ber lter immersed in Dow Corning 704 diffusion pump oil was selected as the best impaction substrate. Further testing showed that the penetration curve was ...
Aerosol Science and Technology 34: 389– 397 (2001) ° c 2001 American Association for Aerosol Research Published by Taylor and Francis 0278-6826=01=$12.00 C .00
Design and Calibration of the EPA PM2:5 Well Impactor Ninety-Six (WINS) Thomas M. Peters,1 Robert W. Vanderpool, and Russell W. Wiener2 1 Research Triangle Institute, Research Triangle Park, 2 National Exposure Research Laboratory, US
North Carolina EPA, Research Triangle Park, North Carolina
The EPA well impactor ninety-six (WINS) was designed and calibrated to serve as a particle size separation device for the EPA reference method sampler for particulate matter under 2.5 ¹m aerodynamic diameter. The WINS was designed to operate downstream of a PM 10 inlet at a volumetric ow rate of 16.7 Lpm. For design simplicity and ease of construction, fractionation of the aerosol in the WINS is provided by a single-stage, single-jet, round-hole impactor. Particles >2.5 ¹m (aerodynamic diameter) have suf cient inertia to be impacted upon a circular 37 mm diameter glass ber lter immersed in 1 mL of a low volatility oil. The relatively large amount of oil is intended to minimize substrate overloading and subsequent particle bounce experienced by some conventional impactors and to represent an easily eld dispensable quantity of de ned tolerance. The novel geometry of the impaction reservoir(or well) is designed to capture any reentrained materialfrom the impaction surface and to prevent loss of oil should the unit be inadvertently turned over or onto its side. The penetration curve of the nal WINS design has a 50% cutpoint diameter equal to 2.48 ¹m and a geometric standard deviation of 1.18. During development, several nozzle designs and well geometries were evaluated to optimize the performance of the WINS. Additionally, two candidate oils (Neovac and Dow Corning 704 diffusion pump oils) and three types of lters (glass ber lters, drain discs, and polycarbonate membrane lters) were evaluated for use as impaction substrates in the WINS. The performance of the WINS was similar for the two oils in combination with a glass ber lter and a drain disc; however, a polycarbonate lter demonstrated elevated penetration values. Based on these tests, a Gelman Type A/E glass ber lter immersed in Dow Corning 704 diffusion pump oil was selected as the best impaction substrate. Further testing showed that the penetration curve was essentially the same when operated with quantities of oil ranging from 0.75 to 3 mL.
INTRODUCTION On July 18, 1997, the U.S. Environmental Protection Agency promulgated a new federal reference method (FRM) for measuring ambient concentrations of particulate matter under Received 6 May 1999; accepted 21 September 1999. Address correspondenceto Robert W. Vanderpool, Research Triangle Institute, Research Triangle Park, NC, 27709.
2.5 ¹m (PM2:5 ) (EPA 1997). This method speci es a reference method sampler that is de ned by design and performance requirements, set forth in the Code of Federal Regulations (CFR). Detailed design and fabrication speci cations, freely availableto any manufacturer, are presented in 40 CFR Part 50, Appendix L describing all components through which the aerosol passes: inlet, downtube, PM2:5 separator, upper portion of the lter holder, and lter cassette. The design goals for the internal separation of PM2:5 in the FRM sampler were as follows: ² ² ² ² ² ² ² ²
operate at 16.7 actual liters per minute (aLpm) downstream of the Graseby-Andersen 246B PM10 inlet, require a single air ow control and measurement system, possess a design which is easy and inexpensiveto manufacture and simple to operate, require no moving parts during sampling, provide a unit that is easy to maintain, require no maintenance over multiple days, provide sustained operation at high loadings, and provide separation of the aerosol by inertia with separation characteristics such that 50% of the particles below 2.5 ¹m in aerodynamic diameter penetrate to the sample collection lter (D50 ) and have a shape described by a geometric standard deviation (GSD) of 1.0 to provide suf cient time for the particles to accelerate to the uid velocity and, if possible, the entrance of the nozzle should be tapered or conical. The well geometry of the impaction surface that was adopted for use in this design led to the use of a nonstandard nozzle design. A single-hole nozzle was viewed as favorable due to the relative ease of construction, adaptation to the well geometry, and lesser propensity to clog. Unfortunately, it is necessary to operate a single nozzle outside the recommended Reynolds number upper limit of 3000 to achieve a D50 size equal to 2.5 ¹m (a Reynolds number of approximately 5500– 6000 is required to achieve this separation). Figure 2 displays several prototype upper housing designs that were evaluated to investigate the in uence of nozzle design on separation characteristics. The critical parameters of each of these prototype designs and the nal design of the WINS are presented in Table 1. Figure 2a presents the longthroat prototype nozzle. The well impaction surface required use of a nozzle with an unusually long throat length, T D 2:5 cm. This design had a nozzle width of 0.452 cm, a T = W ratio equal to 5.5, a S = W ratio of approximately 1.8, and a Reynolds number of 5200. A second nozzle, the bored WINS (shown in Figure 2b) was constructed to evaluate the effect of a reduced
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throat length on separation characteristics. The T = W ratio was reduced from 5.5 to 1.2 in this design. A third nozzle design, the three-hole WINS, was constructed to elucidate the effect of reduced Reynolds number operation on the nozzle design. In this design, the central bore culminated in three holes of W D 0:300 cm to yield a Reynolds number of 2600. Additionally, reduced jet velocity of the three-hole design was believed to reduce particle bounce. The nal design of the WINS has a nozzle width equal to 0.391 cm to yield a Reynolds number of 6000 at a design ow rate of 16.7 Lpm. The jet-to-plate distance was selected to yield an S = W ratio of 3.1 to minimize interaction between the impactor air jet and the potential conical build-up of particles on the impaction surface. The throat length, T D 0:61 cm, was kept short in relation to the jet width (T = W D 1:6). The nonstandard shape of the well presented additional opportunity for particle collection on the walls of the well and the antispill ring. Three con gurations were used in the development of the WINS: a relatively small well with a diameter of 3.0 cm, a larger well with a diameter of 3.7 cm, and a at plate. During the initial phases of development, an oil (Dow Corning 704 diffusion pump oil) saturated glass ber lter was used as the impaction substrate for most of the tests. Alternatively, aluminum surfaces were coated with Apiezon grease to elucidate the effects of surface roughness of the glass ber lter on separator characteristics.
