Pneumatic conveyor is a device that uses air with a certain speed and pressure to drive granular materials to be transported vertically or horizontally in a closed pipeline. The conveying process of materials is completely controlled by the dynamic state of air. When the air velocity is within the critical range, the materials are in a suspended state. That is to say, the gravity of the material reaches equilibrium with the dynamics of the air; Below the critical range, the material decreases: above the critical range, the material is transported. Pneumatic conveyors are widely used in power plants to transport fly ash, cement, alumina powder, raw material powder, kaolin, limestone powder, mineral powder, antimony powder, sawdust, rice husk, straw particles, food grade materials and other powdered materials.

　　

Characteristics of pneumatic conveyor

　　

Pneumatic conveyor is a mechanical device that uses a fan to create an airflow inside a pipeline to transport granular materials. Its conveying principle is to add materials to an air flow with a certain velocity to form a suspended mixture, which is transported to the destination through pipelines, and then separated from the air flow to be discharged. Pneumatic conveyors are mainly used for conveying powders, granules, and small lumps; Small pieces of 30rum material are sometimes transported as individual items. For different materials, different wind speeds should be selected to ensure that the materials remain suspended in the pipeline and do not block the pipeline, while also conveying as much material as possible, achieving both economy and rationality.

　　

The advantages of pneumatic conveyors are: they can improve working conditions, increase production efficiency, facilitate automation, reduce cargo damage, and ensure cargo quality; Simple structure without traction components; High productivity, unaffected by the surrounding conditions and climate of the pipeline; The conveying pipeline can be flexibly arranged to adapt to various loading and unloading processes; Beneficial for achieving bulk transportation, saving packaging costs, and reducing costs.

　　

The disadvantages of pneumatic conveyors are: high power consumption and loud noise; The transported materials have certain limitations and should not be used to transport damp, viscous, and fragile materials; When transporting materials with high wear and tear, components such as pipelines are prone to wear and tear.

　　

Basic structure of pneumatic conveyor

　　

(1) Power unit: It is a general term for airflow boosting and conveying equipment. Commonly used are air compressors, blowers, vacuum pumps, and their driving motors, which provide a certain speed and pressure of air flow to complete the transportation of materials.

　　

(2) Feeder: It is a device used to feed granular materials into the feeder and form a certain mixing ratio with the airflow. Its structure and working principle depend on the physical properties of the conveyed material and the form of the pneumatic conveying device. Generally, a feeding device that can suspend the material in the airflow is used and installed at the beginning of the conveying system.

　　

(3) Conveyor: It includes pipelines and auxiliary fittings for conveying gases and their mixtures with materials, and is a channel for material transportation.

　　

(4) Separators: Pneumatic conveying devices transport a mixture of materials and air. Separators are devices that separate materials from gas-solid two-phase flow when they are transported to the terminal.

　　

(5) Dust collector: After passing through the separator, most of the material is separated, but some small particles or powders are difficult to separate. Therefore, the airflow discharged from the separator will contain some fine materials and a large amount of dust. The dust collector installed after the separator can not only recover this part of the material, but also reduce environmental pollution. For widely used suction pneumatic conveyors, it can also reduce the wear of the blower.

　　

(6) Unloader and ash unloader: Their function is to unload the separated material from the separator and discharge the dust from the dust collector, while preventing the backflow of external air into the conveying system. Therefore, the unloader is installed below the separator, and the ash unloader is installed below the dust collector.

　　

Types of pneumatic conveyors

　　

According to the air pressure inside the pneumatic conveyor pipeline, the conveyor can be divided into three types: suction type, blowing air pressure type, and suction blowing combined type.

　　

1) Suction pneumatic conveyor

　　

The main feature of the suction type pneumatic conveyor is to extract air from the entire pipeline system through a blower, so that the air pressure inside the pipeline is lower than atmospheric pressure, forming a certain negative pressure. As shown in the figure, the material is mixed with air at the suction nozzle, and due to the negative pressure inside the pipeline, it is sucked into the conveying pipeline and transported along the pipeline. After reaching the predetermined discharge point, the air is separated from the material by an air separator, and after dust removal and noise reduction treatment, the air is discharged into the atmosphere of the predetermined space.

