The design method of each parameter of open screw conveyor
The various parameters of the screw conveyor are determined according to the actual conveying requirements, including the screw speed, screw outer diameter and pitch. When determining these parameters, the following requirements need to be met:
The conveying capacity of the conveyor must be greater than the total amount of incoming materials; the material cannot jump and roll perpendicular to the conveying direction; the material moves forward in the axial direction; the material has the largest possible axial conveying speed, and the axial speed at each point must be greater than Peripheral speed.
The design method of each parameter is as follows:
(1) Screw speed
When the spiral speed is low, the influence of the blocked material flow on the material movement is not significant. Once the spiral speed exceeds the critical speed, the material will jump and roll perpendicular to the conveying direction, which plays the role of stirring the material instead of pushing it axially. At this time, the transportation efficiency is reduced, the energy consumption is large, and the equipment is accelerated. In order to avoid this phenomenon, the spiral speed cannot exceed its critical speed. The empirical formula for the critical spiral speed is:
A is the comprehensive characteristic coefficient of the material, D is the outer diameter of the spiral. Under the premise of meeting the requirements of conveying volume, the spiral speed should not be too high.
(2) Spiral outer diameter
When other parameters are determined, the curve of the axial moving speed Vz and circumferential speed Vt of the material in the trough with the spiral radius r of the material is shown in the following figure:
Speed vs. radius curve
Analysis of this curve shows that when selecting the spiral outer diameter, in addition to meeting the requirements mentioned at the beginning of the article, it should also be ensured that the selected outer diameter is on the right side of the line B-B in the figure. For open screw conveyors, if the outer diameter of the screw is too large, it will increase manufacturing costs and is not conducive to cooling. In addition, the value of the outer diameter does not have to be an integer multiple of 500.
According to the curve of speed versus radius under different pitches, it can be seen that there is a corresponding relationship between the selection of the spiral pitch and the outer diameter of the spiral. The larger the pitch, the larger the corresponding outer diameter of the spiral. After the outer diameter of the spiral is determined, there is a minimum pitch corresponding to it, that is, a more suitable pitch.
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