one Calculation of pneumatic conveying parameters
Before conducting calculations, it is necessary to know the properties of the material (including true density, bulk density, particle size, and other physical and chemical properties, etc.), as well as the requirements of the conveying process (including horizontal length, lifting height, number of bends, design conveying capacity, etc.).
The properties of materials determine what conveying form they are suitable for, for example, from the characteristics of titanium dioxide, it can be known that titanium dioxide is suitable for dilute phase conveying but not for dense phase conveying.
After determining the conveying method, the conveying solid gas ratio (i.e. conveying material mass flow rate: conveying gas mass flow rate) can be selected, and then the conveying pipe diameter, conveying pressure, conveying gas volume and other parameters can be determined according to the conveying process requirements.
Under the determined requirements of the conveying process, the larger the diameter of the conveying pipe, the lower the conveying pressure, and the higher the conveying gas volume.
The designed conveying capacity of titanium dioxide for a certain project is 5000kg/h, with a maximum horizontal distance of 65 meters and a vertical height of one5 meters. The conveying elbows are determined to be 6. The local atmospheric pressure is set at one0onekPa, and the gas temperature is simplified to be constant at 20 ℃. The following process is used to determine other conveying parameters:
1) Assuming a solid to gas ratio of 5, the calculated gas volume for transportation is 1three.9 standard cubic meters per minute (at one atmosphere and 20 degrees Celsius);
Solid gas ratio refers to the mass ratio of solid materials to the transported gas during pneumatic conveying. This ratio is crucial for the design and operation of pneumatic conveying systems, as it affects the system's conveying capacity, energy consumption, pipeline wear, and other aspects. Generally speaking, the higher the solid to gas ratio, the less gas is required to transport materials of the same quality, which can reduce energy consumption and pipeline wear. However, it may also increase system pressure loss and material breakage rate.
Gas volume refers to the volume or mass flow rate of gas transported during pneumatic conveying. The size of the gas volume directly affects the conveying capacity and energy consumption of the pneumatic conveying system. When designing a pneumatic conveying system, it is necessary to determine the appropriate air volume based on factors such as the properties of the material, conveying distance, conveying method, pipeline diameter, and pressure.
2) The resistance loss between the dust collector and exhaust pipe is taken as Δ Pjx=three000 Pa.
three) Assuming the conveying pipe diameter is DN125 (inner diameter is 125mm) and the pressure at the outlet of the conveying pipeline is threekPa gauge pressure, the conveying gas velocity at the outlet of the conveying pipeline can be calculated to be 18.4m/s. Based on experience, this gas velocity is appropriate.
4) The resistance loss of stable transportation can be calculated using the following formula
Pe is the outlet pressure of the pipeline,=104000 Pa
M is the solid gas ratio,=5
K is the drag loss coefficient,=1 (this coefficient is between 0.5 and 1.5 and is related to various factors)
λ a is the friction coefficient of the pipeline, DN125 pipeline,=0.03
Leq is the equivalent length of the pipeline, which is 190m
The equivalent length of a pipeline is the length of a vertical pipeline, elbow, other fittings, and valve converted to a horizontal pipeline. Typically, a 1m vertical pipeline is calculated as a 2m horizontal pipeline to determine the equivalent length. For powder, the equivalent length of an elbow is 10m, and for ball valves, the equivalent length is 5m
D is the inner diameter of the pipeline,=0.125m
ρ e is the outlet air density,=1.24kg/m ³
Ve is the outlet air velocity,=18.4m/s
Calculated: Δ Pm=42561 Pa
5) The acceleration resistance loss of materials, formula omitted, ΔPac=1478 Pa
6) The resistance loss of the feeder, formula omitted, the resistance of the rotary valve Δ PN=1330 Pa
7) The above resistance is added together to obtain the conveying resistance loss Δ P=48368Pa=48.4 kPa
8) When using a Roots blower, it is also necessary to calculate the resistance loss at the inlet of the exhaust pipeline of the Roots blower, which is usually small and omitted here.
9) When selecting gas source machinery, the calculated resistance value needs to be multiplied by a margin factor of 1.2-1.5, and the calculated gas volume value needs to be multiplied by a margin factor of 1.1-1.2.
After calculating the transportation resistance loss, it can be determined whether the selection of the transportation pipeline is appropriate. The exhaust pressure of the single-stage Roots blower is below 98kPa, and the calculated resistance value is within this range. Therefore, the pipeline selection above is correct. For the compressor, this value is too low, so the pipeline can be selected smaller.
The above calculation is based on the positive pressure conveying of the rotary valve discharge. If a Venturi injector is used under the rotary valve, further calculation of the resistance of the Venturi injector is required.
