Physics behind gravitational and centrifugal sedimentation and how the relative centrifugal force (RCF) varies with particle size, shapes, distance from rotational axis, mass and the rpm. This will explain the root of today's modern centrifuge systems extensively used in cleaning fuels and lubricants especially in industrial, marine, power and energy sectors.

1. Fixed Angle and Variable Angle Centrifuge:
In VARIABLE ANGLE centrifuge particles can be seperated based on size by changing RPM and the radius of
rotation to increase RCF. This method is also called differential centrifuge
Fixed Angle and Variable Angle centrifuges are commonly
used in laboratory. FIXED ANGLE centrifuge can separate
particles from liquid but it is not possible to separate
particles based on their size.
In centrifugal sedimentation, particles are separated by centrifugal force generated by rotating the liquid
Physics Behind Gravitational and Centrifugal Sedimentation
Development of todays modern centrifugal filters /separators
Apart from filtration, two types of sedimentation process used to separate suspended solids from liquid.
One is gravitational sedimentation and another one is centrifugal sedimentation.
Gravitational Sedimentation:
In gravitational sedimentation, the particles are settle down by the gravity force.
Factors governing the gravitational sedimentation rates are as below.
> Gravity Force
> Particle Size and Shape
> Particle Mass
> Liquid Density
> Liquid Velocity
> Liquid Temperature
Unfortunately, earth's gravitational force is not in our control and we cannot accelerate the sedimentation
rate under normal contition. However, we can increase the settling time to get the desired result. A fixed
volume of liquid containing suspended solids when kept undisturbed under earth gravitational field, after
some time the suspended particles will begin settle down if the gravitional force acting on the suspended
particles becomes larger than the bouyance force.
Presented By: Md. Moynul Islam | Email: mdmoynul@yahoo.com | Cell: +8801816-449869
Centrifugal Sedimentation:
Suspension of lime
powder in water
It took 2 days to achieve
this degree of clarity
Clarity After 24 hours
g
g
FIXED ANGLE CENTRIFUGE
mmi@tekasai.com www.tekasai.com Page 1 of 5

2. Relative Centrifugal Frorce (RCF):
d 500 mm
rpm 120 rpm
RCF 80.64 times
Centrifugal force 80.64 times larger than gravity force
In VARIABLE ANGLE centrifuge particles can be seperated based on size by changing RPM and the radius of
rotation to increase RCF. This method is also called differential centrifuge
VARIABLE ANGLE CENTRIFUGE
mmi@tekasai.com www.tekasai.com Page 2 of 5

3. In previous section we have seen the sedimentation process es for fixed volume of liquid. Now we will see
how to remove particles continuously from liquid and factors gcontrolling sedimentation rates
mmi@tekasai.com www.tekasai.com Page 3 of 5

4. mmi@tekasai.com www.tekasai.com Page 4 of 5

5. 0.00
4.43
8.87
13.30
17.73
22.17
0.00
3.57
7.13
10.70
14.27
17.83
0.00
8.00
16.00
24.00
32.00
40.00
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
0 500 1000 1500 2000 2500
Solids Removed by Centrifuge (kg)
Solids Retained in System (kg)
Total Solids Generated (kg)
Current Turn Rate: turns per hour
Change Turn Rate to see its effects
3.5
mmi@tekasai.com www.tekasai.com Page 5 of 5