EP0016042A1

EP0016042A1 - A blowing device having a low noise level

Info

Publication number: EP0016042A1
Application number: EP79900651A
Authority: EP
Inventors: Hasse Bengt Folke Moss
Original assignee: Ingemanssons Ingenjorsbyra AB
Current assignee: Ingemanssons Ingenjorsbyra AB
Priority date: 1978-06-14
Filing date: 1980-01-29
Publication date: 1980-10-01
Also published as: GR68502B; FI791890A; WO1980000064A1; SE451236B; SE8004994L; SE8305416D0; IT1121094B; NL7904639A; BE876956A; GB2047574A; SE8305416L; SE7806883L; IT7923541A0; DK61980A; DE2952876A1; NO791957L; JPS55500443A

Abstract

Une soufflante comprend un corps avec une connection (8) d'arrivee d'un fluide gazeux sous pression, un ajutage de refoulement (17) et une soupape (11, 13) ayant un passage de fluides reglable situe entre ladite connection et ledit ajutage. L'ajutage possede une pluralite de canaux de refoulement (3) pour le fluide et la zone d'ecoulement de passage du fluide dans la soupape est sensiblement plus grande que la somme des zones d'ecoulement desdits canaux de refoulement empechant ainsi que ce dispositif ne produise un bruit prejudiciable tout en obtenant une grande puissance de soufflage. Les canaux de sortie ou refoulement sont repartis regulierement dans le sans circonferentiel dudit ajutage (17) et la surface du manteau externe de l'ajutage de refoulement est formee de maniere a permettre une co-ejection de l'air exterieur par l'ecoulement fluide ejecte dudit ajutage.A blower comprises a body with a connection (8) for supplying a pressurized gaseous fluid, a discharge nozzle (17) and a valve (11, 13) having an adjustable fluid passage situated between said connection and said nozzle. . The nozzle has a plurality of discharge channels (3) for the fluid and the flow zone for the passage of the fluid in the valve is substantially greater than the sum of the flow zones of said discharge channels preventing as well as this device produce a harmful noise while obtaining great blowing power. The outlet or discharge channels are distributed regularly in the circumferential without said nozzle (17) and the surface of the external mantle of the discharge nozzle is formed so as to allow co-ejection of the outside air by the fluid flow. ejects said nozzle.

Description

A blowing device having a low noise level.

Blowing tools are used within the industry in many connec¬ tions. An example is cleaning by means of air blowing, e.g. at turning and milling operations. Other examples are coo¬ ling, heating and drying, and transport of various details, e.g., at automatic machine tools. It may also be the ques¬ tion of blowing by means of other gases than air, e.g., pro¬ tective gas at welding operations . The noise generated by such blowing tools is frequently so high that levels of impaired hearing are reached.

Typically, the most common blowing tools have a design acc¬ ording to Fig. 1, which is a longitudinal cross-sectional view.

The gas is supplied to the tool via a high pressure hose or pipe 2, which is coupled to a source of gas having a pres- sure above the atmospheric pressure . When the blowing tool is to be used, a hand grip 3 is depressed which causes a valve slide 4 to be displaced so that gas can pass via a groove 5 in the slide and an extension tube 6 out through the mouth 7 of the extension tube .

When a gaseous medium is caused to be exhausted to the envi¬ ronment in this manner, an exhaust velocity of the gas is obtained which depends on the pressure ratio between the counterpressure after, i.e., downstream, the mouth and the pressure before said mouth.

If the mouth is not shaped as a so-called Laval nozzle, a maximum exhaust velocity is obtained at the so-called criti¬ cal pressure ratio. The pressure before the mouth at which the critical pressure ratio is reached is determined by the counterpressure after the mouth, which is in turn influenced by the degree of co-ejection or entraining ejection, i.e., to what extent the air jet leaving the mouth in its movement will entrain surrounding air. Thus, the higher the co-ejec¬ tion the higher supply pressure in the high pressure hose 2

OMPI will be required in order to attain the critical pressure ratio.

