Patent Application
20130248693
The present invention relates to apparatus and methods for detecting airborne particles and more particularly to apparatus and methods for containing particles to be measured within a laminar gas flow within a sensing chamber.
A variety of manufacturing environments require strict control over the presence of foreign debris in the air. Semiconductor manufacturing, for example, has long required “clean-rooms” that use extensive air filtering to reduce the number and size of particles in the air to some acceptable level. Other manufacturing environments have similar but distinct requirements. For example, in pharmaceutical or medical device manufacturing environments, hospitals, and food processing or preparation environments, it is critical to control not only the number of particles in the air, but minimization of biologic particles is of particular importance. Microbial contamination, for example, can render an entire batch of pharmaceutical product unusable leading to significant monetary losses in the manufacturing process. Additionally, it is advantageous to have instantaneous detection of contamination events, including instantaneous information about whether a contamination event is biologic or non-biologic, during the manufacturing process for pharmaceuticals or medical devices.
Various systems and methods exist to detect and determine the size of airborne particles. Systems are also available to detect and characterize detected airborne particles as biologic or inert. For example, systems have been designed to detect the presence airborne particles by measuring the amount and directionality of light scattered by particles to determine particle size and the measurement of fluorescence excited in particles by illumination with a source light to classify measured particles as biological or non-biological.
In most conventional systems, fluid to be sampled (e.g., environmental air), is pulled into the system and introduced into a sensing chamber, for example, through an inlet nozzle. The fluid is then measured for particles in some way, for example, by illuminating the fluid with a beam of light. The fluid is then evacuated from the sensing chamber through an outlet nozzle.
This arrangement poses certain challenges. For example, particles may escape the flow of fluid between the inlet nozzle and the outlet nozzle and enter the sensing chamber. In other cases, particles may back up against the flow of fluid, and enter the sensing chamber though the outlet nozzle. In both cases, such particles can cross the illumination beam and scatter light or fluoresce, just as they would if they were being measured in the interrogation zone between the inlet and outlet nozzle. These particles create spurious optical emissions, which may be detected by the optical detectors in the particle detection system, thereby degrading the accuracy of the measurement data.
The problem is illustrated in
While in interrogation zone 112, the fluid is illuminated by a light beam, generated by a light source, for example, a laser, laser diode or LED, in the vicinity of 135. While illuminated, particles in the fluid scatter light in the direction of 140, where scattered light may be detected by a scatter detector. Additionally, while illuminated, biological particles in the fluid fluoresce, and this fluorescence light is collected by an ellipsoidal reflector 125 and directed to a fluorescence detector in the vicinity of 130.
In certain systems, stray particles 142 can escape the fluid flow in the interrogation zone, leave the vicinity of the interrogation zone, and occupy other areas of the sensing chamber 102. Stray particles 142 are particles that either never reach outlet nozzle 110 to be evacuated from the system, particles that back up from outlet nozzle 110 (as shown in
A conventional solution for dealing with the problem of particles escaping the flow of fluid into a sensing chamber utilizes a “sheath flow” of clean air, which encapsulates the flow of particles under test as they travel through an interrogation zone. In a conventional arrangement an inlet nozzle is provided having an inner or central portion embedded in and surrounded by an outer annular portion. The outer annular portion divided into an upper and lower section. Environmental air from the upper annular section is diverted from an input stream. This “sheath air” is then filtered and accelerated with a sheath pump. The sheath air is then re-introduced into a lower annular section of the inlet nozzle. Meanwhile, air containing particles to be measured proceeds in the inner portion. In the region of the interrogation zone (i.e., near the input aperture to the sensing chamber), the lower annular section of both the central nozzle (carrying the air to be measured), and the outer annular portion taper to accelerate both flows of air. The combined air flows are then introduced to the interrogation zone, sampled, and then evacuated using a total flow pump. The result is that the air containing particles to be sampled is encapsulated in a relatively more quickly flowing sheath of clean, filtered air. This prevents particles from escaping the interior flow of sample air before both flows are drawn from the sensing chamber. A device operating according to this method is described in U.S. Pat. No. 5,561,515 to Hairston, et al., at
Embodiments of the invention prevent particles from escaping a fluid flow under test by raising the pressure in the surrounding sensing chamber. This creates a uniform high-to-low pressure gradient from the interrogation zone into the outlet nozzle, which ensures that all particles are evacuated from the interrogation zone. The sensing chamber is pressurized in a number of ways. In one embodiment, filtered environmental air is drawn into the sensing chamber at a location remote from the interrogation zone using the negative pressure generated at the system's outlet nozzle. In other embodiments, a portion of the flow of air to be sampled is diverted prior to introduction to the inlet nozzle thereby creating a bypass flow. The bypass flow is then filtered, and its flow rate is regulated with an aperture. The bypass flow is then introduced to the sensing chamber at a position remote from the interrogation zone.
