US 3698626 A
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Description (OCR text may contain errors)
United States Patent Kotrappa et al.
[451 Oct. 17, 1972 CENTRIFUGE SEPARATOR  Inventors: Payasada Kotrappa; Max E. Light,
both of Albuquerque, N. Mex.
 Assignee: The United States of America as represented by the United States Atomic Energy Commission  Filed: May 17, 1971  Appl. No.: 144,036
 US. Cl. ..233/2, 233/13, 233/35  Int. Cl. ..B04b 3/00  Field of Search ..233/1 R, 12, I3, 27, 29, 35,
 References Cited UNITED STATES PATENTS 432,719 7/1890 Von Bechtolsheim ..233/28 1,936,524 11/1933 Placek ..233/13 2,622,796 12/1952 Steinacker et al. ..233/19 R 2,893,629 7/1959 Darnell ..233/2 X 2,956,434 10/1960 Donoghue ..233/1 R FOREIGN PATENTS OR APPLICATIONS 987,023 3/1965 Great Britain ..,.,...233/13 Primary Examiner-J0rdan Franklin Assistant Examiner-George l-l. Krizmanich Attorney-Roland A. Anderson [5 7] ABSTRACT A centrifuge separator with spiral channel of expanding width from the center of the spiral, to receive material to be separated at the center of the spiral, and with means for laminarly flowing a stream of gas through the channel to carry the material therethrough and for collecting separated materials at different locations along the outer wall of the channel.
7 Claims, 8 Drawing Figures CLEAN AIR ('IOO SYSTEM DRIVE SYSTEM INVENTORS MAX E. LIGHT PAYASADA KOTRAPPA I PATENTEDBBT 11 I912 3,698,626
SHEET 2 [1F 3 INVENTORS 38 PAYASADA KOTRAPPA MAX E. LIGHT PATENTEDucT 17 I972 SHEET 3 BF 3 FIG.6
2O 3O 4O 5O COLLECTION LOCATION FROM BEGINING OF SPOOL (CENTIMETER) INVENTORO PAYASADA KOTRAPPA MAX E. LIGHT CENTRIFUGE SEPARATOR BACKGROUND OF INVENTION There is a growing interest in the potential hazard associated with inhalation of various types of aerosols or other suspensions of particles in gas in the micron and submicron size ranges of both radioactive and nonradioactive forms, and in other investigations. These hazards may be associated with air pollution, industrial operations and accidents, and the like. It has been found to be very difficult to sample airborne particles to determine their concentration and size-frequency distribution due to variations in particle homogeneity and shape. It has also been found that aerosol particles tend to behave in the environment and deposit in the respiratory tract according to their aerodynamic sizes. It is a common practice to assign a hypothetical diameter to a particle which is called the aerodynamic equivalent diameter or aerodynamic diameter. Thisaerodynamic diameter is defined as the size of a spherical particle of unit density which has the same settling speed in air as the particles being analyzed. Thus, all particles of the same aerodynamic size are particles with the same settling speed in air, regardless of any differences in real size, density or shape.
It is desirable in studies of these airborne particles or aerosols to precisely separate the particles into different size group dependent on their aerodynamic sizes. Such separation is preferably carried out at a relatively high sampling rate over a relatively large range of aerodynamic sizes. In applications such as air pollution monitoring, it would be additionally desirable that such separation be achieved with an instrument which is portable, of relatively simple construction and of low cost. In addition, the instrument should be readily calibrated and the particles collected be in a form, such as in small and concentrated collection zones, which are subject to ready analysis and measurements.
SUMMARY OF INVENTION In view of the above, it is an object of this invention to provide a novel aerosol particle separator.
It is a further object of this invention to provide a centrifuge aerosol separator of simple and low cost construction.
It is a further object of this invention to provide a centrifuge which is capable of separating aerosol particles in a readily detectable, analyzable and measurable form.
It is a still further object of this invention to provide an aerosol centrifuge separator having a relatively high sampling rate and good size resolution of micron and submicron particles.
Various other objects and advantages will appear from the following description of the invention, and the most novel features will be particularly pointed out hereinafter in connection with the appended claims. It will be understood that various changes in the details, materials and arrangements of the parts, which are herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art.
