Pneumatic conveying, also known as air flow conveying, is a technology that uses the power of air flow to transport granular and powdery materials along the direction of air flow in a closed pipeline. Due to the use of sealed conveying pipelines instead of traditional mechanical conveying of materials, pneumatic conveying has the characteristics of good flexibility, fewer vulnerable parts, low maintenance costs, small footprint, and no dust flying problems. At the same time, it can also perform physical or chemical operations such as heating, cooling, drying, and airflow classification of materials during the conveying process. It is currently the preferred choice for powder and particle conveying and is widely used in chemical, plastic, rubber, and other industries Powder transportation in industries such as food, electronics, pharmaceuticals, steel, environmental protection, and power generation.

How to choose suitable pneumatic conveying equipment? Consider based on one's own process specifications and the characteristics of the equipment type. There are various classification methods for pneumatic conveying systems, which can be divided into dilute phase conveying and dense phase conveying according to the flow pattern of materials during pneumatic conveying, or into negative pressure conveying and positive pressure conveying according to the mode of airflow during conveying. The scope of application varies for different types of pneumatic conveying systems. This article will explore how to select pneumatic conveying equipment based on different application requirements.

Dilute phase transportation and dense phase transportation

The flow shape of materials has a significant impact on the efficiency and cost of conveying. In pneumatic conveying, the mixing ratio, conveying concentration, or solid gas ratio R is commonly used to represent the solid concentration in the airflow. According to the ratio of materials and airflow in the pipeline and the characteristics of two-phase flow, pneumatic conveying systems are divided into dilute phase conveying (suspension flow conveying) and dense phase conveying. Choosing the appropriate conveying method can effectively improve production efficiency and reduce transportation costs in different application fields.

Dilute Phase Convey

Dilute phase pneumatic conveying, also known as suspension conveying, is essentially a low-pressure, high-speed material conveying process. The principle is relatively simple, mainly relying on the kinetic energy formed by the high-speed air or inert gas (usually 18-30m/s) provided by the Roots blower and other gas source equipment, to continuously supply the materials from the feeding hopper to the storage bin behind the rotating feeder. High velocity air causes materials to be uniformly distributed in vertical pipelines and suspended in horizontal pipelines, and transported continuously over distances of up to 300m. In the process of dilute phase transportation, the ratio of material to air is very low, and the solid gas ratio R is usually between 1-5.

Commonly used gas source equipment for dilute phase transportation - Roots blower

It can be said that high flow rate is a typical characteristic of dilute phase conveying systems, but it is a double-edged sword for material conveying, which can be reflected in the following aspects:——

advantage:

1. Due to the relatively simple nature of the dilute phase system, there are not many open valves, few vulnerable parts, very low initial investment costs, easy maintenance, and relatively cheap prices.

2. After years of development, the technology has become relatively mature, and it can transport a wide range of materials.

3. The conveying speed of dilute phase pneumatic conveying is fast, which can be applied to high flow conveying and improve production efficiency.

Disadvantages:

1. At high speeds, it is easy to cause material breakage, resulting in loss of transported particles, so it is not suitable for sensitive and fragile materials.

2. In terms of energy consumption, higher conveying speed means higher electricity bills.

3. The requirements for airflow are relatively high, and it is necessary to control the speed and pressure of the airflow well in order to achieve good conveying effect.

Dense Phase Convey

Dense phase transportation is relative to dilute phase transportation, and the gas-solid ratio during the transportation process is usually greater than 25. The material is no longer uniformly distributed in the pipeline, but forms a plug or sand dune shape. Unlike dilute phase transportation, which relies on gas acceleration, it mainly relies on the pressure difference before and after the material plug to achieve material transportation. The airflow velocity is usually 8-15m/s, and the material flow velocity is a high concentration ratio, high mixing ratio, low flow velocity transportation state. The transportation distance is usually over 500m/s.

To a certain extent, the dense phase conveying method allows the conveying speed of materials to be much lower than the suspension speed, thus avoiding the conventional problems caused by high material speed in dilute phase pneumatic conveying. Its main advantages include:

1. A lower flow rate has a lower degree of wear on pipelines and materials, and has almost no impact on material quality;

2. Due to high work pressure, long-distance transportation can be achieved; The use of boosters can achieve ultra long distance transportation;

3. The gas consumption is small, the conveying gas volume is small, the separation of material and gas is easy to achieve, and it is also more energy-efficient.

