In modern factories, there is an invisible“porter”Working day and night non-stop——It is a pneumatic conveying system. Imagine powder or granular materials sitting on it“Air express delivery vehicle”To reach the destination quickly and efficiently in a closed pipeline, with almost no one on duty throughout the entire process, this is pneumatic conveying technology. Powder or granular materials are collectively referred to as bulk materials, so the professional explanation for pneumatic conveying isThe technology of using gas with certain energy to transport bulk materials in pipelines.

1Basic principles of pneumatic conveying

The basic principle of pneumatic conveying is not complicated: using gas as a carrier to transport materials through pipelines. Just like drinking water through a straw, pneumatic conveying creates a pressure difference that causes the material to move in the pipeline with the airflow. That is to say,The power source of pneumatic conveying comes from the pressure difference along the pipeline.When the conveying gas velocity is high enough, the material particles will be suspended under the action of lift and drag, and move in the direction of the airflow. According to the different airflow velocity and material concentration, pneumatic conveying presents different flow states (flow patterns), mainly divided into two categories: dilute phase conveying and dense phase conveying. The boundary between dilute and dense phases can be divided based on the solid gas ratio (the ratio of the mass of dispersed material transported to the mass of gas at the same time, also known as the mixing ratio). Solid gas ratio greater than15kg/kgThe pneumatic conveying process can be identified as dense phase conveying, otherwise it is dilute phase conveying.

As the conveying gas velocity decreases, the possible flow patterns in the horizontal pipe are shown in the figure1As shown. When the conveying speed is high, the material is suspended in the airflow and transported by the airflow. The current flow pattern is calledSuspended flowThe characteristics are high porosity and relatively uniform concentration field, with particle motion dominated by fluid turbulence and weak collision effects. As the conveying gas velocity decreases, the efficiency of gas-solid momentum transfer decreases, the tendency for particle settling increases, and a static or moving particle deposition layer is formed at the bottom of the pipeline. Particle swarmLayered flowThe characteristic of transportation is a significant reduction in porosity, and the particle concentration at the bottom of the pipeline is much higher than that at the top. As the conveying gas velocity further decreases, asymmetric sand dune shaped material plugs are formed in local areas of the sedimentary layer. Particle swarmSand dune flowTransport is characterized by the presence of sand dune like structures significantly higher than the sedimentary layer, accompanied by pulsating transport. If the conveying gas velocity continues to decrease, the sand dune shaped material plug will evolve into a columnar material plug that crosses the pipeline, and the flow pattern will evolve intoslug flowThe head of the material plug produces a particle entrainment effect due to the acceleration of the airflow, while the tail undergoes particle deposition due to velocity attenuation, forming a dynamic balance of the length of the material plug. At this stage, the lower the conveying gas velocity, the longer the length of the material plug, until a blockage flow occursPlunger flowthe transformation When the conveying gas velocity decreases to the critical blockage gas velocity, the suction rate at the head of the material plug continues to be higher than the sedimentation rate at the tail, causing a rapid increase in the length of the material plug and ultimately leading to the pipelineblock.

image1 Flow patterns in horizontal pipes at different conveying gas velocities

Of course, not all powder and granular materials will exhibit the above-mentioned flow patterns. The reason is that the pneumatic conveying flow pattern is not only related to the conveying gas velocity, but also closely related to the material's physical parameters (particle size and distribution, density, permeability, gas storage capacity, etc.) and pipeline parameters.

2The main types of pneumatic conveying

Pneumatic conveying can be mainly divided into three basic types based on its working principle: negative pressure, positive pressure, and mixed pressure. Negative pressure conveying, also known as vacuum conveying, is like a household vacuum cleaner where materials are sucked into a pipeline and transported to their destination. Negative pressure conveying as shown in the figure2As shown, a vacuum pump is generally installed at the end of the system. The negative pressure caused by the vacuum pump draws air and materials into the pipeline and transports them to the separator for separation. The air is discharged from the upper part and sent to the dust collector for purification, while the materials are discharged from the bottom of the separator. Negative pressure conveying has the advantages of simple operation, low cost, low risk of material leakage, and suitability for centralized conveying. The disadvantage is that the conveying distance is relatively short, the solid gas ratio is relatively low, and it is difficult to achieve dense phase conveying.

image2 Negative pressure conveying

Positive pressure conveying as shown in the figure3As shown, at the front end of the conveying system, compressed air is generated by a compressor or blower, which creates a pressure difference between the starting and ending points of the pipeline. The pressure difference drives the material to be conveyed to the separator for separation. The advantages of positive pressure conveying system are longer conveying distance, simple maintenance, and suitability for dispersed conveying. The disadvantage is the risk of material leakage and relatively high cost.

image3 Positive pressure conveying

Hybrid conveying combines the advantages of negative pressure and positive pressure, with some pipelines in a negative pressure state and others in a positive pressure state. This system is suitable for complex process requirements and can meet the transportation needs of special industries.

