By ROBERT F. GIBSON.
Besides separating the crystalline sugar and the sirup, secondary objects are to wash the crystals and to pack them in cakes. The cleansing fluid or "white liquor" is introduced at the center of the basket and is hurled against and passes through the sugar wall left from draining. The basket may be divided into compartments and the liquor guided into each. The compartments are removable boxes and are shaped to give bars or cakes or any form desired of sugar in mass. These boxes being removable cannot fit tightly against the liquor guides, and the liquor is apt to escape. This difficulty is overcome by giving the guides radial movement or by having rubber packing around the edges.
Sugar machines proper are of two kinds - those which are loaded, drained and then unloaded and those which are continuous in their working. The various figures preceding are of the first kind, and what has been said of vibrations applies directly to these.
The general advantages claimed for continuous working over intermittent are - that saving is made of time and motive power incident to introducing charge and developing velocity, in retarding and stopping, and in discharging; that, as the power is brought into the machine continuously, no shifting of belts or ungearing is necessary; and that there are less of the dangers incident to variable motion, either in the machine itself or the belting or gearing. The magma (the mixture of crystalline sugar and sirup) is fed in gradually, by which means it is more likely to assume a position of equilibrium in the basket.
There are two methods of discharging in continuous working - the sugar is thrown out periodically as the basket fills, or continuously. In neither case is the speed slackened. In the first either the upper half of the basket has an upward motion, on the lower half a downward motion (Pat. 252,483); and through the opening thus made the sugar is thrown. Fig. 22 (R.B. Palmer & Sons) is a machine of this kind. The bottom, B, with the cone distributor, a, have downward motion.
Continuous discharge of the second kind may be brought about by having a scoop fixed to the curb (or casing), extending down into the basket and delivering the sugar over the side (Pat. 144,319). Another method will be described under "Beet Machines."
The construction of the basket is exceedingly important. Hard experience has taught this. When centrifugals were first introduced, users were compelled by law to put them below ground; for they frequently exploded, owing to the speed being suddenly augmented by inequalities in the running of the engine or to the basket being too weak to resist the centrifugal force of the overcharge. Increasing the thickness merely adds to the centrifugal force, and hence to the danger, as even a perfectly balanced basket may sever.
One plan for a better basket was to have more than one wall. For example, there might be an inner wall of perforated copper, then one of wire gauze, and then another of copper with larger perforations. Another plan was to have an internal metallic cloth, bearing against the internally projecting ridges of the corrugations of the basket wall. A further complication is to give this internal gauze cylinder a rotation relative to the basket.
The basket wall has been variously constructed. In one case it consists of wire wound round and round and fastened to uprights, commonly known as the "wire basket;" in another case of a periphery without perforations, but spirally corrugated and having an opening at the bottom for the escape of the extracted liquid; in still another of a series of narrow bars or rings, placed edgewise, packed as close as desired. An advantage of this last style is that it is easily cleaned.
The best basket consists of sheet metal with bored perforations and having bands or flanges sprung on around the outside. The metal is brass, if it is apt to be corroded; if not, sheet iron. The perforations may be round, or horizontally much longer than wide vertically. One method for the manufacture of the basket wall (Pat. 149,553) is to roll down a plate, having round perforations, to the required thickness, causing narrowing and elongation of the holes and at the same time hardening the plate by compacting its texture. Long narrow slots are well adapted to catch sugar crystals, and this is not an unimportant point. Round perforations are usually countersunk. Instead of flanges, wire bands have been used, their lapping ends secured by solder.
As to comparative wear, it maybe remarked that one perforated basket will outlast three wire ones.
As to size, sugar baskets vary from 80 inches in diameter by 14 in. depth to 54 by 24. They are made, however, in England as large as 6 feet in diameter - a size which can be run only at a comparatively slow speed.
A peculiar complication of basket deserves notice (Pat. 275 874). It had been noticed that when a charge of magma was put into a centrifugal in one mass, the sugar wall on the side of the basket was apt to form irregularly, too thick at base and of varied color. To remedy this it was suggested to have within and concentric with the basket a charger with flaring sides, into which the mixture was to be put. When this charger reached a certain rotary velocity, the magma would be hurled out over the edge by centrifugal force and evenly distributed on the wall of the main basket.
The spindle as now made is solid cast steel, and the considerations governing its size, form, material, etc., are identical with those for any spindle. In order that the basket might be replaced by another after draining, the shaft has been made telescopic, but at the expense of stability and rigidity. In Fig. 16 is shown a device to avoid crystallizations, which are apt to occur in large forgings, and would prove fatal should they creep into the upper part of the spindle proper in a hanging machine. It consists of the secondary spindle, c.