4. High degree of automation, microcomputer control, simple operation, sensitive response, and complete processing functions.

Although dense phase transportation can solve some problems caused by high flow rates in dilute phase transportation, it has many equipment valves, pneumatic and electric equipment, and the cost is mainly concentrated on these valves. Therefore, it is not recommended to use dense phase transportation systems for transportation distances below 300 meters. In addition, long-distance transportation also requires the installation of special pipeline supports and additional steel structures to compensate for pipeline forces, which invisibly increases some initial costs.

Pneumatic conveying method

Inspiratory pneumatic conveying

Inhalation pneumatic conveying, Also known as negative pressure delivery. Generally, a vacuum pump is connected at the end of the system pipeline, and the front section extends into the feeding bin through a suction nozzle. The typical suction pneumatic conveying system is shown in the figure. When the vacuum pump is started, The air inside the system pipeline is sucked away, creating a pressure difference at both ends, The powder is sucked into the conveying pipe under conditions below atmospheric pressure (negative pressure), Finally, it is transported to the separator by the airflow, where the material is separated from the air. The separated material is discharged from the rotary unloader at the bottom of the separator, while the unseparated fine powder particles enter the dust collector for purification with the airflow. The purified air can be discharged into the atmosphere.

This system is relatively convenient for feeding materials, Suitable for feeding from multiple points to a single point, it can connect several feeding bins at the same time for feeding. At the same time, due to the negative pressure inside the pipe, it can avoid safety hazards caused by material leakage during the conveying process, It is a relatively environmentally friendly and safe method of powder transportation. It can generally be used to transport materials that cause significant environmental pollution or are toxic, Such as coal powder, fertilizers, etc.

However, due to the significant pressure loss and energy consumption along the pipeline as it grows, The requirement for pipeline sealing is high, and it is not suitable for long-distance transportation.

02

Compressed pneumatic conveying

Pressure fed pneumatic conveying, also known as positive pressure conveying, includes various types of pneumatic conveying devices such as low-pressure pressure feeding, high-pressure pressure feeding, fluidized pressure feeding, and pulse plug flow pneumatic conveying devices. However, their overall structure and principles are similar, Generally, the system air inlet pipeline is connected to air source equipment such as blowers, The blower sucks in air through the air inlet and compresses it to form a high-pressure airflow, At the same time, the material is quantitatively fed into the high-speed airflow, and high-pressure air flows along the conveying pipeline, Feed animal materials along the pipeline to the end of the system, Finally, the material is separated in a separator or storage bin and discharged into the bin through a discharge device. The compressed air is then purified by a dust collector before entering the atmosphere.

Due to the fact that the gas source is located at the front end of the system, materials cannot freely enter the feeding pipe, and a sealed feeding device must be used. The structure is complex and cannot be fed from multiple points to a single point like the suction system. However, it is suitable for conveying lighter materials to one or several silos by setting up multiple collection devices. Therefore, the conveying distance is long, the capacity is relatively high, and the production efficiency is high, The requirements for pipeline sealing are relatively low, making it a commonly used powder conveying solution.

Hybrid, also known as vacuum pressure conveying, It has both positive and negative pressure in the same conveying system, The specific working principle is that the air inlet of the system is connected to the material separator (as shown in Figure 2), The air source equipment (as shown in Figure 4) acts as a negative pressure vacuum pump, which can suck materials from multiple feeding bins (as shown in Figure 5) into the separator. After the materials are sent into the separator with the airflow, they are separated from the air and then discharged through the lower unloader (as shown in Figure 3) and sent into the pressure feeding conveying pipe. The separated air enters the air source equipment through the air pipe (as shown in Figure 4, acting as a negative pressure fan), and is compressed into high-pressure airflow by the blower before entering the conveying pipe, driving the material unloaded by the unloader to be sent into the storage bin, and finally the air is discharged through the filter.

Due to its combination of the advantages of suction and compression conveying methods, It can achieve multiple material pick-up and multi-point unloading, And the conveying distance is longer than the above two methods, which can be applied to more complex conveying. But the overall system structure and changes in airflow within the pipelines are more complex, The energy consumption is also relatively high.