Cyclone dust removal is a type of dust removal device. The dust removal mechanism is to make the dusty airflow rotate, use centrifugal force to separate and capture dust particles from the airflow, and then use gravity to make the dust particles fall into the ash hopper. The various components of a cyclone dust collector have size ratios, and any changes in these ratios can affect the efficiency and pressure loss of the cyclone dust collector. Among them, the diameter of the dust collector, the size of the air inlet, and the diameter of the exhaust pipe are the main influencing factors. When using, it should be noted that when a certain threshold is exceeded, favorable factors can also be transformed into unfavorable factors. In addition, some factors are beneficial for improving efficiency, but they can increase pressure losses, so adjustments to all factors should be balanced.
Cyclone dust collectors began to be used in 1885 and have developed into various forms. According to the airflow entry method, it can be divided into two categories: tangential entry and axial entry. Under the same pressure loss, the latter can handle about three times more gas than the former, and the airflow distribution is uniform.
A cyclone dust collector is composed of an intake pipe, an exhaust pipe, a cylindrical body, a conical body, and an ash hopper. The cyclone dust collector has a simple structure, is easy to manufacture, install, and maintain, and has low equipment investment and operating costs. It has been widely used to separate solid and liquid particles from airflow, or to separate solid particles from liquids. Under normal operating conditions, the centrifugal force acting on particles is 5-2500 times that of gravity, so the efficiency of cyclone dust collectors is significantly higher than that of gravity settling chambers. A cyclone dust removal device with a dust removal efficiency of over 90% has been successfully developed based on this principle. In mechanical dust collectors, cyclone dust collectors are one type. It is suitable for the removal of non viscous and non fibrous dust, mostly used to remove particles larger than 5 μ m. The parallel multi tube cyclone dust collector device also has a dust removal efficiency of 80-85% for particles larger than 3 μ m. The cyclone dust collector constructed with special metal or ceramic materials that are selected, corroded, and corroded can operate under conditions of temperature up to 1000 ℃ and pressure up to 500 × 105Pa. From the perspectives of technology and economy, the pressure loss control range of cyclone dust collectors is generally between 500-2000Pa. Therefore, it belongs to the category of medium efficiency dust collectors and can be used for purifying high-temperature flue gas. It is a widely used dust collector, mainly used for boiler flue gas dust removal, multi-stage dust removal, and pre dust removal.
Characteristics of cyclone dust collector:
① Cyclone dust collector, with a smaller cylinder diameter, is used to separate finer dust and has a dust removal efficiency of above;
② A high flow cyclone dust collector with a large cylinder diameter is used to handle large gas flow rates, and its dust removal efficiency is above 50-80%;
③ The universal cyclone dust collector has a moderate air processing capacity, but due to different structural forms, the dust removal efficiency fluctuates between 70-85%,
④ The cyclone dust collector is equipped with a valve and has functions.
According to the structural form, it can be divided into long cone, cylindrical body, diffusion type, and bypass type.
According to the combination and installation situation, it can be divided into internal cyclone dust collectors, external cyclone dust collectors, vertical and horizontal cyclone dust collectors, and single tube and multi tube cyclone dust collectors.
According to the airflow introduction situation, the flow path of the airflow after entering the cyclone dust removal, as well as the form with secondary air, can be summarized into the following two types:
① Shear flow reverse cyclone dust collector ② Axial flow cyclone dust collector
Efficiency factor:
Air intake
The air inlet of a cyclone dust collector is a key component that forms a rotating airflow and is the main factor affecting dust removal efficiency and pressure loss. The area of tangential intake has a significant impact on the dust collector. When the intake area is relatively small compared to the cross-section of the cylinder, the tangential velocity of the airflow entering the dust collector is high, which is beneficial for dust separation.
Diameter and height of cylindrical body
The diameter of the cylindrical body is the basic size that constitutes a cyclone dust collector. The tangential velocity of a rotating airflow is inversely proportional to the centrifugal force generated by dust and the diameter of the cylinder. At the same tangential velocity, the smaller the diameter D of the cylinder, the smaller the radius of rotation of the airflow, and the greater the centrifugal force experienced by particles, making it easier for dust particles to be trapped. Therefore, a smaller cylinder diameter should be chosen appropriately, but if the cylinder diameter is chosen too small, the wall and exhaust pipe will be too close, and particles will easily escape; The diameter of the cylinder is too small and can easily cause blockage, especially for viscous materials. When dealing with high air volume, due to the small diameter of the cylinder, the dust containing air volume is limited. Therefore, several cyclone dust collectors can be operated in parallel to solve the problem. The air volume processed by parallel operation is the sum of the air volumes processed by each dust collector, and the resistance is only the resistance of the portion of air volume that a single dust collector is responsible for processing. However, parallel use is more complex to manufacture and requires more materials. Gas is easily blocked at certain locations, increasing resistance. Therefore, the number of units used in parallel should not be too large. The total height of the cylinder refers to the sum of the heights of the cylindrical and conical parts of the dust collector. Increasing the total height of the cylinder can increase the number of rotations of the airflow inside the dust collector, increasing the chances of separating dust from the airflow in the dusty airflow. However, as the total height of the cylinder increases, the radial velocity of the centripetal force in the outer vortex also increases the chances of some small dust entering the inner vortex, thereby reducing the dust removal efficiency. The total height of the cylinder is generally 4 times the diameter of the cylindrical body. For the conical cylinder part, due to its decreasing radius, the tangential velocity of the airflow continues to increase, and the distance for dust to reach the outer wall also decreases. The dust removal effect is better than that of the cylindrical part. Therefore, in the case of the total height of the cylinder, appropriately increasing the height of the conical cylinder part is beneficial for improving efficiency. Generally, the height of the cylindrical part is 1.5 times its diameter, and when the height of the conical cylinder is 2.5 times the diameter of the cylindrical body, a relatively ideal dust removal efficiency can be obtained.
