High concentration standard aerosol generator

Abstract

An apparatus which produces an aerosol having a narrow size distribution. e apparatus includes a supply means for providing a primary aerosol stream having a wide size distribution. Concentration means are positioned to receive the stream and this concentration means includes means for reducing volume flow rate without disregarding a proportionate fraction of the particles to provide a reduced flow rate stream. Venturi means are provided to monitor the stream and transfer the stream. Impactor means are provided for receiving the stream and introducing a core of clean air into the center of the stream. Means are provided for adjusting the relative velocities of the stream and the core to exclude particles having an inertia below a predetermined size, thereby eliminating smaller particles. Outlet impactor means are positioned to receive the stream and the core, for removing particles larger than a predetermined size. Exit means are provided for delivering the primary aerosol stream having the narrow size distribution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference is hereby made to the drawings, in which

FIG. 1 is a schematic block diagram showing one embodiment of the present invention; and

FIG. 2 is a more detailed schematic showing particular features of the embodiment shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The aerosol generator of this invention is a unique combination of several components as illustrated in FIG. 1. Compressed air feeds about 425 slpm (liters per minute at standard conditions) 10 to a polydisperse aerosol generator (PAG) 12. Aerosol from the PAG passes through two virtual Impactors 14 and 16 to the primary device 20 for the removal of the small size fraction. The major purpose of impactors 14 and 16 is to concentrate the primary aerosol stream by reducing the volume flowrate to a level acceptable for the device 1 without discarding a proportionate fraction of the particles of interest. The flow split in both 14 and 16 is 10% so that 4 slpm exists through the token flow of impactor 16. A venturi 18 is utilized to accurately adjust and monitor the flow from 16.

20 is a virtual impactor in which a core of clean air, 21 is introduced at the center of the aerosol stream before entering the impactor jet. All of the token flow is comprised of air from this clean core. Only particles with inertia large enough to penetrate into this token flow are thus retained. Retention of small particles in the primary aerosol stream is far below that which can be achieved with traditional virtual impactors. Clean sheath air 23 is also utilized to produce a sharper retention (or collection) efficiency than would otherwise occur and wall losses are probably reduced also. Limits on the proportions of .sup.Q CORE' .sup.Q 2T and .sup.Q SH are identified as: ##EQU1## where Q is the total flowrate. This virtual impactor 20 was tested in a test investigation at total flowrates from 5 to 30 lpm, using air supply means 37.

As shown in FIG. 1, upon exiting 20 the aerosol stream (Q.sub.3T +3 lpm) enters the outlet impactor (OP) 22 for removal of the large particles. OP is a classical impactor except that sheath air is added prior to the jet to maintain the desired size cut and reduce wall losses. The primary aerosol stream exits the system through a pressure let-down orifice 24. This component is necessary because of the significant pressure head across the system which is necessary to establish the desired cutpoints around 1.5 .mu.m.

FIG. 2 gives a more detailed diagram of components of the system. The liquid supply subsystem shown at the lower left of FIG. 2 consists of 3 reservoirs (1, 2, and 3) in addition to the polydisperse aerosol generator (PAG) canister. The aerosol liquid (DuoSeal Vacuum Pump Oil at this time) is poured into the top (No. 1) reservoir at the fill tube. Opening the valve under reservoir No. 1 allows liquid to flow into reservoir No. 3 which feeds the PAG. Compressed air at pressure P.sub.oil is utilized to augment gravity in transferring liquid to the PAG. this pressure regulator is located at the bottom right of the front panel where it can be adjusted to keep the oil level constant as viewed through the site glass adjacent to it. Once that setting is established .DELTA.P.sub.oil (measured at the gauge adjacent to the site glass) is an accurate parameter for reproducing a desired fluid level. Overflow from reservoir No. 3 flows to reservoir No. 2 until the levels in reservoirs No. 2 and 3 equalize. Then reservoir No. 2 is emptied by pumping liquid back to reservoir No. 1 via the hand operated peristaltic pump.

Aerosol leaves the PAG through two 1 inch tubes 30 to the entrance chamber of VP1, above the PAG. These 1 inch tubes reach to within 3/4" of the top wall of this chamber, thus impacting large drops which may be lofted in the PAG. liquid thus impacted drains to a catch bottle. Note that the unit composed of VP1 and VP2 has a slight tilt to assure proper drainage to the catch bottles.

The pressure P at the VP1 inlet and the pressure drop across the jets of VP1, .DELTA.P, are monitored on the front panel. The flowrate through the 8 jets of VP1 is given by

Q.sub.1 =29.1.sqroot..DELTA.P.sub.1 T/(P.sub.1 +P.sub.BAR)

for Q.sub.1 in alpm (liters per minute at actual conditions), .DELTA.P.sub.1 in PSI, T (temperature) in .degree.R, P.sub.1 in PSIG, and P.sub.BAR in PSIA. Most of this flow is exhausted through 6 lines symmetrically placed around the exterior wall of VP1. The lines enter a "ring" shaped polyvinyl chloride (PVC) plenum which exhausts through a 11/2 inch tygon tube to a high efficiency coalescing filter.

