Clean Air Flow Research Articles

Instrument for the in situ measurement of depositing particles

Contaminant particles peculiar to a type of equipment emanate inside and around a semiconductor, or thin-film-transistor–liquid-crystal-display device fabrication front-end equipment. To analyze such deposition phenomena and implement contamination control measures, it is necessary to observe in situ deposition behavior over a long period. This article describes a measuring instrument for this purpose. Deposition and tearing off of particles or mist take place on the surface of a transparent plate (of glass or quartz) and Mie scattering occurs when a laser beam is irradiated onto the particles or mist from below. The scattered light is detected in situ while the detection signals are accumulated and kept track of. In this instrument, a semiconductor laser with beam focusing optics and an ellipsoidal mirror are integrated into a small, portable unit. The laser beam is scanned onto the glass surface over a maximum area of 250 by 250 mm in one or two dimensions. It is possible to adjust the scanned area, scanning speed, and laser beam diameter. One-spot, fixed irradiation is also possible for detection of particle flow or tearing on the surface. The minimum detectable particle size was about 0.3–0.4 μm for polystyrene latex sphere (PSL) particles deposited on the glass surface with a beam diameter of 35 μm and a scanning speed of 0.3 m/s (0.14 μm for a silicon wafer surface). Two kinds of experiments were carried out by the use of one glass with PSL particles deposited on the surface and another glass on whose surface PSL particles were spouted so they would roll without deposition. By using this instrument, it became clear that the response curve for the rolling of the particles agreed well with the Mie equation, whereas the curve for the deposited particles deviated from the equation, but only in the range of minute particle sizes. Another phenomenon clarified through in situ measurement was that once deposited, real-world particles were subjected to the influence of the down flow of clean air (0.3 m/s), and some were torn off in a time period on the order of hours. A comparison of the real-world particle deposition rate was made between the grounded and ungrounded states using a glass plate with a transparent conductive thin film. An increase in the deposition rate was observed for the ungrounded state, while a remarkable tearing-off rate was seen for the grounded state.

A new experimental wind tunnel facility for aerosol sampling investigations

A new experimental wind tunnel facility for aerosol sampling investigations has been built and its performance evaluated. Subsequently, an experimental methodology using a polydisperse test aerosol of glass beads to measure entry, transmission and overall sampling efficiencies has been developed and tested. The new facility is composed of a horizontal cylindrical pipe of 5 m long and 30 cm in diameter. The measurement zone is located just at the exit, allowing to take benefit of the whole cross-sectional area inside a stabilised aerosol flow. The working air velocity range is 0.5–4.5 m s −1 Air velocity and turbulence profiles are uniform within 10%. Turbulence in the working section is controlled with a square mesh grid. The test aerosol is generated by a fluidized-bed generator and dispersed into the clean air flow upstream of the horizontal part. Generated particles are within a size interval extending from a few μm to about 80 μm in aerodynamic diameter. Tests of time and space stability of the test aerosol in the working section were carried out. They have shown a reasonably uniform spatial distribution and time stability considering the size range of generated particles. The experimental method allows to obtain, simultaneously with the same technique entry, transmission, and overall sampling efficiencies of samplers from several μm up to 70 μm in particle aerodynamic diameter with a good accuracy. It is based on the measurement of the distribution of particle number concentration vs particle aerodynamic diameter of deposited and sampled aerosols in a reference probe and in the test sampler. To evaluate both the new wind tunnel facility and the methodology, measurements of the different efficiencies were achieved using a cylindrical sharp-edged thin-walled probe as a test sampler. This evaluation was performed in three steps. At first, the reproducibility of transmission efficiency measurements of the probe working in isokinetic conditions was determined. It appears fairly good between 10 and 70 μm in particle aerodynamic diameter. Then, the methodology was applied to the assessment of the aspiration efficiency of a probe working in subisokinetic conditions. Finally, a consistency test of the data was proposed and applied to our data; it consists in comparing the mass fractions of collected samples (deposited on the internal sampler walls, collected onto filters) calculated from the efficiency data and the distributions of particle concentrations, with those which are directly recovered after each experiment and weighed. This test yields an indicator of the quality of the whole efficiency data set.