The invention relates to a centrifuge separator having a rotor, a generally spiral channel carried by and radiating from the center of the rotor with a smoothly and continuously expanding width from the center,
means for rotating the rotor and channel about its center, means for feeding material to be separated to the channel at the center, means for laminarly flowing a stream of gas through the channel from the center to carry the material therethrough and means for collecting size separated materials at spacially separated locations along the channel.
DESCRIPTION OF DRAWING The invention is illustrated in the accompanying drawing wherein:
FIG. 1 is a cross-sectional side view along line A-A of FIG. 2 and FIG. 5 of the aerosol centrifuge separator of this invention;
FIG. 2 is an elevation top view of the rotor and channel of the separator of FIG.,1 with the cover removed;
FIG. 3 is an enlarged side view of the laminator used in the rotor and channel of FIG. 2;
FIG. 4a is an enlarged side view of the material feeder which is positioned adjacent the laminator;
FIG. 4b is a cross-sectional view of the material feeder along line BB of FIG. 4a;
FIG. 5 is a top elevation view of the rotor support showing the clean gas input and depleted gas output conduit arrangement;
FIG. 6 is a pictorial view of the particles collected on a collector foil of the centrifuge of FIG. 1 showing the particle separation and distribution patterns; and
FIG. 7 is a graph of particle size vs collection location along foil for different revolution rates of the separator.
DETAILED DESCRIPTION The centrifuge separator of this invention is shown in the cross-sectional side view of the overall apparatus in FIG. 1 and by the separate views of some of the elements of the separator in FIGS. 2, 3, 4a, and 4b. Certain of the details of the centrifuge separator are shown in simplified manner for purposes of illustration to show the overall operation and theory of the separator and it is understood that these features may be somewhat modified, if desired, within the skill of the art to optimize these features, such as in certain of the airflow passages and couplings thereof.
The separator includes a rotatable rotor 10 which carries or has incorporated therein a suitable spiral channel 12 which is formed or shaped in accordance with this invention to provide the desired particle separation. Rotor 10 is of generally cylindrical shape and may be fabricated by machining the appropriate channel and passageways from a solid cylindrical member shown or it may be made from an appropriately fabricated channel enclosure and other forming members to provide support for the other associated elements of the centrifuge separator. It will be appreciated that channel 12 may be machined completely through the rotor 10 material using the rotor cover 14 and rotor support 16 as the upper and lower extremities of the channel or it may be machined partly therethrough leaving a portion at the base at certain segments of the rotor, as shown. Channel 12 is formed with an inner generally arcuate or curved wall 18 which spirals outwardly from the center of rotor 10, at least for a portion of the channel 12 length. Spiral channel 12 also includes an outer generally arcuate wall 20 which spirally expands outwardly from an initial location spaced from the center of rotor with an expanding dimension with respect to the inner wall 18. The respective inner and outer walls 18 and of channel 12 may be formed as true spirals or as modifications thereof so long as the width of channel 12 expands continuously and smoothly. With inner and outer walls 18 and 20 as true spirals, the channel width will expand uniformly from its inner location to the termination of the channel.
In the embodiment shown, the various segments of the channel walls are formed from arcs of circles to effect the desired channel width expansion. For example, inner wall 18 is formed from a 180 arc of a first radius and a 276 W are of an increased radius while the outer channel wall 20 is made or formed from a 180 arc of a first radius greater than the radius of the first arc of inner wall 18, a second 180 arc of a greater radius than the first arc of outer wall and of the second arc of inner wall 18, and a third are of 96 of a radius greater than the radius of the second arc of outer wall 20. The respective radii are selected or chosen so as to give some desired relative expansion from the initial width of the channel 12 to its terminal width, such as about from a one to three expansion in the embodiment shown. It is understood that other ratios of expansion may be selected, within the limitations of the overall size of the rotor 10, with the increasing ratio of expansion increasing the range of sizes of particles which may be separated on the wall of the channel with some decrease in resolution between sizes. The one to about three expansion shown provides a desirable compromise between these factors.
The particle separation and collection may be achieved directly on outer wall 20 of channel 12 or a removable foil or sheet 22 or the like may be positioned along outer wall 20 and held in place by a suitable bracket member 23. The foil 22 may be made of any appropriate material to which the particles may adhere, such as stainless steel, copper, paper, or the like, and from which suitable analysis of the particles may be made. If it is desired, provision may be made for the continuous removal of one or more particle sizes from a location or locations along outer wall 20 for the continuous sampling or separation of a desired particle or particle size.
