[Fig. 1087.] shows the structure of a single vacuum-pan. The horizontal diameter of the copper spheroid A, is not less than 5 feet; the depth of the under hemisphere is at least 18 inches from the level of the plane; and the height of the dome-cover is 2 feet. The two hemispheres (of which the inferior one is double, or has a steam-jacket,) are put together by bolts and screws, with packing between the flanges to preserve the joints tight against atmospheric pressure. The jacket of the lower hemisphere forms the case of the steam, which communicates heat to the syrup enclosed in the inner hemisphere. In general, the pans contain, when filled to the flange, 100 gallons of syrup, and yield about 11 cwt. of granulated sugar, at every charge.
A, represents the vacuum spheroid; B, the neck with the lid. From the side of B, a pipe passes into the lower extremity of the bent pipe C, D, which terminates in the vertical pipe E, connected with the vacuum main-pipe K, proceeding horizontally from the air-pump (not shown in the figure). At the top of E, a valve, movable by a screw H, is placed for establishing or cutting off the connexion with the air-pump at pleasure. Behind F, is the measure cistern, from which the successive charges are admitted into the pan. This measure is filled with the clear syrup, by opening the stopcock I, on the pipe under the ceiling, which communicates with the filter-cistern placed above. G is the valve or plug-hole, at the bottom of the pan, for discharging the granulating syrup. This plug is opened by means of a powerful lever attached to it; the connexion with the air-pump being previously intercepted. L, is the barometer, or manometer, for showing the state of the vacuum corresponding to the temperature. N, N, is a cistern-pipe for receiving any little syrup which may accidentally boil over the neck B. Its contents are let off by a stopcock at its bottom from time to time. M shows the place of the proof-stick, an ingenious brass rod for taking out a sample of syrup without admitting air. See [infrà].
The charging-cistern contains about 20 gallons. This quantity of syrup being first admitted, and brought to a certain pitch of concentration, a second measure is introduced, the inspissation of which is supposed by some refiners to cause an agglomeration of saccharine matter round the first crystalline particles. The repetition of this process for two or three times is imagined to produce the large brilliant grain of vacuum-pan sugar. This hypothesis is more specious than sound, because the granulating syrup discharged from the pan is subjected to a heat of 180° or 190° in the subjacent steam-cased receiver, whereby the granulations are again reduced to a very small size. Into this receiver, two or three skippings or discharges of the pan are admitted in succession, and the whole are diligently mixed and agitated by a stirring oar. It is by this process that the granulating tendency is promoted and determined. From this receiver (absurdly enough called a cooler) the moulds are filled in the usual way, by means of copper basins or large ladles.
The case of the under hemisphere of the vacuum-pan is filled with steam, generated under a pressure of four or five pounds on the square inch; the heat of which causes the interior syrup to boil rapidly while the air-pump is kept in action. A small escape-pipe for waste steam must be placed at the opposite side of the case or jacket, to ensure its equal distribution; as also a stopcock below, to let off the water of condensation. The pans are mounted on iron feet, or short pillars, which insulate them from the floor, and allow their whole surface to be inspected, and any flaw to be repaired. The air-pump usually stands in a cold-water cistern, to favour the condensation of the aqueous vapour, which it draws out of the pans; and it is kept in constant action by the steam-engine, being attached to the working-beam of its piston.
[Fig. 1088.] exhibits the general arrangement of the vacuum-pans, and their subsidiary apparatus. Here are shown, on the ground floor, the heaters e, e, (miscalled coolers), into which the concentrated syrup is let down. These heaters are made of copper, in one piece, surrounded with a cast-iron jacket, bolted at the flange or brim to it. Each pan contains, when full, about 350 gallons, equivalent to nearly 35 cwt. of crystallized sugar. They are furnished with steam-cocks and waste steam-pipes. Under the level of the spheroids d, d, the horizontal main-pipe is seen, for supplying the cases with steam. In the face of each pan, above the line b, b, the handle of the proof-stick appears, like that of a stop-cock. The distribution of the measure cisterns, and some other parts of the pans, is slightly varied in this representation from the former. From the bottom of the liquor cisterns C, C, pipes descend to the charging measures a, a, below. The cisterns C, C, are made of copper, and contain each about 400 gallons. Six tons of refined sugar can be turned out daily in a three-pan house.
[Fig. 1089.] represents in section another form of the vacuum-pan, a is the spheroidal copper vessel, supported by four iron columns b, b. It may be discharged by means of the pipe c, which is secured with a conical valve d. This may be opened or shut, by acting on the lever e. The lower of the two hemispheres of which the pan is composed is double, and the interstitial space f, f, is filled with steam by the pipe g, as the heating and evaporating agent. h, is the steam valve; i, the pipe for the efflux of the condensed water. k, a tube for the escape of the air at the commencement of the operation. l, is an apparatus inserted air-tight into the cover of the vacuum-pan, and which dips down into the syrup; serving to take out a sample of it, without allowing air to enter, and hence called the proof-stick. The construction of this instrument is exhibited in [figs. 1091], [1092], [1093], [1094], [1095.], which will be presently explained. m, is the thermometer, which is also plunged into the sugar; behind it, is the barometer. n, is the charger or gauge-vessel, filled with the filtered syrup, which it discharges by the pipe n′. o, is the cover or capital of the vacuum-pan. o′, is a safety-valve, through which the air may be admitted, after the completion of the process. p, is a bent pipe, slanting downwards, with a stopcock q, at its end, to receive the superfluous syrup. The vapour, which is disengaged from the syrup during its concentration, is extracted from the top of the pan into the pipe r, passes from this into the vessel s, which is divided by a plate of copper into two compartments. The syrup forced over accidentally in the ebullition, goes into the vessel s, and passes by the glass tube t, into the pipe p. The glass tube serves to show the quantity of the syrup that has boiled over, so that it may be drawn off when necessary. For this purpose, the stopcock u, of the vessel v, must be closed, and q must be opened, in order to fill v, while the air contained in it escapes into the pan. The stopcock q, being then shut, and u, with the little air-cock x, opened, the syrup will flow into the large receiver placed beneath it, commonly but erroneously called a cooler; because it is a double copper basin, with steam in the interstitial space. The hot steam rushes from s, into the cast-iron vessel y, where it is condensed. z, is a pipe for introducing the water of condensation through the copper rose a′. The condensed water flows through the pipe b′, and the valve e′, to the air-pump, which receives motion from the shaft of the steam-engine.