In another embodiment, a method and system for flushing a sensing chamber to dislodge and evacuate deposited particles in provided. In that embodiment, filtered environmental air, or bypass flow air, is introduced into a sensing chamber at more than one remote location. As this introduced air is drawn out of the sensing chamber, particles deposited on the interior of the sensing chamber are dislodged and removed.
In one embodiment, a particle detection system for detecting particles in an environmental gas is provided. The system includes a sensing chamber having an inlet flow nozzle fluidly coupled to the environmental gas, and an outlet flow nozzle. The inlet flow nozzle and the outlet flow nozzle define an interrogation zone. The system also includes an extra flow port located in the sensing chamber remote from the interrogation zone, the extra flow port being fluidly coupled between the sensing chamber and a source of extra gas.
In another embodiment, the system includes a source of negative pressure fluidly coupled to the outlet flow nozzle and capable of drawing environmental gas through the interrogation zone at a defined sample flow rate. In another embodiment, the extra flow port is fluidly coupled to the source of negative pressure through the exit nozzle such that the source of negative pressure is capable of drawing extra gas at a defined extra gas flow rate.
In certain embodiments, when the source of negative pressure is in operation, the ratio of the extra gas flow rate to the sum of the extra gas flow rate and the sample flow rate exceeds 0.21. In another embodiment, the source of negative pressure is in operation, the extra gas flow rate equals the sample flow rate. In another embodiment, when the source of negative pressure is in operation, a pressure in the sensing chamber exceeds a pressure at the outlet flow nozzle.
In certain embodiments, the extra flow port is fluidly coupled to a bypass flow path, which connects the inlet flow nozzle and the sensing chamber. In some embodiments, the bypass flow path comprises an adjustable orifice and a particle filter.
In another embodiment, the extra flow port of the system is fluidly coupled to a source of filtered environmental gas. In some embodiments, the system includes a second extra flow port located in the sensing chamber remote from the interrogation zone and the second extra flow port is fluidly coupled between the sensing chamber and an source of extra gas, and fluidly coupled to the first extra flow port.
Certain embodiments include a method of detecting particles in an environmental gas. The method involves drawing environmental gas to be sampled into a sensing chamber, measuring the environmental gas to be sampled to detect particles therein, evacuating environmental gas from the sensing chamber through an outlet nozzle, and pressurizing the sensing chamber such that its pressure is higher than that at the outlet nozzle.
In some embodiments, pressurizing the sensing chamber such that its pressure is higher than that at the outlet nozzle includes drawing extra gas into the sensing chamber. In other embodiments, measuring the environmental gas to be sampled to detect particles therein occurs in an interrogation zone, and drawing extra gas into the sensing chamber includes diverting a portion of the environmental gas to be sampled before it is introduced to interrogation zone and introducing the extra gas at a position in the sensing chamber remote from the interrogation zone.
