Fundamentals of pneumatic conveying technology

Fundamentals of pneumatic conveying technology

Pneumatic conveying device is a completely enclosed conveying equipment that uses gas as the carrier medium in the pipeline to transport materials from one place to another. It has the advantages of simple equipment, compact structure, small footprint, safety and reliability, and high conveying efficiency. However, its high energy consumption has certain requirements for the particle size and viscosity temperature of the transported materials, which affects its widespread use. For general loose granular powder materials, pneumatic conveying can be used. The characteristics of materials have a significant impact on the success and efficiency of pneumatic conveying. Different types of materials have different characteristics, and the same material may not necessarily have the same pneumatic conveying characteristics. For example, the pneumatic conveying behavior of the same powder with different particle sizes can be completely different. So in engineering design, it is necessary to pay attention to the characteristics of the transported materials and select the best, reasonable, and economically applicable pneumatic conveying system

1、 Material conveying characteristics

The material conveying characteristics are established within the range of material conveying capacity by the pressure drop line of the equal conveying pipe and the equal material to gas ratio line on the coordinate chart of material mass flow rate and air mass flow rate. Among them, the pressure drop line of the conveying pipeline is very important, and the shape slope and quantity value of the given pipeline pressure drop line can vary greatly with different types of materials.

The characteristics of material pneumatic conveying are influenced by the following factors:

1. Geometric conditions of the conveying pipeline.

2. The physical or chemical properties of the material.

3. Different granulation and processing methods or materials contain different additives.

4. The flow pattern of materials

From this, it can be seen that the various characteristics of the material not only affect the transportability of the material, but also affect the normal use of all equipment in the pneumatic conveying system. Therefore, when conducting feasibility analysis on pneumatic conveying engineering, it is necessary to comprehensively analyze the characteristics of the equipment, pipelines, and materials to be conveyed, and evaluate the suitability or matching of the materials with the selected conveying device form. Usually, the pneumatic conveying characteristics of materials consist of two parts, namely particle characteristics and bulk material characteristics

（1） Particle properties

1. Particle size.

The particle size is expressed in terms of particle size, which is related to the unit example and also to the characteristics of the bulk material. Li can be divided into a single particle size representing a single particle and an average particle size representing a group of particles composed of many different sized lychees. The average particle size requires a single particle size for each particle to be determined first and then averaged. There are various calculation methods for this. For spherical particles, their diameter can be defined directly, but for irregular particles, their diameter must be defined according to the following equivalents, hence it is also called equivalent diameter.

The definition of two-dimensional equivalent is divided into inner circle diameter, outer circle diameter, and the diameter of circles with the same circumference.

2. Three dimensional equivalence is used to define the diameter of a sphere with the same surface area, volume, and mass, as well as the dimensions of a hole that passes through a circle or a square.

3. Classified by statistical diameter. The diameter is randomly averaged. The distance between the endpoints of the two poles is measured in a fixed direction. The equal division of area refers to the division of three-dimensional particles into two straight lines of equal area in a fixed direction. Or Yan Guifen can divide the length of the particle projection area into two equal parts. When using an optical or electron microscope, it is an example of observing from above and projecting onto a plane. Therefore, the particle size is determined based on the projection map of the particles. Besides. Beyond the diameter, there are also directional maximum diameter projection source equivalent diameters and other perimeter element equivalent diameters.

2. Particle density

The mass of a single particle divided by its volume. The value of is the particle density. There are three ways to represent particle density:,

1. The true density and particle volume divided by the particle volume excluding open and closed pores

2. The apparent density and particle mass are divided by the volume of particles excluding open pores but including closed pores.

3. The effective density and particle mass are divided by the volume of particles containing open and closed pores. Obviously, the actual density is difficult to measure because the pores inside the particles cannot be removed. It can also be seen that the actual density is greater than the apparent density, and the density is greater than the effective density. In engineering applications, the apparent density is mainly used. Particle density is used to calculate the conveying airflow velocity, which affects the minimum conveying speed of pneumatic conveying and the required pressure drop for conveying.

3 particle shapes

Particle shape is a performance parameter that is difficult to define because there are various shapes, and these descriptive terms are only relative and subjective rather than objective. Therefore, in order to introduce particle shape into the analysis model, shape should be defined on a quantitative basis. Multiple shape coefficients have been proposed for individual particles, such as spherical circular area shape coefficient and specific surface area shape coefficient.

As mentioned earlier, for materials with uneven particle size, particles are usually regarded as spherical, and the basic theory of determining their equivalent diameter and pneumatic conveying assumes that the example is spherical for analysis. This is because the ratio of surface area to volume of spherical particles is the smallest, and theoretical analysis is easy to cut. Spherical particles have no directionality, and experimental results are also easy to obtain online. Therefore, spherical particles are commonly used as reference shapes, Provide a specific physical characteristic with a practical example. Urgent action is defined as the surface area of the same volume as the particle divided by the actual surface area of the particle.