The token flow of VP1, carrying the primary aerosol, passes to the inlet chamber of VP2. The flowrate through the single jet of VP2 is given by

Q.sub.2 =3.2 .sqroot..DELTA.P.sub.2 T/(P.sub.2 +P.sub.BAR)

where units are the same as in Equation (1). P.sub.2 and .DELTA.P.sub.2 are monitored on the front panel. Most of this flow is exhausted through a coalescing filter and rotameter in a manner similar to the exhaust of VP1. Q.sub.2 is monitored by a rotameter located on the front panel. The token flow of VP2 passes to the Hochrainer impactor (VP3) through a venturi which provides an accurate determination of flowrate:

Q.sub.2T =0.83 .sqroot..DELTA.P.sub.V /(P.sub.3 +P.sub.BAR)

for Q.sub.2T in alpm, .DELTA.P.sub.V (pressure drop across the venturi) in in PSlG and P.sub.BAR in PSIA. .DELTA.P.sub.V and P.sub.3 are monitored by gauges on the front panel. The aerosol stream is merged with two other flows Q.sub.CORE and Q.sub.SH in the Hochrainer impactor, Q.sub.CORE and Q.sub.SH are monitored by rotameters on the front panel. The flowrate through the jet of VP3 is given by

Q.sub.3 =0.61 .sqroot..DELTA.P.sub.3 T/(P.sub.3 +P.sub.BAR)

for Q.sub.3 in alpm (upstream conditions), .DELTA.P.sub.3 inches of water, P.sub.3 in PSIG, and P.sub.BAR in PSIA. Meters giving .DELTA.P.sub.3 and P.sub.3 are located on the front panel. The sample stream passes through a series of 0.8 mm holes to establish laminar flow. These holes can become covered with liquid due to wall losses. Cotton threads to serve as a wick are used to reduce the buildup of liquid. The wick extends into the drain bottle for this region. This wick was found necessary for long term (hours) of operation. Most of the flow of VP3 is exhausted through a coalescing filter, a metering valve, and a rotameter. The valve and rotameter are located on the front panel. This valve is important for adjusting the flow split between the exhausted air, Q.sub.HD' and the token flow, Q.sub.HT. The primary aerosol (without the small fraction) passes into the token flow of VP3. This flowrate Q.sub.HT is determined by the difference (Q.sub.3 -Q.sub.HD). The token flow of VP3 is merged with a sheath of clean air in the outlet impactor (OP) shown in schematically in FIG. 2. This sheath air flowrate Q.sub.1 is monitored by a rotameter on the front panel. Q.sub.1 is set at a value for which OP will remove the large size fraction. An air supply means 37 provides the sheath of air in the outlet impactor OP. In principle the exact value depends upon the trade-off between the desired width of the distribution versus the desired particle rate.

As shown in FIG. 2, a substantial part of the HCSAG hardware is designed to drain liquid collected on the walls away from the flowstreams. These losses are undesirable but unavoidable. This feature is necessary for continuous operation of the device.

Flowrates needed to obtain the desired output aerosol are obtained empirically. Adjustments of valves is needed to obtain the desired flows and once these adjustments are performed, operation of the system requires only that liquid be added to reservoir No. 1 and the main air supply valve be opened.

Tests of the devices described above demonstrated continuous operation providing a stable aerosol with geometric standard deviation of 1.3 MMD (mass median diameter) of 1.7 micron, and total concentration of 100 mg/m.sup.3 3.8 cfm. This combination of characteristics is unique and cannot be obtained with existing aerosol generators.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows:

Claims

1. An apparatus for producing an aerosol having a narrow size distribution, comprising:

supply means for providing a primary aerosol stream having a wide size distribution;

concentration means positioned to receive said stream, including means for reducing the volume flow rate of said stream without discarding a proportional fraction of the particles to produce a reduced flow rate stream;

venturi means for receiving said reduced flow rate stream, including means to monitor said stream and transfer said stream;

impactor means for receiving said stream and introducing a core of clean air into the center of said stream, including means for adjusting the relative velocities of said stream and said core to exclude particles having an inertia below a predetermined size;

outlet impactor means positioned to receive said stream and core, for removing particles larger than a predetermined size; and

exit means for exiting a primary aerosol stream.

2. The apparatus of claim 1 wherein said concentration means include a pair of vertical impactors connected to sequentially receive said stream and transfer said stream through a plurality of jets to a coalescing filter, to thereby produce said reduced flow rate stream.

3. The apparatus of claim 1 wherein said impactor means further includes air supply means for providing sheath air to said means to thereby produce sharper retention efficiency.

4. Apparatus of claim 1 wherein said outlet impactor further includes means for providing sheath air prior to adding the jet, to maintain the desired size cut and reduce wall losses.