Method And Apparatus For Generating Monodisperse Aerosol

Abstract

A vibrating orifice monodisperse aerosol generator usable as a basic aerosol standard having a liquid feed system, a droplet generator, a droplet dispersion system and an aerosol flow system. The liquid feed system is a syringe pump operable to feed liquid into the droplet generator. The liquid is discharged from the droplet generator through a small orifice as a liquid jet. The jet disturbed at a constant frequency with an oscillating piezoelectric ceramic breaks up into uniform droplets. The droplets are dispersed by a turbulent air jet and diluted into a monodispersed aerosol.

Description

BACKGROUND OF THE INVENTION

Basic aerosol standards, consisting of monodispersed aerosols of a high degree of monodispersity and accurately known particle size are needed for a variety of purposes ranging from fundamental aerosol research, calibration of aerosol sampling and measuring instruments, testing of particulate control devices such as cyclones, filters, scrubbers, and the like, to the study of health, meteorological and other effects of particulate air pollutants. To be generally useful, the aerosol standard must be sufficiently flexible to permit aerosols to be generated from a variety of solid and liquid materials having desired physical properties.

Conventional monodisperse aerosol generators, as the spinning-disc aerosol generator, the atomizer-impactor generator, the condensation generator and the polystyrene latex aerosol generator, all have severe limitations for use as a basic aerosol standard. The spinning-disc generator, while flexible in terms of the aerosol material, can produce aerosols with geometrical standard deviations of not better than 1.07 or a standard deviation of approximately 7 percent of the mean particle size. The spinning-disc generator is also a relatively complex piece of apparatus. The atomizer-impactor generator can only produce a moderately monodisperse aerosol with a geometrical standard deviation of not better than 1.40. The polystyrene latex generator can produce aerosols from only one material and only of the discrete sizes provided by the polystyrene latex manufacturer. This generator also produces residue particles which are undesirable for many applications.

Vibrating orifice aerosol generators are based on the principle of the breakup of a liquid jet. It is known that cylindrical liquid jets are unstable to rotationally symmetric disturbances. These disturbances will grow until the jet breaks. The known droplet generators are not usable as a basic aerosol standard in their present state of development as they need a droplet dispersion system in an aerosol flow system of a well defined geometry to provide stable operation.

SUMMARY OF THE INVENTION

The invention relates to an apparatus and method for generating a monodispersed aerosol of any liquid or any solid material that can be put in solution form. The apparatus for generating a monodispersed aerosol has a housing means having at least one outlet opening for discharging the aerosol. Mounted on the housing means is an apparatus for generating liquid droplets. The apparatus has a discharge orifice generally aligned with the outlet opening the housing and spaced from the opening. The housing means includes means providing a chamber between the apparatus and the outlet opening. The liquid is supplied under pressure to the apparatus to generate droplets. The apparatus has means subjected to a vibratory force which breaks the liquid jet into substantially equal size droplets which are discharged into the chamber. Air under pressure is supplied to the chamber, whereby the droplets emanating from the apparatus are dispersed into the air and discharged with the air through the outlet opening. The dispersed aerosol then enters the aerosol flow system which acts to dilute and transport the aerosol.

IN THE DRAWINGS

FIG. 1 is a schematic view of the aerosol generator flow system of the invention;

FIG. 2 is an enlarged fragmentary sectional view of the aerosol generator of FIG. 1 located in the flow system;

FIG. 3 is a sectional view of the aerosol generator taken along line 3--3 of FIG. 2;

FIG. 4 is an enlarged sectional view of the droplet generator forming part of FIG. 2; and

FIG. 5 is an enlarged sectional view taken along line 5--5 of FIG. 4.

Referring to the drawings, there is shown in FIG. 1 the flow system for the aerosol generator of the invention, indicated generally at 10. System 10 includes an aerosol generating assembly, indicated generally at 11, containing the aerosol generator 12 for generating droplets and dispersing the droplets in carrier air in the form of a monodispersed aerosol.

