A Brief Discussion on the Conditions and Mechanisms for the separation of Dust from Air

Abstract

Pure air is one of the important environmental factors for the survival of human beings and all living things. Separating dust from the air is also an important technical measure to improve product quality (such as ensuring the gradation accuracy of asphalt mixtures). There are seven types of dust collectors, namely gravity and inertial dust collectors, cyclone dust collectors, wet dust collectors, filter layer dust collectors, bag dust collectors, electrostatic precipitators and combined dust collectors. They all have common separation conditions for separating dust from the air. However, due to the different dust removal principles of various dust collectors, there are significant differences among them in terms of structure, performance, and application scope. This article starts with an introduction to the "conditions for dust to separate from the air", and then elaborates on the dust removal machines and application scopes of commonly used dust collectors, so as to select or design suitable dust collectors for various road construction and maintenance projects and the specific conditions of dusty air.

Key words:

Dust-air separation mechanism

Dust removal is a specialized technology for separating and capturing solid particles such as dust in dusty air flows for environmental protection or to improve product quality. Dust collectors, dust collectors, dust filters, and filters can be collectively referred to as dust collectors. They are specialized equipment used to separate and collect dust particles suspended in air or gas.

The conditions for the separation of dust from gas

Dust and other solid particles move forward along with the gas. When the dusty gas enters the separation zone, under the action of one or several forces, the dust particles will deviate from the airflow and, after sufficient time, move to the separation interface and adhere to it. The dust particles retained at the interface are constantly removed, which can create conditions for new dust particles to continue adhering to the separation interface. The repetitive cycle is the operation process of the dust collector. It can be seen that the conditions for the separation of dust particles from the gas are:

1) It has forces that cause the movement trajectories of dust particles to be different from those of gas flow lines. Common ones include gravity, centrifugal force, inertial force, diffusion (force), static electricity (force), direct interception (force), thermal cohesion, sound waves and light pressure, etc.

2) There are separation interfaces that allow dust particles to adhere to them, such as the walls of containers, the surfaces of certain solids, the surfaces of large-diameter particles, the surfaces of fabrics and fibers, liquid films or fog droplets, etc.

3) There should be sufficient time for the dust particles to move to the separation interface, which requires the dust remover to have a certain space and be able to control the gas flow rate.

4) It is necessary to continuously remove the dust particles that have adhered to the separation interface without them returning to the airflow. This requires a cleaning and ash discharge process.

The mechanism of dust separation in gas

2.1 The main mechanism of dust separation in gas

2.1.1 Gravity separation of dust

Based on natural sedimentation due to gravity, dust particles are separated from the slowly moving airflow, which is the simplest but least effective dust removal method. Because in a gravity dust collector, the gas medium is in a turbulent state, even if the dust particles stay in the dust collector for a long time, they cannot be effectively separated from the dusty gas flow in fine particle size. However, it has a better separation and capture effect on coarser dust particles. Therefore, the gravity separation mechanism of dust and gravity dust collectors are mainly suitable for dust particles with diameters greater than 100 to 500μm. To achieve better dust removal effects, a slow air flow rate must be maintained during this process.

2.1.2 Centrifugal separation of dust

Due to the rapid rotation of the gas medium, the suspended dust particles in the gas reach a very high radial migration speed, thus effectively separating the dust particles. To ensure the dust removal effect, the structure of the centrifugal dust collector must make the retention time of dust particles in the dust collector short. Correspondingly, the rotational diameter of the airflow inside the dust collector should be small; otherwise, many dust particles cannot reach the container wall during the short retention time in the rotating dust collector. For instance, in a cyclone dust collector with a diameter of 1 to 2 meters, dust particles larger than 10μm can be effectively separated and captured. If a large-sized cyclone dust collector is required, it can only separate and capture dust with a particle size greater than 70 to 80μm, that is, the performance of such a cyclone dust collector is relatively poor.

The prominent advantage of cyclone dust collectors is that they can handle the dust removal of high-temperature gases and are relatively inexpensive. However, the dust removal efficiency for high-precision purification of gases is not high.