If the interior of the blowing tool is designed so as to achieve low losses, i.e., if all passages for the gas befor the nozzle have a substantially greater cross sectional are than that of the nozzle 7, the pressure of the gas immedia¬ tely before the nozzle would be substantially the same as the pressure, for instance 6 to 8 bars, of the gas supplied by the pipe 2.

The pressure of the gas after its expansion after the nozzl is never lower than the critical pressure, p , i.e. not low than 0.528 times the pressure immediately before the exit o the gas. Thus, in such tools the pressure of the gas after the expansion is normally at least 3 bars.

In such cases it is important that the nozzle has such pro¬ perties that it provides a co-ejection sufficient for the counterpressure after the nozzle 7 to deviate only slightly from the pressure immediately after the expansion zone, i.e. in the contraction zone. For the gas having the pressure p would otherwise meet a subpressure and expand in an explosi manner and the gas particles would be accelerated laterally so that a subpressure would be created in the jet core whic would have a retarding effect on the gas particles. This would be repeated periodically so that a standing pressure wave would be obtained consuming the energy of the gas belo the critical pressure while generating a strong noise. Also the effective blowing power of the jet would be considerabl reduced.

At a predetermined pressure upstream of the mouth no reduc- tion of the blowing force will be obtained, since the co- ejection will be increased. This fact is valid, in spite of, e.g., when subcritical pressure ratio is present, the exhaus velocity and the mass flow will then be reduced. In other word, although the expansion work in the apertune is reduce an increased co-ejection will cause that the gas jet through admixture of surrounding air will maintain or increase its total, kinetic energy.

When the gaseous medium at a high velocity, near the veloci- ty of sound, leaves the mouth 7 of the extension tube 6, forceful vortices, turbulences, are formed, when the rapidly exhausted gas is mixed with the external stationary air or gas. From these turbulences a strong noise is emitted. With an increased ejection said turbulences may be reduced, where- by the radiation of sound is reduced.

In order to provide, at a predetermined supply pressure, a restriction of a mass flow obtained at a fully open valve, the flow area at the valve must be reduced. If the degree of restriction is such that the flow area at the valve is less than 0.52 times the flow area of the mouth, indepen¬ dently of the supply pressure and the counter pressure, the flow velocity at the valve is at least 1.2 times higher than the flow velocity at the mouth given by the pressure ratio. At such a restriction that the flow velocity at the valve is higher than the flow velocity obtained from the pressure ratio at the outlet, the noise generation at the valve is higher than the noise generation at the mouth.

Furthermore noise generated at the two noise sources seldom have the same composition of frequencies. If the valve assembly is located at a smaller distance than a few meters from the mouth, the acoustic resistance is so small that the sound power is propagated out into the environment:around the mouth proper, so that the sound power obtained at the valve will totally or partly determine the sound level around a blowing tool in operation. To what extent this condition will occur is determined by the flow velocity actually present at the mouth. For if the velocity losses within the extension tube 6 are small the flow velocity ob¬ tained in practice at the mouth will be determined by the flow velocity at the valve. Thus, the flow velocity at the valve can give rise to two sound sources, which, besides that they may be added each one cause a higher sound level as compared with the case in which a reduced mass flow had been obtained by a reduced supply pressure, e.g., at a com- pressor equipment.

The noise level increasing effect may be 3 to 10 dB(A) . The difference depends on the degree of reduction. Even in the cases where the flow area at the valve or at the connection of the tool is larger that the through-flow area at the mout the flow velocity at the valve or the connection may indirect ly cause an increased exhaust velocity and thus a more power ful noise generation as compared to the case, in which the valve or connection area is considerably larger than the out let area. For in a flow at subsonic velocity through a tube with losses, the pressure drop will cause a reduction of. the density of the flowing medium and, consequently, a correspon ding increase of the flow velocity. In order to obtain a suf ficient blowing force, i.e., mechanical work with predeter¬ mined media, preferably air, a mass flow of such a magnitude is required that the velocity of the medium within a blowing tool having practical dimensions will be extremely high. At the connection between the supply conduit and the supply connection of the blowing tool, and especially at the gas medium control means normally required and the subsequent outlet channel, forceful velocity changes will occur locally Turbulences caused thereby will influence the flow on one hand within the tool and on the other hand within the outlet proper, but also after the air has left the blowing tool. The turbulences outside the nozzle will become still more forceful as well as the noise generation.