Sphericity represents the deviation between the shape of particles and the shape of balls of the same volume. For any shape of a sphere, the value of particles is smaller than that of bulk particles. The overall shape and structure of the particles are important for the design of pneumatic conveying devices. If the particles are polygonal, they are prone to breakage and fiber like structures during litigation. This means that examples may be tangled together, causing difficulties in material discharge and blockage in pipelines. Customized and robust particles with sharp angles may cause internal scratches and patterns When designing equipment and selecting equipment pipelines and fittings in powder engineering. This must be carefully considered.

4. Particle brittleness

Particle brittleness cannot be easily defined by a single indicator or numerical values obtained from experiments. Therefore, the grinding of materials during pneumatic conveying can only be accurately evaluated by simulating the situation in pipelines and conducting a series of wear tests such as free fall.

5 particle hardness

One of the main drawbacks of pneumatic conveying is the wear and tear on pipelines and components, which is a function of the particle transport speed in the Indus River. Currently, India is still classified according to the Indian classification of ink. Based on the Mohs hardness values of some materials, the possibility of using pneumatic conveying can be roughly evaluated. Wear resistance measures that should be taken.

6. Specific surface area

The specific surface area of particles is expressed in terms of the total surface area per unit mass and volume, and is commonly used as a measure of material fineness. In pneumatic conveying of materials, the permeability data measured by an air permeameter is usually used to estimate the surface value. When comparing. The increase in surface size means that the permeability of the material decreases while the pneumatic capacity increases, and the mass flow rate of the material can be appropriately increased.

（2） Bulk material characteristics

Loose material is composed of a large number of individual particles, which are often of different sizes and shapes, and may not even have the same chemical composition. Each chestnut is surrounded by free space and gaps, so loose material can be seen as a combination of particles and spaces that ignore rules. In pneumatic conveying, most of the air is occupied by space and the conveying airflow, so these examples are related to the needs that include them,. This determines the behavior of urination and its pneumatic conveying characteristics. Some characteristics of loose materials can be related to individual particles, but more importantly, they are related to the bulk stacking state of the material. The overall material performance in the chasing state is called loose material characteristics.

1. Loose density

Loose density, also known as bulk density, is the value of the mass of the bulk material divided by the volume occupied by the bulk material. Since loose materials are composed of many irregular sets. The composition of material particles includes the volume of particles and the gaps between particles, so it has a surface loose density, which depends on the density, shape, particle loading method, and coordination with each other. For a certain loose density of bulk materials, it does not have a single value. It varies greatly with the density of materials, and is also related to the method of chestnut filling and container. It is usually more appropriate to provide the range of loose density rather than a single value. When measuring any loose density, the test conditions should be the first mock examination or as close to the actual situation as possible. The following three methods are commonly used to determine the loose density of bulk materials.

(1) When the state is loose, the density is loose. The density is when a pile of goods is piled up and poured into a container without applying any external force to tighten it.

(2) Compact, lightweight, and loose density. This is the loose density obtained from the applied visual strength such as impact and vibration of the container,

(3) The loose density particles in Chongqing are only used for the design of fine rod vermicelli due to the looseness of air films when they are separated from each other. When pneumatic conveying, data on loose density is essential for determining the following important parameters.

The approximate displacement obtained from the feeder.

2. The approximate mass of loose materials for known volume of materials and cutting materials in Cangdong.

3. The approximate volume of the hopper and material factory required to store a certain amount of bulk material. It must be pointed out that when determining the size of a certain section of equipment in the system, loose density should be used and should be as close as possible to any given time state of the bulk material at that location in the system.

2 porosity.

There is a certain difference between the density of any given bulk particle and the loose density. Generally speaking, the loose density in all states is about half of the particle density value. As the loose density support depends on the particle density, shape, and the stacking and filling state of the bulk particles, the method of associating these factors is as follows: the porosity is equal to the total volume of bulk divided by the void space in the three materials

3. Liquidity.

Shiny has a wide range of volumetric strength, ranging from free flowing to very tight. The position of a specific loose material between these two extreme situations, which represents its fluidity, must be understood from the beginning when designing pneumatic conveying devices, as it affects the form of scattered material devices and system components. The main factors that you want to remove liquidity are particle size and chestnut, particle shape, water content, electrostatic and other factors, as well as temperature and chemical composition.

(2) The particle size exhibits natural attraction and increases with decreasing particle size. A loose material actually remains unchanged in all other aspects, but its beneficial size decreases and its fluidity deteriorates

(2) The particles with regular shapes cannot be filled together to form a mechanical connection, so they will not hinder a powder

The particle moves freely relative to its neighboring particles. Irregular shapes or fibrous particles can be interconnected, thereby increasing the impact of loose materials on each other

Resistance to flow.

