The hazards of dust and classification of dust collectors

Classification of dust:

    1》Based on physical and chemical properties, dust can be divided into inorganic dust (mineral dust, such as coal, asbestos; metallic dust, such as iron, zinc; artificial inorganic dust, such as diamond, cement), organic dust (vegetable dust, such as cotton, flax; animal dust, such as animal hair, feathers; artificial organic dust, such as organic dyes) and mixed dust

    2》According to the hazards of dust to human health, it is divided into complete dust and respirable dust. Complete dust: refers to all dust including various particle sizes obtained during dust sampling. Respirable dust: refers to fine dust particles with a particle size of less than 5 microns. It can enter the alveolar area through the upper respiratory tract of the human body, which is the main cause of pneumoconiosis and is very harmful to the human body.

    3》Divided according to the free SiO2 content in mineral dust: Silica dust: refers to dust with a free silica content of more than 10%. It is the main factor causing silicosis. Rock dust in mines is generally silica dust. Non-silica dust: refers to dust with a free silica content of less than 10%. For example, coal dust in coal mines is generally non-silica dust.

    4》Divided according to the existence state of mineral dust: floating mineral dust refers to dust suspended in the air, referred to as floating dust. Deposited mineral dust: refers to dust that settles from the air, referred to as fallout.

    5》Divided by dust particle size: Coarse dust has a particle size greater than 40 microns, which is equivalent to the minimum particle size for general screening and can easily settle in the air. Fine dust: Its particle size is 10 to 40 microns. It can be seen with the naked eye under bright light and undergoes accelerated settling motion in still air. Fine dust: its particle size is 0.25~10 microns, which can be observed with an optical microscope and undergoes isokinetic settling motion in still air. Ultrafine dust: Its particle size is less than 0.25 microns, which can only be observed with an electron microscope and diffuses in the air.

 Density of dust:

       Including bulk density and true density, bulk density: also called apparent density or bulk density, refers to the mass of particulate matter per unit bulk volume of dust. True density: refers to the mass of particulate matter per unit volume (excluding the spaces between particles, the volume of external openings and internal closed pores). ρb=ρp(1-ε)  

Dispersion of dust:

       Dispersion refers to the ratio of mass or quantity of dust particles of various sizes in dust. It is called mass dispersion in terms of mass (that is, the mass of dust particles of each particle size accounts for the percentage of the total mass), and it is called quantitative dispersion in terms of quantity (that is, the number of particles of each particle size accounts for the percentage of the total number of particles). The percentage of small particles is called high dispersion

Classification of toxic and harmful gases:

       According to the harmful effects on the human body, toxic and harmful gases are divided into the following five categories: 1. Simple asphyxiating gases. The gas itself is not toxic, but as its concentration increases, the oxygen content in the air decreases accordingly, thus suffocating people, such as carbon dioxide, methane and nitrogen. 2. Chemical asphyxiating gases. This type of gas undergoes a chemical reaction after entering the human body with breathing. Its binding capacity with red blood cells in the blood is greater than the binding capacity of oxygen with red blood cells, thus causing the human body to be hypoxic and causing suffocation. Such gases, such as carbon monoxide, hydrogen cyanide, etc., irritate the upper respiratory tract. Such gases can irritate the nose, throat, etc. and cause inflammation, such as ammonia, sulfur dioxide, etc. 4. Gases that irritate the lungs. Such gases can strongly irritate the lungs and cause pneumonia, pulmonary edema and other symptoms, such as phosgene, nitrogen dioxide, etc. 5. Gases that damage the central nervous system. After entering the human body, these gases will paralyze and anesthetize the central nervous system, causing damage to the central nervous system, such as gasoline.

Classification of dust removal systems:

① Classification according to the scale and configuration characteristics of the dust removal system (on-site dust removal system, decentralized system and centralized dust removal system)

②Classification according to the type of dust collector (dry dust removal system, wet dust removal system)

③Classification according to the section where the dust collector is installed (single-stage dust removal system, multi-stage dust removal system)

④Classification according to the position of the dust collector in the dust removal system (negative pressure dust removal system (dust collector before the ventilator), positive pressure dust removal system (dust collector after the ventilator)

 
The basic form of air collecting hood:

       According to the flow mode, it is divided into two categories: suction hood and blowing-suction air collection hood. Suction hoods are divided into (closed hoods, cabinet-type semi-closed hoods, external gas collecting hoods) according to their sealing conditions and relative positions. Sealed hoods can be divided into (partial closed hoods, overall closed hoods, large volume closed hoods) according to structural characteristics. Suction hoods: A row or a slit-shaped air blowing port is set opposite the external gas collecting hood. It is combined with the external gas collecting hood and is called a blowing suction hood.