Wind tunnel measurement of boundary-layer response to sediment transport

This paper presents an empirical analysis of theboundary-layer response to sediment transport in awind tunnel with working section 0.7 × 0.76 × 12.5 m.With equilibrium transport, both the boundary-layer depth anddisplacement thickness increase relativeto clean air. The logarithmic expression of the Lawof the Wall provides a good description of thevelocity profile within the lower 20% of theboundary-layer depth, at least. Shear velocitymagnitudes (u∞) are similar to and scalewith those measured in the equivalent clean airflow, though this relation is dependent to someextent on particle shape and size. There is noempirical evidence to suggest that u*drops either to or below threshold, as hypothesizedin some deterministic models of saltation. Departurefrom the Law of the Wall is evident in the flowabove the saltation curtain at high velocities(u* > 9 m s-1). Though similar inappearance to an outer wake effect, direct solutionof Coles' wake parameter indicates that this is nota wake phenomenon. This departure is most likelyascribed to a wind tunnel constraint on the downwindadjustment of a relatively thin boundary layer tothe effective roughness associated with thesaltation cloud. Froude numbers, computed for thehigh incident velocities where this departure ismost evident, exceed the conservative limit of 10suggested by White and Mounla (1991) for anequilibrium boundary layer.

Effect of Clean Air Core Geometry on Fine Particle Contamination And Calibration of a Virtual Impactor

ABSTRACT The optimum diameter and position of the clean air core in a clean air core virtual impactor were experimentally determined. It was found that the clean air core diameter should be at least twice as large as the converging nozzle diameter and that the clean air core should be positioned so that the ratio of the clean air flow velocity to the aerosol flow velocity ranges from 1.5 to 5.0 at the outlet of the clean air tube. Use of a laminator in a virtual impactor was found to increase the particle loss while maintaining almost the same collection efficiency curve.

On wind velocity profile measurements taken in wind tunnels with saltating grains

ABSTRACTThis paper reviews some aspects of wind tunnel experiments on sand‐transporting winds. It follows previous papers that have discussed the influence of the outer region of the boundary layer on wind velocity measurements. This influence was quantified with the use Coles’Wake function. In this paper this correction is applied to six previously described wind velocity profiles. An attempt is made to calculate the profile parameter II from these measurements. The values found were not consistent with the expected II, which was determined by Coles for clean air flow. This value (II=0.55) was assumed to be valid in previous analyses for sand‐transporting winds. Evidence for a mutual dependency of friction velocity and profile parameter is presented and the difficulty in determining u. is pointed out. It is suggested that the constant stress region of the boundary layer should be kept large enough for measurements when Coles’Wake function is not to be used in the data analysis.

A Virtual Impactor with Clean Air Core for the Generation of Aerosols with Narrow Size Distributions

An improved virtual impactor with interchangeable nozzles was designed and fabricated. One impactor nozzle, of 1.98-mm diameter, was calibrated with monodisperse polystyrene latex spheres. The results show that the separation efficiency of this impactor can be predicted on a semiempirical basis. With a clean air core in the center of an aerosol flow, this impactor has a better separation characteristic than the conventional virtual impactor. Aerosols with narrow size distributions were obtained by passing Arizona road dust particles through this impactor. The mass median diameter (MMD) of these aerosols varies from 10.0 to 3.3 μm with the geometric standard deviation (GSD) ranging from 1.16 to 1.48. The mass concentrations (Ma) of these aerosols ranges over several orders of magnitude from 0.03 to 13 mg/m3. Aerosols with the MMD values ranging from 2.0 to 0.98 μm were obtained with this virtual impactor followed by a real impactor. The GSD values of these aerosols vary from 1.44 to 1.28 and the M a range from 0.07 to 3.57 mg/m3. The effects of varying total flow rate (Q t), clean air flow ratio (q), and minor flow ratio (r) were investigated. As Q i decreases, the MMD becomes larger and the GSD is decreased. With increasing q, the distribution curve becomes narrower and the Ma decreases. In addition, the fine particle contamination increases as r increases.

A Predictive Model for Vapor Concentration in a Nose-Only Inhalation Chamber

A unique nose-only inhalation chamber was designed and constructed to deliver uniform concentrations of gas, vapor, and aerosol contaminants to mice. This research investigated the fluid dynamics of a vaporous contaminant in the vertical flow chamber. The vapor was introduced by allowing the liquid phase of the contaminant to evaporate freely into the chamber interior. A contaminant mass transfer model was developed to predict concentrations generated by the system. The mathematical model of the system used clean airflow, liquid surface area, thickness of the stagnant air layer covering the liquid, system pressure, contaminant diffusion coefficient, and contaminant vapor pressure to compute the vapor concentration delivered to exposure ports. The equation was verified by placing various containers of methyl isobutyl ketone in the chamber and determining with a photospectrometer the resulting equilibrium concentrations. Vapor pressure, diffusion coefficient, and system pressure were held constant while airflow, surface area, and stagnant air layer thickness were varied systematically within the chamber. The resulting empirical data points were compared to the curves predicted by the theoretical model. Empirical concentrations fell within 0 to 48% of the theoretical values, showing that the equation can be used to choose values for airflow, surface area, and stagnant air layer thickness that will result in chamber concentrations in close proximity to the target concentration. If an exact concentration is essential, parameters may be individually adjusted to converge on the target concentration.