As shown in FIG. 2, aerosol generator 12 is mounted on a support or porous plate 13 located over an opening in a housing 14. The outer peripheral edge portion of the plate 13 rests on and is attached to the top of housing 14. The housing 14 surrounds an absolute filter 16 through which the air passes prior to its flow through the porous plate 13. Positioned on top of plate 13 is an elongated tube or cylinder 17 having an elongated chamber 18. The upper end of the cylinder is tapered or cone-shaped and has an outlet opening 19, providing an outlet passage for the discharge of monodispersed aerosol, indicated by arrow 21. A plurality of fasteners 22, as bolts, extend through suitable holes in an outwardly directed flange 17A on the tubular member 17 and aligned holes in plate 13 to secure the tube 17 and plate 13 to the top of housing 14.

Air is supplied to the bottom of housing 14 with a blower 23 having an electric motor (not shown) controlled with a variable transformer 24. Air can also be obtained from a compressor having appropriate pressure regulators. The outlet of blower 23 is connected to a line or tube 26 leading to the bottom of housing 14. A control valve 27 interposed in line 26 regulates the flow of air into housing 14. A differential pressure gauge 28 is connected in line 26 before valve 27 and an outlet portion of blower 23 to provide means for determining the pressure of the air being supplied to the housing 14.

The liquid feed system for the aerosol generator 12 has an infusion pump or syringe 29 which forces the liquid under pressure through a membrane filter 32 interposed in a line 31 connecting the pump 29 to the aerosol generator 12. Other types of pumps can be used to provide the generator 12 with a substantially constant supply of liquid under pressure.

Aerosol generator 12 has a droplet generator or apparatus operable to form liquid droplets, indicated generally at 33 in FIGS. 2 and 4. The air supply system for the generator 33 is diagrammatically shown in FIG. 1. Droplet generator 33 is connected to a high pressure air line 34 providing a supply of air to a pressure regulator 36 and an absolute filter 37. The air flows from filter 37 through a rotameter 38 into a feed line 39 connected to the aerosol generator 12. The droplet generator 33 is subjected to a vibratory force generated with a signal generator 41 coupled via line or conductor 76 to a vibrating portion of the droplet generator 33, as shown in FIG. 4.

As shown in FIG. 2, the aerosol generator 12 has a housing comprising a generally cup-shaped base 42 mounted on the center portion of plate 13. A plurality of fasteners 43, as bolts, attach base 42 to plate 13. Base 42 has an upwardly directed cylindrical side wall 44 surrounding a chamber 46. The top of base 42 is closed wih an inverted, cup-shaped housing member 47 having a generally flat top wall or metal plate 48. The center portion of top wall 48 has an outlet hole or outlet opening 49 located along the axial or longitudinal center line of the chamber located along the axial or longitudinal center line of the chambr r 18. Member 47 has a cylindrical side wall 51 of electrical non-conductive material telescoped over the side wall 44. Fasteners 52, as bolts, secure the walls 44 and 51 together. An annular seal 53 is interposed between walls 44 and 51. The top wall 48 is spaced from and located above the top of the droplet generator 33 forming therewith a space or chamber 54. An annular stop washer 55 secured to the inside of side wall 51 rests on top of the side wall 44. Secured to side wall 51 is an inlet nipple 56 connecting the air line 39 to the generator 12 so that air indicated by arrow 57 is introduced into the chamber 54 and mixed with the droplets emanating from the generator 33. The air is introduced into the side of chamber 54 and is in a turbulent state as it mixes with the droplets. The air and droplet mixture in chamber 54 is discharged through opening 49 into the chamber 18 and is dispersed in a conical pattern, as shown in FIG. 2. The dispersed aerosol is carried with the clean air flowing through porous plate 13 in the chamber 18 toward the outlet opening 19. This clean air functions to dilute and transport the aerosol. In the case of a nonvolatile solute in a volatile solvent, the clean air also evaporates the solvent. The monodispersed aerosol 21 is discharged through the opening 19 at the top of the tube 17. When it is desired to generate a monodispersed aerosol having a unipolar electrical charge, a DC voltage can be applied to the metal plate 48 to induce a charge on the generated droplets.

