even in the changing temperature environment of a fuel oil burner in which it is incorporated.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. An apparatus for atomizing a liquid, comprising in combination (a) a transducer, comprising in combination (1) two piezoelectric crystals,

(2) an electrode between said crystals,

(3) a stepped horn formed of a large diameter segment and a small diameter segment joined through a fillet, said large diameter segment being in driving contact with the face of one of said crystals,

(4) an opposed mass stub being in driving contact with the face of the other of said crystals,

(5) liquid delivery means adapted to deliver the liquid to be atomized to the end face of said small diameter segment of said horn,

(6) clamping means adapted to maintain said stepped horn, said opposed mass stub and said crystals with their intervening electrode in said driving contact to form said transducer; and

(b) power supply means adapted to supply high frequency A.C. power to saidcrystals thereby to drive said transducer in the ultrasonic frequency range, said power supply means comprising in combination (1) a pentode amplifying means,

(2) power transforming means adapted to deliver DC. current to said pentode amplifying means,

(3) phase shift means adapted to effect a -degree phase shift between the input and the output of said amplifying means; and

(4) resistor means associated with the grid of said pentode and adapted to alter the bias of said grid whereby said output of said amplifying means sustains the amplitude of oscillations required to drive said transducer over a predetermined optimum frequency range.

2. An apparatus in accordance with claim 1 wherein said stepped hornand said opposed stub have flanges and said clamping means comprise means for joining said flanges with said crystals therebetween.

3. An apparatus in accordance with claim 1 wherein said clamping means comprise collars in holding engagement with said stepped horn and said opposed stub, and bolts joining said collars.

4. An apparatus in accordance with claim 1 wherein said stepped horn, said opposed stub and said clamping means are aluminum.

5. An apparatus in accordance with claim 1 wherein said liquid delivery means is a conduit Within said horn which terminates in said end face of said small diameter section of said horn.

6. An apparatus in accordance with claim 1 wherein said opposed mass stub is of such a length that an integral multiple of one-quarter wavelengths of the resonant fre quency of said transducer occurs between the end of said stub and the nodal plane at said electrode.

7. An apparauts for atomizing a liquid, comprising in combination (a) a transducer, comprising in combination 1) two piezoelectric crystals,

(2) an electrode between said crystals,

(3) a stepped horn formed of a large diameter segment and a small diameter segment joined through a fillet, said large diameter segment being in driving contact with the face of one of said crystals,

(4) an opposed mass stub having a length such that an integral multiple of one-quarter wavelengths of the resonant frequency of said transducer occurs between the end of said stub and the nodal point at said electrode and being in driving contact with the face of the other of said crystals,

(5) liquid delivery means adapted to deliver the liquid to be atomized to the end face of said diameter segment of said horn,

(6) clamping means adapted to maintain said stepped horn, said opposed mass stub and said crystals with their intervening electrode in said driving contact to form said transducer;

(b) support means adapted to position said transducer;

and

(c) power supply means adapted to supply high frequency A.C. power to said crystals thereby to drive said transducer in the ultrasonic frequency range, said power supply means comprising in combination (1) a pentode amplifying means,

(2) power transforming means adapted to deliver DC. current to said pentode amplifying means,

(3) phase shift means adapted to effect a 180-degree phase shift between the input and the output of said amplifying means; and

(4) resistor means associated with the grid of said pentode and adapted to alter the bias of said grid whereby said output of said amplifying means sustains the amplitude of oscillations required to drive said transducer over a predetermined optimum frequency range.

8. An apparatus in accordance with claim 7 wherein wavelength-s of the resonant frequency of said transducer occurs between the end of said stub and said nodal point at said electrode.

9. An apparatus in accordance with claim 7 wherein said supporting means is attached to said opposed mass stub at a nodal point.

10. An apparatus in accordance with claim 7 wherein said opposed mass stub has at least three recesses in its surface in a plane corresponding to a nodal point, and said supporting means comprises a supporting ring surrounding said opposed mass stub and pins in said supporting rings adapted to engage said recesses and move radially therein, whereby said transducer is supported at a nodal point in a manner to permit free radial oscillations of said transducer.

11. An apparatus in accordance with claim 7 wherein said opposed mass stub is a second stepped horn equivalent in dimensions to said stepped horn.

References Cited by the Examiner UNITED STATES PATENTS References Cited by the Applicant UNITED STATES PATENTS 2,453,595 11/ 48 Rosenthal. 2,481,620 9/49 Rosenthal. 2,834,158 5/ 58 Petermann. 2,504,831 4/50 Griss. 2,512,7 43 6/50 Han-sell. 2,881,092 4/59 Sedlacsik. 3,016,233 1/ 62 Olmstead.

EVERETT W. KIRBY, Primary Examiner.

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Classifications
U.S. Classification239/102.2, 310/325, 239/4, 310/316.1
International ClassificationF23D11/00, H02N2/00, F23D11/34
Cooperative ClassificationF23D11/345
European ClassificationF23D11/34B