2.1.3 Inertial separation of dust

The inertial separation mechanism of dust is that when the airflow bypasses an obstacle of a certain structural form, the inertia can be utilized to separate the dust particles from the airflow. The larger the cross-sectional size of the obstacle, the earlier the airflow path deviates seriously from the straight line direction when bypassing the obstacle, and correspondingly, the earlier the dust particles suspended in the airflow start to deviate from the straight line direction. It should be noted that the separation of dust by using the inertial mechanism will inevitably cause a certain pressure loss to the airflow. However, it can achieve a very high separation and capture effect, thereby compensating for this shortcoming. Therefore, inertial dust collectors can efficiently separate and capture dust particles a few micrometers in size, and their efficiency is close to that of bag filters and Venturi tube dust collectors. In practical applications, inertial dust collectors are usually used in conjunction with gravity settling devices.

2.1.4 Electrostatic (force) separation of dust

The principle of electrostatic (force) separation of dust is to utilize the interaction between the electric field and the charged particles. Therefore, it requires the dust particles to be charged. In order to generate the force that separates charged particles from the airflow, an electric field must be present. Because the electrostatic force acting on charged particles is relatively small, only by allowing dust particles to remain in the electric field for a long time can the purpose of collecting dust be achieved. Therefore, electrostatic precipitators are generally very large in size, and accordingly, the equipment cost is relatively high. However, compared with bag-type dust collectors of the same size, its unique advantage is that the electrostatic purification device does not cause significant pressure loss and thus consumes less energy. Another important advantage of electrostatic purification is that it can be used to handle gases with working temperatures up to 400℃. In addition, there is no limit to the minimum size of electrostatic precipitators.

2.2 Auxiliary Mechanism for Dust Separation in Gas

2.2.1 Diffusion Effect

The vast majority of dust particles suspended in the gas remain on the solid surface after touching it, and are thus separated from the total number of dust particles near the surface. Therefore, a concentration gradient of dust particles is generated near the deposited surface. Because dust particles participate in the Brownian motion of the molecules around them to some extent, they constantly move towards the deposition surface, making the concentration difference tend to balance. The greater the concentration gradient of dust particles, the more intense this movement becomes. The smaller the size of the dust particles suspended in the gas, the stronger the degree of their participation in the Brownian motion of molecules, and the more intense the movement of the dust particles towards the deposition surface accordingly.

The diffusion and sedimentation of dust particles play a particularly significant role when fabric filters are used to capture fine dust

2.2.2 Thermal sedimentation effect

The temperature difference between the walls of containers and pipes and the suspended dust particles in the airflow will affect the movement of these particles. If there is a small dust particle near the arm surface of a hot pipe, it is the result of rapid and uneven heating of this dust particle. Molecules closer to the hotter side, after colliding with dust particles, fly away from the particles at a speed greater than that of the molecules closer to the cooler side. As a result, the intensity of the pulses acting on the particles varies, causing the particles to move in a direction away from the heated tube wall, thereby triggering the particle sedimentation effect.

When the dust-accumulated surface inside the dust collector is cooled manually, the effect of thermal sedimentation is particularly obvious.

2.2.3 Coagulation effect

Coagulation is the phenomenon where suspended dust particles in a gaseous medium adhere to each other during the process of contact (collision). When the velocity of the gas medium changes locally, the coagulation effect that occurs is particularly obvious in turbulent pulses. This is because after the dust particles are blown away by the flowing gas medium, due to their own inertia, they cannot keep up with the rapid changes in the movement trajectory of the gas unit, resulting in collisions between the dust particles.

The external force causing coagulation can be the gravity of the dust particles or the electric force of the charged dust particles.

If it is polydisperse dust, fine particles coagulate with coarse particles. Moreover, the more fine particles there are, the greater the difference in size between them and the coarse particles, and the faster the coagulation process occurs.

The coagulation effect of dust particles provides favorable conditions for dust collection for dust removal equipment of various structures and principles.

The basis for the selection of dust collectors

Among the physical properties of dust, the particle size of dust is a key characteristic parameter, and many other properties of dust are closely related to the particle size. Therefore, the primary basis for choosing a dust collector is the size of the dust particle. The following figure shows the relationship between the physical properties of dust particles and their particle size range and the corresponding dust collectors, which can be used as a reference when selecting the appropriate dust collector for different types of dust.