Purification of oil (lub or fuel) is nothing but the separation of impurities like water and sludge from oil. This separation is done on the basis of density difference. Denser substance goes lower by gravity force and lighter substance goes up, and this is the basic fact behind gravity separation. Separating Force is defined by Stoke’s law. In gravitation separation, g is responsible for separation speed, which is fixed.Now if we replace the gravitational acceleration g with centrifugal acceleration (ω² r) then we can increase the separation force by increasing angular velocity and effective radius. Thus if settling tank is replaced by a rotating cylinder the separating force and hence the speed of separation can be increased. This is called centrifuge.