Working mechanism of inertial dust collector:

       In the inertial dust collector, the main purpose is to make the air flow turn rapidly or impact on the baffle and then turn rapidly. Due to the inertial effect, the movement trajectory of the dust particles is different from the air flow trajectory, thus separating them from the air flow. The higher the airflow speed, the greater this inertia effect and the higher the dust removal efficiency.

Working principle of cyclone dust collector:

       After the dusty airflow enters from the air inlet at a high speed of 15~25m/s, it is restricted by the upper cover of the outer cylinder and the wall of the inner cylinder, forcing the airflow to rotate from top to bottom. This movement is usually called external swirling flow. A large centrifugal force is formed during the rotation of the airflow. Under the action of the centrifugal force, the dust particles are gradually thrown toward the outer wall, and then rotate along the outer wall under the action of gravity and fall to the ash storage box. The rotating and descending external swirling flow gradually converges toward the center due to the shrinkage of the cone. When it drops to a certain level, it begins to return and rise, forming a bottom-up rotational movement. This movement is generally called internal swirling flow. The internal swirling flow does not contain large particles of dust, so it is relatively clean and can be discharged to the atmosphere through the inner cylinder. However, due to the mutual interference and penetration of the internal and external rotating airflows, it is easy to bring up the dust that settled at the bottom, and some of the fine particles are taken away. In order to improve the dust removal efficiency, an air blocking and dust exhaust device is often installed at the lower part of the cone.

The principle of bag dust collector:

       When dust-containing gas passes through a clean filter bag, due to the larger mesh of the filter material itself, most of the fine dust will pass through the mesh of the filter bag with the air flow. Therefore, the dust removal efficiency of the credit filter bag is low. Coarse dust particles are trapped due to inertial collision, interception, Brownian diffusion, static electricity, gravity sedimentation, etc. and produce a 'bridging' phenomenon in the mesh. As the dust-containing gas continues to pass through the fiber gaps of the filter bag, the 'bridging' phenomenon of dust between fibers continues to strengthen. After a period of time, After a while, an initial layer of dust will be formed on the surface of the filter bag. In the subsequent dust removal process, the initial layer of dust will interact with the airflow dust to form the main filter layer. As dust accumulates on the filter cloth, the dust removal efficiency will increase accordingly, thereby achieving the dust removal effect.

Dust removal mechanism of wet dust collector:

       During the relative motion between water or other liquids and dust-containing gas in the wet dust collector, the dust particles are captured by the liquid medium under the action of inertial collision, interception, diffusion and condensation between the liquid medium and the dust particles, thereby achieving the purpose of dust removal.

Venturi scrubber working principle:

       The venturi tube consists of a constriction section, a throat and a diverging section. After the dust-containing gas enters the contraction section, the flow velocity increases and reaches the maximum value when it enters the throat. The washing liquid is added from the contraction section or throat, and the relative flow rate between the gas and liquid phases is very large. The liquid droplets are atomized under the high-speed air flow, the gas humidity reaches saturation, and the dust particles are moistened by water. Violent collisions and agglomeration occur between dust particles and liquid droplets or dust particles. In the diffusion section, the gas-liquid velocity decreases, the pressure rises, and the condensation effect with dust particles as condensation nuclei accelerates, condensing into dust-containing droplets with larger diameters, and then being captured in the demister.

Working principle of electrostatic precipitator:

       It mainly includes four basic processes: corona discharge, dust particle charging, migration and capture of charged particles in the electric field, and dust removal. Corona discharge, in a non-uniform electric field, when the DC voltage is large enough, the free electron cup in the gas accelerates random collision to ionize the neutral molecules of the gas to form new free electrons and positive ions. The new free electrons and positive ions are accelerated and collide with the neutral molecules for ionization, producing a large number of ions, which then causes the dust particles to be charged (electric field charge - greater than 1n m, diffusion charge - less than 0.4um), the combined effect of electric field charging and diffusion charging is mainly suitable for particles in the middle particle size range. The collection of particles is the uneven power plant generated between the electrodes, the gas is ionized, and then the dust is charged, and reaches the dust collection pole under the action of the electric field force. Finally, the dust is shaken to the ash hopper through the dust cleaning device, and the dust is removed. Electrostatic precipitators generally use electromagnetic vibration or hammer vibration to remove dust, while corona stages generally use mechanical vibration to remove dust.

