The difficulty of boiling dense liquids is well known. The cause of this difficulty is the lessened ability of the vapourized water to overcome the pressure of the atmosphere, normally amounting to about 15 lb. a sq. in. By relieving the liquid of this pressure, the " boiling " (i.e. the driviug-off the watery vapour) can be effected at far lower temperatures, reducing the consumption of fuel, and lessening the danger of burning the liquor. To apply these principles to the con centration of sugar-syrups, the various forms of vacuum-pan have been introduced, in all of which the boiling proceeds in vacua.
The first step with beet-syrup is to boil the watery liquor in a " double-effect " or " triple effect" apparatus till it marks 25° B., then known as " thick juice." It next goes to a cistern where it is heated to boiling, and is again filtered through animal charcoal, by which more colouring matter is removed, as well as some albuminous bodies that are more readily absorbed from dense than thin liquors. After this second filtration, the juice is brilliant, transparent, and almost colourless, but still contains much water. This is finally removed by boiling in vacua.
In the matter of concentration, the treatment of beet-sugar and cane-sugar are precisely similar ; but there are a few variations in the apparatus, the forms employed in the cane-sugar industry being largely of English manufacture, while those used in beet-sugar factories are essentially Con tinental.
The vacuum-pan and its accessories are shown in elevation and plan in Figs. 1321 and 1322. The pan a is mounted en a east-iron framing, carried by 8 cast-iron columns. Boarding or iron plates form a staging round the pan. The pan is fitted with thermometer b, vacuum-gauge c, sight glasses d, proof-stick e for extracting samples of sugar, slide f for discharging sugar, cock g to admit steam to clean out the pau, and arm-pipe and receiver k to catch any sugar that may boil over. The receiver is fitted with delivery-cock and air-cock for destroying the vacuum when necessary. The condenser i is fitted with a perforated pipe and stop-cock, a lever, and an index-plate, to regulate the supply of water for condensing the vapour from the pan ; j is the measure for regulating the supply of syrup, fitted with stop-cock and inlet-pipe from the filtered-juice tank, a glass gauge to indicate the quantity of syrup, an outlet-pipe with stop-cock opening into the pan, and an air-pipe having a cock communicating with the pan for forming a vacuum in the measure. k is a valve for the supply of steam from the expansion-vessel to the
worm of the pan ; /, a pipe for carrying off condensed water from the steam-coil of the pan to the condense-box m, which communicates by a pipe with a brick tank from which the feed-water is supplied to the boiler ; n, air-valve mounted on the air-main, for regulating the communication between the air-pump and the pan ; o, dividing-box for distributing the flow of air from the pan to the air-pumps ; p, two 16-in. air-pumps, 1 ft. 9 in. stroke ; q, 10-H.P. high-pressure beam engine fitted on diagonal frames, with 11-in. cylinder, 3 ft. 6 in. stroke, and 12-ft. diam. fly-wheel with 6-in. elliptic rim, 4-in. plunger feed-pump, stop-cock, clack-box, copper air-vessel, and feed water supply-pipe r to a Cornish boiler ; s, pipes for supply of steam to the engine and pan through the expansion-vessel ; t, sluice-cock for regulating the supply of steam to the mercurial regulating valve u, by which the supply of steam is regulated to the expansion-vessel v, fitted with a whistle valve and safety-valve, to prevent excess of pres sure in the worm of the pan ; the steam passes from the expansion-vessel through the pipe to to the stearu-valve h, which regulates its admisssion to the pan.
The pan is shown in section in Fig. 1323. The copper pan a is fitted in a cast-iron steam-case b, with steam-space left between, and is surmounted by a copper dome c. The copper and iron pans and the done are bolted together through their flanges with a wrought-iron ring and bolts so a,s to be air- and steam-tight throughout. A man-hole d, with a ground gun-metal oover, is attached to the top of the dome, from whioh proceeds the arm-pipe opening into the receiver 7i. A steam valve k opens into the copper steam-worm y. This worm gradually diminishes in diameter from the entrance-point at the steam-valve to the exit at the bottom of the pan. A wrought-iron pipe x is fitted into the cast-iron pan b, to carry off the water from the steam-case ; the slide-valve x at the bottom of the pan is for discharging the sugar. The dome of the pan is mounted with n vacuum-gauge, thermometer, " sight-glass," and " proof-stick " for testing the concentration of the liquor.