As stated above, if a liquid has mixed with it another liquid or a solid of different specific gravity from the liquid itself, separation of the substances will take place if the mixture is left undisturbed. This separation of two immiscible liquids, or of a solid from a liquid, is due to the difference in the force of gravity on the materials separated. Gravity settlement, as it is called, has been practised in arts and manufactures since time immemorial. In gravity settlement, the force acting on the particles which have to be separated is very small and the distance through which the particles have to move often comparatively large.

Centrifugal purifiers are machines in which a separating force is produced which is thousands of times greater than the force of gravity and in which the distance through which the particles have to move is considerably less than in a settling tank. A Centrifugal Purifier is thus similar in principle to gravity settlement, but on account of the force being greater and the distance less the time required for separation is very greatly reduced. Whereas liquids have often to be left in settling tanks for many weeks or even months, by means of a Centrifugal Purifier separation can be effected almost instantaneously, thus enabling continuous operation to be obtained.

A Centrifugal Separator can effect a very high degree of separation and purification, provided that there are not two or more liquids present, soluble in each other, and that there is a difference in their specific gravities. With modern types of these machines, a solid can be separated from a liquid, or two liquids from one another (provided these are not soluble in one another) when the difference in their specific gravities is only 1 per cent. This when duly considered will be admitted to be a very high degree of separation indeed. These Centrifugal Separators are used for such purposes as purifying lubricating oil from Internal Combustion Engines, also from Steam Engines, Insulating Oils, Fuel, Oils, Varnishes Dehydration of Tar, etc., etc. Messrs. Alfa-Laval Co., Ltd., state that the oil used for the lubrication of Internal Combustion Engines becomes contaminated with impurities quicker than the lubricating oil in any other prime mover.

Impurities found in these engines can be classified under three main headings :

Carbon

In all Internal Combustion Engines, except very large engines of the crosshead type, the lubricating oil becomes contaminated with carbonised oil from the cylinders and pistons.

In the case of large engines of the crosshead type where conditions are such that the lubricating oil may not become contaminated with carbon, it has been found that the piston cooling oil sometimes becomes badly carbonised, and the removal of this carbon from the cooling oil is just as important as the removal of the carbon from the lubricating oil.

It has been proved by experience that when carbon is first formed in an Internal Combustion Engine, the particles are relatively large in size. If means are provided for the removal of the particles as soon as possible after they are formed, the majority of the carbon can be extracted before it becomes dispersed in the oil in a colloidal form.

If, however, the carbon particles are allowed to become broken up, they then form a very active emulsifying agent which may render the subsequent purification of the oil more difficult.

Mechanical Impurities

During its continuous circulation through the lubricating or cooling system, the oil picks up particles of dust, dirt, grit, rust, scale, etc., and as these impurities are abrasive in nature, they cause rapid wearing of the moving parts. On account of the dangerous nature of these impurities, it is desirable that they should be removed from the oil as soon as possible.

Water

Many Internal Combustion Engines are water-cooled, in which case there is often a possibility of slight leakage of the cooling water into the lubricating oil.

In all Internal Combustion Engines, steam is formed as the result of combustion of the fuel. As the piston rings are never perfectly tight, there is always the possibility of a slight leakage of steam into the crank chamber. When this steam comes into contact with cooler surfaces, it condenses into water which then finds its way into the lubricating oil.

Based on many tests which have been made on oils taken from the crank case of such engines, the presence of free water in large or small quantities has been established.

Water in the oil of Internal Combustion Engines not only reduces the lubricating properties of the oil, but also, in the presence of colloidal carbon, may result in the formation of emulsion. Some of these emulsions are very thick jelly-like substances, and are thus liable to cause restriction or stoppage in the lubricating system.

Batch System

In the case of a number of small engines where it is not a practical proposition to instal a separator with each, and in certain other cases, it has been found that satisfactory results are obtained by treating; the oil on the Batch System.

For treating the oil on this system, as large a quantity as possible of the oil is drained from the engine at suitable intervals and run into an oil heating tank. The temperature of the oil should then be raised to about 180°F. and its purification effected by either of the following methods :

(1) Straight Centrifugal Method

The oil at a temperature of about 180°F. is run slowly through a De Laval. Experience has shown that with certain types of oils, this simple treatment enables practically all the carbon to be removed as well as all abrasive matter and water.

(2) ;Water Washing Plus Centrifugal Purification