Conveyor differential speed

The scroll rotates slower than the bowl at a differential speed of 1/20 to 1/160 of the bowl speed this differential speed causes translation of the solids along the bowl. Particularly, soft solids can be conveyed with low conveyor differential speeds should higher differential speeds cause resuspensions. Units will have g forces up to 6000 and range in diameter from 6" to The solids discharge outlet is usually smaller than the liquid... 

The scroll usually is carried on a hollow axial hub, through which the slurry feed tube passes to the feed zone. The diameter, the number, and the pitch of the conveyor flights are chosen to match the needs of the slurry being treated — as are the depth of the pond, the length of the bowl, the conveyor differential speed, and the angle of slope of the beach. 

The back-drive system is a means of controlling the speed of the gearbox pinion shaft (and thereby the conveyor differential speed) using, for instance, a motor or a brake. This could be offset from the gearbox shaft, in the same manner as the main drive, and connected by a belt. This belt would be a timing belt because of the accurate control required. Normally the back-drive is connected directly and in line with the gearbox pinion. 

The difference in rotational speed between the bowl and the conveyor is commonly referred to as the conveyor differential speed, N. Conveyor differential speed is calculated from a knowledge of the rotational bowl speed, S, the gearbox pinion speed, Sp, and the gearbox ratio, Rob ... 

Test data may indicate that the size of test machine is adequate for the duty envisaged. Alternatively, the machine tested would be too big, which is not usual. More usually the test data have to be scaled to a larger size of decanter. When the data need to be scaled to another decanter size, other calculations may need to be performed for each run, such as conveyor torque/volume, feed rate/Sigma, wet solids rate/conveyor differential speed, as well as the Sigma value itself if the data involve changes of bowl speed. As will be seen, these intermediate calculations help with the scale-up. 

From equation (4.70) it will be seen that torque is proportional to feed rate and inversely proportional to differential. Therefore, the data of Figure 6.6 should be re-plotted as torque against feed rate/conveyor differential speed. This is done in Figure 7.3. Here it will be seen that most of the data, for the lowest two differentials tested, form a straight line 1.1 kNm/(m /h/ rpm diff). 

Power for each Hquid and the soHd phase must be added to get Pp. P, the soHds process power, = T -AN for scroU decanters, where = conveyor torque and AN = differential speed between bowl and conveyor. Pp is the friction power, ie, loss in bearings, seals, gears, belts, and fluid couplings. P, the windage power, = K and fi = viscosity of surrounding gas p = density of gas D = rotor outside diameter N = rpm and K = shape... 

A new 2-gear-drive was developed which accurately controls the differential speed between bowl and conveyor screw by precise electronic torque control. This new drive instantly accounts for fluctuations in the feed liquid and adjusts the differential speed between bowl and conveyor screw for optimal sludge concentration. This new control device also improves the overall performance of the decanter (WESTFALIA SEPARATOR (10)), [2, 18-19]. 

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