US3491850A - Sound silencing apparatus - Google Patents

Description

Jan. 27, 1970 l. HEITNER SOUND SILENCING APPARATUS Filed July 12, 1967 FIG. 2

FIGJ

United States Patent 3,491,850 SOUND SILENCIN G APPARATUS Irving Heitner, New York, N.Y., assignor to Pullman Incorporated, Chicago, Ill., a corporation of Delaware Filed July 12, 1967, Ser. No. 652,809 Int. Cl. F01n 1/10; F02m 35/00 U.S. Cl. 18135 16 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method for filtering and silencing sound. This invention also relates to a novel apparatus employing this method for filtering and silencing sound.

In particular, this invention relates to a method and to an apparatus for attenuating, silencing and filtering noises from high pressure and high velocity fluid flow through a confined exhaust passage.

As herein employed, sound silencing and noise attenuation shall mean the silencing, filtering, attenuating, blocking and reducing of audible noise levels to such intensity as to eliminate physiological damage from prolong exposure thereto.

Most devices available for silencing the sound of a gas in flow embody two considerations within their design, these being acoustical and aerodynamic. The acoustical considerations are those concerning the attenuation of incident noise through diffusion and absorption; aerodynamic considerations are those concerned with the amount of noise regenerated by the expanded gas stream as it is discharged from the silencer into the atmosphere.

These' considerations have generally resulted in sound silencing apparatus based upon the principles of sound absorption and diffusion in conjunction with exhaust deflecting elements. The diffusion elements frequently embody one or more multiple apertured tubular members. As the fluid passes through the tubular members, the apertures act as traps to reduce the noise level. Gas flow is normally not through the apertures but rather the apertures serve as openings to receive the pressure waves which constitute the noise. The space beyond the apertures is generally packed with noise absorbing material.

However, such apparatus is limited as to its effective noise attenuation in several respects. If empty space is provided beyond the apertures, the apparatus is limited as to the frequencies it can attenuate. If, on the other hand, the space beyond the apertures is filled with a noise absorbent material, the high velocity fluid flow tends to nullify the effectiveness of the absorbent material, either by forcefully ejecting it or by compacting it. Further, the providing of a space beyond the apertures necessitates the excessively large diameter of the silencer.

Accordingly, it is the object of this invention to provide a sound silencing method and apparatus applicable to a fluid passing through an exhaust passage.

Another object of this invention is to provide a sound silencing method and apparatus which effectively attenuate sound over a wide range of frequencies.

" ice It is another object of this invention to provide a sound silencing method and apparatus which employ the energy of the flowing fluid to attenuate the sound.

It is still another object of this invention to provide a sound silencing method and apparatus which attenuate sound by establishing an aerodynamic barrier to the passage of sound.

Yet another object of this invention is to provide a sound silencing method and apparatus which attenuate sound by establishing critical pressure drop relationships within the fluid system of flow.

These and other objects and advantages of the present invention will be more easily understood from the following discussion of the principles of the method and features of the apparatus as illustrated herein.

There are several factors which must be considered in designing a silencer for high velocity fluid flow. The total exhaust noise from a silencer is the result of both the attenuated incident noise transmitted through the silencer, plus that regenerated in the silencer and that created at its exhaust to atmosphere. Noise does not emanate from a high velocity jet itself but rather from the extended mixing region where this jet shears the relatively still air of the atmosphere. Noise regenerated at the outlet of an open cylindrical silencer is proportional to V where V is the velocity of the exhaust jet and n is constant having a value of approximately 8.

Similarly, the level of silencer exhaust noise regeneration is a function of the exhaust pressure of the gases emanating from the silencer. For a silencer which exhausts into air at atmospheric pressure, any pressure relationship between the exhaust pressure of the silencer and the atmospheric pressure at the discharge greater than about 1.89 results in a marked increase in the percentage of mechanical energy converted into acoustical energy.