       Main factors affecting the performance of electrostatic precipitator: 1. Smoke properties: ① Dust characteristics (a specific resistance influence: α low specific resistance dust is not applicable. High specific resistance dust is suitable, the higher the specific resistance, the higher the dust removal rate. b. The higher the particle diameter, the greater the dust removal efficiency. After the dust removal efficiency max, the particle size increases and the efficiency becomes lower. Dust with small accumulation density has low efficiency, large adhesion and low efficiency.) ② Smoke Gas properties (the humidity of the flue gas is high and the efficiency is high. The temperature is low and the composition is different, and the carrier migration is different. The flue gas composition has a great influence on the negative corona discharge. The flue gas pressure is high and the discharge voltage increases. The dust concentration in the flue gas can be purified within a certain range. If it exceeds the range, the efficiency decreases.) 2. Equipment condition ① Installation quality of the equipment: Uneven electrode wire thickness, sharp spikes, uneven curling edges will reduce efficiency ② Air flow distribution At the bottom, the air flow speed is low and the efficiency is high, and vice versa. 3 Operating conditions ① Air flow speed is the same as above ② Vibration cleaning: other dust cleaning methods such as cam vibration and electromagnetic vibration will reduce the dust removal efficiency ③ Secondary dust emission will reduce the dust removal efficiency

       Technical measures to prevent coal dust explosions mainly include three aspects: a. Measures to reduce and reduce dust; water injection for each layer (water injection for short holes 2-3.5m, water injection for deep holes 5-25m, long hole water injection oblique to the working face and tunnel drilling 30-100m long) b. Prevention Coal dust ignition measures; eliminate sources of detonation fire (clear underground open flames, blasting flames, electrical explosions, other fire sources, and prevent gas ignition) c. Limit the expansion of the scope of coal dust explosions: remove falling ash and scattered rock powder, set up rock powder sheds, set up water sheds, and set up automatic explosion-proof sheds.

    Coal seam water injection method: short hole water injection: water is injected in the vertical coal wall of the mining face or by drilling a hole diagonally across the coal wall. The length of the water injection hole is generally 2 to 3.5m. Long hole water injection: from the transport lane or return air lane of the mining working face, drill upward holes or downward holes parallel to the working face along the inclination direction of the coal seam, and the hole length is 30 to 100m; water injection in the tunnel borehole: drill water from the upper lane adjacent to the coal seam to the lower coal seam, or drill water from the floor tunnel to the coal seam.

      The mechanism and process of coal dust explosion are mainly manifested in the following aspects: a. Suspended coal dust is carbonized into flammable gas under the action of high-temperature heat source. b. Combustible gases mix with air and burn. c Combustion releases heat and transfers it to nearby suspended coal dust, causing the combustion cycle to continue and its reaction speed to become faster and faster. Through violent combustion, an explosion finally forms.

       A coal dust explosion must meet three conditions at the same time: a. The coal dust must be suspended in the air and reach a certain concentration; b. Generally speaking, the lower limit concentration of coal dust explosion is 30~50g/m3, and the upper limit concentration is 1000~2000g/m3. C. There is a high-temperature heat source that can ignite coal dust explosions

    Design of the dust removal system: 1. Draw an isometric diagram of the ventilation system, label each pipe and note the flow rate and length of each pipe section (Q and D do not become one pipe section, numbered from far to near). The length of the pipe section is calculated according to the length of the key center line, without deducting the length of the pipe fitting itself. 2 Select the calculation loop (generally starting from the long pipe section), that is, the most unfavorable pipe section 3 Select the flow rate (select the lowest wind speed according to the situation and location) 4 Calculate the pipe diameter and friction resistance 5 Calculate the local resistance and dust removal equipment resistance 6 Parallel pipeline resistance balance calculation and pipe diameter 7. The total resistance and total air volume of the dust removal system 8 Select the ventilator and motor

    The design of the gravity settling chamber: 1. The settling speed of dust particles 2 The residence time of dust particles in the settling chamber 3 The cross-sectional area of ​​the settling chamber 4 The height, width and length of the dust collector 5 Dust removal efficiency 6 The resistance of the dust collector

    Pneumoconiosis: a systemic disease dominated by diffuse fibrosis in the lung tissue caused by long-term inhalation of production dust in professional life and direct flow in the lungs. Silicosis: a lung disease dominated by pulmonary fibrosis caused by long-term inhalation of large amounts of dust containing free sio2 during production.