Laminar Airflow Systems: Issues Surrounding Their Effectiveness

he operating room team must take all possible steps, including structural T improvements in operating room design, to remove extraneous harmful factors during surgery. When changes are as expensive as the wholesale installation of laminar clean airflow systems, however, O R team members must consider if the changes will reduce wound infection rates. The American College of Surgeons Committee on the Operating Room Environment published statements on OR air issues in 1972, 1973, 1976, and 1977.' For the most part, these four statements are as valid today as when they were originally written. In the intervening decade, there have been a number of poorly controlled studies and a few good singleand multi-institutional clinical trials that were done with as much control as possible. When we participated in planning a new hospital and when we prepared program presentations, we reviewed the literature and our own experiences. Even the most recent studies did not

The calibration of a timbrell aerosol spectrometer

An experimental method for the calibration of a Timbrell aerosol spectrometer is described and the influence of the aerosol and winnowing air flow rates on performance investigated. The Timbrell spectrometer size classifies airborne particles according to their aerodynamic diameters, winnowing them in a recirculating flow of clean air while they fall under the influence of gravity in a horizontal settling chamber. Size separation takes place under Stokesian conditions and the particles deposit on to a series of microscope slides. The results of the calibration with monodisperse polymer latex microspheres and polydisperse spherical particles of polyvinyl acetate show that the instrument achieved acceptable size resolution (∼12%) at an aerosol sampling flow rate of 1 cm 3 min −1, with a ratio of aerosol to winnowing flow rate of 6.7 × 10 −3. Under the conditions used the instrument was shown to be most sensitive for size classifying particles in the range 4–12 μm aerodynamic diameter. The Timbrell aerosol spectrometer is a useful device both as a reference method for the determination of aerodynamic diameters of airborne particles and as a means of collecting such particles for subsequent examination by other aerosol analysis equipment.

Classification performance of a rectangular jet virtual impactor—effect of nozzle width ratio of collection nozzle to acceleration jet

The classification performance of a rectangular jet virtual impactor has been studied with particular attention to the effect of the nozzle width ratio. Numerical calculations showed that a smaller cut-size would be expected when the width of the collection nozzle was set wider than that of the acceleration jet. The sharper classification was also expected for larger nozzle width ratio. The experimental results showed, however, that if the nozzle width ratio was larger than about 1.7, the cut-size data disagreed with the theoretical values. The data for the sharpness showed a similar tendency. Flow visualization experiments showed that the flow in the impactor of large nozzle width ratio was separated from the wall of the collection nozzle and large eddies were generated there. Some of the particles separated into the collection nozzle would be caught by the eddies and exhausted to the separation zone, resulting in a larger cut-size and less sharp separation than those expected by the numerical calculations. The undeflected flow rate was increased so as to eliminate the eddies, and also the clean air flows were introduced. Then the smaller cut-size and the sharper classification were obtained.

Size classification of industrial sub-micron powder by a virtual impactor

The size classification of sub-micron particles has been investigated both theoretically and experimentally by use of a large-scale virtual impactor. The newly constructed impactor is a two-dimensional type, and clean air is introduced in order to attain the sharp-cut in the classification of industrial powders such as the raw material for ceramics. It was found that the particles were successfully classified in the sub-micron range. Classification efficiencies, based on a centrifugal size analysis, were well predicted by numerical calculations. The classified particles were also examined by electron micrographs, showing that the classification was very sharp. The sharpness index (D p 75/D p 25) attained in the experiment was as small as 1.69, which was adjustable by changing the flow rate ratio between the clean air flow and the total flow. It was confirmed that both the sharpness of separation and the cut size could be controlled by changing the classification parameters such as the flow rate ratios between the clean air and the aerosol flow, the jet width, the jet velocity, or the ratio of the slit gap to the jet width.

Development of Compact 500 kV 8000 a Gas Insulated Transmission Line-Dust Control during Field Jointing and Method for Detecting Conductive Particles

This paper describes the results of studies made on dust control during field jointing and a method for detecting conductive particles after installation in the development of a compact 500 kV 8000 A gas insulated transmission line (GIL). The study on dust control during field jointing proved that field jointing of the compact GIL can be done like conventional GIL in an easily fabricated vinyl shelter without a clean air flow. Harmful conductive particles can be detected with an Acoustic Emission (AE) sensor. This sensor is effective in improving the reliability of the compact GIL when used with a suitable ac voltage during field test. A 120 m long test line of compact 500 kV 8000 A GIL was constructed and, in a long-term field test, proved to have properties sufficient for practical use.

