Conveyor torque

Conveyor torque, T, is a measure of the force exerted by the conveyor in moving the separated solids through the bowl, up the beach and out of the decanter. It equals the pinion torque, T, times the gearbox ratio  

It is not easy to measure conveyor torque directly, whereas pinion torque can usually be obtained from instrumentation on the pinion braking system. This signal is often given to a PLC for control purposes. Conveyor torque is a vital measure in the control of modern decanter systems. 

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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... 

The stress resulting from the axial forces must be considered in analy2ing the parts which resist axial separation, such as bolts, nuts, rings, etc. On scroU centrifuges the axial force owing to the axial component of the conveyor torque must also be considered. 

In some old designs a beach is not used at all. A simple flat rear hub is used with cake discharge holes. The conveyor is tapered as if there were a beach, and the cake then forms its own beach. This design can work reasonably well with stiff cohesive cakes, but very high conveyor torques can result. 

For both designs the conveyor torque equals the pinion torque times the gearbox ratio whatever the differential. 

With large diameters, large conveyor hubs are possible allowing high conveyor torques. Generally these decanters have been used where clarification is easy and where high solids volume or torques are required,... 

A typical positive rake angle of a conveyor flight would be 1°. Sometimes this is automatically provided by the design of the tile wear protection. The reasoning behind such a raked flight is that it wilt tend to lift the cake from the bowl and in so doing reduce conveyor torque. Thus, this would not be used on soft cake/low torque applications but more on torque-producing, stiff, cohesive cakes. 

It has just been shown that the perft)rmance of a dry solids decanter is related to conveyor torque achievable, bowl speed, pond depth, and Uoccuiant usage. Once the cake dryness has been fixed, it is useful to be able to assess the maximum capacity possible on a given decanter. 

Time for particle to traverse decanter Thickness of bowl shell Conveyor torque Motor torque... 

The decanter needs to be steadfastly mounted, level, on a firm base, and sufficiently high to allow good access to couple up discharge off-take facilities. Within the decanter s own start-up and control gear usually will be instruments for continuously monitoring the bowl speed, the brake speed, and the brake or conveyor torque. Ideally the conveyor torque or differential will be automatically controlled by a simple PLC controller. 

Light/heavy phase diameter (three-phase) Conveyor torque ... 

Figures 6.5 and 6.6 show the effect of conveyor differential and feed rate on conveyor torque. From these graphs it can be appreciated that increasing feed...

Figure 6.5. Gr p/i-Conveyor Torque v Feed Rate-Spent Grain.

Early decanters using eddy current brakes were not able to indicate conveyor torque continuously. A reading of brake speed, and another of brake current, had to be recorded and used when referring to a brake calibration chart, to obtain the brake torque. Conveyor torque could be obtained by multiplying this figure by the gearbox ratio. A typical eddy current brake calibration chart is shown in Figure 7.1. 

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. 

While the majority of applications will be scaled by Sigma, there are occasions when high conveyor torques are experienced during test work. Then it will be necessary to scale the torques experienced, to the proposed larger machine, not only to ensure that this will not be limiting but also to estimate the power required for the drive motor. 

In Figure 7.5 test data of cake dryness are plotted against torque/volume. It shows that a torque/volume figure of 2.0 N/cm will be sufficient to produce a cake of 30%. Thus, for no. 1 machine, conveyor torque ... 

It should be noted that if drier cakes will be obtained in the future with development of equipment, techniques or chemicals, the conveyor torque will increase. Reserve gearbox torque capacity will permit such improvement and also result in longer gearbox life. 

Figure 7.5. Dryness I s, Conveyor Torque Volwne Ration...

This concludes two very satisfactory scale-ups. The scale-ups have shown that the required dryness can be achieved with both of the two larger machines at the desired capacity. The calculations have shown what conveyor torques and what differentials would be needed on each machine. 

Conveyor torque today is an essential part of decanter control. However, direct reading of conveyor torque is very difficult to achieve. Even direct reading of pinion torque is difficult, but could be done using strain gauges on the pinion shaft. However, the most usual method is to use a calibration of the braking device. The control device for the brake will give a read-out, on request, of the braking torque. 

The conveyor torque developed by the heel after feeding and the bowl has emptied, apart from the heel... 

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