Measuring and testing – Sampler – sample handling – etc. – With constituent separation
Reexamination Certificate
1999-07-23
2002-08-13
Noland, Thomas P. (Department: 2856)
Measuring and testing
Sampler, sample handling, etc.
With constituent separation
C073S028050, C073S863030
Reexamination Certificate
active
06431014
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a cascade aerosol impactor for classifying aerosol particles that includes pressure sensors for monitoring the functions of the impactor, and further includes mounting structure that permits rotating impaction plates without mechanical drives that require seals. The nozzles used are also constructed to reduce cross flow effects.
Inertial impactors are widely used for measuring the size distribution of aerosols. For purpose of this invention, particles suspended in a gas are referred to as an aerosol. The gas is usually air, but other gases such as nitrogen, oxygen, argon, helium, etc. can also be the suspending gaseous medium. The particles can be a solid, a liquid, or a mixture of both. The particle size is usually between 0.002 &mgr;m and 100 &mgr;m.
Inertial impactor, aerosol impactor, or impactor all refer to an aerosol sampling or collection device that separates aerosol particles from the gaseous medium in which they are suspended by the inertial effect of the particles. The device usually uses a nozzle to accelerate the gas to a high velocity and direct the gas jet against an impaction plate to cause particle impaction on the plate. Particles will impact only when their size is larger than a certain critical value, while smaller particles with insufficient momentum or inertia will be carried by the gas flow around the plate to an exit and escape collection.
The critical particle size at which particle collection occurs is referred to as the impactor cut-point. The cut-point particle diameter of an impactor can be varied by varying the nozzle size and gas velocity. Smaller nozzles and higher gas velocities will produce smaller cut-point particle diameters. The impactor cut-point is also affected by the gas viscosity, the shape of the nozzle, as well as the nozzle-to-plate distance. In an ideal impactor all particles larger than the cut-point are collected with 100% efficiency while smaller particles are not collected. In a real impactor, particle impaction does not occur ideally at a single particle size. The transition from zero to 100% particle collection usually occurs over a range of particle sizes. The narrower this range, the sharper the impactor cut-size characteristics. The ideal impactor is then an impactor with a perfect cut value. In a real impactor with less than a perfect sharpness-of-cut, the cut-point is usually defined as the particle diameter at which 50% of the particles are collected.
The prior art impactor just described is a single stage impactor. It consists of a single nozzle plate carrying one or more nozzles in parallel and an adjacent impaction plate. Several single-stage impactors can be arranged in series to form a cascade impactor. Cascade impactors are designed so that large particles are collected first, followed by smaller and smaller particles between an inlet and an outlet. There is usually a final filter to collect small particles below the cut-point of the last impactor stage. For instance, in a three-stage impactor with cut-point diameters of, say 10, 3 and 1 &mgr;m, particles larger than 10 &mgr;m are removed by the first, 10-&mgr;m cut stage. Subsequently, particles in the 3-to-10 &mgr;m and the 1-to-3 &mgr;m ranges are removed by the 3-&mgr;m and 1-&mgr;m cut stages, respectively. The final filter then collects particles smaller than 1 &mgr;m.
FIG. 1
is a schematic diagram of a prior art three-stage impactor with a final filter, in which a single nozzle of a progressively smaller diameter is used in each stage to increase the gas velocity to higher and higher values to collect smaller and smaller particles.
The cascade impactor is very useful for size distribution analysis of aerosol particles. Particulate air pollutants, aerosols in the work place environment, as well as other aerosols of practical interest are usually polydisperse, with particle sizes spread over a wide range of values. Cascade impactors can be used to separate particles by size into narrower intervals for analysis. The size-fractionated particles can then be analyzed gravimetrically to determine their mass size distribution. Alternatively, the particles can be analyzed chemically to determine the chemical composition of the particles as a function of particle size. Cascade impactors are widely used in air pollution studies to determine the physical and chemical properties of the airborne particles, assess their potentially harmful health effect, or determine the origin of the particles for pollution abatement or control purposes.
In the schematic prior art diagram of
FIG. 1
, an aerosol source
6
is connected to an inlet of a cascade impactor
8
. Impactor
8
has a single nozzle
9
A,
9
B and
9
C in each impactor stage
8
A,
8
B and
8
C. Impaction plates
10
A,
10
B and
10
C are provided and a filter
11
follows impactor stage
8
C. An outlet
12
leads to a pump or a blower
13
. In practical impactors, each impactor stage is usually comprised of a number of nozzles formed in a nozzle plate. The flow through each nozzle can be quite small, but the total flow through all the nozzles can be quite high by using a large number of nozzles in parallel. A high sampling flow rate is needed to increase sample collection so that the quantity of particulate matter collected is sufficient for analysis.
In recent years, demands for increased accuracy and precision for aerosol measurement have led to the development of cascade impactors with a high volumetric flow rate and large number of impactor stages. For instance, the Micro-Orifice Uniform Deposit Impactor, sold under the trademark MOUDI™, manufactured by the MSP Corporation of Minneapolis, Minn., the assignee of this application, comprises eight (8) or ten (10) impactor stages with nominal cut-point particle diameters that range between 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.1 and 0.056 &mgr;m. In the final stages, nozzle diameters as small as 50 &mgr;m are used. To provide the needed 30 liter-per-minute sampling flow rate, as many as 2,000 nozzles are used in some stages.
The need for increased measurement sensitivity in air pollution research and for other applications has created the need for impactors with flow rates larger than the 30 liters-per-minute. To design impactors with higher flow rates, even larger number of nozzles need to be used. To create a cascade impactor with, say, 90 liters-per-minute sampling flow rate, and similar operational pressure drop characteristics, the number of small nozzles needs to be increased by a factor of 3. Thus, 6000 nozzles need to be used in the final stages of such high flow MOUDI™ cascade impactors.
In designing impactors with large numbers of very small nozzles, it is important to consider the effect of cross flow in the impactor. As will be explained, when a nozzle plate with a large number of nozzles is provided, the gas flow through the outer nozzles, must pass radially outward across the surface of the nozzle plate between it and the impaction plate. This outward radial gas flow is referred to as the cross-flow. The cross flow can cause the gas jets through the nozzles in the cross flow path to be deflected side ways and change their cut-point characteristics. The sharpness-of-cut of the impactor as a whole will then decrease. The cross flow effect is the greatest for nozzles located near the outer edges of the nozzle cluster. To obtain good sharpness-of-cut characteristics, it is important to consider the cross flow effect in designing high flow impactors.
The use of large number of very small nozzles also creates the practical issue of nozzle plugging during use. As aerosols are sampled by the impactor, some accumulation of particulate matter around the edge of the nozzle is unavoidable. Over time, enough material can accumulate to partially block the flow and cause the cut-point of the impactor to change. This effect, if not monitored, can lead to measurement errors. The nozzles can usually be cleaned, but cleaning, if done improperly, for instance, by using a high intensity ultrasonic clean
Liu Benjamin Y. H.
Marple Virgil A.
MSP Corporation
Noland Thomas P.
Westman Champlin & Kelly P.A.
LandOfFree
High accuracy aerosol impactor and monitor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High accuracy aerosol impactor and monitor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High accuracy aerosol impactor and monitor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2945150