In addition to the drawback that the turbulences affect the gas flow and consequently the efficiency thereof they also give rise to noise within the tool. Said noise can be equall disturbing as the noise generated at the exit of the gas fro the nozzle, owing inter alia to the differences existing bet ween the composition of frequencies of the two types of noise The object of the- present invention is to provide a blowing tool having a low noise level, a great blowing power and a high mechanical efficiency. The tool provides a flow, which is continuously controllable within a large range.

A blowing device or a blowing tool, respectively, according to the present invention has the characteristic features stated in the claims. In regard of low properties the device according to the invention may be considered as two series connected nozzles, which are separated by a chamber. The valve is a first restriction of the gas passage and the end nozzle is a second restriction. The flow channel connected to these restrictions is so wide that the velocity energy there can be disregarded, whereby the chamber, after the first restriction, will cause the kinetic energy at the first restriction to be lost. At the second restriction a state is obtained, which will be equivalent with the flow therethrough only but from a reservoar having a lower pres¬ sure.

The connection 8, the valve 11, 13 and the nozzle 17 of the blowing device have been provided with such through-flow areas that when the tool is in operation and the valve is fully open the pressure within the chamber 16 is at least equal to 0.9 times the gas pressure fed to the blowing de¬ vice.

When for a given feeding pressure the flow of gas is to be reduced, i.e. when the through-flow area of the valve shall be reduced, the expansion work within the valve opening is increased. The velocity of the gas within the valve opening is increased. In such cases it is especially important that said chamber is provided, in combination with a nozzle cau¬ sing apressure drop, so that the energy corresponding to the velocity of the gas at the first throttling section, i.e. in the valve, is brought to disappear before the gas reaches the nozzle.

O PI

Λ ° Since the end nozzle is thus formed with a considerable pre sure drop in the inlet thereof, the chamber will also opera te as a pressure equalizing zone, thus reducing turbulences occurring at the first restriction. The end nozzle is desig¬ ned so that it will provide a high co-ejection of external 5 gas. Hereby the media jet will obtain a higher total mass flow but the noise generation will also be reduced.

In order to still reduce the risk of the turbulences gene¬ rated within the blowing device affecting the gas flow afte the mouth of the end nozzle, the nozzle comprises a plurali 10 ty of elongated gas exit channels the through-flow areas an also the added or total through-flow areas thereof being substantially less than the through-flow area of said chambe

Owing to the combined effect the gas velocity through exit channels of the nozzle being considerably greater than the

15 velocity of the gas flow within the chamber and the respec¬ tive exit channels having a small through-flow area, the re maining gas turbulences or vortexes, if any, within the cha ber will be drawn out into an elongated gas bubble the lat¬ ter rapidly losing its energy corresponding to the gas velo

20 city.

The higher counter pressure after the end nozzle, caused by the increased co-ejection, will allow a higher supply pres¬ sure to be utilized before a critical flow velocity will occur in the end nozzle. The dimensions of the valve are 25 such that at fully open valve the critical pressure ratio thereacross cannot be obtained, until after the critical overpressure has been obtained across the end nozzle.

In the accompanying drawings Figs. 2 to 10 show embodiments of a device according to the invention. Fig. 2 shows a longi

30. tudinal section of a first embodiment of the tool. Fig. 3 and 4 show the nozzle in longitudinal section and in end view respectively. Fig. 5 and 6 correspond to Fig. 2 but with the tool valve in half open and fully open position

' respectively. Figs. 7 and 8 shows a longitudinal section and an end view respectively of a second embodiment and Figs. 9 and 10 show correspondig views of a third embodi¬ ment of the invention.

