US3429187A - Ring slit conifuge - Google Patents

Description

United rates arent f 3,429,187 RING SLIT CNIFIJGE Werner Stber, Penlield, N.Y., assigner to the United States of America as represented by the United States Atomic Energy Commission Filed Nov. 22, 1967, Ser. No. 685,003

US. Cl. 73--432 Int. Cl. GOll 19/00 Claims ABSTRACT OF THE DISCLOS Background of the invention The investigation of aerosols and the evaluation of particulates contributing to radiation hazards frequently requires the separation of particles according to their size. A common instrument suitable to some degree for this purpose is the cascade impactor. The different impaction stages of this device produce deposits of defined size ranges which can be analyzed separately. As the need for a more detailed analysis grows, however, increases in the number of stages in this type of device to meet this requirement raises the cost as well as mechanical problems to the level where other approaches to the problem become of interest.

One such alternative approach is that of obtaining a continuous size spectrum of the particulate present in an aerosol. The most convenient way of providing this is a dynamical arrangement wherein a laminar ow of air passes through a transverse mechanical force iield which elutriates the air-borne particles according to their kinetic diameter. If these diameters do not fall below 1 micron, as in case of coarse aerosols and dusts, particle separation can easily be effected by the gravitational field acting on a laminar flow of air passing through a horizontal duct. For smaller particles, however, the Brownian motion limits the applicability of gravitational devices. In this case, it is necessary to apply more powerful force fields, as can be produced in centrifuges.

In size-separating centrifugal devices the air ducts participate in the rotation of the centrifuge rotor in such a Way that the air flow is almost perpendicular to the centrifugal force field. Thus, the same principle of size separation as in gravitational devices is maintained. Correspondingly, a laminar air flow is required and, as with gravitational instrument, there are two methods of aerosol deposition, both of which have been realized in actual centrifuge designs. In one case, the aerosol enters the duct over the whole cross section, while in the other case the aerosol is released from an inlet nozzle into a laminar flow of clean air.

In the former arrangement, the centrifuge has the advantage of a relatively high sampling capacity, but suffers from the drawback that particle separation remains incomplete and only tedious mathematical evaluation procedures give approximate results for the actual particle size distribution of an aerosol deposit.

In the latter arrangement, which is sometimes referred to as a conifuge, a true particle size spectrum is obtained. Basically, a typical construction for carrying out this operation consists of a funnel-shaped lid and an inner rotating cone between which a lminar flow of clean air AZQW Patented Feb. 25, 1969 is forced down from the top to the bottom by centrifugal forces. The ow rate is controlled by outlet jets in the rotor base. The aerosol is fed into the duct by a nonrotating inlet nozzle at the tip of the cone. Here the particles are entrained in the laminar ow of clean air and become subject to the centrifugal field. Thus, the trajectories of all particles of like size end up at the same annular ring on the inner wall of the funnel-shaped lid where they can be investigated. For a polydisperse aerosol the deposition pattern results in a true particle size spectrum between the top and the base of the cone.

A major drawback, however, of the above-described device is the relatively low sampling rate. In case of nongaseous atmosphere pollutants, which usually occur in rather low concentrations, the conifuge requires very long sampling times. For this reason, the conifuge has been utilized only in studies of high concentrations of airborne particles as they are found in mining operations and in cigarette smoke.

Summary of the invention The invention described herein was made in the course of, or under, a contract with the U.S. Atomic Energy Commission.

The present invention makes it possible for the iirst time to utilize the conifuge effectively and efficiently for the sampling of non-gaseous atmospheric pollutants under relatively low concentrations of air-borne particles at sampling rates heretofore considered to be unattainable.

In accordance with this invention higher rates of laminar flow are accomplished by utilizing capillary openings to accelerate the clean air. ln addition, the ratio of clean air ow to aerosol iiow can be modified for optimum conditions under all liow rates without affecting the rate of clean gas flow.

It is thus a principal object of this invention to produce a continuous sampling spectrum of particles from low concentration aerosols such as is found in atmospheric conditions.

Other objects and advantages of this invention will hereinafter become readily apparent from the following description of a preferred embodiment of this invention taken with reference to the accompanying drawing.

Brief description of the figures FIG. 3 is a graph showing typical calibration curves for the preferred embodiment.