As shown in FIG. 2, droplet generator 33 is located in chamber 46 of the base 42. An annular O-ring 59 of resilient material, as rubber, surrounds generator 33 to position the generator in chamber 46. The O-ring is in frictional engagement with an annular outer portion of the generator. The outer peripheral portion of O-ring 59 is located in an annular groove 60 in the top of the inside face of side wall 44.

Referring to FIGS. 4 and 5, there is shown droplet generator 33 operable to produce monodispersed liquid droplets. Generator 33 has a housing having a second member including generally flat top plate 61. A center portion of plate 61 has a hole or outlet opening 62. Extended downwardly from the bottom side of plate 61 is a cylindrical side wall 63. The plate 61 and side wall 63 form an inverted, generally cup-shaped housing member having an outwardly directed top flange or lip 61A. The inner face of side wall 63 is threaded to accommodate a threaded plug or cap 64 forming a first housing member. Plug 64 has a longitudinal passageway 66. Line 31 is connected to the plug 64 to provide the inlet port or opening of passageway 66 with fluid under pressure from the infusion pump 29. The inside wall of plate 61 has a circular recess 67 accommodating a removable disc or plate member 68. The center portion of disc 68 has a small discharge orifice or opening 69. The disc 68 is held in recess 67 with an annular resilient member or an O-ring 71. A compressive force is applied to the O-ring 71 by turning plug 64 into the side wall 63. The top face of plug 64 engages O-ring 71 and holds it in a compressive state against disc 68. The O-ring 71 surrounds a chamber 72 located at the upper end of passageway 66 adjacent the disc 68. The orifice 69 is in general axial or longitudinal alignment with passageway 66 and outlet opening 49.

An annular means 73 surrounds the side wall 63 and is operable to vibrate the generator 33 at a frequency that breaks the liquid jet 75 into substantially equal sized droplets 77. Annular means 73 is an annular member or ring of piezoelectric ceramic secured to the underside of flange 61A with an epoxy 74, or similar fastening means. Signal generator 41 is electrically coupled to flange 61A and annular means 73 with a line or conductor 76. Epoxy 74 is an electrically conductive fastening adhesive. Other suitable attaching or bonding means can be used to couple the vibrating means to the flange 61A. The inside surface of vibrating means 63 is spaced, indicated at 78, from the outside surface of side wall 63, whereby all the vibratory forces are applied to the plate 61.

In operation, liquid is supplied to generator 33 via the line 31 on operation of pump 29. The liquid flows through passageway 66 into the chamber 72. The liquid is discharged as a jet 75 through the orifice 69 which directs the liquid through hole 62. The signal generator 41 provides a constant frequency AC voltage to the piezoelectric ceramic 73. This AC voltage vibrates the cup-shaped housing structure 61-63 and disturbs the liquid jet 75. Liquid jet 75 breaks up into equal size droplets 77 at the frequency of the AC voltage. The droplets 77 enter the chamber 54. Air 57 enters the chamber 54 in a turbulent state, thereby dispersing the droplets 77. The dispersed droplets and air are discharged into chamber 18 through opening 49 in a conical pattern, indicated at 58 in FIG. 2. The dispersed aerosol is diluted by the moving clean air in chamber 18 and transported to the outlet opening 19. The clean air flows from housing 14 through porous plate 13 in a generally linear configuration whereby the dispersed aerosol is carried generally linearly in the center or core section of chamber 18.