Provision has also been made for receiving and carrying a laminator 24 (shown in detail in FIG. 3) and a material or particle feeder 26 (shown in detail in FIGS. 4a and 4b) at the beginning or central portion of channel 12 by suitable formed recesses or mountings on rotor 10 and cover 14. For purpose of illustration, the laminator 24 and material feeder 26 are not shown in FIG. 1 to facilitate illustration of the respective supporting recesses in rotor 10 and cover 14.
Laminator 24 includes a channel 28 aligned with the central location or beginning of channel 12 and an air or gas inlet groove 30 in rotor 10. Channel 28 includes a plurality of thin strips or plates 32 which are held or fastened to laminator 24 within channel 28. The passageways between sheets 32 are of generally equal cross section throughout their width, length and height so as to provide laminar flow of gas to the beginning of channel 12. The smoothly expanding width of channel 12 assures that such laminar flow continues throughout the length of channel 12 with a decrease in gas velocity along the channel inversely proportional to the increase in channel width. It is important that the channel 28 be substantially aligned with the beginning of channel 12 to eliminate or minimize any turbulence which may be caused by any discontinuities in the walls of channels 12 and 28 at their mating junctions. The recess 34 in rotor 10 which receives and carries laminator 24 is shaped so as to assure the correct alignment of channel 28 with inlet 30 and channel 12. The laminator strips or sheets may be made of stainless steel, brass or copper foil of about 0.006 inch thick and spaced apart about 0.05 inch, and be secured to laminator 24 by appropriate adhesives or solder. The laminator body may be made of a single block of material having channel 28 machined therefrom or it may be made of four pieces, as shown in FIG. 3, of materials like brass or the like appropriately formed and connected together. Additional smoothing and finishing of surfaces and mating parts may be achieved by filling portions of the device with a soft material like lead or solder which may then be more readily finished to the desired shape.
The material or particle feeder 26 is positioned adjacent laminator 24 in a recess 36 in rotor 10 and suitably shaped to insure the desired relative positioning of the parts of material feeder 26 with that of channel l2 and the laminar gas flow emanating from laminator 24, to again minimize production of turbulent gas flow within channel 12. Feeder 26 includes a passageway or bore 38 which is aligned with the center axis of rotor 10. Feeder 26 also includes a surface 40 which is aligned with and a continuation of the inner wall 18 of channel 12 and is formed with the same radius as the first arc of wall 18 and continues to and communicates with passageway 38. Another portion 42 of feeder 26 forms a part of the inner wall 18 of channel 12 and terminates just short of portion 40 and spaced therefrom to provide a gap and passageway 44 coupling passageway 38 with channel 12. A segment of passageway 38 and portion 42 may be made as a separate piece which is fastened to the remaining part of feeder 26 as a fillet 46, if such is desired to facilitate fabrication of the overall feeder. Also, the separation may be modified and results varied by using fillets with different gap and passageway sizes. In addition, feeder 26 may be made with separate or unitarily machined end plates 48 and 50 which fit respectively within recess 36 and an appropriately shaped recess 52 in cover 14. Portion 42 is preferably shaped so as to minimize any abrupt curvatures between it and portion 40 and inner wall 18 so as to minimize turbulent flow therealong. In addition, portion 42 is shaped so as to minimize the length of gap 44, preferably about 0.05 inch or less, to prevent or substantially prevent deposition of particles or materials in gap 44. Gap 44 is preferably located, because of the relative positions of portions 42 and 40 with respect to inner wall 18, so as to minimize centrifugal forces to which material or particles within passageway 38 and gap 44 are subjected, as shown in FIGS. 4a and 4b.
The gas flow to laminator 24 and channel 12 is transmitted to inlet 30 by a passageway 54 in rotor support 16, as will be described in more detail below with respect to FIG. 5, while the material or particles to be separated are supplied to passageway 38 by a bore 56 in rotor cover 14. The passageway 56 is aligned with passageway 38 and is coupled to a'sample material intake nozzle 58 which is appropriately mounted on cover 14 by support 60. In the embodiment shown, nozzle 58 may be held from rotating and its central bore coupled to an appropriate aerosol or other material or particle feed supply, such as directly to the atmosphere where particulate pollution is being monitored, by suitable bearings 64 and 66. The sample or material feed supply may be coupled to bore 62 in any desired manner to effect the desired flow therethrough to passageway 38. In the case of small particles and the like, the particles may be carried by an appropriate gas, such as air, through the bores and passageways to channel 12.