Performance of a Modified Virtual Impactor

The performance of a modified virtual impactor with a clean air core in the center of an aerosol flow is described. With this feature, the modified virtual impactor has overcome the inherent disadvantage of conventional designs which have fine particle contamination in the coarse particle fraction. Furthermore, the separation efficiency of this impactor can be predicted from the efficiency curve of the conventional type virtual impactor and the ratio of clean air flow to the total flow rate. With a higher clean air flow, the 50% cutoff Stokes number slightly increases and the efficiency curve has better separation characteristics. Internal wall losses in the impactor are a function of particle size and reach a local maximum near the 50% cutoff size.

Air flow patterns in the operating theatre.

Bacteria-carrying particles and exhaled anaesthetic gases are the two contaminants found in the air flow patterns of operating rooms. Their origin, direction and speed were illustrated by a motion picture using Schlieren photography and smoke tracers. Compared with a conventionally well air conditioned operating theatre, it was shown that a downward flow of clean air reduced the number of bacteria-carrying particles at the wound site by sixty times. The Exflow method of achieving this without the restriction of any side panels or floor obstruction was described. The total body exhaust worn by the surgical team was shown to reduce the bacteria count by a further eleven times. Clinical results show that when both these systems are used together, patient infection was reduced from 9 per cent to between 0.3 per cent and 0.5 per cent, even when no pre-operative antibiotics were used. Anaesthetic gas pollution was measured and shown to be generally 1000 p.p.m. at the head of the patient, in induction, operating and recovery rooms, also in dental and labour rooms. A high volume low pressure active scavenging system was described together with its various attachments including one specially for paediatric scavenging. Results showed a reduction of nitrous oxide pollution to between zero and 3 p.p.m. The economy and cost effectiveness of both these pollution control systems was shown to be good due to the removal of health hazards from patients and theatre staff.

An improved virtual impactor for particle classification and generation of test aerosols with narrow size distributions

The virtual impactor performance has been improved with regard to its separation characteristics by a new design embedding the aerosol flow between a core and an enveloping sheath of clean air flow. With this feature, the new impactor has successfully overcome the intrinsic disadvantage of conventional designs which always have finite separation efficiencies, even if zero inertia particles are sampled. Furthermore, various and high degrees of precision in size separation were established by adjusting the clean air flow rates. Test aerosols with narrow size distributions were obtained by passing paraffin aerosols, coal dust, slate powder and asbestos fibers through a series of two units of the new virtual impactor.

An inertial spectrometer for aerosol particles

An inertial spectrometer is described which measures the size distribution of aerosols. It consists of a rectangular duct with a 90° bend through which clean air flows. A thin aerosol sheath is injected upstream of the bend and the overall flow is sucked downstream through a wall of the duct which is made of a membrane filter. The particles leave the original streamline by a distance which is a function of the aerodynamic size, while remaining airborne: the separation is magnified by projection to the filter which then contains the complete size information of the aerosol. An example of the characterization of polydisperse aerosols is given and possible applications of the spectrometer are discussed.

Aerosol classification by electric mobility: apparatus, theory, and applications

An improved version of the Hewitt (differential) electric mobility analyzer was developed and its classifying characteristics were determined theoretically. The central mobility of the classified aerosol was found to be ( q c + q m )/4 πΛV, where q c and q m are the clean air and main outlet flows, respectively, Λ is a geometric factor, and Λ is the center rod voltage. The half-width of the mobility band was found to be ( q a + q s )/4 πΛV, where q a and q s are the aerosol and sampling outlet flows, respectively. These expressions were verified by the tests with a monodisperse aerosol of known size and low charge. A major advantage of this device is that the classified aerosol fraction remains airborne, ready for further use or analysis. Thus, one application of the device is the production of monodisperse test aerosols of known size, charge, and concentration by classification of a polydisperse aerosol. Other current applications include accurate measurement of particle electric mobility moments and high-resolution particle size analysis for polydisperse aerosols in the 0.005–1.0 μm size range.

Zur Abscheidung von Aerosolteilchen nach ihrer grösse mittels einer neuartigen Impaktordüse

This paper describes a new impactor jet which separates particles according to their size, that is their aerodynamic diameter. In this apparatus a central flow of clean air forms a shielding flow over the impactor surface. The aerosol particles, contained in a layer adjacent to the clean air, are carried outward by the shielding flow as they approach the impaction surface, and are thus separated. A simplified theoretical model based on plane stagnation point flow indicates that a useful resolution should be achievable for a limited range of particle radius. The resolution, r/ Δr, is considerably reduced as particle size increases because of the diminishing influence of the shielding flow on the trajectories of the larger particles. The useful range of this apparatus will therefore lie between the lower particle size impaction limit and an upper particle size limit fixed by the resolution requirements. Based on the ideas of the theory an impactor was built and tested for size separation capability. Particle size separation was possible for example in the interval between 0·5 and 2 μm dia, but resolution was not as high as expected by the simplified theory.