Although in the majority of cases straight centrifugal treatment is the only method necessary, it has been found that with certain types of oils, particularly when used in certain types of engines, improved results can be obtained by mixing the oil with an equal volume of water at 180°F. and thoroughly stirring at this temperature for a period of about half an hour. The oil should then be slowly run through a De Laval, the oil and water in the tank being thoroughly mixed during this process so that, as far as possible, oil and water in equal proportions are passed through the machine. In certain cases it has been found that practically all the carbon in the oil is then carried out with the water and passes out of the water spout of the De Laval. The oil issuing from the clean oil spout will then be perfectly fit for re-use.

(3) ;Special Decarbonising Treatment plus Centrifugal Purification

The processes mentioned in paragraphs 1 and 2 above produce oils which are perfectly fit for re-use, but which are sometimes dark in colour compared with a sample of the unused oil. If for anv reason it is desired to produce a purified oil having the same colour as the unused oil, this can be done by the use of a chemical plus Centrifugal treatment. To the oil at 180°F. a definite quantity of De Laval decarbonising solution is added, and, after thoroughly stirring, the mixture is run through the machine. With this treatment it has been found possible to remove all traces of carbon from certain classes of used oil and to restore the oil to its original colour and to free it entirely from all solid and liquid impurities."

Fig. 135 shows the construction of a De Labal oil purifier bowl. It is in this part of the machine that separation and purification take place. All parts shown in the figure are rotated by means of a spindle at about 9,000 revs, per minute in the case of small size machines and about 6,000 revs, per minute in the case of the larger. The centrifugal force generated by revolving the liquid in the bowl causes the heavier particles to travel in a direction at right angles to the axis of rotation. The action of the bowl will be clearly understood by reference to the arrows shown in the figure and from the following description.

Fig. 135.

Assume that the mixture to be separated consists of water, oil and finely divided sand. This is fed into the top of the bowl at the part marked " Oil to be purified." As it passes down the centre of the bowl, it quickly acquires the same speed as the bowl and when it reaches the bottom, it passes outwards, as indicated by the arrows. The liquid then rises between holes punched in the conical discs. These discs separate the liquid into a large number of very thin films and it is between these discs that purification takes place.

Under the action of the centrifugal force, the sand and water, being heavier, will move outwards, while the oil, being lighter, will move inwards. When the heavier particles have moved a short distance, about ^ of an inch, they strike the underside of one of the conical discs and are then kept separate from the upward stream of lighter particles. Under the action of the centrifugal force, the heavier particles, on coming into contact with the underside of one of the conical discs, are forced down the disc and on reaching the edge are thrown off into the sediment space.

If the bowl is assembled for use with a water seal, as shown on the right hand side of Fig. 135, the water separated from the mixture displaces some of the sealing water which then flows out of the water spout of the machine. If the bowl is assembled as a clarifier, i.e., without a water seal, as shown on the left hand side of the figure, the water has to be retained in the sediment space of the bowl. In both cases the sand collects in the sediment space, from which it is removed, when the bowl is taken to pieces for cleaning. The oil, being the lightest component of the mixture, passes inwards, flowing along the upper side of the conical discs.

By means of the special construction employed in all De Laval bowls, the two streams, i.e., the upward stream of purified oil and the downward stream of water and sand, are kept separate, thus assuring a high degree of purification without re-mixing.

The purified oil, when it reaches the top of the cones, passes through suitable ducts into the cylindrical portion marked " Purified Oil." From this part of the machine, it is discharged into the collecting tray and flows from the clean oil spout of the De Laval.

In connection with the above the following additional notes may also be of interest :-

There are two different methods of separation :-

A. Purifying.-The question here is to split an original liquid formed of two separate ones of different specific gravities, and these two liquids-being separated by centrifugal force in the bowl.-require to be discharged continuously from their individual outlets. The original liquid may contain quantities of impurities, and these are also separated by centrifugal force and are caught in the sediment space of the bowl.

The two separated liquids flow from the bowl at different distances from the axis of rotation : the heavier liquid comes out through the centre hole of the gravity disc, and the lighter one through the openings in the neck of the top disc (see illustration on page 227).