Evaluation of the WINS Impaction Substrate Two oils and three lters were selected following a limited survey of available oil and lter candidates. The criteria for selection of the oil were reasonable cost, low vapor pressure to minimize volatilization,and a viscosity great enough to prevent migration from the well while maintaining the ability to wick through the particulate deposit. Selection of the oil was limited to diffusion pump oils because of their low vapor pressures (typically 2.5 ¹m have suf cient inertia to be impacted upon a circular 3.7 cm diameter glass ber lter which is immersed in 1 mL of a low volatility oil. The relatively large amount of oil is intended to minimize substrate overloading and subsequent particle bounce experienced by some conventional impactors (refer to Vanderpool et al. (2001) for more information). The novel geometry of the impaction reservoir (or well) is designed to capture any reentrained material from the impaction surface and to prevent loss of oil should the unit be inadvertently turned over or onto its side. The nonstandard geometry of the long-throat prototype WINS led to a rather shallow separation curve. Several additional prototype nozzles and wells of varying sizes were used to demonstrate that a combination of nozzle throat length, well size, and surface roughness of the glass ber lter caused the deviation from the ideal case. As a result, the nal WINS used a nozzle possessing a throat-to-nozzle width of 1.2 and the well size was increased from 3.0 cm to 3.7 cm. Additional testing of atter impaction substrates revealed that the glass ber mat possesses desirable oil retention characteristics and was therefore maintained as the impaction substrate. The nal WINS design was shown to have a cutpoint of 2.48 ¹m with a GSD of 1.18. This design meets all speci ed design objectivesand should thus serve as an effective particle size separator during conditions of its intended eld use. ACKNOWLEDGMENT The authors gratefully acknowledge the valuable comments provided by Mr. Robert Gussman of BGI, Inc. and Dr. Dale Lundgren of the University of Florida during the development of the WINS. This work was conducted by Research Triangle Institute with support provided by the U.S. Environmental Protection Agency through contract no. 68-D5-0040. It has been reviewed in accordance with the Agency’s peer and administrative review policies
DESIGN AND CALIBRATION OF WINS IMPACTOR
and approved for presentation and publication. Mention of trade names or commercial products does not constitute endorsement or recommendation by RTI or the Agency. REFERENCES Biswas, P., and Flagan, R. C. (1988). The Particle Trap Impactor, J. Aerosol Sci. 19:113– 121. EPA. (1997). National Ambient Air Quality Standards for Particulate Matter; Final Rule, Federal Register, Vol. 62, No. 138, July 18, 1997, pp. 38651– 38752. John, W., and Reischl, G. (1980). A Cyclone for Size-Selective Sampling of Ambient Air, JAPCA 30:872– 876. Kenny, L. C., and Gussman, R. A. (1997). Characterization and Modelling of a Family of Cyclone Aerosol Preseparators, J. Aerosol Sci. 28:677– 688. Loo, B. W., and Cork, C. P. (1988). Development of High Ef ciency Virtual Impactors, Aerosol Sci. Technol. 9:167– 176. Markowski, G. R. (1984). Reducing Blowoff in Cascade Impactor Measurements, Aerosol Sci. Technol. 3:431– 439. Marple, V. A., Liu, B., Behm, S., Olson, B., and Wiener, R. (1989). A Personal Environmental Monitor (PEM). Presented at The American Association for Aerosol Research Annual Meeting, October 10 – 13, 1989, Reno, NV. Marple, V. A., and Rubow, K. L. (1986). Theory and Design Guidelines. In Cascade Impactor, edited by J. P. Lodge, Jr. and T. L. Chan. American Industrial Hygiene Association, Akron, OH, pp. 79 – 101. Marple, V. A., Rubow, K. L., Turner, W., and Spengler, J. D. (1987). Low Flow Rate Sharp Cut Impactors for Indoor Air Sampling: Design and Calibration, JAPCA 37:1303– 1307. Marple, V. A., and Willeke, K. (1976). Impactor Design, Atmos. Environ. 10:891– 896.
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