Rotor is attached to rotor cover 14 and rotor support 16 by any appropriate means, such as by threaded bolts about their periphery. Rotor support 16, and consequently rotor 10, is supported on a rotatable shaft 70 which in turn is supported by bearings 72, 74, and 76 on a base housing 78 and rotatably driven by a suitable drive system 80. Rotor support 16 includes a tapered or other suitably formed bore which mates with a cor-' respondingly shaped portion of shaft 70. Shaft 70 and rotor support 16 may becoupled together by any appropriate arrangement such as by a threaded nut 84. Shaft 70 includes a plurality of equally spaced parallel bores 86 arranged symmetrically about the axis of shaft 70 and coupled via a plurality of radial bores 88 to an annular groove 90 which communicates with bore 54 in rotor support 16 and air inlet 30 of rotor 10. The bores 86 are appropriately positioned about the shaft 70 so as to insure a balanced arrangement with respect to the rotation thereof and plugged at their exposed end. Bores 86 also communicate, at their other end, with an annular groove 92 of ring 94 which in turn communicates via radial passages 96 with an air inlet port 98 through base 78. Ring 94 is positioned between bearings 72 and 74 and sealed thereby from other portions of the device or from the atmosphere. Clean air or gas may be supplied by an appropriate filtered pressurized system 100 to port 98 to effect the desired laminar flow through channel 12.
Rotor support 16 and shaft 70 may also be provided with a groove or recess 102 (see also FIG. 5) which extends from the center of shaft 70 to a position below an air or gas exit chamber 104 at the enlarged or expanded end of channel 12. Chamber 104 and groove 102 are coupled together by a series of holes 106 in rotor 10 through the base or bottom wall of chamber 104. The exit gas from chamber 12 may be appropriately filtered to collect any particles or materials which are not collected along locations on the outer wall 20 of channel 12 by an appropriate filter 108 mounted in bracket 110. Filter 108 may be any conventional paper or the like filter material or it may be removed in those applications where all particles conveyed into channel 12 are collected along wall 20. The material or particle depleted air or gas may be carried through groove 102 and a central bore 112 and passageway 1 14 in shaft 70, and a ring manifold 1 16, similar to ring 94, to an outlet port 118 through base 78. The flow of gas or air through the system may be enhanced and controlled using a pump 120 to draw not only the clean air from system 100 but also the aerosol sample from bore 62 through the respective passageways, bores and channels of the centrifuge separator.
Drive system may be any conventional single or variable speed drive arrangement, such as a single or variable speed motor or conventional motor driven centrifuge arrangement. For applications where particular and predictable particle sizes are being collected or separated, a single speed motor of relatively small size, low power and low cost may be used to drive the centrifuge without requiring repetitive calibration of the centrifuge. The centrifuge may be balanced at its operation speed or speeds by removal or addition of material to the rotor or its cover or support in a well known manner with relative ease.
FIG. 6 shows a typical material collection foil 122 with a representative pattern or distribution of particles therealong. The particles are separated according to sizes in distinct bands of monodisperse particles or materials in which each band includes particles of a certain narrow and restricted aerodynamic size. Using a rotor 10 of seven inch outside diameter, an expanding spiral channel constructed as described above was produced having an outer wall 20, and consequently foil 122, length of l8 inches and heigth of 1.25 inches. With a total flow of about 5 liters per minute (lpm) and a controlled aerosol or sample supply of about 0.4 lpm and a clean air supply of about 4.6 lpm and operating at about 4,500 revolutions per minute (rpm), the bands of deposition of spherical latex particles of unit density are shown in FIG. 6 on foil 122. The first band 124 includes particles of 3.0 microns size (aerodynamic diameter), the second band 126 of 2.0 microns size, the third band 128 of 1.0 micron size, the fourth band 130 of 0.8 micron size, fifth band 132 of 0.66 micron size and the sixth band 134 of 0.56 micron size particles with the first band occurring at a location about 5.0 centimeters from the beginning of channel 12 and the last band beginning at about 25 centimeters from the beginning. Particles of sizes less than 0.35 microns were collected in distinct bands on filter 108. Particles may be collected from about 5 or more microns in size to less than about 0.3 microns.