From the foregoing, it will be seen that two important features should be incorporated into any silencer. First, the exit velocity from the silencer should be reduced to as low a value as practicable; second, the exit pressure of the exhausting gas should simultaneously be reduced to a value not appreciably greater than that pressure into which the silencer exhausts.

In attaining these objectives when silencing a high pressure, high velocity gas flowing within an exhaust passage, a novel principle is employed in the method and apparatus of this invention. It has been found that when the pressure and velocity characteristics of the gas are established at certain levels, it is possible to create within the system a zone which acts as a barrier to the transmission of sound within the exhaust passage.

In accordance with this invention, there is provided a method of attenuating sound from high pressure and high velocity fluid flowing through a confined passage which comprises establishing a noise barrier within the flow of gas and exiting the gas from the silencer at a low exit velocity and at a low exit pressure, both of which are so related to the pressure into which discharge is eflected as to produce negligible noise.

Similarly, in accordance with this invention, there is provide-d apparatus for silencing noises from high pressure and high velocity fluid flow through a confined passage which comprises means for establishing a noise barrier within the path of flow, and means for reducing the velocity of the gas and the pressure of the gas to an exit value approximately that of atmospheric pressure.

The principle of this novel method and the operation of the apparatus of this invention will be more easily understood when explained in conjunction with the attached drawings, in which:

FIGURE 1 is an elevation view of one embodiment of this invention; and,

FIGURE 2 is an elevation view of a second embodiment of this invention.

Referring now to FIGURE 1 showing in elevation one embodiment of the apparatus of this invention, silencer has inlet 11 and outlet 12. Insulation 14 is provided on the outer surface of silencer 10. Inlet of the flowing fluid is made to silencer 10 through tubular inlet connection 18 opening into space 15 through opening 23, inlet connection 18 entering through bottom plate 13 of space 15. In let nozzle 18 may be of any suitable type, such as flanged or welding connection, a welding connection being llldlcated.

Inlet nozzle 18 has disposed therein plate 19 which is apertured, containing either a single aperture or a multiplicity of apertures 20, as hereinafter discussed. Generally speaking, apertures 20 of plate 19 are sized to produce a critical pressure drop and to produce a frequency which can be attenuated by space 15.

Space 15 is bounded on its upper extreme by plate 21. Plate 21 is apertured, containing either a single aperture or a multiplicity of apertures 22 as hereinafter discussed. Apertures 22 are peripherally located within plate 21 with respect to outlet opening 23 of inlet nozzle 18; that is, preferably none of the apertures 22 of plate 21 are in vertical alignment with outlet 23 of inlet nozzle 18.

Space 16 is bounded on its upper extreme by solid plate 30, which also serves as the lower boundary for space 17. Providing communication between space 16 and space 17 is tube or conduit 31.

Tube 31 has a solid bottom plate 33, lower apertures 32, upper apertures 35 and solid upper plate 34. Communication is provided between spaces 16 and 17 through apertures 32 opening into tube 31 in space 16 and out of tube 31 through apertures 35 opening into space 17. Bottom plate 33 of tube 31 may be apertured.

Tube 31 is of any suitable length as hereinafter described and apertures 32 and 35 are of any suitable number and configuration.

As previously mentioned, solid plate 30 serves as the lower boundary for space 17. Supported on plate 30 there can be placed sound absorbent material 36 retained in place by any suitable retainer such as a screen or a retainer plate 37. Compacted stainless steel shavings of any suitable depth have been found satisfactory for use as sound absorbent material 36.

It is to be understood that the apparatus of this invention, while illustrated and described as operating in a vertical position is similarly satisfactory for operation in a horizontal position.

The unique method of this invention will now be described in conjunction with the apparatus as previously described.

