Thread Content
1. Dust removal 1.1 Classification of dust collectors There are many types of dust removal equipment; based on the main mechanisms used for dust removal, they can generally be divided into five categories: mechanical dust collectors, filter-type dust collectors, electrostatic dust collectors, wet dust collectors, and combined dust collectors. 1.1.1 Mechanical dust collectors: These types of collectors use forces such as mass forces (gravity, inertia, and centrifugal force) to separate dust particles from dusty air streams. They include gravity settling chambers, inertial dust collectors, and cyclone dust collectors. (1) Gravity dust collector: A dust-laden airflow enters a settling chamber with a cross-sectional area much larger than that of the pipeline; as a result, the flow velocity decreases suddenly. The dust particles settle naturally under the influence of gravity and fall into the ash hopper – this is a simple type of dust collector. Figure 1.1 is a schematic diagram of a simple horizontal gravity sedimentation chamber. Figure 1.1 Simple horizontal gravity dust collector: When L/v ≥ H/vs, the dust can settle into the ash hopper. (II) Inertial dust collectors: These are devices that utilize the inertial force of dust particles to separate them from the dusty airflow, by causing a sudden change in the direction of the airflow or by causing it to collide with obstacles such as baffles or louvered plates. (III) Cyclone dust collector: A cyclone dust collector is a dust removal device that utilizes the centrifugal force generated by the rotation of dust-containing air streams to separate dust particles from those streams; it is one of the most widely used dust removal devices at present. 1. Working principle of cyclone dust collectors A conventional cyclone dust collector consists of an air inlet pipe, a cylinder, a cone, and an air outlet pipe; the flow pattern of the air is shown in Figure 1.2. When the dust-laden airflow enters the cyclone dust collector through the inlet pipe, its motion changes from linear to circular. The vast majority of the rotating airflow moves downward in a spiral along the wall and the cylinder, toward the cone, and is commonly referred to as an outward rotating flow. As the dusty gas rotates, centrifugal force is generated, which throws particles with a density greater than that of the gas toward the wall of the vessel. Once in contact with the wall, these particles lose their inertial force and fall along the wall due to the downward-flowing gas currents and gravity, eventually entering the ash discharge pipe. As the rotating downward counter-rotating airflow reaches the cone, it is drawn toward the center of the dust collector due to the conical narrowing, resulting in an increasing tangential velocity. When the airflow reaches a certain point at the lower end of the cone, it moves upward within the cyclone dust collector in the same rotational direction, continuing its spiral flow. Finally, the purified gas is discharged outside the device through an exhaust pipe; this is commonly referred to as internal swirl flow. Some of the uncaptured particles are also carried away. Figure 1.2 Schematic diagram of the working principle of a cyclone dust collector. 2. Commonly used cyclone dust collectors. There are dozens of types of cyclone dust collectors in use in China at present; here are some of the most commonly produced ones. (1) BH-type woodworking cyclone dust collector (2) XCX-type cyclone dust collector (3) XDP-type cyclone dust collector (4) XLD/H-type cyclone dust collector (5) XZZ-type cyclone dust collector (6) XZD-type cyclone dust collector (7) CLK-type diffusive cyclone dust collector (8) CLT/A-type cyclone dust collector (9) GQX-F-type high-efficiency multi-tube cyclone dust collector. 1.1.2 Filter-type dust collectors Filter-type dust collectors are highly efficient types of dust collectors, and they are generally divided into bag filters and particle bed filters. (1) Bag filter: A bag filter is mainly composed of several key components, including filter bags, the filter housing, an ash hopper, a dust cleaning mechanism, and an ash discharge mechanism. 