Description of the preferred embodiment Referring to the drawing, conifuge 10 for use in sampling atomspheric air is seen to consist of a closed vessel 12 formed from a hollow cylindrical body 14 and a pair of upper and lower end walls 16 and 18. Within vessel 12 is a conically shaped rotor 22 mounted on a vertically suspended shaft 24 which extends out through opening 18a of end wall 18. Shaft 24 is provided with a dust shield 24a and a key fitting extension 24b to which would be connected as understood in the art a motor or other drive source (not shown) to rotate Shaft 24 and hence operate conifuge 10.

Rotor 22 has a flange 25 on which is mounted a funnel shaped lid 26 which has a flange 28 which may be bolted or otherwise attached to flange 25. Lid 26 which is lined with a removable foil 27 forms with rotor 22 a conical, annular space or channel 29 whose function is to be further described below. Lid 26 has an upper cylindrical section 32 which is internally threaded at 32a as illustrated to receive a cylindrical laminator 34 which is externally threaded. A ange 36 at the top of laminator 34 overlaps the end of lid 26. Laminator 34 is provided with a downwardly extending extension 38 annularly spaced inwardly from the inside wall of cylindrical extension 32 of lid 26 forming an annular channel 39. In addition, as best shown in FIG. 2, laminator 34 is provided with several annularly arranged rows of spaced openings, described as capillaries 42. Typically each ring of capillaries would contain about l() holes of 0.06" diameter. Hence, as indicated by arrows A, gas entering channel 39 would do so by way of capillaries 42, as will be further explained below.

The lower end of rotor 22 is provided :with a number (such as six) of radially directed outlet jets 44 which are connected by way of chambers 46 and passageways 48 to channel 29. The diameters of jets 44 may, if desired, and for reasons explained further below, be reduced by inserting sleeves for that purpose or by making the bottom portion of rotor 22 containing the jets removable so that another section with different sized jets may be substituted.

Extending down through and supported by end wall 16 of vessel 12 is a stationary duct member 52 which is held in place by a cap 54 which is in turn supported by an annular plate S6 resting on wall 16, a rubber O-ring 58 and a ring 62 which is threaded on a vertical lip 64 extending from plate 56. A threaded nut 66 engaging a threaded portion of member 52 holds the assembly in place. The function of O-ring 58 is to make up for slight differences in dimensions between the described rotor mounting and housing l2.

Cap 54 is provided with coolant passageways 68 which are aligned with coolant passageways 72 extending down through member S2. The bottom portion of the latter is provided with cylindrical extension 74 with a ared portion 76. The inside of extension 74 is threaded at 75 to support a port member '78 which has a bottom solid cylindrical extension 82 which is threaded at 83 to receive a closure 84. A cavity 86 is formed between closure 84 and extension 82 for a purpose to be later described. Cap 54 is also provided with one or more holes 87 for a purpose to be later described.

Duct member 52 also has an axial aerosol duct 88 extending its length and which is aligned with a V-shaped opening 92 in port member 78. Opening 92 is connected to an annular space 94 between flared extension 76 and port member 78 by way of a plurality of passageways 96. Port member 78 is also provided with coolant passageways 98 which cooperate with coolant passageways 72 in duct member 52. As 'will be more particularly described later, aerosol enters duct 88 as indicated by arrow B, passes into V-shaped opening 92, iiows through passageways 96, and enters annular space 94. The aerosol ows out into channel 29 by passing through the annular jet formed by the lower tip of extension 38 and annular tip 78a of port member 78 as illustrated. Flared extension 76 does not form a jet but functions to prevent particulate in the aerosol from depositing on the inside of extension 38 by reducing or eliminating sharp changes in direction of ow of the aerosol leading up to the jet opening.

In the operation of the apparatus just described, removable foil 27 is placed along the outer wall of channel 29 (lining the inner Wall of lid 26). Shaft 24 and rotor 22 are then rotated externally through extension 24b by suitable drive means such as a motor (not shown). The rotation of rotor 22 establishes as is understood in the art a centrifugal force field in channel 29 and acts to pump gas through the system.