Flow of the high pressure, high velocity gas enters silencer 10 through inlet nozzle 18 and passes through the apertures of plate 19. Plate 19 serves as the first of a number of systematic steps in reducing the pressure of the flowing gas. While a single apertured plate 19 is shown, it will be understood that a plurality of apertured plates may be used and similarly, that any suitable means for systematically producing pressure drop may be used, without limiting such means to a single apertured plate or a series of apertured plates. It is only desirable that each means employed for reducing the pressure do so in such a manner that the pressure downstream of the means be no less than the critical pressure when referring to the pressure upstream of the pressure drop-producing means; that is to say, any pressure drop-producing means employed should be such as to produce a pressure, a further reduction of which will not result in a further increase in the mass flow through the pressure drop producing means. Accordingly, it will be seen that the number of apertured plates or pressure-reducing means prior to the entry of the gas into the tube will be largely dependent upon the original pressure of the fluid flowing through the silencer.

Apertured plate 21 serves a function similar to previously-mentioned apertured plate 19, since, in the present instant, systematic pressure reduction necessitates two plates. The number of apertures in the plates will depend, in part, upon the quantity of gas flowing through the silencer in relation to the pressure drop attainment as previously discussed. The total number of apertured plates provided is selected to produce a final pressure such as to allow the gas to exit the silencer to the atmosphere at a minimum velocity and may be selected to produce high frequencies capable of being attenuated in the body of the silencer. The minimum size of the apertures is chosen in consideration of the likelihood of plugging by extraneous matter and of retaining the silencer within practical size limits.

As previously mentioned, communication is had between space 16 and space 17 by means of conduit or tube 31. Flow into tube 31 is through apertures 32 which are located to provide opposed, impinging flow into tube 31 while simultaneously providing a change in direction of the fluid flow and a critical pressure drop. An apertured plate could be satisfactorily used to serve a portion of the function of tube 31 but the use of a tube permits incorporation of a multitude of functions into one element. For example, opposed and mutually impinging jets are incorporated into tube 31 by reason of the location of the apertures 32 therein. These opposed jets create an area of extreme turbulence which minimize the likelihood of pressure recovery, as is possible in the use of apertured plates, and also establish a region of turbulence, which seemingly acts as a barrier to noise produced on the upstream side thereof.

A second barrier to noise produced upstream of the flow Within the silencer is created within tube 31. The diameter of tube 31 is selected to produce within the gas flow a velocity approximating sonic velocity. The pressure drop through tube 31 should be maintained at a practical value while providing a tube length within practical limitations. By so doing, there is provided over a relatively extensive region a sound barrier and there is similarly provided a tube length which will avoid the production of high frequency noises.

Flow of the gas out of tube 31 into space 17 is through apertures 35. Flow from tube 31 directly out of outlet 12 is prevented by solid plate 34 which also serves as a noise-reflection barrier. Apertures 35, while shown as slotted, may be of any configuration which imparts low velocity to the exiting gas while further reducing the pressure of the gas to a low value but not to the extent of creating a critical pressure drop in relation to that pressure existing exterior to the silencer through exit 12.

Some further reduction in velocity of the fluid flowing is provided by that region between apertures 35 and exit 12. The walls of the silencer also reflect sound produced by the gases exiting from the silencer. Sound absorbent material 36, held in place by retainer plate 37 acts, by absorption, to further minimize sound. The location of the sound absorbent material out of the main flow of the gas stream aids in retaining the material intact, uncompressed and operative.

As previously mentioned, insulation 14 is provided on the exterior of the silencer. This acoustical insulation attenuates any noise radiating at right angles to the direction of gas flow through the silencer.

FIGURE 2 is an elevation view of a second embodiment of the apparatus of this invention employing the principals of the method of this invention.

As shown, silencer 40, having inlet 41 and outlet 42, is composed of three sections of varying diameter. Inlet section 43 contains apertured plates 47 and 48; midsection 44 contains apertured plate 49 and apertured tube 50; outlet section 45 contains apertured tube 56 and sound absorbent material 57. The silencer 40 is insulated externally with sound insulation material 60.