Exhaust pipe diameter and
The diameter and insertion of the exhaust duct have a significant impact on the dust removal efficiency of the cyclone dust collector. Choose a suitable value for the diameter of the exhaust duct. Reducing the diameter of the exhaust duct can reduce the rotation range of the internal vortex, making it difficult for dust to be discharged from the exhaust duct, which is beneficial for improving efficiency. However, at the same time, increasing the speed of the air outlet increases the resistance loss; If the diameter of the exhaust duct is increased, although the resistance loss can be significantly reduced, due to the close proximity of the exhaust duct to the cylindrical wall, it is easy to form a "short circuit" phenomenon between the inner and outer vortices, which causes some of the dust that is not covered in the outer vortices to directly mix into the exhaust duct and be discharged, thereby reducing the dust removal efficiency. It is generally believed that the diameter of the exhaust duct should be 0.5-0.6 times the diameter of the cylindrical body. Inserting the exhaust duct too shallowly can easily cause dusty airflow from the inlet to directly enter the exhaust duct, affecting dust removal efficiency; The deep insertion of the exhaust duct can increase the friction surface between the airflow and the pipe wall, resulting in increased resistance loss. At the same time, it shortens the distance between the exhaust duct and the bottom of the cone body, increasing the chance of secondary dust mixing and discharge. It is generally advisable to insert the exhaust duct slightly below the bottom of the air inlet. Due to the relatively high steel consumption per unit of cyclone dust collector, a better method in the design scheme is to gradually reduce the material thickness from the upper part of the cylinder downwards from thick to thin!
Operation process
In the case of fixed size and structure of cyclone dust collectors, the key to their dust removal efficiency lies in the influence of operating factors.
velocity of flow
The cyclone dust collector uses centrifugal force to remove dust, and the greater the centrifugal force, the better the dust removal effect. The centrifugal force experienced by dust in circular motion (or curved motion) is F=ma, where F - centrifugal force, N; m - mass of dust, kg; a - centrifugal acceleration of dust, m/s2。 Because, a=VT2/R， In the formula, VT represents the tangential velocity of dust particles, m/s； R - the rotation radius of the airflow, m， So, F=mVT/R。 It can be seen that with a fixed structure (R constant) and the same dust (m stable) of the cyclone dust collector, increasing the airflow velocity of the cyclone dust collector will increase the centrifugal force of the cyclone dust collector.
The gas volume of the cyclone dust collector is Q=3600AVT, where Q represents the gas volume of the cyclone dust collector, m3/h； A - cross-sectional area of the cyclone dust collector, m2. Therefore, in the case of a fixed structure (R remains constant, A remains constant) and the same dust (m is stable), the airflow velocity of the dust collector is proportional to the air volume, and the air volume of the cyclone dust collector is determined by the intake air volume of the induced draft fan.
It can be seen that increasing the inlet airflow velocity can increase the tangential velocity of the airflow inside the dust collector, which increases the centrifugal force on the dust and is beneficial for improving its dust removal efficiency. At the same time, it can also increase the amount of dust treated. But as the inlet airflow velocity increases, the radial and axial velocities also increase, and the impact of turbulence increases. For each specific dust cyclone dust collector, there is a critical inlet airflow velocity. When this velocity is exceeded, the influence of turbulence increases compared to separation, causing some separated dust to be carried away again and affecting the dust removal effect. In addition, as the airflow increases, the dust removal resistance will also sharply rise, pressure loss will increase, and power consumption will increase. Taking into account the dust removal effect and economy of the cyclone dust collector, the airflow velocity at the inlet should be controlled between 12-20 m/s, with a maximum of 25m/s. Generally, 14m/s is recommended.
