1 introduction
Using pneumatic conveying system to transport dry bulk granular material basically no restrictions on the ability,almost all material can be delivered through high flow and long-distance transportation. But in fact it also subjects to some restrictions. The main limitation is that as a transmission medium, the gas is compressible. These parameters mainly limit the economy scale and the power demand. Conveying capacity relies mainly on the five main parameters, pipe diameter, transmission distance, effective pressure, gas velocity and material performance. Most of these parameters can be predicted.Now, the conveying theory has not yet fully understood.
1.1 pipe diameter. The pipe diameter mainly influence the material flow. If you need to increase the material flow,you should increases diameter regardless of other parameters. Increasing the pipe diameter, its cross-sectional area also can be increased, which is equivalent to increase the transmission capacity of the material.
1.2 Conveying distance is the same as the liquid and the gas single-phase flow.The pipeline pressure loss is proportional to the distance, long-distance conveying, particularly the high material flow transportation tends to be in high pressure.The transmission length exceeds to be more than a mile, breaking through the limit method of other sections of this manual leaves to be discussed, this section is only about the basic principle.
1.3 Effective pressure Although air and other gas can be compressed to high pressure, it often does not have the high pressure transmission.That is because the compressed gas volume and flow increases unceasingly, and the pressure is decreasing. With more than 2000 LBF/in2 (13.79 MPa) single-way of hydraulic conveying material can be more than 70 miles for its delivery. The reason is that the water is incompressible, the hydraulic is under long distance transmission, the speed change is not important. Mentioned in the first chapter, using the pneumatic conveying, the gauge pressure is about 15 LBF/in2 (0.1 MPa) the air pressure is usually attributed to high pressure. For example, releasing 15 LBF/in2 air to the standard atmospheric pressure,its speed is 2 times of the straight pipe's. Although by piecewise expanding the pipe diameter making the air inflation changed to some extent.The calculation procedure is quite complicated. So the air gage pressure reaching at the 100 lb/in2 (0.67 MPa) receiver is at standard atmospheric pressure.The pneumatic conveying system for conveying material is not common. Using the pneumatic conveying system to convey material must maintain at a certain pressure.While using 300 lb/in2 (2 mpa) high air pressure is not common. It is rarely to see the critical high back pressure conveying, if any,you need the pipe segment. Based on the section to design the long-distance pneumatic conveying system.
1.4 conveying gas velocity Blower,compressor, or negative pressure of the pneumatic conveying system can be introduced the volume flow parameters in addition to the air source pressure. Although the conveying air velocity, especially the conveyor line inlet velocity or picked up speed decide the pneumatic conveying design parameters. Under the single pipe diameter,regardless of the positive pressure or negative pressure conveying system, the wind speed of the feeding point of the front pipeline is always the smallest.
At the end of the pipeline in a single pipe diameter,it has the maximum speed.The minimum air speed decides the key to run the pneumatic conveying system.Of course, by the multiplication of air volume flow rate and pipe cross-sectional area, The proper use of velocity must be considered, as shown in the next chapter "air demand quantity".
The minimum speed of the conveyor largely depends on the nature of the bulk material transportation and the transportation way. Typical dilute phase conveying speed is about 3000 ft/min (about 15 m/s). The relation between the conveying velocity and particle size, shape and density will be discussed. The minimum speed of dense-phase conveying is about 600 ft/min (3 m/s).The powder can use dense-phase conveying powder. The minimum air velocity is related to the material-air ratio or the solid fill rateThis chapter will give you a detail description.
Dilute-phase conveying particles suspended in the air has a relatively high value. The density between the particles and air has a huge difference between.Typical liquid's suspension conveyor speed is only 300 ft/min (about 1.5 m/s), The density difference of water and grain is very small, water and air density difference is about 800:1 Conveying medium speed is about 10:1.From these we can see that the air pressure and density is not the main influencing factor in the pneumatic conveying gas velocity of the minimum value in general.
The nature of the material is an important parameter of the pneumatic conveying system.It decides the minimum speed of the traditional dilute phase conveying system.Most materials have different conveying parameters.Its apparent features are the important factor of the material nature.