In certain embodiments, drawing extra gas into the sensing chamber comprises supplying filtered environmental gas to the sensing chamber. In other embodiments, drawing extra gas into the sensing chamber comprises providing an extra gas flow path. In some embodiments, the extra gas flow path connects the sensing chamber to an inlet nozzle where environmental gas to be sampled is introduced to the sensing chamber. For some embodiments, the extra gas is filtered and flow-regulated prior to being introduced to the sensing chamber.
In some embodiments, environmental gas to be sampled is drawn into the sensing chamber at a first predefined rate and extra gas is drawn into the sensing chamber at a second predefined rate. In certain embodiments, the ratio of the first predefined rate and the sum of the first predefined rate and the second predefined rate exceeds 0.21.
Systems and methods according to the invention have certain advantages. For example, in contradistinction to the sheath flow method described above, systems and methods disclosed herein are entirely passive, relying only on the suction pump in fluid communication with the outlet nozzle to provide additional clean air flows to the sensing chamber. Thus, unlike in the sheath flow method, there is no need for an additional sheath flow pump. Additionally, systems using sheath flow generally use much higher total flow rates in the sheath air than in the sample air.
For example, Applicant understands that in one commercially available particle detection system that uses the sheath flow method, the Aerodynamic Particle Sizer® Model 3321, available from TSI Incorporated of St. Paul, Minn., the sample flow rate is 1 L/min and the sheath flow rate is 4 L/min, resulting in a total flow of 5 L/min. This results in a ratio of sheath flow to total flow of 0.8, meaning that 80% of the air being moved by the total flow pump is “wasted”. This means that the total flow pump, tasked with evacuating all the air from the interrogation zone, must work much harder than it would otherwise have to work to simply draw into and evacuate from the interrogation zone the 1 L/min flow of air to be measured. The consequence of this is the need for larger pumps, generating greater amounts of waste heat, which is disadvantageous both from the standpoint of the physical package size of the particle detection system, and the ability of the system's sensitive optical and electronic systems to manage noise.
Systems in accordance with embodiments of the invention enable this additional pumping capacity to be used to pull more air to be sample, i.e., to achieve higher levels of throughput for air to be measured. Applicant has discovered that systems in accordance with the invention can eliminate the escaping particle problem by introducing an auxiliary or bypass flow to the sensing chamber that is smaller than the sample flow. This allows the majority of pumping power, which is a limitation in such systems, to be devoted to moving sample air.
Additionally, systems according to embodiments of the invention use a much simpler mechanical design, which does not rely on a complicated inlet nozzle geometry (i.e. inner nozzle embedded inside of outer nozzle).
This invention is described in preferred embodiments in the following description with reference to the Figures, in which like numbers or indications represent the same or similar elements. References throughout this specification to “one embodiment,” “an embodiment,” “a related embodiment,” or similar language mean that a particular feature, structure, or characteristic described in connection with the referred to “embodiment” is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. It is to be understood that no portion of disclosure, taken on its own and in possible connection with a figure, is intended to provide a complete description of all features of the invention.
The described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are recited to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
The present invention is useful in a variety of detectors for detecting particles. Examples are particle size detectors and biologic particle detectors that detect both particle size and fluorescence to determine if a particle is a biologic or inert particle.
The present invention relates generally to methods for detecting airborne particles (generally, fluid-borne particles) in an air flow to be sampled, and more particularly to apparatus and methods that reduce particle deposition or escape from the sample air flow into the sensing chamber.
An example of a detection system with which the present invention can be used comprises one or more of the following elements:
a light source (for example, a laser, a laser diode or a LED);
optical components for the collection and focusing of scattered light and/or excited fluorescence on detectors;
one or more detectors for detecting scattered light (for example, a photodiode) and or one or more fluorescence detectors (for example, a PMT); and
one or more optical filters.
Detection systems may further comprise software, DAQ interface, and computer processor.