This silencer was designed for a mass flow of approximately 95,000 pounds per hour of a gas having a molecular weight of about 9.0. At inlet 41 to the silencer, the gas was at a pressure of about 275 p.s.i.a. and at a temperature of approximately 100 F.

Inlet nozzle 46 comprises a diameter pipe, approximately 36 in length. Apertures 53 in plate 46 consist of about one hundred twenty /2" diameter holes. The gas pressure after passing through these apertures is approximately 135 p.s.i.a. and the gas velocity is approximately 275 feet per second.

Plate 48 contains apertures 54 which consist of about one hundred twenty-five /2" diameter openings. The gas pressure after passing through these apertures is about 70 p.s.i.a. and its velocity is approximately 500 feet per second.

Mid-section 44 is fabricated from a diameter pipe approximately 50" in length. Tube 50 is a 10" diameter conduit approximately 40" in length.

The velocity of the gas after passing from inlet section 43 to mid-section 44 is reduced to approximately 130 feet per second prior to plate 49. The apertures 61 in plate 49 consist of about one hundred /1 diameter openings. The gas pressure after passing through these apertures is approximately 35 p.s.i.a. and its velocity is approximately 250 feet per second.

Tube 50 is closed by plate 51 having apertures 55 therein. These apertures 55 are seven 1% diameter openings. Tube 50 has apertures 52 in its side walls, these being about forty /2" diameter holes.

The gas passes at a pressure of approximately 35 p.s.i.a. through the apertures 52 into tube 50 in such a manner and in such a multiplicity of directions as to create an area of extreme turbulence within the tube. The velocity of the gas through each tube aperture 52 is approximately 1,995 feet per second. Under the conditions of the gas at this point, sonic velocity is approximately 2,000 feet per second. The pressure of the gas within tube 50 is reduced to approximately 20 p.s.i.a. and its velocity is approximately 1,800 feet per second.

Outlet section 45 is approximately 42" in diameter and 60" in height. It contains apertured tube 56 which is approximately 20 in diameter and 39 in height. There are six apertures 62 in tube 56, each being approximately 6" x 12''. Tube 56 is closed at its top by plate 63. Sound absorbent material 57 in outlet section 45 consists of an 18" deep bed of stainless steel shavings maintained in place by apertured plate 64.

The gas, upon leaving tube 50 at a pressure of approximately 20 p.s.i.a. and a velocity of approximately 1,800 feet per second is reduced in velocity within apertured tube 56 to about 530 feet per second and a pressure of about 17 p.s.i.a. The gas flows through apertures 62 of tube 56 at about this same velocity and exits from outlet section 45 through outlet 42 at a velocity of approximately 120 feet per second and a pressure of approximately 15.5 p.s.i.a. under which conditions it is exhausted to atmosphere.

It will be evident from the above discussion that many variations to the embodiments of both the method and the apparatus as described herein can be made without departing from the scope of either invention.

As pertaining to the unique and novel method disclosed herein, it appears that both the establishment of the turbulent fluid mixing area within the tube and the establishment of a fluid velocity at, or essentially at, sonic velocity within the tube are effective 'as sound barriers. Each, in and of themselves, 01' in combination with each other, seems to underlie the effectivenes of the method described herein for attenuating and silencing noises from high pressure and high velocity fluid flow through a confined passage.

In respect to the apparatus, wide variations may be made from the embodiments described herein while still remaining within the scope of the invention. For example, any number of apertured plates may be used as conditions may permit. The number and size of such apertures may be varied. The size, arrangement and configuration of the tubes can be altered as can the number of tubes employed. Configurations of both inlet and outlet apertures of the tubes can be similarly modified as can the various areas existing within the silencer. All of these and other alterations can be made without departing from the scope of the invention.