A nipple 8 (Fiq. 2) for the connection of a supply conduit for pressurized gas to the tool has a centre channel 9 hav¬ ing a cross sectional area A. Onto the nipple 8 there is screwed a first sleeve 20 upon which a second sleeve 21 is screwed which encloses a chamber 10 in which the valve unit of the tool is provided. Gas passes from the nipple 8 to the valve unit, which has a circular valve seat 11 in the cham¬ ber 10. A circular valve body 13 is secured to a control arm 12, said valve body being pressed towards the valve seat by means of a spring 14. The valve seat has a larqe diameter, whereby the flow velocity through a wholly open valve will be low, thus reducing the turbulences after the valve. As stated above the noise after the end nozzle will increase with increased turbulence of the gas reaching said nozzle.

After the valve there follows a conical or tapering extension sleeve 15, wholly or partly formed of rubber or other elastic material, enclosing a cylindrical chamber 16. Inside this chamber, which separates the valve from the exhaust of end nozzle 17 of the tool, a pressure stabilized zone is obtai¬ ned. By this the mutual dependence between the valve nozzle and the end nozzle is reduced. A precondition for this is that the length of the chamber 16 is sufficient and that the smallest through-flow area thereof is greater than, suitably at least 1.5. times and preferably at least 2.5.times the through-flow area of the valve 11, 13, or, in the case the latter is greater than the through-flow area of the inlet channel 9, in a similar manner greater than the lastmentioned through-flow area. Furthermore the smallest through-flow area of the chamber 16 ought to be greater than, suitably at least 2 times and preferably at least 3 times the sum of the thrσugh- flow areas of the outlet channels 30. At restrictions obtai¬ ned by means of the valve the chamber will operate so that

OΛ'Pl kinetic energy generated att the first restriction, i.e., the valve, is lost. Through the end nozzle a flow state is obtained which is equivalent to the flow state that would be present if the flow reduction had been performed at a reduced supply pressure before the blowing tool, for insta with a compressor plant, from which said pressurized mediu is obtained. The chamber should have a lengt of at least five (5) times its crosssectional diameter. The exhaust or end nozzle 17 follows after the chamber 16. The end of the control arm 12 has a stud entering a bore 18 in the nozzle. At a manually affected, laterally directed pressure on the extension sleeve 15 the valve cone 13 is displaced oblique so that a smaller or larger partial opening of the valve is obtained as illustrated in Fig. 5.

Owing to the fact that the control arm 12 is coupled to th end nozzle 17 the control arm maintains its concentric or coaxial position within the chamber even when the extensio sleeve 15 is displaced obliquely. This is essential in ord to avoid turbulences around the control arm.

By the fact that the control arm 12 is, in the manner desci bed, coupled to the end nozzle acoustical resonances in th form of standing sound waves between the wall surfaces of the chamber 16 are also eliminated. In this manner differe ces in dynamic pressure at the movable valve member slight offset at the through-flow of the gas will not come into resonances with strong acoustical pressure maxima and in this manner so called "shriek" sounds often occurring in water-taps are avoided.

The end nozzle 17 is circularly cylindrical and near its periphery it has a series of annularly located, cylindrical channels 30 having a small diameter d in relation to the mouth diameter D of the nozzle. The location of the channel adjacent the periphery, in combination with the conical shape of the extension sleeve, provides a high co-ejection of gas externally of the end nozzle. The combined or total ϋi cross-sectional area of the channels must be smaller than all through-flow areas within the tool, i.e. smaller than the flow area A at the channel 9 but also smaller than the flow area of the valve as wholly opened. This is important in order that a critical flow shall notbe obtained at these restrictions before a critical flow occurs in the end nozzle. In order to provide efficient co-ejection the channels should have a lengt L, which is at least 10 times the diameter d. By this also the contraction zone, i.e. the cross-section 0 where the gas is contracted in order thereafter to expand adiabatically, will come to exist shortly before the exit, i.e. within the respective outlet channel, so that the de¬ gree of expansion of the respective air jet is more uniform than in the case when the contraction occurs somewhat out-

15 side, i.e. downstream the outlet jet.

