The settling velocity and suspension speed of the pneumatic conveying system
When the grain materials moves in the airflow, its settling velocity and suspension speed are its most basic properties. When the spherical object with a diameter d fall from the stationary state free in the air, due to the effect of gravity, the falling speed will become faster. At the same time, the object under the resistance of the air also will increase gradually. The object weight of G and in the air by the buoyancy R and resistance P and balance according to the following relations, namely: G - P = R.
Objects will sink at the same speed of V because of the inertia effect. This velocity is called the settling velocity. Set the object's settling velocity at V and put it in the well-distributed airflow at the vertical upward velocity. Then its absolute speed will be V material=V- V sink.
At this point, if the V=V sink, then the object's absolute speed will be zero, namely the object will remain at the same place, neither rise nor decline. Usually at this time the airflow speed is called the suspended velocity (V suspended). The object's suspension speed and settling velocity are equal on the figure, namely the V suspended =V sink.Thus, when the object is at the airflow greater than its suspension speed, the object will be pushed by the air.
In the vertical pipe, the air power and the material gravity are at the same straight line. To make the material flow in the same direction with the airflow, the air velocity must be greater than the material suspension velocity. Therefore, the suspension speed is the proof to confirm the air velocity of the pneumatic conveying.However the material movement in the pipeline is very complex, influenced by all kinds of factors;In the mean time, the shape of the material to be delivered are often highly irregular, so the actual suspension speeds of various materials need to be measured by experiments.
In a horizontal pipe, because the air dynamic direction is perpendicular to the gravity direction of the material particles, therefore,the suspension and the motion state are much more complicated. When choosing the air velocity, usually it is still on the basis of the suspension velocity in the vertical pipe.
In the actual pneumatic conveying pipeline, due to the friction, collision of material between each other and with the wall and the uneven airflow in pipeline such various reasons, the actual suspension speed of air velocity is greater than the material needed
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