The size of dust particles is different, its physical and chemical properties are different, which not only shows different hazards to people and the environment, but also has a great impact on the dust removal performance of the dust collector, so the size of the dust is the basic characteristic in the dust removal technology. The meaning and representation of dust size should be clearly defined.
1. The structure and shape of the dust.
Dust has different shapes and structures depending on the way it is generated.
a) The structure and morphology of a single particle.
Only in a few cases are dust particles spherical (plant pollen, bracts, etc.) or other regular shapes. For irregularly shaped dust particles, they can be divided into:
1) Particles with the same linear scale in all directions, such as regular polygons, regular cubes, etc.
2) Plate-like particles are much longer in both directions than in the third direction, such as flakes, leaves, scales.
3) Needle-like particles are much longer in one direction than in the other two.
b) The shape of the polymer.
Polymers are generally formed by the polymerization of two or more or even millions of particles. The smaller the primary dust particles, the more obvious the aggregate is in the gas**. With the decrease of the particle size of the primary particles, the greater the possibility of condensation due to the random wave movement of the particles, the strength of the polymer after condensation also increases, and the effect of anti-turbulent diffusion is also very strong.
1) Same length in each direction.
2) Linear chains.
iii) Spherical coefficient.
When determining the average particle size of a particle population and studying the aerodynamic behavior of the particles, the particles are generally assumed to be spherical in shape. For non-spherical irregular particles, the concept of "spherical coefficient" is usually used to indicate the degree to which they are inconsistent with spherical particles, or to make necessary corrections to the rationale obtained by spherical particles.
Spherical coefficient: Refers to the ratio of the surface area of a spherical particle of the same volume to the actual surface area. For spherical particles 1, while for non-spherical particles it is always less than 1. For example, the octahedron 0846, regular cube 0806, tetrahedron 0670, regular cylinder 262(l d)2 3(1+2l d), where d represents the diameter of the cylinder and l represents the length of the cylinder. The values of some of the materials measured by the experiment are shown in Table 3-1.
Table 3-1 Sphericity of particles.
Second, the particle size of the dust.
a) The particle size of a single particle.
The shape of dust particles is generally very irregular, and only a few are in the shape of regular crystals or spheres. For spherical particles, the diameter of the ball can be used as a representative size of the particle size and is called the particle size. For irregularly shaped particles, the particle size is determined according to the determination method to determine the best representative size of the particle.
The methods for determining and defining particle size can be summarized into two categories: one is directly determined and defined according to the geometric properties of the particles, such as microscopy and sieving; The other type is indirectly determined and defined according to a certain physical property of the particle, such as sedimentation method and light scattering method. Depending on the method of measuring and defining particles, the particle size values obtained are also different, and it is difficult to compare them with each other. In practice, the method of particle size determination and definition is mostly selected according to the purpose of the application.
When observing the projection size of dust particles with a microscope, the directional diameter df, the equal area diameter dm or the equal circle projection area diameter da can be used. The particle size referred to when analyzed with a sieve is the width of the sieve hole through which the particles can pass; There are also equal volume diameters, equal surface area diameters, and perimeter diameters in the geometric equivalent diameters, all of which are expressed in the equivalent relationship with the diameters of the corresponding spherical particles.
Among the common gravimetric particle size measurements, the physical equivalent size, sedimentation particle size and aerodynamic diameter are the most common applications.
1) Sedimentation particle size DS: the diameter of a sphere with the same density and sedimentation velocity as the particle in the same fluid, also known as Stokes particle size.
2) Aerodynamic particle size da: the diameter of a sphere with a unit density (p 1g cm3) equal to the sedimentation velocity of the particles in air. Due to p, p 1g cm3