Under this head are particularly included those evaporators which depend upon the principle of exposing thin films of liquid to the action of a heated surface in the open air. They are genericslly known as "wetzels" among planters, and comprise the "pans" bearing the names of Gadsden, Wetzel, Schroeder, and Baur, and many modifications, some of which, such as Murdock's, have steam-heated coils. The original form was Aitchison's simple cylinder revolving with partial immersion in the liquid, and heated internally by steam. In its revolution, the cylinder carries on its surface a film of liquor, whose water is soon evaporated. In the Gadsden pan, the cylinder is replaced by a skeleton cylinder, consisting of 2 metallic discs connected by a series of metallic rods fixed at short intervals around the periphery of each disc. Here the drawbacks are the churning of the liquor (except at very low speeds), and the insufficiency of the heat derived from the steam-jacket of the pan.
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 steam-heated discs would increase the evaporating surface, and produce better grain. A front elevation of his pan is shown in Fig. 140; and vertical and transverse sections of the disc on an enlarged scale in Fig. 141. a is the steam engine; b, exhaust-pipe to heat the revolver; c, revolver consisting of 10 copper discs; d, copper pat; 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 of air and uncondensed 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 the discs e are small buckets 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, from 32 1/2° Tw. (20° B.), as taken from the battery, the temperature never exceeding 170° F. (77° C). The distributing-cups churn the liquor excessively.
One of the most recent modifications is Pontifex's, shown in Fig. 142. The to them at a density of 44°-46 1/4° Tw. (26°-27° B.).
Another apparatus on similar principles is Fryer's "concretor," made by Manlove, Alliott, Fryer, & Co., Nottingham and Rouen, shown in Fig. 143. It consists of a series of shallow trays A, placed end to end, and divided transversely by ribs running almost from side to side. At one end of these trays is a furnace B, the flue of which runs beneath them; and at the other end are a boiler C and an air-heater D, which utilise the waste heat from the flue, employing it both to generate steam and to heat air for the revolving cylinder.
The whole series of trays A is placed on a slight incline, the upper end being next the furnace. The topmost 3 trays are. made of wrought iron, since the intense heat here would render cast iron liable to fracture. The clarified juice from the pipe M flows first upon the tray nearest the furnace; it runs down the incline towards the air-heater D, meandering from side to side in a shallow stream. Thus it has to pad a contains the liquor to be evaporated, within which revolves a coil of steam-pipe 6. Thus a large heating surface is obtained, without the drawback of churning up the liquor.
It is to be observed that all these forms of film evaporator are destined only to finish the concentration begun in the battery. The liquor is brought traverse a channel 400 ft. long before it can leave the trays at the end adjacent to the air-heater, although the distance between the furnace and the air-heater in a direct line is not quite 50 ft. While flowing over these trays, the juice is kept rapidly boiling by means of the heat from the furnace; and although it only takes 8-10 minutes cylinder is full of scroll-shaped iron plates, over both sides of which the thickened syrup Sows as the cylinder bevolves, and thus exposes a very large surface to the action of hot air, which is drawn through it by means of a fan G. Motion is given to the whole apparatus by means of a small engine. In this cylinder, the syrup remains for about 20 minutes, and at the end of that time flows from it at a temperature of about 195°-200° F. (91°-94° C), and of such cooling. By the use of dampers, the hot gases from the flue may be directed either under the boiler, returning through it to the heater, or direct to the heater.
At J is an auxiliary furnace for raising steam, when the heat from the concretor flue is insufficient or not forthcoming, - as, for instance, when beginning to crush canes, and before the juice has covered the trays. K is a smoke-door for cleaning out the boiler-tubes. L is a chimney, either of brick or iron, for the hut escape of the gases.
The vacuum-pan is shown in section in Fig. 144. 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 dome are bolted together through their flanges with a wrought-iron ring and bolts so as to be air- and steam-tight throughout. A man-hole d, with a ground gun-metal cover, is attached to the top of the dome, from which proceeds the arm-pipe opening into the receiver h. A steam-valve k opens into the copper steam-worm y. This worm gradually diminishes in diameter from the en-trance-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 6, to carry off the water from the steam-case; the slide-valve x at the bottom of the pan is for discharging the sugar.
Glycerine (introduced here on account of its syrupy nature, though it bears no relation to sugar) is generally concentrated in a modification of the "Wetzel" evaporating-pan, constructed by Chenaillier, Paris. This ecoporatour universel, as he terms it, which is very economical and effective, is shown in Fig. 145, and consists essentially of pairs of soucers set edge to edge upon a hollow central revolving shaft, through which, steam passes to the interior of the saucers (the waste steam from a high-pressure engine will do); the lower edges of the saucers dip in a jacketed trough of the liquid to be evaporated, and when they are revolved, layers of this are brought up and speedily concentrated cm their surface. It may also be worked in a vacuum, as shown in Fig. 146.