In the purification of fuel, centrifugal force is the fundamental principle of operation. Centrifugal force is force that is exerted upon a body or substance by rotation. Centrifugal force impels the body or substance outward from the axis of rotation.

A centrifugal purifier is essentially a container which is rotated at high speed while contaminated fuel is forced through, and rotates with, the container. However, only materials that are insoluble in the fuel can be separated by centrifugal force. For example, JP-5 or naval

Figure 9-6.-Disc-type centrifugal purifier (DeLaval).

distillate cannot be separated from lubricating oil, nor can salt be removed from seawater by centrifugal force. Water, however, can be separated from fuel because water and fuel do not form a true solution when they are mixed. Furthermore, there must be a difference in the specific gravities of the materials before they can be separated by centrifugal force.

When a mixture of fuel, water, and sediment stands undisturbed, gravity tends to form an upper layer of fuel, an intermediate layer of water, and a lower layer of sediment. The layers form because of the specific gravities of the materials in the mixture. If the fuel, water, and sediment are placed in a container which is revolving rapidly around a vertical axis, the effect of gravity is negligible in comparison with that of the centrifugal force. Since centrifugal force acts at right angles to the axis of rotation of the container, the sediment with its greater specific gravity assumes the outermost position, forming a layer on the inner surface of the container. Water, being heavier than fuel, forms an intermediate layer between the layer of sediment and the fuel, which forms the innermost layer.

The separated water is discharged as waste, and the fuel is discharged to the service tank or the day tank. The solids remain in the rotating unit. Separation by centrifugal force is further affected by the size of the particles, the viscosity of the fluids, and the time during which the materials are subjected to the centrifugal force. In general, the greater the difference in specific gravity between the substances to be separated and the lower the viscosity of the fuel, the greater will be the rate of separation.