Fig. 1. How the filtration works

Fig. 1 shows diagrammatically how the filtration works.  The centrifugal force jams the sand particles against the sides of the tapered holes, restraining them.  However, the oil has no trouble passing through the filter.  This phenomenon is similar to a one-door room full of people and rats.  If all the people try to leave at once, they will jam up at the door and no one can leave.  The rats, on the other hand, can easily escape between the legs of the people.  

Fig. 1. How the filtration works.

          © Ben DEMAYO 2017