In terms of method of generating a monodispersed aerosol, the droplet generator 33 functions to discharge a liquid jet 75 through orifice 69. This jet is disturbed with a vibratory force to break the jet into substantially uniform droplets 77. The liquid jet 75 is disturbed at a frequency established with an AC voltage on a member, as piezoelectric ceramic ring 73, which will vibrate when subjected to an AC voltage. The droplets 77 are directed into a mixing or dispersion chamber 54 in a direction in general alignment with the outlet opening 49 for the chamber. During the time the droplets are in the chamber 54, they are subject to a flow of turbulent air 57. Both the turbulent air and the dispersed droplets are discharged from chamber 54 through outlet opening 49 into chamber 18 which forms a passage to the outlet opening 19. A stream of clean air in chamber 18 dilutes the aerosol and carries the dispersed aerosol to the outlet opening 19.

The aerosol generator 12 produces particles that do not have to be measured. The aerosol diameter can be calculated from the liquid flow rate, the disturbance frequency, and non-volatile solute concentration within 0.1 percent error by the following formula:

Dp = [ 6QC/.pi.f] .sup.1/3

where

Q = volumetric flow rate,

C -- volumetric concentration of non-volatile solute in volatile solvent,

f = disturbance frequency.

The particles are substantially uniform in that no size difference can be detected between them by using an optical microscope.

While there have been shown and described a preferred apparatus and method for generating a monodispersed aerosol, it is understood that changes in the size, materials, electrical circuits, fluid and air flow systems may be made by those skilled in the art without departing from the spirit of the invention.

Claims

The embodiments of the invention in which an exclusive property or

1. An aerosol generator comprising: housing means having at least one outlet hole, an apparatus for generating liquid droplets mounted on said housing means, said apparatus having a discharge orifice generally axially aligned with said hole and spaced from said hole, said housing means including means providing a chamber between the apparatus and the outlet hole, means for supplying the apparatus with liquid under pressure whereby the apparatus generates droplets, and means operable to provide air under pressure into the chamber whereby droplets emanating from the apparatus into the chamber are dispersed into the air and discharged with the air

2. The generator of claim 1 wherein: the apparatus for genereating liquid droplets includes means operable to vibrate the apparatus at a frequency

3. The generator of claim 2 wherein: said means to vibrate the apparatus is

4. The generator of claim 1 including: means to apply a constant frequency AC voltage to said apparatus to vibrate the apparatus whereby the jet of liquid moving from the orifice breaks into substantially equal size

5. The generator of claim 1 wherein: said apparatus for generating liquid droplets includes housing means having a first member with a first passage and a second member having an outlet opening attached to the first member, and means having an orifice located between the first member and the

6. The generator of claim 5 including: an annular member engageable with said first member and said means having an orifice to position said means

7. The generator of claim 6 wherein: the annular member is a resilient

8. The generator of claim 1 in combination with: a porous support means, tube means having a chamber extended from said support means for receiving aerosol from the generator, said generator being mounted on said porous support means within said tube means with the outlet hole positioned to direct aerosol into the chamber of the tube means, and means to supply air to said porous support means whereby said air transports the dispersed

9. The aerosol generator of claim 8 wherein: said generator is located generally along the longitudinal axis of the tube means and on the support means so that a substantially uniform cylindrical core of air surrounds

10. The generator of claim 1 wherein: the housing means has a side wall surrounding a chamber, said apparatus for generating liquid droplets being located in said chamber, and resilient means mounting said apparatus on

11. The generator of claim 10 wherein: the resilient means is an annular

12. An apparatus usable with a source of liquid under pressure to generate droplets comprising: housing means having a passageway, an inlet opening, and an outlet opening in communication with the passageway, disc means having an orifice mounted on said housing means, said orifice being open to liquid from said passageway whereby a jet of liquid is discharged from the orifice, means secured to the housing means operable to vibrate the disc means at a frequency that breaks up the jet of liquid moving from the

13. The apparatus of claim 12 wherein: said means secured to the housing

14. The apparatus of claim 13 wherein: said material of the annular member

15. The apparatus of claim 13 including: means to apply a constant frequency AC voltage to said annular member to cause said housing means to vibrate whereby the jet of liquid moving from the orifice breaks up into substantially equal size droplets related to the frequency of the AC

