The size of dust particles is different, and its physical and chemical characteristics are different. Not only does it show different hazards to people and the environment, but it also has a great impact on the dust removal performance of the dust collector. Therefore, the size of dust is the basic characteristic of dust removal technology. The meaning and expression method of dust size should be clearly defined.
The structure and shape of dust Dust has different shapes and structures due to the different ways of generation.
A. Structural form of a single particle
Dust particles are spherical (plant pollen, bracts, etc.) or other regular shapes only in a few cases. Dust particles with irregular shapes can be divided into:(1) Particles with the same linear scale in each direction – such as regular polygons, regular cubes, etc.
(2) Flat particles – the length in two directions is much longer than that in the third direction, such as flakes, leaves, and scales.
(3) Needle-like particles – much longer in one direction than in the other two.
B. The shape of the polymer
Aggregates are generally formed by the aggregation of two or more particles or even millions of particles. The smaller the primary dust particles, the more obvious the presence of aggregates in the gas. As the particle size of the primary particles decreases, the possibility of particles agglomerating due to random Wavelang motion increases, and the strength of the aggregate increases after aggregation, and the effect of anti-turbulent diffusion is also strong. Generally speaking, high-dispersion primary Particle systems are all aggregated into aggregates, and few continue to exist as single particles. The shapes of aggregates generally fall into two categories.(1) same length
(2) Linear chain
C. Spherical coefficient
When determining the average particle size of the particle group and studying the aerodynamic behavior of the particles, the particles are generally assumed to be spherical for analysis. For non-spherical irregular particles, the concept of "spherical coefficient" is usually used to indicate the degree of their inconsistency with spherical particles, or to make necessary corrections to the theoretical formula obtained by spherical particles.
Spherical coefficient: refers to the ratio of the surface area of spherical particles with the same volume to the actual surface area. = 1 for spherical particles and always less than 1 for non-spherical particles. For example, regular octahedron = 0.846, regular cube = 0.806, regular tetrahedron = 0.670, regular cylinder = 2.62 (l/d) 2/3 (1+2l/d), where d represents the diameter of the cylinder and l represents the cylinder length. sand 0.543~0.628
gravel 0.630
iron catalyst 0.578
silica 0.554~0.628
bituminous coal 0.625
pulverized coal 0.696
Subacetyl plastic cylinder 0.861
The particle size of the dust
(1) 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 spherical. For spherical particles, the diameter of the sphere can be used as a representative dimension of the particle size, and is called the particle size. For irregularly shaped particles, it is necessary to determine an optimal representative size representing the particle size according to the measurement method as the particle size of the particle.
The measurement and definition methods of particle size can be classified into two categories: one is directly measured and defined according to the geometric properties of particles, such as microscope method and sieving method; the other is indirectly measured and defined according to certain physical properties of particles , such as sedimentation method and light scattering method. The measurement and definition methods of particles are different, and the obtained particle size values are also different, so it is difficult to compare with each other. In practical applications, the determination and definition methods of particle size are mostly selected according to the application purpose.
When observing the projected size of dust particles with a microscope, the directional diameter dF, the equal-area diameter dM, or the equal-circle projected area diameter dA can be used; the particle diameter referred to in the sieve analysis is the sieve width that the particles can pass through; the geometric equivalent Among the diameters, there are equal-volume diameters, equal-surface-area diameters, and perimeter diameters, all of which are representations of the equivalent relationship with the diameters of the corresponding spherical particles.
In the common gravimetric particle size measurement, the physical equivalent diameter, sedimentation particle size and aerodynamic diameter are the most commonly used.