FIG. 7 shows the effect of varying the speed of rotation of the centrifuge separator wherein curves 136, 138 and 140 represent collection of particles at speeds of 4,500, 3,000 and 1,500 rpm using a total air flow of about 5 liters per minute and an aerosol flow rate of about 0.4 liters per minute. As the rotation speed increases, the particles are collected closer to the beginning of the spiral.
The aerosol or input sample may be provided at any desired rate, such as from about 1 to 1.5 liters per minute, limited only by the size of passageway 38 and gap 44 in feeder 26. Clean air may be supplied at rates from 5 to about 25 liters per minute with increasing flows decreasing the range of sizes which may be collected along a given foil length at a given rotation speed of the centrifuge. The centrifuge may be rotated at speeds from about 1,000 to 10,000 rpm and higher, preferably from about 1,500 to 6,000 rpm, with the higher speeds providing a larger range of particle sizes deposited along the foil. With a given clean air supply rate and speed of rotation of the centrifuge, the centrifuge repeatably deposits the same particle sizes at the same locations along the outer wall 20 of chamber 12 permitting ready calibration of a particular centrifuge system.
When the respective fiows are instituted and the centrifuge rotated, aerosol particles having finite inertia may move across the clean air stream in channel 12 because of centrifugal forces provided by the spinning of the spiral channel and be collected on the outer wall of the channel. While moving across the clean air stream, the particles experience different drag resistance depending upon their size and characteristics. Particles with a larger terminal settling velocity (or aerodynamic diameter) will move out across the stream earlier than those with the smaller aerodynamic diameters and will be collected on the foil a shorter distance along the channel. The laminar air flow spreads the particles along the outer wall of the spiral channel according to their aerodynamic diameters. Particles having an aerodynamic diameter too small to cross the stream will deposit in a pattern on filter 108 at the exit of channel 12 and will be separated on the filter according to their aerodynamic diameters. Since the spiral channel 12 moves further from the center of rotation with length, the aerosol sample encounters a continuously increasing centrifugal force with distance along the channel. Also, since the channel widens, velocity of the laminar air stream decreases along the channel. Both of these characteristics tend to enhance the collection of particles of relatively small aerodynamic diameters along a relatively short length of channel. The deposited particles form a visible band of generally parabolic profile determined by the dimensions of the gap or slit 45 in material feeder 26.
The relatively narrow bands of deposited particles permits ready analysis thereof through any conventional measuring system, including radioactivity measuring systems or the like. The respective collection bands of particles may be separated by cutting the tape and the sections of tape subjected to analysis or the particles removed for separate analysis. The narrowness of the bands and the consequent high concentration of particles therein provides a ready form for radioactive or the like analysis in conventional measuring systems. In addition, the arrangement of elements of this centrifuge separator minimizes losses of particles from deposition on elements of the arrangement other then the collection foilitself. Further, the respective parts of the arrangement, particularly the larger parts, may be made from relatively light weight materials such as aluminum or alloys thereof and of relatively small size with conventional and low cost fabrication procedures.
What is claimed is:
1. A centrifuge separator comprising a rotor with a spiral channel radiating from the center of said rotor of continuously expanding width from said center, means for feeding material to be separated to said channel at said center, means for laminarly guiding a stream of gas through said channel from said center for carrying material therethrough, and means for collecting separated materials at different locations along the length of said channel.
2. The separator of claim 1 wherein said channel includes an inner arcuate wall and an outer arcuate wall which smoothly recedes from said inner wall along the lensgth of said channel.
. The separator of claim 2 wherein said collecting means includes a foil disposed along said outer wall.
4. The separator of claim 3 wherein said collecting means also includes a filter disposed across the end of said channel in said stream of gas.
5. The separator of claim 2 wherein said feeding means includes a member having a passageway aligned with the center of rotation of said rotor and a wall forming a part of the inner wall of said channel beginning at said passageway.
6. The separator of claim 1 wherein said means for laminarly guiding includes a plurality of parallel sheets aligned with said channel.
7. The separator of claim I wherein said channel traverses about one revolution about said rotor.