Clean air within vessel 12 is circulated in a closed cycle through the apparatus in the following manner. Air enters capillaries 42 as shown by arrows A -where it is accelerated in velocity and is then discharged into channel 39. The air, under high velocity, is then carried into channel 29 and ultimately is discharged through nole 44 back into the space `surrounding lid 26. With conifuge located in the aerosol environment to be sampled, aerosol enters duct member 52 as indicated by arrow B and as previously described is fed through the adjustable annular slit formed by the lower tip of extension 38 and tip 78a of member 78 into the clean air flowing from channel 39 into channel 29. As already noted, rotor 22 acts as a centrifugal pump which causes the air and aerosol ow as described.

As the aerosol air mixture travels down channel 29 the particulate present is deposited as a result of the centrifugal force eld established on removable foil 27 in a true particle size spectrum between the top and base of lid 26. Excess air accumulating within vessel 12 leaks out through opening hole 87 in cap 54 and also the space around shield 24a.

The apparatus just described is capable of sampling rates which are unattainable by devices of this type previously in use. It can be operated at a variety of rotational speeds up to at least 12,000 r.p.rn. with appropriate adjustment in size of outlet jets 44 being made for the selected speed. As already noted, this can be accomplished by providing a removable annular section (not shown) for changing jet size or using inserts in over sized outlet jets 44 for reduced sie at slower speeds.

At speeds above 6,000 r.p.m. cooling may be necessary. In such a case, cooling water is fed into and through some of the coolant passageways 68, 72 and 98 into chamber 86, from whence the coolant returns by way of the remaining passageways.

Capillaries 42, consisting of a large number of parallel holes of relatively small diameter, permit the clean air to accelerate without turbulence to the rate dictated by the rotational speed of rotor 22. This maintains conditions of laminar flow in annular channel 39, which is required before a meaningful impaction spectrum can be obtained. In addition, capillaries 42 tend to precipitate residual aerosol particles which may have escaped prior precipitation in channel 29 because of a size smaller than the critical size defined by the operating conditions.

The annular jet formed by the lower tip of extension 38 and annular tip 78a of port member may be adjusted in size by rotating laminator 34 which is threaded into cylindrical section 32. In this way it is possible to obtain by theoretical considerations and/or by trial and error the most advantageous relationship of aerosol to clean air from the point of View of maximum capacity and/ or particulate distribution along foil 27.

An example of calibration curves prepared for a device built in accordance with this invention is shown in FIG. 3, where L refers to the location of the precipitation on the foil, D is particle size, and F refers to the total flow rate.

It is thus seen there has been provided an improved size-separating centrifugal particle sampler in which higher sampling rates than heretofore obtainable are made possible. While only a preferred embodiment of the invention has been described it is understood that many variations thereof may be made without departing from the principles of this invention. Thus the invention is not to be limited thereby but is to be defined only by the scope of the appended claims.

I claim:

1. Particle size separating apparatus comprising:

(a) gas pumping means for establishing an annular channel subject to a transverse force iield of continuously increasing effect along its length;

(b) means for delivering particulate carrying gas into said channel;

(c) means extending through said channel for collecting particulate impacted out of said gas as a result of said force field and distributed continuously according to size; and

(d) containment means for receiving the clean gas discharging from said gas pumping means and containing said clean air in a closed system surrounding said gas pumping means;

(e) said delivering means including inlet means for accelerating said clean gas and delivering the latter to said channel while simultaneously maintaining laminar flow conditions, and

(f) adjustable means for injecting into said clean air at the entrance to said channel an aerosol containing particulate to be collected.

2. The device of claim 1 in which said gas pumping means is a conically shaped rotor surrounded by a spaced, Stationary lid forming said channel therebetween.

3. The device of claim 2 in Which said 'mlet means forms and includes a plurality of parallel arranged capillaries for maintaining laminar ow as the clean gas is accelerated up to the velocity reached at the entrance to said channel.

4. The device of claim 3 in which said injecting means includes a ring slit formed by an annular stationary wall surrounded by an outer spaced, slidable cylindrical exten- References Cited UNITED STATES PATENTS 1,945,786 2/1934 Peltzer et al. 233-28 3,074,627 1/1963 Goetz 73-432 3,092,583 6/1963 Wolif et al. 73-28 X 3,141,337 7/1964 Hoffstrom 73-432 TIM R. MILES, Primary Examiner.

US. Cl. X.R. 209-473; 233-28

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