Although the nature of the bulk material, like particle size, particle size distribution, particle shape, shape distribution, material density are important, and some natures can be known by measuring in time, including the gas-solid interaction, like gas and air permeability. In general,those materials which have good air permeability or save gas are more suitable for the conventional low-speed dense-phase conveying system.If the material neither with well gas reservation nor good permeability ,is limited to the dilute phase conveying.
1.5.1 dense-phase conveying
There are two kinds of low-speed dense-phase conveying device. the material with good air reservation tends to be of fluidization conveying, flowing like water in the tube bottom in a horizontal pipe and less gas in the material. When the solid filling rate is about 150, the pipeline is approximately full. The dense phase conveying has obvious pulse flow, material flows after a second or two to a sudden stop, and then flows smoothly, it is also for pulse in the vertical upward mobility, and again to stop the flowing material falling back to the bottom of the vertical pipe at any time.The material has good air permeability, it tends to form the embolism by pipeline, embolization is the inner diameter of the pipeline, and is separated into a short gap. Such a conveying air velocity is decreasing, along the pipeline the gap is filled with the material. The bolt is eventually moved like a wave in the way of fixed bed along the top. The air quantity reduction will lead to the conveying gas air flow speed decrease and the material flow will also be reduced.
It is completely composed of a single large particles of the material, such as polyethylene, nylon pellet, peanuts, some grains and seeds, which is very suitable for plug flow. For dilute phase conveying, the nylon, polymer are damaged due to the need of high-speed transmission,Due to the high air permeability, the air will easily penetrate into the material. The maximum filling rate of the material conveying is about 30.
2 characteristics of the materials conveying
A mark of success is a pneumatic conveying system design to meet the running requirements, which reaches at the maximum output. You must correctly handle the conveying material's characteristics. Transmission characteristics will tell the material conveying speed, the most suitable for material conveying speed, pipe diameter, the ratio of air flow and material flow and transmission distance to the designer.
In other words, a running pneumatic conveying device, its reasonable transport parameters will tell the designers what is the expected flow, whether it is suitable for transporting different materials, whether the gas flow is satisfied. The transmission parameters can also be used to check whether the conveyor device is the most optimized, the running is satisfied or not, the pipe diameter and parameters of the compressor are appropriate for the existing conditions.
If before this,you have enough experience for the material, and the material conveying parameters have been established, then you can design on these basis. If there is no experience for material and no enough complete scientific research, you must test on the material delivery. This scheme will provide us with the material flow and gas flow ratio, conveyor line pressure loss, amplifying the conditions and the data needed to complete the material transfer.
This experiment can provide the material minimum conveying air velocity and influencing conditions of conveying. The huge difference between the dense-phase conveying and dilute-phase conveying conveying parameters are especially important.
The test scope should include the material transfer mode. When the past experience is rarely available, you increase the air pressure, pipe diameter, transmission distance, using existing data to predict pipeline parameters in proportion. If you cannot increase the transmission model proportionally,then you can deduce the scale of long distance, high solid filling rate and the minimum conveying air velocity. Don't put it into practice suddenly unless you can prove that the material is like this conveying capacity.
High pressure is commonly seen in the dilute-phase conveying. If the materials are suitable for dilute-phase pneumatic conveying mode. the material natures will affect the delivery form.; the material flow is different under the same transmission condition. The successful design of material conveying which have not tried before, the conveying test is very important. So there must have the additional high pressure air in the conveying trials to set up the transmission line and a very wide range of conditions.
In addition to the material nature, the conveying distance is also an important parameters. For solid filling rate, what kind of material can be delivered and the way of conveying can directly influence the pressure gradient and limit the transmission capacity. More transmission performance is seen in chapter 8.
2.1.1 pure air information
In order to illustrate how to apply the transfer parameters.You must first establish and investigate the main parameters affecting the delivery. Figure 4.1 is a picture of pressure loss and air flow diagram considering the air through pipeline
It indicates that the material flow is 0 lb/h.
As you can see in figure 4.1 , the air pressure loss increases significantly with the increase of air flow. When joining pipe materials, its pressure will increase as well under a given air flow rate. This is because the results of air resistance when the particles going through the pipeline. It also should have a high enough air speed to convey the material, or the particles can't be transported and gradually block the pipeline.
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