1. Dust removal mechanism: When the dust-containing airflow passes through the filter bags, the dust is trapped on them, while the purified gas is discharged from the outlet. After a period of time, the air reverse-blowing system is activated, and the dust inside the bag is blown into the ash hopper by the reverse airflow. When dust-laden gas enters the dust collector and passes through the clean filter bags, due to the larger pores in these clean filter bags, both the gas and most of the fine particles can pass through the pores between the fibers; however, larger dust particles are retained, causing a \"bridging\" effect between those pores. As the dusty gas continuously enters the gaps between the filter bag fibers, the amount of dust trapped in those gaps also increases. After some time, a layer accumulates on the surface of the filter bag; this layer of dust is known as the initial dust layer. During subsequent filtration processes, this initial dust layer becomes the main filtering layer of the filter bag. Thanks to the effect of the initial dust layer, a high dust removal efficiency can be achieved even when filtering very fine dust. A bag filter uses the filtering property of fibrous fabrics to retain dust in dusty gases on the filter bags. This filtering effect is usually achieved through the combined action of several dust removal mechanisms. (1) Screening effect: When the particle size of the dust is larger than the pores in the filter bag’s fiber mesh or those in the dust layer already attached to the filter bag, the dust particles cannot pass through the filter bag and are thus retained. (2) Inertial collision effect: When dust-laden gas approaches the filter bag fibers, the air flows around them; however, larger dust particles deviate from the flow path due to their inertia, collide with the fibers, and are thus trapped. (3) Hooking (trapping) effect: When dust-laden gas approaches the fibers of the filter bag, if the part of the dust particle near the fiber protrudes into the edge of the fiber, the dust particle will be hooked by that edge. (4) Diffusion effect: When the diameter of dust particles is below 0.3 micrometers, they deviate from the flow path due to continuous collisions with gas molecules, undergoing irregular Brownian motion similar to that of gas molecules. This increases the chances of contact between the dust particles and the fibers in the filter bag, thereby facilitating their capture. (5) Electrostatic effects: Dust particles and filter media may acquire static electricity for various reasons. When the charge of the dust particles is the same as that of the filter media fibers, the filter bags repel the dust particles, reducing the dust removal efficiency. If the charge of the dust particles is opposite to that of the filter media, the dust particles will adhere to the filter bags. 2. Commonly used bag filters: (1) Pulsed jet bag filter; (2) FSF type bag filter; (3) SDC type double-layer bag filter; (4) MC type bag filter; (5) GC type wave fiber flat bag filter; (6) DMC-SH ring-jet pulsed bag filter; (7) JH type bag filter; (8) LDB type, LYDZ type, LCPM type bag filters; (9) XL type bag filter; (10) Small mobile dust removal units; (11) XCS, FX12 type dust removal units. (II) Particle bed filters: Particle bed filters are devices that use solid particles with a certain particle size range as the filtering medium to separate dust from dusty gases. Its dust removal mechanism is similar to that of bag filters. The particle layer dust collectors and bubbling particle layer dust collectors that are widely used in domestic production currently consist of components such as an air inlet, a sedimentation chamber, a particle layer, an ash hopper, an ash discharge port, a back-blowing port, a clean air outlet, an electric actuator valve, and an air outlet. 1.1.3 Electrostatic precipitators: Electrostatic precipitators are dust removal devices that use the electrostatic force generated by a high-voltage electric field to charge dust particles, thereby separating them from the airflow. 1. Working principle: A static electric precipitator is mainly composed of a discharge electrode and a collection electrode. The discharge electrode (corona electrode) is a thin, exposed wire with a very small radius of curvature; its upper end is connected to the first stage of a direct current power supply, while its lower end is fixed by a weight ； The collection electrode is a tube or plate with a certain area, which is connected to the other pole of the power supply. When a higher voltage is applied between the two poles, the electric field strength near the dust collection electrode is relatively low; therefore, the electric field between the poles is not a uniform electric field. Its principle involves four processes: gas ionization, dust particle charging, dust particle sedimentation, and cleaning by shaking. (1) Gas ionization: High-voltage direct current is applied to the corona electrode, resulting in corona discharge that ionizes the gas and generates a large number of positive and negative ions. (2) Particle charging: If the area near the corona electrode is negatively