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1、 Pneumatic conveying is usually divided into dilute phase conveying and dense phase conveying
The gas source equipment of dilute phase conveying system is usually roots blower, and the conveying pressure is relatively low. Due to the small solid gas ratio, high wind speed and large amount of gas-solid separation, the materials are easy to break, the pipeline is easy to wear, and the energy consumption is high. The gas source of dense phase transportation system is usually compressor, which can be divided into non plug type and plug type according to the flow state of the materials transported in the pipeline.
Plug pneumatic conveying is usually sand dune or plug flow conveying. Large solid gas ratio and low gas velocity can transport more materials with less gas, and the wear of materials is small, and the crushing rate is relatively low. The material flow state of non plug and plug dense phase pneumatic conveying is usually divided into pulse gas knife type and automatic plug forming type. Compared with pulse gas knife type plug conveying system, self plug forming conveying system has the following advantages
1. The material flow rate is low, the friction with the pipeline is smaller, and the material crushing rate is low; 2. The automatic bolting equipment is simple, the wearing parts are few, the maintenance rate is low, and the system operation is safe and reliable; 3. The continuous transportation and automatic operation are realized, and the operation cost is low;
The polymer particles after cutting, drying and screening need to be transported to the finished product bin for storage and packaging. In order to reduce the phenomenon of damaged wire drawing, the self forming bolt dense phase pneumatic conveying scheme is generally adopted.
2、 Self forming dense phase pneumatic conveying system
1. The working principle of dense phase transportation is as follows:
(1) The material continuously enters the gas material mixing acceleration chamber through the rotary discharge valve, and the opening of the control valve at the compressed gas inlet of the conveying pipeline is adjusted by the signal of the pressure transmitter, so as to adjust the pressure and flow of the conveying pipeline.
(2) When the material at the blanking place is fully filled with the conveying pipeline, the flow of compressed gas will increase, and the material at the blanking place of the rotary valve will be transported instantly, thus forming a plug flow one by one. The conveying pressure is generally 0.2 ~ 0.3MPa, which varies according to the distance and the size of the solid gas ratio.
(3) The material in the conveying pipe mainly moves forward depending on the static pressure difference between the two ends of the bolt, which has the characteristics of low speed, dense phase and low dynamic index.
2. Design points of main equipment and piping
(1) Rotary valve
The standard rotary valve is generally used in the place with low pressure air lock, and the working pressure is about 0.03 PA, while the working pressure of the self forming plug dense phase conveying system is generally 0.2 ~ 0.3 MPa, so the rotary valve must be high pressure rotary valve, and the maximum pressure can reach 0.6 MPa.
According to the experience of similar projects in the past, if the rotary valve is not selected properly, it may often lead to overload, poor feeding and other faults, which will affect the stable operation of the device; it may lead to large air leakage and high energy consumption of the system; it may also lead to breakage and wire drawing of granular materials, which will affect the product quality.
Therefore, The high-pressure rotary valve is one of the key equipment to successfully realize the dense phase transportation of self forming plug. The rotary valve is to evenly and continuously add the material into the gas material mixing acceleration chamber connected with it according to a certain speed, and also bear the role of locking gas. In this way, the dense phase transportation gas will not leak from the impeller of the rotary discharge valve and take away a large amount of gas, which will affect the normal transportation and increase the gas consumption 。
The maximum working pressure of the rotary valve can reach 0.6MPa. The shaft end and top plate of the rotary valve shall be equipped with the cleaning device of the shaft and end plate. The design of the valve includes the tangential feeding and anti shearing design of the non coincidence nozzle specially designed for the granular material conveying system. The valve shall have the characteristics of high efficiency, low power consumption, low maintenance rate and low wear rate. The bearing of the rotary valve is generally self-lubricating external design, which can effectively avoid the damage of dust to the bearing; the clearance between the rotor and the shell of the rotary valve is generally not more than 0.2mm, and small clearance can effectively reduce the air leakage of the product and reduce energy consumption.
(2) Directional valve
Generally, there are two types of directional valve, i.e. flap type and rotary plunger type. The valve body and rotor shall be closely matched, without material accumulation inside and without moving parts outside the valve body. It is required that the reversing valve should not be stuck, blocked or not in place in the process of reversing.
(3) Gas mixture
In the accelerating chamber, the material entering the volume is fully mixed with the gas, so that the material can obtain a certain movement speed and energy and move along the pipeline.
(4) Air control
Unit air control unit is the core equipment used to realize the pneumatic conveying system of rotary valve, which includes pressure regulating valve, flowmeter and lafar throttle. Through the real-time monitoring of the conveying pressure of the conveying system, adjust the amount of gas into the conveying system, so as to realize the stable conveying pressure and ensure the stable conveying of the system. Generally, the terminal speed is used as the set value of flow, the flow detected by the flowmeter is used as the measured value, and the opening of the regulating valve is adjusted by the controller.
(5) Piping
The pipeline should be as short as possible, and anti blocking measures should be taken to prevent pipeline blockage. If the pipeline route is short, the energy consumption for material transportation is small, and the elbow of the pipeline is worn the most. Try to choose materials with smooth inner wall and not easy to wear, such as polyethylene, ceramics, etc., which can not only reduce the wear of the elbow, but also ensure the quality of the material.