Basic theory of pneumatic conveying Principles of air flow conveying in horizontal pipelines
In horizontal pipes, since the direction of air resistance is opposite to the flow direction and perpendicular to gravity, appropriate force must be generated to suspend the fiber medium, or at least make the fiber medium rise intermittently, in order to successfully complete the transportation. In horizontal pipes, there are two forces that make the fiber medium suspend. One is that the Reynolds number in general pneumatic transportation is large and it is in a turbulent flow state. The turbulent flow has a vertical component. The other is that due to the irregular shape of the fiber medium, when the air flow causes the fiber medium to rotate or sink to the bottom of the pipe, the flow velocity above is larger and the static pressure is smaller, and the difference in static pressure between the upper and lower sides generates lift.
During the transportation process, when the fiber medium is suspended, gravity tends to make the fiber medium sink to the bottom of the pipe, which causes uneven distribution of fiber materials in the pipe. When the fiber material sinks to the bottom of the tube, it rubs against the bottom of the tube, slows down, slides or even rolls along the tube wall, and then jumps up again, forming a spiral leisure. This leisure reduces the critical Reynolds number, increases energy consumption and produces stratification. If the flow rate is further reduced, a fibrous deposit layer will be formed at the bottom of the pipe, and the deposit layer will gradually thicken and cause similar surges and blockages.
The airflow speed when the cotton block begins to slide along the horizontal pipe is called the starting speed; during the fiber descent process, the speed when the air resistance and gravity are equal is called the terminal speed; the cotton block is maintained while being transported in the horizontal pipe The speed of airflow in the air is called airborne speed. Generally, the starting speed is close to the terminal speed. The airborne speed can be calculated according to the theoretical formula. When transporting fibers or cotton blocks, the theoretical airborne speed can be calculated according to the Davis formula, or it can be calculated according to the empirical formula:
Airborne speed vs = (2.4～3.7)vt
Where: vs — airborne speed, m/s;
vt — terminal speed, m/s.
Therefore, when transporting in horizontal pipelines, the air flow velocity should be larger than that in vertical pipelines.
In horizontal pipeline transportation, the leisure speed of the fiber medium and the relative speed of the fluid have a certain impact on pipeline transportation. In general theoretical analysis, it is believed that the leisure speed of the fiber medium should be close to the air flow speed. In fact, because there are many cotton pieces of different sizes being transported, and the flow rate at each point of the airflow is inconsistent, coupled with the influence of gravity, the cotton pieces collide with each other and rub with the pipe wall on all sides, which reduces the leisure speed of the fiber medium. . According to theoretical derivation and practical summary, the conveying speed and mixing ratio of several materials are shown in the table below.
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