University:
California State University, Northridge
Course:
EOH 466B/L | Evaluating the Occupational Environment and Lab
Academic year:

2008
Views:

120

Pages:

10
Author:

fumuindvtr

Evaluating the Occupational Environment – Particle Behavior Particle Behavior • Excellent reference: Aerosol Technology. Properties, behavior and measurement of airborne particles, Second Edition by William C. Hinds, Wiley-Interscience, 1999 1 Particle Behavior • The physical diameter and density of a particle are important factors affecting behavior of an aerosol. It is convenient to consider particles as spherical. Particle Behavior • Stokes diameter is the diameter of a sphere that has the same density and settling velocity as the particle. 2 Particle Behavior • Aerodynamic diameter is the diameter of the unit density sphere that has the same settling velocity as the particle. Particle Behavior • Terminal settling velocity: can be determined if the size and density of a particle is known. Stokes' law allows an estimation of Vts, for particles between 1 and 100 µ gd 2 ( ρ − ρ a ) Vts = 18η – – – – g = gravitational acceleration, 9.8 x 102 cm / sec d = diameter of particle in cm ρ = density (air = 1.17 x 10-3 g/cm3) η = coefficient of viscosity of air 1.828 10-3 g / cm-sec 3 Particle Behavior • This can be simplified to – V ts = 0.003 x SG x d2 at NTP. – This equation gives V ts in cm/sec using d in microns. Compare Physical Diameter and Aerodynamic Diameter 4 Particle Behavior • The equation should be adjusted by the Cunningham slip correction factor, CC to provide accurate prediction of V ts for particles smaller than 1 micron. gd 2 ( ρ − ρ a )Cc Vts = 18η 5 Particle Behavior • For non spherical particles, Vts must be calculated using a dynamic shape factor, chi, for particle sizes described by the equivalent volume diameter, de gd ( ρ − ρ a )Cc Vts = 18ηχ 2 Compare Different Shape and Aerodynamic Diameter 6 Particle Behavior • Stopping distance: when a particle is released at high speed, how far will it travel? – This can be calculated, and is related to resistance to travel experienced by the particle. For small particles, generally this is less than 1 cm. Can be significant for denser and larger particles. Particle Behavior • Particle size selective samplers apply the principles in V ts to remove particles from an air stream. – Cyclones (centrifugal force) – Vertical Elutriators (settling velocity) – Cascade Impactor (stopping distance) – Horizontal Elutriators (settling velocity) 7 Particle Behavior • Particle amount: how much material is present: most often mass, but can be number of particles, radioactivity, surface area, volume or chemical content. Choice of unit depends on application. Presentation of Particle Size Information • Cumulative probability plots. – Calculate the fraction of total amount in each size class, and sum this fraction from the small to large size classes. • Graph this on 'log-probability' paper: a straight line will indicate a lognormal distribution. 8 Analysis of Particle Count Data Lower Interval (µm) Upper Interval (µm) 0.46 0.54 0.66 0.74 0.86 1.0 1.2 1.4 1.6 1.8 2.2 2.5 0.54 0.63 0.74 0.86 1.0 1.2 1.4 1.6 1.8 2.2 2.5 2.9 Midpoint Number in Fraction Cumulative Interval Interval Interval Fraction less di (µm) ni (%) Than di (%) 0.50 3770 0.74 0.585 13,000 2.56 0.685 55,100 10.84 0.80 62,900 12.38 0.93 98,800 19.44 1.1 109,000 21.45 1.3 98,800 19.44 1.5 37,200 7.32 1.7 17,000 3.35 2.0 9710 1.91 2.35 2850 0.56 2.7 52 0.01 508,182 0.74 3.30 14.14 26.52 45.96 67.41 86.85 94.17 97.52 99.43 99.99 100 9 Sample method development • NIOSH Applet is important to review. • Concepts: – Precision, bias, accuracy – Detection limit – 95 % accuracy – Sample stability – Walk through the applet 10

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