Schroeder overcomes the churning by having a, jacketed pan fitted with a set of revolving solid metallic discs strung upon a square shaft, and fixed about 6 in. apart. The apparatus has the additional advantage of cheapness, but the heat derived from the steam-jacket requires to be supplemented by a coil of steam-pipe winding between the discs, which constitutes an evil.
Bour, observing that larger grains of sugar are produced on the discs in Wetzel's pan than on the pipes, concluded that hollow eteam-heated disca would increase the evaporating surface, and produce better grain. A front elevation of his pan is shown in Fig. 1364 ; and vertical and transverse sections of the disc on an enlarged scale in Fig. 1365. a, is the steam-engine ; 6, exhaust-pipe to beat the revolver ; c, revolver consisting of 10 copper discs ; d, copper pan for holding the liquor under treatment, and discharged by the valve e at bottom; f, pipe for carrying off the condensed water from pan ; g, pipe for carrying off air and uncondensod steam; h, safety-valve. The discs are mounted on an axis which allows the steam to communicate freely with them, at the same time collecting the condense-water and carrying it off at one end. Inside each disc are 2 spoons k, running from the extreme diameter and terminating in the axis, into which the water Is delivered. Outside tbe discs c, are small buckete i, which lift the liquor as the discs move round, and spread it'as a thin film over the surface which is not immersed. The speed of the revolver is 10-20 rev. per minute. Where steam is plentiful, equally good sugar is produced by the quicker speed, and nearly double the work is performed in the same time. One pan cooks 12 cwt. of sugar per hour, frorn 20° B., as taken from the battery, the temperature never exceed ing 77° (170° F.). The distributing-cups churn the liquor excessively.
One of the most recent modifications is Pontifex's, shown in Fig. 1366, The pan a contains the liquor to be evapomted, within which revolves a coil of steam pipe b. Thus a large heating-surface is obtained, without tho drawback of churn ing up the liquor.
It ie to be observed that all these forms of film evaporator aro destined only to finish tho conceutration begun iu the battery. The liquor is brought to them at a density of 26°-27° B.
d. Vacuum-pans.—The principles controlling the boiling of juices in vacuo, and the details of the construction of vacuum-pans and their accessories, have been already given under Beet-sugar (see pp. 1856-7).
Figs. 1367-1370 show vaeuum-pans as used on nearly all larg,e sugar plantations. The grain formed from syrups boiled in vacuo is larger aud more solid than that from syrups simply concen trated to crystallizing-point in open batteries. A Cuban hogshead will contain only 1600 lb. of sugar made in a copper-wall, but 1800 lb. of vacuum-pan sugar. By the use of the vacuurn-pan also, the planter is enabled to boil his molasses, and to extract from 1 gal. some 4-5 lb. of sugar, still having a second molasses for the distillery.
Working the Vacuurn-pan.—The air-pump is started, and so soon as the vacuum reaches 26-27 in., the feed-cock on the side of the pan is opened, and sufficient liquor is drawn in to com pletely cover the first coil ; steam is next turned in, and the liquor rapidly concentrates ; fresh supplies are admitted at short intervals, the feed-cock being opened say for 15 aconds at a time, until the mese commences to show " grain." The grain is fed carefully, the cock being opened frequently, aud each time the quantity adulated is increased. As the amount of sugar in the pan continues to augtnent, steam is turned into the 2nd and 3rd coils, until, at tho completion of the charge, the pan is nearly full, or just below the sight-glass. In this way, the grain " grows" in size. On the conclusion of the boiling, the vacuum is destroyed, and the charge is run out into a tank, and allowed to stand for au hour or two, when a further crystallization takes place.
It is customary to draw in as much syrup as will cover the bottom coil (when reduced by concentration), called " graining low •down." Some prefer to grain higher ; some when the pan is half-full. An objection to graining high is that the grain has not so much time to grow, but it does not always hold good. A pan taking 7 hours to boil a strike of 8 tons of mas.,c-euite (concentrated juice) grained low, will only take 6 hours if grained highcr. The crystals in the second case will not be so large, but, in an 8-ton pan, they will be of fair size, even by the quicker method. The drawing-in is conducted thus :—The charging-cock is opened, and shut off again as soon as the liquid boils up to the "bull's-eye " on the opposite side. The contents quickly boil down ; the cock is opened again, and shut off as before when the liquor boils to the same height. This is kept on until the syrup intended to form gmin has been taken in : roughly speaking, 2000 gal. of good 18°-20° B. syrup to a 5-ton pan is about the correct amount.