Measuring and testing – Sampler – sample handling – etc. – With constituent separation
Reexamination Certificate
1998-02-09
2001-01-09
Raevis, Robert (Department: 2856)
Measuring and testing
Sampler, sample handling, etc.
With constituent separation
Reexamination Certificate
active
06170342
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is used for sampling particles, particularly airborne particles (aerosols). There are many reasons for the sampling of aerosols including the assessment of air pollution in the atmosphere, indoors and in workplaces. Because the effects of aerosols on human health, materials, visibility, etc. depend on particle size, it is necessary to sample the aerosols with particle size selection in order to properly assess the effects. For example, inhaled particles beyond a certain size will not reach the pulmonary region of the human lung.
For some applications, public agencies and other organizations have adopted size-selective sampling criteria specifying sampling efficiency vs particle size. Apparatus for size-selective particle sampling generally consists of an inlet followed by a means to remove unwanted large particles from the airstream. The remaining airborne particles may then be collected on a filter or detected by an electronic sensing device.
A principal deficiency in prior samplers is excess penetration of solid particles compared to liquid particles because of particle bounce from collection surfaces. Some prior samplers attempt to minimize the problem by greasing or oiling collection surfaces. However, the grease loses its effectiveness as it becomes coated with particles. There is also the danger of the grease or oil migrating to the filter, and contaminating the particle deposit. The greased or oiled surfaces necessitate frequent maintenance that, if not done timely and properly, results in poor sampling performance. Cyclones are an exception in that they are free of particle bounce problems. However, their geometry is inconvenient for many applications.
SUMMARY OF THE INVENTION
The invention, named a SPIRAL SAMPLER, is a sampler for airborne particles. A unique feature is a curved channel or spiral groove in the groove plate. Air containing suspended particles enters the inlet tube attached at 90° to the front plate. Air leaving the inlet tube turns sharply by 90° and enters the curved channel or spiral groove, such as, a channel in the groove plate.
The front plate covers the channel, and a gasket between the plates provides a vacuum seal. At the end of the spiral groove, the air turns another 90° and exits the groove plate. The air then passes through a filter held by the filter back plate, which is attached to the groove plate. Particles are deposited on the filter, which may be removed for analysis of particle mass and/or chemical composition.
Another preferred alternative is to replace the filter back plate with the APS (Aerodynamic Particle Sizer) back plate to conduct the airborne particles to the entrance of an electronic particle detector, an existing device not part of this invention. The particle detector may count and size the particles in real time. A number of other existing particle detectors may be used.
The present invention may be used as a particle size selector at the inlet of other devices as well. Air is drawn through the sampler by a vacuum pump attached to the outlet tube when the filter back plate is used or by the vacuum pump in the electronic particle detector when the APS back plate is used.
The inlet tube produces a jet of air that impinges on the surface of the groove plate. That causes particles that are considerably larger than those of interest to impact on the surface. Some are retained on the surface, others may bounce and enter the spiral groove along with the smaller, suspended particles.
The air flow in the spiral groove follows the curve of the groove, resulting in a centrifugal force toward the outside wall. That further results in the formation of two counter-rotating eddies. Thus, the overall air motion consists of two counter-rotating spirals within the spiral groove. The secondary flows have a major effect in augmenting particle deposition on the outer walls of the spiral groove.
For given dimensions of the spiral groove and a given air flow rate, the percentage of particles penetrating to the filter (i.e., leaving the groove plate without depositing) depends on the particle's aerodynamic diameter. The graph of percentage of particles penetrating vs. particle aerodynamic diameter will have a 50% efficiency (the cutpoint) and the curve will have a characteristic shape.
Three preferable prototype SPIRAL SAMPLERS have been constructed and tested in the aerosol laboratory. One sampler, with the 4 &mgr;m groove plate, when operated at a flow rate of 2.8 liters/minute, has been shown to sample particles with a penetration curve closely following that of the ACGIH respirable mass criteria (American Conference of Governmental Industrial Hygienists).
A prototype with the 2.5 &mgr;m groove plate, when operated at a flow rate of 2.4 liters/minute, has been shown to sample particles with a penetration curve closely following that of the 1997 PM-2.5 Federal Reference Method for the sampling of ambient airborne particles by the U.S. Environmental Protection Agency.
The utility of the sampler is in its ability to sample particles according to a desired penetration curve. An additional important requirement is that sticky or liquid particles and solid, or bouncy particles of the same aerodynamic diameter be sampled with the same efficiency. Testing of the prototype with particles of oleic acid, a liquid, and particles of latex, a solid, verify that the SPIRAL SAMPLER satisfies that requirement.
The present invention causes the particles to deposit relatively gently and at grazing angles on the walls, which inhibits solid particle bounce. In addition, the particle deposition is distributed over a large area.
Some existing particle samplers use oil or grease on the deposition surfaces to reduce particle bounce. However, that has the disadvantages that oil or grease may be dislodged and deposited on the filter. Also, vapors from the oil or grease may contaminate the particle deposit on the filter.
The present SPIRAL SAMPLER is operated without oil or grease on the surfaces, thus eliminating the above disadvantages. A further advantage of the SPIRAL SAMPLER is the relatively small pressure drop because of the absence of any small orifices as in many other types of particle samplers.
By varying the dimensions of the inlet tube and the spiral groove as well as the number of turns of the spiral, SPIRAL SAMPLERs may be designed for sampling at any desired flow rate and with chosen particle size cutoffs. Preferred SPIRAL SAMPLERs may be made in a physically small version, such as the prototype, suitable for wearing on the person in combination with a small, battery-powered pump. The small, low flow rate version may also be used as a portable area sampler. Larger versions may be operated as area samplers with electrically-powered pumps.
The SPIRAL SAMPLER is a sampler for airborne particles. The curved channel or spiral groove removes large particles from the air stream before it reaches the filter or the APS, depending on which back is used, as shown in the drawings.
Three different preferred groove plates are specified, with different channel/groove widths, but with the same groove depth. The spiral grooves are formed from arcs of circles with various radii. The critical tolerances are on the depth and width of the spiral grooves. Note that the absolute radii are not as important as the width of the groove. Also, the transitions between different radii should be smooth.
The other critical tolerances are on the O-ring groove and the beveled surfaces of the filter back or APS back. Note that the O-ring groove is not conventional. The O-ring fits the inner diameter of the O-ring groove. On the outside, the O-ring seals against the beveled edge on the opposing surface, i.e., the filter or APS back. That configuration seals against the filter and also against the outside. For that configuration to be effective, the dimensions must be accurate.
In preferred embodiments, all parts are of suitable material, including but not limited to, aluminum or conducting plastic, and preferred materials for the O-r
Creighton Wray James
Narasimhan Meera P.
Particle Science
Raevis Robert
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