charged, positive ions are attracted and lose their charge. Free electrons and negative ions, driven by the electric field force, move toward the collection electrode where they collide with the particles in the dust-laden airflow and bind to them, thereby charging the particles. (3) Particle deposition: Charged particles lose their charge upon reaching the collection electrode; once neutral, they deposit on the surface of the collection electrode. (4) Vibration cleaning: When the dust particles on the surface of the dust collection electrode reach a certain thickness, it affects the neutralization process; therefore, a vibration device is used to shake the electrode, causing the dust particles to fall off and into the ash hopper automatically. 2. Types of electrostatic precipitators There are many types of electrostatic precipitators; based on their various characteristics and classification methods, the following are some products manufactured by domestic companies: (1) SZD type combined electrostatic precipitator; (2) BS type and BWD type electrostatic precipitators; (3) GL type vertical tubular electrostatic precipitator. 1.1.4 Wet dust collectors Wet dust collectors are high-efficiency devices for removing dust particles from gas streams, utilizing the close contact between gas and liquid, as well as mechanisms such as inertia, interception, diffusion, and coagulation to achieve dust separation. Furthermore, it can remove some gaseous pollutants while dust removal is taking place. The dust removal mechanism of wet dust collectors involves effects such as inertial collision, diffusion, adhesion, diffusion drift, and coagulation. (1) Inertial collision: As the airflow moves and encounters liquid droplets, it changes direction and flows around the objects; however, dust particles, due to their inertia, maintain their original direction of motion, breaking away from the airflow’s streamline to collide with the liquid droplets. When a dust particle collides with a liquid droplet, if the particle can be wetted by that liquid, it enters the liquid ； If it cannot be wetted, it adheres to the surface of the droplet. (2) Diffusion: For dust particles with a particle size of less than 0.3 micrometers, these particles move randomly due to thermal motion, just like gas molecules; during this movement, they come into contact with liquid droplets and are thus captured. (3) Adhesion: When the radius of the dust particle is greater than the distance from the center of the dust particle to the edge of the droplet, the dust is adhered to the droplet and thus captured. (4) Diffusion drift and thermal drift: If the air flow contains saturated vapor, it will condense on the surface of colder droplets when in contact with them, creating an additional airflow that moves toward the droplets. This additional airflow drives smaller dust particles to move toward the droplets, where they settle on their surfaces and are captured. (5) Coagulation: The exhaust gas system typically contains water vapor, sulfur trioxide, and gaseous organic compounds; when the temperature drops, these condensing components adhere to the surface of dust particles, causing them to coalesce into larger secondary particles that are easier to be captured by liquid droplets. There are many types of wet dust collectors; below are some products manufactured by domestic environmental protection equipment manufacturers, as well as spray and impact types that are easy to manufacture. (6) WMC type Venturi marble water film dust collector (7) SCX type wet dust collector (8) CCJ/A type wet dust collector (9) Simple wet dust collector. Meanwhile, the water curtain cabinets produced by our company are also a type of wet dust collector. 1.1.5 Modular dust collector: This type of dust collector relies on the combined use of various purification mechanisms for cleaning. Generally done through machinery and filtration ； Combinations such as mechanical and electrostatic, wet and electrostatic, etc. 2. Ventilation system The basic components of a ventilation system include exhaust hoods, ducts, and fans, etc. 2.1 Exhaust hoods: Exhaust hoods are generally used to capture harmful substances. There are many forms of them, and based on their working principles, they can be classified into the following types: enclosed hoods, fume hoods, external suction hoods, receiving exhaust hoods, and blow-suction exhaust hoods. 2.2 Pipes Pipes are one of the main components in a ventilation system; their function is to transport air. 2.3 Fans In air pollution control projects, the commonly used ventilators can be classified into centrifugal and axial flow types based on their function.