Other examples of detection systems that the current invention can be used with, include but are not limited to, those described in commonly owned U.S. patent application Ser. No. 12/642,705 Babico et al. and U.S. Pat. No. 7,430,046 to Jiang et al., the disclosures of which are incorporated herein by reference.
In the sensing chamber, particles are detected when particles present in the laminar air (or gas) flow between the inlet and outlet nozzles 210, 215, pass through a light beam 225 (from a non-illustrated light source) in the particle interrogation zone 230 region and interact with the light beam 225. The light source may be a laser beam, a laser diode, an LED or other type of light source. The wavelength of the light beam 225 is chosen to interact with particles that may be present in the air flow and produce a signal that can be detected by one or more detectors. In preferred embodiments the light beam 225 is collimated or near collimated. For example, particles may interact with a laser beam to scatter light from which their particle size can be determined, or absorb one or more photons from the light and auto-fluoresce to determine if they are biologic in nature. In each instance one or more non-illustrated detectors are used to detect scattered light, fluorescence, or other signal caused by the interaction of the particles with the light beam.
To overcome the problem of particles escaping from the particle interrogation zone between the inlet and outlet air gap, the embodiment of
Bypass flow path 235 is connected to sensing chamber 205 at a location remote from interrogation zone 230. Thus, the interior of the sensing chamber, taken as a whole, receives a total flow rate equal to that present in the inlet flow path 255 and the bypass flow path 235. Accordingly, total flow path 260, which is connected to outlet nozzle 215, carries air (or other gas) at a rate equal to the sum of the inlet flow path and bypass flow path rates. The system of
The flow rate across interrogation zone 230 is typically a boundary condition for optimizing the components of the system of
Experimental and analytical results indicate that this effect is scalable, and that other flow rates may also be used. In particular, Applicant has discovered that the effectiveness of preventing escape of particles from the interrogation zone or outlet nozzle depends on the ratio of the bypass flow rate to the total flow rate. In particular, a ratio of bypass flow rate to total flow rate greater than 0.21 has been found to be effective in preventing escaping particles. In one embodiment the bypass flow ratio used is 0.23.
The escape of particles from the particle interrogation zone between the inlet and outlet nozzles is controlled by pressure balance. The pressure in the sensing chamber is raised by making a hole (port) in the sensing chamber and introducing bypass flow though the hole. The pressure increase in the sensing chamber provides a pressure balance between the internal sensing chamber volume and the outlet nozzle, thereby significantly reducing or preventing the back-flow of particles, or the escape of particles from the particle interrogation zone.
Modeling of the invention by Computational Fluid Dynamics (“CFD”) illustrates the advantages of the bypass flow of the present invention. This is illustrated in
Thus far, the phrase “bypass flow” has been used to describe the flow of additional air, apart from sample air, which is introduced to the sensing chamber of a particle detection system in order to pressure balance the sensing chamber and prevent backflow and escape of particles from the interrogation zone. Indeed, the embodiment of
Applicant has determined, both experimentally and through CFD modeling, that systems according to embodiments of the invention are remarkably insensitive to the location of the bypass flow port, i.e., the precise location where the bypass or extra flow is introduced to the sensing chamber. In certain embodiments, the inlet port to the sensing chamber for the bypass flow may be connected to a manifold that provides for two or more air inlets inside of the sensing chamber, or for the bypass air to be introduced at specific locations in the sensing chamber. For example, in one embodiment the bypass flow can be introduced in the region of the particle in interrogation zone. In additional embodiments, bypass air flow is introduced to the sensing chamber at two different points along two different paths, which may originate from a single bypass port. An example of the latter arrangement is shown in
The embodiment of
Additional CFD modeling of systems according to embodiments of the invention is shown in
While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.
This application claims priority to U.S. Provisional Patent Application No. 61/614,395, filed on Mar. 22, 2012, which application is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61614395 | Mar 2012 | US |
