PRINCIPLES OF SIZING

Hydraulic sizing (process)
The sizing of the diffusion drum is based on the following parametres: • influent flow rate • piping diametre The water coming out of the diffusion drum to the sedimentation zone must have a very low kinetic energy in order to guarantee a good separation and settling of the solids.
Moreover, the bottom scrapers must have a continuous and logarythmic profile in order to successfully convey the sludge into the central well in less than a complete rotation of the shaft, 270° (¾).

Mechanical Sizing
The sizing of the bottom scraper is based on the type of sludge to be evacuated from the tank bottom. Normally the central shaft and the scrapers are designed to bear a stress of 20 Kg. per linear metre. The torque to transmit is calculated by 
T [kgm] = r² x K
r è is the tank radius 
K is the load on the scrapers per linear metre (20 kg for biological clarifiers)
For a reliability purpose, the drive unit shall be capable of transmitting a 1.8 higher torque than the design value.

Torque limiting device (dynamometric cell) 
The dynamometric cell measures the torque transmitted by the drive unit.
This value is evident directly on the instrument and allows to evaluate the stresses on the trestle during the rotation.
The device is equipped with limit switches set at two different torque values, one for alarm and one for the machine shutdown.

Selection of the electric motor
The electric motor size is selected so that, in case of shutdown, its static torque is lower than the one which can be borne by each downstream unit. For this reason the electric motor is usually very small. Exuberant motors may cause, in case of shutdown, serious damages to the structures if the torque limiting device fails to intervene or if it is not properly connected. 

Example of calculation

Tank diametre 20 m.
K = 20
Scrapers peripheral speed 1,5 m/min
Slow shaft rpm 0,023
Torque r² x K = 10² x 20 = 2000 kgm 
so the drive unit shall be designed to transmit a 1.8 higher torque.

Electric Motor

To transmit a torque of 2000 kgm at 0.023 rpm, 64 watt are necessary. If we hypothize a drive unit output of 60%, the electric motor shall have a power of 107 watt.
Normally it is difficult to find electric motors which are smaller than 0,18 kw - 180 watt.
If the selected motor has got a power of 180 watt, in case of shutdown it might give a 2.6 times higher power for a few seconds, i.e. 468 watt.
The torque transmitted by the motor to the torsion trestle, with a drive unit output of 60%, would be equal to 5700 kgm.
In this case it should be evaluated whether to install a shear pin or a dynamometric cell to limit the torque.