The basic form of a Venturi is to have a channel that first narrows and then expands, with the smallest part called the throat. The airflow velocity at the throat is the highest, usually around 200m/s, even close to the speed of sound. According to the Panoulli formula of energy conservation, if the airflow velocity is high, the gas pressure will be low. Therefore, through design, the gas pressure can be reduced to be equal to or slightly lower than atmospheric pressure at the throat, so that the material can flow freely into the throat because there is no leakage in the opposite direction of the feeding direction. In the expansion section, the rapid decrease in gas velocity is converted into pressure to push the material. The structure of the Venturi injector/feeder is simple, often due to the free flow of small particles in short distance transportation. However, Venturi also has great limitations. In the diffusion section, only about one-third of the kinetic energy is converted into pressure energy to transport materials, and its own resistance loss is significant. This involves complex thermodynamic calculations, which will be skipped here. However, it can be qualitatively said that if we want to achieve atmospheric pressure at the throat, the resistance of Venturi itself is approximately equal to the total conveying resistance of the subsequent conveying pipeline. Returning to the above calculation results, we can know that if we want to achieve this effect, the total resistance loss should be 96.8 kPa. That's why Venturi is not used for long-distance transportation. The farther the distance, the greater the resistance loss of the pipeline, and the higher the resistance loss of Venturi itself.
Using Venturi conveying, the electrical power consumption is usually twice that of ordinary positive pressure conveying. Of course, if the pressure at the throat of the Venturi does not need to drop to equal or lower than atmospheric pressure, its own resistance will not be as high, but this deviates from the purpose of using it.
Based on the above description, the conclusion can be drawn that in order to achieve a conveying capacity of 5t/h, the positive pressure conveying of Roots blower with rotary valve can choose DN125 pipeline and use compressed air bin pump for conveying. Pipelines smaller than DN125 can be selected, and the conveying method of Venturi must choose pipelines larger than DN125, and the energy consumption is much higher than that of positive pressure conveying.
2 Selection of dust removal equipment
The specifications of the dust collector are related to the processing air volume and the selected filtering air speed. The specifications of the dust collector can be distinguished by the filtering area, and the filtering air speed=processing air volume (m ³/min)/filtering area (m ²). Note that the unit of filtering air speed is m/min.
The selection of filtering wind speed is mainly related to the material of the filtering element, the dust concentration of the inlet gas, the properties of the dust, and so on. For situations where the dust concentration in the inlet gas of workshop dust removal is low, the filtration wind speed of bag filter can be selected as 1-3m/min; For pneumatic conveying applications, due to the extremely high dust concentration at the inlet of the dust collector, the filtration wind speed needs to be reduced. Bag dust collectors usually choose 0.7~0.9m/s, and filter cartridge dust collectors usually choose 0.5~0.7m/s. If the material dust is fine and sticky, this value should be taken as the lower limit.
For the above calculation, the gas volume is 13.9Nm ³/min. The pressure at the dust collector is close to atmospheric pressure, and the temperature is assumed to be 20 degrees Celsius. Therefore, the processing gas volume of the dust collector is basically 13.9m ³/min. The bag filter requires a filtration area of 20m ², and the filter cartridge filter requires 28m ². A 20m2 bag filter requires 50 bags with a diameter of 125 and a length of 1m, or 25 bags with a diameter of 125 and a length of 2m.
A foldable filter cartridge with the same volume has a filtration area 3-5 times larger than a bag filter, but it has the disadvantage of difficulty in cleaning dust in the folding seams. Therefore, for the same air volume, the filtration area of the filter cartridge dust collector should be larger than that of the bag filter. In the application of titanium dioxide dust removal, shallow bottomed pleated filter cartridges should be selected to reduce the difficulty of dust cleaning.
3 Comparison between Rotary Valve with Roots Fan and Warehouse Pump with Compressor
1) Using a rotary valve and Roots blower, the conveying pipeline, switching valve, and dust collector are large, while using a bin pump and compressor requires a smaller size However, the cost of the compressor system is much higher than that of the Roots blower, so the cost comparison between the two should be similar, or the compressor scheme should be slightly higher.
2) From the power of the gas source machinery, the Roots blower scheme may be more energy-efficient. The reason is that titanium dioxide cannot be transported in a dense phase. Under the design conditions of this project, compressed air is used for transportation, and the transportation resistance is only about 0.15~0.2 MPa. The exhaust pressure of the compressor is 0.7 MPag, which is much higher than the exhaust pressure of the Roots blower that is compatible with the transportation pressure drop. Another clear point is that compressed air is required for pneumatic valves, dust collectors, arch breakers, etc. For Roots blower solutions, a set of compressed air systems is also needed.
3) From the perspective of its impact on product quality, the compressor solution can easily remove water and oil, providing higher quality conveying gas than the Roots blower solution.
4) From the perspective of system adaptability, the higher the exhaust pressure of the Roots blower after compression, the more likely the exhaust is to reach supersaturation, while titanium dioxide is very fine, and moisture will cause trouble for transportation. The post-processing system of the compressor can provide dry and clean conveying gas. But when the system unfortunately gets stuck, the compressor scheme can use higher pressure to remove the blockage, while the Roots blower does not have this convenient condition. For this project, the transportation distance is relatively long, especially in the case of many forks, and the situation of material blockage should be fully considered.
