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:

       当含尘气体通过洁净的滤袋时，由于滤料本身的网孔较大，大部分微细粉尘会随着气流从滤袋的网孔中通过，因此，信用滤袋的除尘效率较低。粗大的尘粒因惯性碰撞、截留、布朗扩散、静电、重力沉降等作用被阻留并在网孔中产生‘架桥’现象，随着含尘气体不断通过滤袋的纤维间隙，纤维间粉尘‘架桥’现象不断加强后，一段时间后，会在滤袋表面形成粉尘初层，则在以后的除尘过程中，粉尘初层与气流粉尘进行一系列作用后而形成主要过滤层，随着粉尘在滤布上的积累，除尘效率相应增加，从而达到除尘效果。

湿式除尘器的除尘机理：

       在湿式除尘器内的水或其他液体与含尘气体作相对运动的过程中，液体介质与尘粒间在惯性碰撞，拦截，扩散及凝并等效应的作用下，尘粒被液体介质所捕获，从而实现除尘目的。

文丘里洗涤器工作原理：

       文丘里管包括收缩段、喉管和扩散段。含尘气体进入收缩段后，流速增大，进入喉管是达到最大值。洗涤液从收缩段或喉管加入，气液两相间相对流速很大，液滴在高速气流下雾化，气体湿度达到饱和，尘粒被水湿润。尘粒与液滴或尘粒之间发生激烈碰撞和凝聚。在扩散段，气液速度减小，压力回升，以尘粒为凝结核的凝聚作用加快，凝聚成直径较大的含尘液滴，进而在除雾器内被捕集。

静电除尘器的工作原理：

       主要包括电晕放电、尘粒荷电、带电粒子在电场中的迁移和捕集、粉尘清除四个基本过程。电晕放电，在非均匀电场中，直流电压足够大时，气体中的自由电子杯加速随机碰撞使气体中性分子电离形成新的自由电子和正离子，新的自由电子和正离子被加速与中性分子碰撞电离，产生大量的离子，随即引起尘粒荷电（电场荷电-大于1nm，扩散荷电-小于0.4um），电场荷电和扩散荷电的综合作用主要适用于中间粒径范围的粒子，粒子的捕集是电极间产生不均匀电厂，气体被电离，接着粉尘荷电，在电场力作用下达到集尘极，最后通过清灰装置粉尘震落至灰斗，粉尘清除。静电除尘器一般用电磁振打或锤式振打清灰，电晕级一般用机械振动来青灰。

       影响静电除尘器性能的主要因素：1.烟尘性质：①粉尘特性（a比电阻影响：α低比电阻粉尘不适用。高比电阻粉尘适用，比电阻越高除尘率越高。b粒径直径约高除尘效率越大，除尘效率max之后，粒径增大，效率变低。堆积密度小的粉尘，效率低，粘附力大，效率低。）②烟气性质（烟气湿度大效率大。温度大效率低，成分不同，荷载体迁移不同，烟气成分对负电晕放电有很大影响，烟气压力大，放电电压增大。烟气含尘浓度在一定范围内，可净化，超出范围，效率降低）2.设备状况①设备的安装质量：电极线粗细不匀尖刺不平卷边会降低效率②气流分 部，气流速度低，效率高，反之则反。3操作条件①气流速度同上②振打清灰：其他清灰方式比如凸轮振打电磁振打均会降低③二次扬尘会降低除尘效率

       预防煤尘爆炸的技术措施主要包括三个方面：a.减、降尘措施；每层注水（短孔注水2-3.5m，深孔注水5-25m长孔注水斜于工作面孔长30-100m巷道钻孔注水）b.防止煤尘引燃措施；消灭引爆火源（清除井下明火，爆破火焰，电气失爆、其他火源、防止瓦斯引燃）c.限制煤尘爆炸范围扩大：清除落灰散布岩粉，设置岩粉棚，设置水棚，设置自动隔爆棚。

    煤层注水方式：短孔注水：在回采工作面垂直煤壁或与煤壁斜交打钻孔注水，注水孔长度一般为2～3.5m。长孔注水：从回采工作面的运输巷或回风巷，沿煤层倾斜方向平行于工作面打上向孔或下向孔注水，孔长30～100m；巷道钻孔注水：由上邻近煤层的巷道向下煤层打钻注水或由底板巷道向煤层打钻注水。

      煤尘爆炸机理及过程主要表现在以下方面：a悬浮的煤尘在高温热源的作用下被干馏成可燃性气体。b.可燃性气体与空气混合而燃烧。c燃烧放出热量，传给附近悬浮的煤尘，使燃烧循环的进行下去，其反应速度越来越快。通过剧烈燃烧，最后形成爆炸。

       煤尘爆炸必须同时具备三个条件：a.煤尘必须悬浮于空气中，并达到一定的浓度；b.一般说来，煤尘爆炸的下限浓度为30～50g/m3，上限浓度为1000～2000g/m3。C.存在能引燃煤尘爆炸的高温热源

    除尘系统的设计：1.绘制通风系统轴测图，对各管道标号并注上各管段流量和长度（Q、D不变为一个管段，由远到近编号）管段长度按关键中心线的长度计算，不扣除管件本身长度。2选择计算环路（一般从长的管段开始计算）即最不利管段3选择流速（根据情况地点来选取最低风速）4计算管径和摩擦阻力5计算局部阻力和除尘设备阻力6并联管路阻力平衡计算与管径7.除尘系统总阻力和总风量8选择通风机和电动机

    重力沉降室的设计：1.尘粒的沉降速度2粉尘颗粒在陈静室内停留时间3沉降室的横截面积4除尘器高度宽度长度5除尘效率6除尘器的阻力

    尘肺病：由于在职业生活中长期吸入生产性粉尘并在肺中直流而引起的肺组织弥漫性纤维为主的全身性疾病矽肺病：由于生产过程中长期吸入大量含游离sio2的粉尘引起的以肺纤维化为主的肺部疾病。