Page 124 - Advanced Mine Ventilation

Characteristics of Respirable Coal 8

Dust Particles

Chapter Outline

8.1 Settling Velocity of Small Particles Due to Gravity (Stoke’s Formula) 106
8.1.1 Derivation of Settling Velocity When r [ [ 107
8.1.2 Settling Velocity When r [ 108
8.1.3 Settling Velocity When r z [ 108
8.2 Aerodynamic Shape Factor for Dust Particles 109
8.3 NonSettling Fraction of Respirable Dust 110
8.4 Size Distribution of Respirable Dust Particles 112
8.4.1 Normal Distribution 113
8.4.2 The Log Normal Distribution 113
8.4.3 The RosineRammler Distribution 114
8.5 Determination of Mass Distribution for Fine Coal Dust Particles 116
8.5.1 Experimental Procedure for Mass Distribution Measurement 117
8.5.2 Calculation of Stoke’s Diameter for a Given Time, t 118
8.6 Chemical Composition of Respirable Coal Dust 119
References 122

Although fine dust particles behave like gases, there are significant differences that
warrant a separate treatment. Respirable dust particles range from 10 3 to 10 mm.
(1 mm ¼ 10 4 cm). We need to define a few terms here before we proceed further.
1. Fine particles: These particles have an aerodynamic diameter of 10 mm or less. Micron is the
unit most commonly used in particle mechanics.
2. Stoke’s diameter: It is the diameter of a spherical particle of the same density and the same
settling velocity as the nonspherical particle.
3. Aerodynamic diameter: It is equal to the diameter of a sphere of unit density that has the same
settling velocity as the “Stoke’s diameter” of the particle with a specific gravity of r.

1
Aerodynamic diameter ¼ Stoke's diameter ðrÞ 2 :
4. Settling velocity: It is the displacement of a particle under gravity per second. It will be
derived and discussed later.
5. Diffusivity: Small particles have a diffusivity coefficient like gases. It is given by D in the
equation

dQ dc
¼ D dy$dz (8.1)
dt dx

Advanced Mine Ventilation. https://doi.org/10.1016/B978-0-08-100457-9.00008-0
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