Half of the cone angle, 0t according to Fig. 3, of the exter¬ nal mantle of the nozzle should be less than 20°, preferably less than 15°, e.g. 4 to 10°. The mantle surface 32 should be smooth along all or at least a substantial portion of the

20 axial length thereof, and in any case free from discontinui¬ ties or roughness, which could substantially reduce the co- ejection or cause turbulence, and it should end very near the channels 30 at the plane end surface 33 of the nozzle, which should be generally perpendicular to the centre axis

25 31 of the tapering mantle surface. The tool is also provided with means for a fixed adjustment of a predetermined gas flow. Such an adjustment is illustrated in Fig. 6. On a stud 19 connected to the valve body 13 wings 23 are provided, which, when the extension sleeve 15 is screwed in a direc-

30 tion outwards from the sleeve 20, are hooked onto a shoulder 22 on the sleeve 20, whereby the valve is opened in propor¬ tion to the screwing outwards^' of the extension sleeve 15.

In a blowing tool of a conventional size the diameter d of the bores 30 may be about 0.3 to 1.5 mm and the bores may 35 have a mutual distance between centres of at least twice the bore diameter d. The centre axes of the bores are loca- ted on a circle with the diameter ^rDy, wherein Dy may be la ger thanlD-6d. An inner series of bores may also be provid in a circle with the diameter Di, which is preferably larg than 2d. In the centre of the nozzle no bore corresponding to the bores 30 should be present.

The maximum opening section of the valve 11, 13 shall be greater than, suitably at least 1.2 times and preferably at least 1.5 times the total cross sectional area of the outlet of the nozzle, i.e. the sum of the cross sectional areas of the outlet channels of the nozzle. At a smaller than maximum opening section of the valve this section can represent the narrowest section and cause noise. Since the valve seat has a large circumference, the distance is ve small between the valve body and the valve seat at the cir cular arc at which the valve opens whereby the noise produ ced will be substantially above the audible frequency rang The valve 11, 13 of the blowing tool is therefore suitably designed in such a manner that, when the valve opening has a through-flow area of about 0.5 times the total through- flow area of the outlet openings 30 of the nozzle, the dis tance between the movable valve member and the valve seat does not at any point exceed 0.2 millimeters and preferabl does not exceed 0.1 millimeter.

In order to prevent direct skin contact between the tool o rator and the outlet openings of the channels 30 the nozzl 17 is provided with one or more projections 40. Figs. 7, 8 and 41 Figs. 9 and 10, extending from the end surface or plane 33 of the nozzle. The lengts M of said projections is substantial and at least 1.2 times, preferably at least 2 times the diameter of the respective outlet channels 30, and the projections can be placed between the channels 30 as shown in Figs. 7 and 8. Alternatively a single projec¬ tion 41 can be provided centrally of said surface 33, and surrounded by the outlet openings 30. Said projections are designed so that the co-ejection referred to above will no be materially disturbed and so that the noise level at the nozzle outlet is substantially increased. A prototype of the device according to the present invention in accordance with the embodiment illustrated in Fig. 2 has been subjected to practical testing and has been compared on one hand with many commercially available so-called silent blowing nozzles having a body of porous, sintered metal in¬ serted into the exhaust tube and has also been compared with many other conventional blowing tools. In all cases the con¬ ventional tools gave rise to higher air consumption and con¬ siderably higher noise levels and blowing power than the blo- wing tool according to the invention.

Also, a comparison between a tool according to the invention and a similar tool provided around its nozzle with a radial collar having an outer diameter of 3D and a thickness of 0.5 mm, said collar being fixed around the mantle 32 at a distance 10 mm from the end surface 33 of the nozzle, gave as a result that the tool according to the invention has a lower noise level and a higher blowing power..The same result was obtained at a comparison with a tool similar to said em¬ bodiment except that the channels were distributed irregular- ly over the cross sectional area of the nozzle and 20°.