1. Easily hygroscopic materials form hydrates on the surface of particles and clump, preventing loose materials from shearing and flowing, such as sugar; (3) Moisture content affects flow performance in the following aspects:

2. Non absorbent materials, the water adhering to the surface of particles will form water bonds to pull the particles together. Therefore, humidification can improve the cohesive ability of loose materials until the highest value, such as sand;

3. Materials that are easy to absorb water, any water content will be sucked into the particles until saturation. This form of water content will not increase viscosity. Once there is too much water, it will appear as surface water content, promoting viscosity.

(4) Static charged bulk materials may be subjected to other processes before pneumatic conveying or during pneumatic conveying, which may cause particles to acquire static charges and result in surface stickiness. In addition, static charges can also increase pressure loss inside the pipeline.

4. Breathability

Loose material is a combination of material particles and voids, and the ability of airflow to pass through these voids is called breathability The permeability of bulk materials can be expressed as the relationship between the apparent gas velocity through a continuous bed and the pressure drop along the flow direction, which is one of the important characteristics for determining the suitability of bulk materials for pneumatic conveying

5. Degassing and gas storage properties

The air contained in fully inflated bulk materials will gradually decrease over time at certain intervals, and this behavior is called "degassing". The ability of loose materials to retain air for a period of time is called gas retention., Gas retention is evaluated based on time, meaning that the longer the degassing time, the greater the gas retention. Degassing is one of the main characteristics of the suitability of bulk materials for dense phase pneumatic conveying.

6. Friction angle

During the flow and pneumatic conveying of bulk materials, there is frictional resistance between particles and between particles and walls, and one of the static and dynamic mechanical properties of bulk materials is the friction angle. In order to more accurately express the characteristics of loose materials, the following four angles have been defined.

(1) The angle of rest (angle of rest, angle of rest) is the angle between the natural slope of the pile and the horizontal plane. Usually, the smaller this angle, the easier it is for loose materials to flow. It is related to particle size and shape.

(2) The angle of internal friction is the arctangent value of the ratio of the shear force acting on a surface of the loose material layer to the vertical force when shear sliding occurs on one side.

(3) The wall friction angle is the friction angle between the bulk layer and the adjacent solid plane

(4) The sliding angle is the minimum angle at which loose materials can slide off an inclined surface. The sliding angle when all the loose materials slide off is usually more than 10 'larger than the angle at the beginning of sliding. For fine powder, due to adhesion, the sliding angle may even be greater than 90. Therefore, in reality, 90% of the sliding angle is defined as the sliding angle.

Generally speaking, the sliding angle is greater than the wall friction angle; For non adhesive materials, the internal friction angle is greater than the wall friction angle, and the resting angle is greater than the sliding angle.

The wall friction angle and internal friction angle have the greatest impact on pneumatic conveying. During dilute phase pneumatic conveying, due to the low material to gas ratio,

The spacing between particles is relatively large, so the internal friction angle can be ignored, and the friction coefficient between particles and the pipe wall is also small. During dense pneumatic conveying, loose particles are densely packed into clusters or plugs, making the effect of friction angle an extremely important factor. The larger the friction coefficient of loose materials, the higher the required conveying pressure, which increases pipeline wear and transportation energy consumption, and the conveying distance is greatly limited. For plug flow dense phase pneumatic conveying, it is required that the internal friction angle of loose materials must be greater than the wall friction angle, otherwise no material plug can be formed and only the moving material bed can be formed. In addition, the larger the internal friction angle of materials, the shorter the material plug can be, thereby reducing the pressure loss of the conveying pipeline, If the wall friction coefficient is small, transportation is easy.

7. Particle size distribution

Due to the fact that bulk materials contain a large number of individual particles, and most cases have a wide range of particle sizes, particle size distribution curves are used to depict the particle size characteristics of bulk materials. Particle size distribution (also known as particle size distribution) is the proportion of the mass of particles of various sizes in the total mass of bulk materials. It is particularly important for pneumatic conveying and is a parameter that can better define the essence of bulk materials. Usually, two types of curve graphs are used for representation, namely cumulative curve graph and fractional size curve graph. The particle size coordinate lines in the graph can be expressed on a linear or logarithmic scale.

Figure 11-2 (a) shows the cumulative particle size distribution curve, which can be plotted based on either a percentage larger than the specified particle size or a percentage smaller than the specified size (i.e. below the sieve). The particle size corresponding to a 50% value is called the "median size" and is one type of average particle size. In addition, there is "boundary particle size", which is composed of a pair of characteristic particle sizes, used to represent the particle size distribution range of the material sample. For example, dio and dgo indicate that 80% of the particles in the bulk material are within the indicated particle size range. However, it should be noted that two types of bulk materials with the same median diameter but completely different particle size distributions [Figure 11-2 (b)] may have very different flowability and storage characteristics. Therefore, both particle size distribution and average particle size are important performance indicators.