# Centrifugal Purifiers Basic Principle and Working

Before going to the centrifugal purifier we will discuss basic gravity separation.

Gravity separation

Consider a tank having a mixture of water, sludge, oil dirt etc, over a period of time solid particles settle at the bottom and oil at the top. Which means lighter liquids will be at the top and heavier liquid will be at the top.

Separating Force is defined by Stoke’s law as : Fs = π/6 × D³ (ρw – ρo) × g

In gravitation separation, g is responsible for separation speed, which is fixed.

Centrifugal Purifiers

Centrifuging is the process by which the effects of gravity can be amplified by the use of centrifugal force to the extent that the separation process becomes rapid and continuous.

Centrifuges work by rapidly spinning a bowl containing the liquid, thus producing the required centrifugal force to produce separation, so we can replace the gravitational force (g) to centrifugal force (ω² r). So the separating force is defined  Fs = π/6 × D³ (ρw – ρo) × ω² r

The principle of operation of the centrifuge is simple. When a bowl containing impure fuel is rotated, centrifugal forces will throw any item with density greater than the fuel oil density (solids and free water) to the periphery of the bowl.

Working of Marine Purifiers

The centrifugal purifiers or separator consists of a bowl containing a set of discs stacked one on top of the other. The dirty oil flows down the distributor and up through the holes in the discs.

After passing down the central passage, the untreated oil is carried by centrifugal forces towards the periphery of the bowl and then passes up through the disc stack. Here is where the actual separation takes place, in the channel formed between two discs. Two forces act on each solid or liquid particle. The particle is pushed upwards with the oil stream towards the center while the centrifugal force directs it to the periphery. The residual force on denser particles (impurities) will drive them towards the periphery, while the less dense particles (oil) will be directed towards the center of the bowl and raise to the outlet connection. Separated heavy liquid continuously going to the sludge tank through heavy liquid outlet and light liquid get separated out to service tank. In particular intervals Bowl opens and sludge will be ejected from the bowl.