The invention is not limited to the shown and described embodiments since the latter can be varied in many respects within the scope of the invention.

OMPI

Claims

-1. A blowing device comprising a body part having a con nection for pressurized gaseous fluid, an exhaust nozzle fo said fluid and a valve located between said fluid connectio and said exhaust nozzle and comprising an adjustable fluid

5 passage determining the rate of flow of fluid through said device, characterized in that said nozzle is provided with a plurality of exhaust channels for said fluid and that the through-flow area of the fluid passage of said valve when the latter is wholly open is considerably greater than the

10 sum of the throughflow areas of said exhaust channels.

2. A blowing device as claimed in claim 1, characterize in that said exhaust nozzle has a generally circular cross section and comprises a plurality of exhaust channels sub¬ stantially evenly distributed in the circumferential direc-

15 tion of said nozzle and located adjacent the outer mantle surface of said nozzle.

3. A blowing device as claimed in claim 2, characterize in that the outer mantle surface of said exhaust nozzle is substantially frusto-conical and substantially everywhere

20 forms an acute angle with the center axis of the nozzle as seen in a plane through said center axis.

4. A blowing device as claimed in any of the preceding claims, characterized by an elongated, substantially cylind rical chamber located between said nozzle and said valve,

25 the length of said chamber being at least 5 times the cross section diameter of said chamber.

5. A blowing device as claimed in claim 4, "characterize in that said chamber has a minimum through-flow area which is substantially greater than the sum of the through-flow

30 areas of the exhaust channels of said nozzle.

6. A blowing device as claimed in any of the preceding claims, characterized in that the exhaust channels of said nozzle have a substantially circular cross section and a length which is at least 10 times the cross sectional dia-

— 35 meter thereof.

7. A blowing device as claimed in any of the preceding claims, characterized in that the flmid passage of said valve has a shape such that the noise generated at the valve falls substantially wholly above the audible frequency range even at a relatively slight opening of the valve.

8. A blowing device as claimed in any of the preceding 5 claims, characterized in that said valve comprises a gene¬ rally circular valve seat which in combination with a corre- spondig valve body in the form of a solid of revolution de¬ fines an annular fluid passage, said valve body being adap¬ ted to be displaced by means of manually operable means to

10 open or close the annular fluid passage along part of or the entire periphery thereof.

9. A blowing device as claimed in claim 2 or 3, characte¬ rized in that said exhaust nozzle at he outer side thereof is free from discontinuities and other formations which can 15 prevent to a considerable degree co-ejection of atmospheric air with the fluid ejected from said nozzle.

10. A blowing tool as claimed in claim 9, characterized by such a design of the exhaust channels of the nozzle that the ratio between the counter pressure generated by said co-

20 ejection immediately after the mouth of the .exhaust nozzle and the air pressure before said mouth attains the critical pressure ratio (0.528), not until the latter air pressure is higher than 6 times the external atmospheric pressure.

11. A blowing device as claimed in claim 4, characterized •25 in that said chamber is defined by a manually bendable sleeve connecting said valve with said nozzle and being coupled to said valve in such a manner that the valve may be opened and closed by bending said sleeve laterally.

12. A blowing device as claimed in claim 11, characterized 30 in that said sleeve, for fixed adjustment of a desired gas flow through said valve, is adapted to be screwed relative to the part of the tool body comprising said connection for pressurized air.

13. A blowing tool as claimed in claim 11 or 12, charac- 35 terized in that the valve body is centrally connected to one end of an elongated, rigid member, the other end of which being centrally connected to said exhaust nozzle.

14. A blowing device comprising an exhaust nozzle having an end surface in which a plurality of exhaust channels for pressurized air terminate, characterized by one or more pro tecting means projecting from said end surface for preven¬ ting direct skin contact between the operator of the device and the end surface of said nozzle.

15. A blowing device as claimed in claim 14, characteri¬ zed in that the nozzle is provided with a plurality of annu larly arranged exhaust channels surrounding said protecting means which is the form of a single projection centrally of said end surface.