16. The apparatus of claim 12 wherein: said housing means includes a first member having a first passageway and said inlet opening, and a second member having said outlet opening attached to the first member, said disc

17. The apparatus of claim 16 wherein: said second member has a cylindrical wall and the first member has a cylindrical portion attached to said

18. An apparatus usable with a source of liquid under pressure to generate droplets comprising: housing means having a passageway, an inlet opening, and an outlet opening in communication with the passageway, means having an orifice mounted on said housing means, said orifice being generally axially aligned with said outlet opening to direct a jet of liquid through the outlet opening, said housing means including a first member having a first passageway and said inlet opening and a second member having said outlet opening attached to the first member, said means having an orifice being located between said first member and said second member, and annular means engageable with said first member and said means having an orifice to position said means having an orifice on the housing means.

19. The apparatus of claim 18 wherein: the annular means is a resilient

20. The apparatus of claim 18 wherein: said second member has a recess surrounding the inlet opening, a disc means being located in said recess.

21. The apparatus of claim 12 wherein: said orifice in the disc means is a

22. The apparatus of claim 12 wherein: the housing means comprises a first member having a generally flat portion, cylindrical side wall portion and an outwardly directed flange portion, said outlet opening being in said generally flat portion, and a second member attached to the side wall portion, said disc means being located between the first member and second member, and said means operable to vibrate the housing means being secured

23. The apparatus of claim 22 wherein: the flat portion has an inwardly open recess, said disc means having an orifice is located in said recess, and resilient means engageable with the disc means and second member

24. The apparatus of claim 23 wherein: the resilient means is an O-ring.

25. The apparatus of claim 22 wherein: the said means operable to vibrate the housing is a piezoelectric ceramic ring surrounding the side wall

26. A method of generating a monodispersed aerosol comprising: discharging a liquid jet through an orifice in a member, vibrating the member to break up the liquid jet passing through the orifice into substantially uniform droplets, subjecting the droplets to a flow of turbulent air to disperse the droplets in the air, and discharging the droplets to a selected

27. The method of claim 26 wherein: the droplets are discharged into a

28. The method of claim 26 wherein: the member is vibrated at a frequency

29. The method of claim 26 wherein: the liquid jet is discharged into a chamber having an outlet opening, said turbulent air being introduced into said chamber, and said droplets and turbulent air being discharged through

30. The method of claim 26 wherein: the liquid jet is discharged in a direction in general alignment with the outlet opening for the chamber.

31. The method of claim 26 wherein: the droplets are subjected to a DC

32. An apparatus usable with a source of liquid under pressure to generate substantially equalized droplets comprising: housing means having a passageway, an inlet opening and an outlet opening in communication with the passageway whereby the liquid can flow through said housing means, means having an orifice mounted on said housing means between said inlet opening and said outlet opening, said orifice being open to liquid from said passageway whereby a jet of liquid is discharged fom the orifice, means secured to the housing means operable to vibrate the means having an orifice at a frequency that breaks up the jet of liquid moving from the orifice into substantially equal size droplets, said means secured to the housing means being an annular member of piezoelectric material

33. The apparatus of claim 32 wherein: said means having an orifice is a

34. The apparatus of claim 32 wherein: the housing means comprises a first member having said outlet opening and a second member secured to the first member having the inlet opening, said means having an orifice being

35. The apparatus of claim 34 including: resilient means engageable with one of said members and the means having an orifice to position the means

36. The apparatus of claim 34 wherein: one member has an inwardly open recess, said means having an orifice being located in said recess, and means engageable with said means having an orifice and the other member to

37. The apparatus of claim 36 wherein: said means is a resilient O-ring.

38. The apparatus of claim 34 wherein: said one member has an outwardly directed flange, said annular member of piezoelectric material being secured to said flange.