The condition of dust
The size of dust particles is a key factor affecting the outlet concentration. The dust outside the cyclone dust collector is subjected to two forces in the radial direction simultaneously. One is the centrifugal force generated by the tangential velocity of the rotating airflow, which causes the dust to be pushed outward; The other is the centripetal force generated by the radial velocity of the rotating airflow, which causes the dust to be pushed inward. At the interface between the inner and outer vortices, if the centrifugal force generated by tangential velocity is greater than the centripetal force generated by radial velocity, the dust will move towards the outer wall under the push of inertial centrifugal force and be separated; If the centrifugal force generated by tangential velocity is less than the centripetal force generated by radial velocity, the dust will enter the inner vortex under the push of centripetal force and be discharged through the exhaust duct. If the centrifugal force generated by tangential velocity is equal to the centripetal force generated by radial velocity, that is, the external force acting on the dust particles is zero, theoretically, the dust should rotate continuously at the interface. In fact, due to the turbulent airflow and various random factors, dust in this state has a 50% chance of entering the inner vortex and a 50% chance of moving towards the outer wall. The dust removal efficiency should be 50%. The critical dust particles separated at this time are called the segmented particle size. At this point, the interface between the inner and outer vortices is like a sieve with a divided particle size. Dust larger than the divided particle size is intercepted and captured by the sieve, while dust smaller than the divided particle size is discharged from the exhaust pipe through the sieve.
The smaller the particle size of the dust captured by the cyclone dust collector, the higher the dust removal efficiency of the dust collector. The magnitude of centrifugal force is related to dust particles, and the larger the particles, the greater the centrifugal force they are subjected to. When the particle size and tangential velocity of dust are larger, and the radial velocity and exhaust pipe diameter are smaller, the dust removal effect is better. The ash concentration in the gas is also a key factor affecting the outlet concentration. When the dust concentration increases, the dust tends to agglomerate, causing smaller dust particles to agglomerate and be captured. At the same time, larger particles will also be carried to the wall or separated by impact during their movement towards the wall. However, due to the high-speed downward rotation of the airflow inside the dust collector, the pressure at its top decreases. Some of the airflow also carries small dust particles and rotates upward along the outer wall to reach the top, then rotates downward along the outer wall of the exhaust pipe and is discharged through the exhaust pipe, resulting in the dust removal efficiency of the cyclone dust collector being impossible to reach 90%.
According to the formula for calculating dust removal efficiency, η=(1-So/Si) × 90%, where η - dust removal efficiency; So - the flow of dust at the exit, kg/h； Si - the amount of dust flowing into the area, kg/h。
Because the dust removal efficiency of a cyclone dust collector cannot be 90%, as the amount of dust flow increases, although the dust removal efficiency improves, the amount of dust discharged from the exhaust pipe will also increase. So, to reduce the dust concentration at the discharge outlet, it is necessary to lower the dust concentration at the inlet. A multi-stage dust removal method using multiple cyclone dust collectors in series can be adopted to achieve the goal of reducing emissions.
Operating procedures
preparation
1. Check if all connection parts are securely connected.
2. Check the sealing and leakage of dust collector and flue, dust collector and ash hopper, ash hopper and ash discharge device, ash conveying device and other joint parts.
3. Close the baffle valve, start the ventilation fan, and gradually start it after there are no abnormal phenomena.
technical requirement
1. Pay attention to changes in easily worn areas such as the inner wall of the outer cylinder.
2. Pay attention to the adhesion, blockage, and corrosion of dust when the temperature or humidity of dusty gases changes.
3. Pay attention to changes in pressure difference and the condition of smoke discharge. Because wear and corrosion can cause perforation of the dust collector and result in dust emissions, the dust removal efficiency decreases, the exhaust smoke color deteriorates, and the pressure difference changes.
4. Pay attention to the airtightness of each part of the cyclone dust collector, and check the changes in the gas flow rate and dust concentration of the cyclone tube.
The tightness of the lower part of the cyclone dust collector is another important factor affecting the dust removal efficiency. After entering the cyclone dust collector, the dusty gas rotates in a spiral motion from top to bottom along the outer wall. This downward rotating airflow reaches the bottom of the cone and then turns upward, rotating upwards along the axis. The pressure distribution inside the cyclone dust collector is determined by the distribution of axial and radial velocities of the airflow, with relatively small pressure changes in each axial section and large pressure changes in the radial section (mainly referring to static pressure). The airflow moves in a circular motion inside the cylinder, with higher pressure on the outer side than on the inner side, higher static pressure near the outer wall, and lower static pressure at the axis. Even if the cyclone dust collector moves under positive pressure, the axis is still under negative pressure, and the negative pressure extending all the way to the ash discharge port is large. If it is not tight enough, it will produce significant air leakage, and the dust that has settled will inevitably be carried out of the exhaust pipe by the rising airflow. So, in order to achieve the design requirements for dust removal efficiency, it is necessary to ensure the tightness of the ash discharge port, and timely discharge the dust at the bottom of the dust collector cone under the tightness of the ash discharge port. If the dust cannot be continuously and timely discharged, it will circulate at the bottom, causing excessive wear of the cone.