3Components of pneumatic conveying system

Pneumatic conveying systems typically consist of the following components: a gas source (positive pressure conveying using a blower or compressor, negative pressure conveying using a vacuum pump) as the power source for pneumatic conveying; Feeding device (with pressure feeding tank or rotating)valve）Used to control the rate at which materials enter the conveying pipeline; Transport pipelines (horizontal pipes, vertical pipes, and bent pipes) serve as channels for gas-solid flow; Material receiving and separation device (material receiving tank, dust collector); Control system（PLCTheDCS）Used to achieve automated control and fault monitoring of pneumatic conveying systems, ensuring stable and efficient operation of the system.

4Why choose pneumatic conveying?

Compared with other mechanical conveying methods such as screw conveyors, belt conveyors, bucket conveyors, etc., pneumatic conveying has the following significant advantages, which is also the reason why it is widely used in various industries:

①Sealed environmental protection: Materials are transported in fully sealed pipelines, and there is no dust leakage during normal operation, fundamentally improving the working environment and meeting the strict requirements of modern industry for environmental protection.

②Flexible layout: The conveying pipeline can be flexibly arranged according to the structure of the factory building, and can be installed horizontally, vertically, or diagonally. The conveying distance is long (can exceed)1000m）Capable of adapting to various complex terrains and process layouts.

③High degree of automation: The pneumatic conveying system is easy to achieve automated control, greatly reducing labor costs and intensity.

In addition, pneumatic conveying also has the advantages of small footprint, multiple process operations (such as mixing, drying, cooling, etc.) that can be carried out simultaneously during the conveying process, and low maintenance costs.

5Application scenarios of pneumatic conveying

Pneumatic conveying technology is widely used, covering almost all industries related to the processing of powder and granular materials. In the gasification industry, powdered gasification raw materials (coal powder, biomass, etc.) are commonly transported to gasification furnaces through pneumatic conveying; In the power industry, pneumatic conveying is widely used to transport coal powder and fly ash; In the food industry, the transportation of food raw materials such as flour, sugar, and milk powder relies on pneumatic conveying systems; The chemical industry uses it to transport resinsPVCChemical raw materials such as powder. In the building materials industry, pneumatic conveying technology is widely used for the transportation of cement and lime powder; In the pharmaceutical industry, the transportation of pharmaceutical powder requires high hygiene conditions, and the sealing characteristics of pneumatic conveying precisely meet this demand. In the grain and oil industry, important adsorbents such as clay and bentonite, as well as filter aids such as diatomaceous earth, can achieve high solid to gas ratios and high concentrations of dense phase transport due to their excellent fluidized dense phase transport capabilities. In recent years, pneumatic conveying has even been widely used in emerging high-tech fields such as lithium-ion battery materials and polymer materials, demonstrating its strong adaptability and progressiveness. The market demand for pneumatic conveying in the future will continue to expand.

6Challenges Faced and Future Development Trends

Although pneumatic conveying has many advantages, it also faces some challenges, such as relatively high energy consumption and difficulty in conveying materials with strong viscosity.

In the future, pneumatic conveying technology is developing in multiple directions:

①Intelligentization, through sensors and advanced control algorithms to achieve system self optimization and continuously improve the level of automation operation.

②Low energy consumption, based on the physical properties and transportation environment of powder particles, targeted improvement of transportation methods to enhance the level of dense phase transportation with low wear, low gas consumption, high solid gas ratio, and high transportation volume.

③Standardization, establishing unified standards to promote the development of industry norms. for example2024In, China initiated the development of the national standard for "General Requirements for Pneumatic Conveying Systems of Conveying Machinery", which will provide important support for the healthy development of the pneumatic conveying industry.

With the continuous development of industrial technology, the intelligence level of pneumatic conveying systems will become increasingly high. The future system will integrate big data and IoT technology to achieve self-control, self diagnostic analysis, and ultimately achieve fully unmanned operation. The pipeline layout may be as flexible as building blocks, with modular design of core components such as feeding devices, conveying pipelines, and control systems, allowing for free combination according to different material characteristics and conveying requirements.