　　

The biggest advantage of a suction type pneumatic conveyor is that it is easy to feed materials. It can be fed with powder particles simultaneously from one or several feeding points through one or several suction pipes, and there is less floating dust. Its disadvantage is that it has significant limitations on the particle size and density of the transported particles, and the transportation distance is limited because as the distance increases, the resistance increases, and the requirement for negative pressure vacuum degree is high. However, after the negative pressure vacuum degree reaches a certain value, the air becomes thin, the air flow capacity decreases, and the transportation force decreases. Ensuring a certain degree of negative pressure is crucial for suction pneumatic conveyors. Except for the blower, the pipeline system should be strictly sealed to avoid affecting the pipeline pressure. The more bends and joints there are, the poorer the reliability and safety.

　　

2) Blow air pressure pneumatic conveyor

　　

Unlike the suction type pneumatic conveyor, the pressure in the pipeline of the compression type conveyor is higher than atmospheric pressure. As shown in the figure, the air is compressed by the blower and enters the conveying pipeline. The material enters from the hopper, mixes, and is blown along the pipeline until it is separated by the separator at the discharge point. The material is discharged from below, and the air is discharged into the atmosphere through dust removal and noise reduction.

　　

The biggest advantage of a pressure fed pneumatic conveyor is its long conveying distance and the ability to continuously apply pressure; Its disadvantage is that the structure of the feeder is complex, and the density and particle size of the powder particles, as well as the permeability of the powder in the feeder, must be considered. Because the feeder needs to deliver the material into the high-pressure pipeline, it must prevent high-pressure air from rushing out of the pipeline. Pressure fed pneumatic conveyors are widely used in the loading and unloading of bulk cement.

　　

3) Combined pneumatic conveyor with suction, blowing and compression

　　

Sometimes we encounter situations where there is no dedicated loading and unloading equipment at the unloading location, and the conveying distance is long. Using suction or compression pneumatic conveyors alone cannot be effective, and the advantages of both must be combined: suction pneumatic conveyors are easy to feed, while compression pneumatic conveyors are suitable for long-distance conveying. Combining the two can achieve good results when designed and used together, as shown in the figure.

　　

The material from nozzle 1 to separator 3 is the suction part, and from the bottom of separator 3 to unloader 5 entering the conveying pipeline is the pressure feeding part. The hybrid pneumatic conveyor has the advantages of suction and pressure feeding, but its structure is complex. Most of the air entering the pressure feeding part of the blower is separated from the suction part, so the dust content is relatively high.

　　

Application of pneumatic conveyor

　　

Due to the close relationship between the application scope of pneumatic conveyors and the physical properties of the conveyed materials, the following analysis examines the impact of several main material characteristics on the application scope of pneumatic conveyors.

　　

(1) The particle size of the material. The particle size of the material is too large, which can easily block the feeding device and prevent the material from automatically entering the conveying pipe. Therefore, it is generally required that the particle size should not exceed 50mm, or the maximum particle size of the material should not exceed 0.3-0.4 times the diameter of the conveying pipe.

　　

(2) The bulk density of materials. The packing density of materials greatly affects the structural dimensions and energy consumption of pneumatic conveyors. Because with the increase of material packing density, it is necessary to increase the airflow velocity in the pipe, thereby increasing power consumption and accelerating pipe wall wear. Therefore, pneumatic conveying is not suitable for materials with high packing density.

　　

(3) The humidity of the material. The humidity of materials is closely related to the reliability of pneumatic conveyor operation. Due to excessive humidity, not only will the loose nature of the material be destroyed, but the material will also adhere to the inner walls of the device components, resulting in uneven feeding, increased energy consumption, reduced conveying capacity, and even blockage of the entire system. So, for various materials, it is necessary to determine the maximum humidity at which their transportation is not damaged.

　　

(4) The abrasive properties of materials. The so-called abrasion resistance refers to the wear and tear of moving materials on other objects. The size of the grinding property depends on the hardness, surface characteristics, and shape size of the object particles, which affects the power consumption and service life of the pneumatic conveyor.

　　

(5) The temperature of the material. The combustible dust of the transported material can cause dust explosions and serious accidents at a certain concentration and temperature. Therefore, in pneumatic conveying, the temperature of the material should not exceed its ignition point (usually below 400 ℃, otherwise inert gas conveying should be used instead).

　　

At present, the widely used method in ports is to utilize the airflow velocity inside the pipes to be between 10 and 10; A pneumatic conveyor with a high-speed airflow of around 30m/s that transports materials in a suspended state. But it consumes a lot of energy and the working components wear out quickly. Therefore, increasing the mixing ratio of materials in the air and reducing the conveying airflow velocity are effective ways to overcome the above disadvantages. Some new types of pneumatic conveyors are currently being developed, which operate at high mixing ratios and low airflow speeds (usually only 4— A pneumatic conveyor that uses gas pressure to transport high concentration materials under the condition of 6m/s.