Having described my invention, I claim:

1. An apparatus for exhausting a high pressure fluid at a safe noise level, which comprises:

a first conduit;

a second conduit of larger cross sectional area than said first conduit;

first means for providing fluid flow between said first and second conduits at sonic velocities; and

second means for producing in the fluid flowing therethrough at least a critical pressure drop such that any further reduction of pressure will result in no further increase in the mass flow through said second means, said second means being located upstream from said first means.

2. An apparatus according to claim 1 in which the first means comprises an elongated conduit having apertured inlet and outlet portions.

3. The apparatus according to claim 1, in which the second means is an apertured plate.

4. The apparatus according to claim 1, in which sound absorbent material is disposed within said second conduit.

5. The apparatus of claim 2 in which at least a portion of the apertures of said inlet portion of the elongated conduit is disposed in impingement relationship with respect to the fluid flowing through said apertures.

6. The apparatus of claim 2 in which the total flow area of the apertures of the outlet portion is substantially greater than that of the apertures of the inlet portion.

7. The apparatus of claim 2 in which at least a portion of the apertures of the outlet portion is peripherally disposed therein to change the direction of the flow of the fluid upon its withdrawal from said elongated conduit.

8. The apparatus of claim 3 in which at least two apertured plates are located upstream from said first means, each apertured plate producing at least such critical pressure drop.

9. The apparatus of claim 3 in which the diameters of the apertures of said plate are such that upon passing the fluid through said apertures a high frequency sound is generated.

10. An apparatus for exhausting a high pressure fluid at a safe noise level which comprises:

a first conduit;

first means within said first conduit for producing in the fluid flowing therethrough at least a critical pressure drop such that any further reduction of pressure will result in no further increase in the mass flow through said first means;

an intermediate conduit of larger cross sectional area than said first conduit in open communication therewith;

a second conduit of larger cross sectional area than said first conduit; and

means for providing fluid flow between said intermediate and second conduits at about sonic velocities.

11. The apparatus of claim 10 in which a second means for producing at least a second such critical pressure drop is provided in the intermediate conduit.

12. A method of exhausting a high pressure fluid at a safe noise level which comprises:

reducing the pressure in at least one step to produce a pressure drop in the fluid in each of such steps amounting to about at least a critical pressure drop such that any further reduction in pressure will result in no further increase in mass flow;

passing the resulting fluid of reduced pressure in a subsequent step to an elongated conduit at about sonic velocity;

Withdrawing the fluid from said conduit at a substantially reduced velocity; and exhausting the fluid at a further reduced velocity. 13. The method of claim 12 in which said resulting fluid of reduced pressure is passed to the elongated conduit as impinging jets.

14. The method of claim 12 in which the flow direction of said fluid is changed upon withdrawal from said conduit.

15. A method according to claim 12 in which attenuable high frequency sound is generated in said reducing of the pressure of the fluid and thus generated sound is absorbed in a subsequent step.

16. A method of exhausting a high pressure fluid at a safe noise level which comprises:

introducing the high pressure fluid to a first zone; passing said fluid through means within said first zone for producing at least a critical pressure drop such that any further reduction in pressure will result in no further increase in mass flow therethrough;

reducing the velocity of the fluid of reduced pressure in a second zone;

passing the fluid of reduced pressure and velocity through means within said second zone for producing at least a second critical pressure drop;

passing the resulting fluid of further reduced pressure through means for producing fluid flow at about sonic velocities and for still further reducing the pressure of the fluid;

reducing the velocity of the resulting fluid of still further reduced pressure in a third zone; and

exhausting the fluid from said third zone.

References Cited UNITED STATES PATENTS 1,061,775 5/1913 Newton et a1 181-57 XR 1,578,682 3/ 1926 Raymond. 2,872,998 2/1959 Tinker 181-56 XR 2,882,881 4/ 1959 Nedley. 2,995,199 8/1961 Myers 181--53 XR FOREIGN PATENTS 801,318 5/1936 France. 765,639 1/ 1957 Great Britain. 606,952 7/ 1960 Italy.

370,281 8/ 1963 Switzerland.

US. Cl. X.R.

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