b. PLUM heated by Steitin.—The simplest form of steam cvaporating-pan consists of a rectangular wrought-ireu tank, at the bottom of which is a series of copper steam-pipes, connected by gun-metal bands brazed to them, and carried on wrought-iron aupports. The tank is fitted at the side with a ateam-valve at ono end of the steam-pipe mnge; at the other side, is a cast-iron box, fitted with a wrought-iron pipe, for the escape of the condense-water to a condense-box. This form of evaporator presents a large heating surface, with facility for cleaning. By pasaing, the ends of tho steam-pipe range through stuffing-boxes, tho pipes cau be turned up, aud all parts of the interior of the tank be readily cleaned, a matter of great importance.
Under Pressure.—The 5 atom. conceutrating-pans erected at Aba-el-Wakf receive the juice when it has fallen to a temperature of about 71° (160° F.) Each consiats of a copper tray, 23 ft. long and 6 ft, wide, heated by a steam-boiler beneath, and covered by a sheet-iron casing which confines the ateam evolved from the juice. The steam-boilera work under 60 lb. pressure. The heating surface of each tray is increased by 495 vertical nozzles screwed into it; these are of brass, oak very thin, and alightly tapered. Their mean external diameter is 2* in., and they project 4 in. above the plate. If the juice is iu good order, it makes very little foam ; if not properly tempered, a thick froth soon forma, but appears to condense against the cover, and drop back into the boiling fluid. Each particle of juice takes about 18 minutes to pass through the tray, and though exposed to the temperature due to 3-4 lb. pressure of ateam on its ,,urface, the syrup gains hardly more colour than would be due to the increased density. The ateam generated from the juice is collected into a wrought-iron main, and taken by one branch to the vacuum-pans, and by another to the vacuum-pumps and centrifugal-engines, which it actuates, supplying all the power necessary for boiling to grain, curing, and raising the water required throughout the mill. A great drawback to the use of steam from the juice is its low pressure (3-6 lb.).
The advisability of concentmting syrup under pressure in this manner has been the aubject of much discussion. It is usually held that any temperature above 60° (140° F.) is prejudicial to sugar solutions, and that above 74°-77° (165°470° F.) the proportion of sugar inverted to the unerystallizable condition is very large. A perfectly white refined sugar expoaed to a temperature of 107° (224° F.) for 3 hours becomes quite yellovv. The normal boiling-point of Byrup at 10° B. is about 101° (214° F.). In these pans, the extra pressure of 3-6 lb. of steam means an increase
of 8°-16° F. in the temperature in order to arrive at the boiling-point, which would seem to be highly injurious. Long exposure, however, is quite as mischievoua as high tempemture. It is easy to avoid ene by incurring the other ; the difficulty is to avoid both. Perhaps the chief harm of rapid concentration at a high temperature is the violent ebullition of the mass, whereby portions of heated surface are momentarily left dry. The Aba pans, working with a steam temperature of 143° (290° F.) on the under side, and the juice being- at 105i2 (222° F.), actually made less molasses (i. e. inverted and charred sugar) than some more generally-recognized plans. Still the system cannot be recommended for adoption where there is no necessity for using the water evaporated from the juice.
c. Film Evaporators.—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 generically known as " wetzels " among planters, and comprise the " pans " bearing the names of Gadsden, Wetzel, Schroeder, and Bour, 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 two 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.
Wetzel's improvement upon this is the substitution of steam-pipes for the solid rods. This overcomes the deficiency of heat, and has been very generally adopted, though the churning is not reduced. Fig. 1363 shows the Wetzel pan and its special engine, as made by Fawcett, Preston, & Co., Liverpool. The pan a contains the liquor ; the pipes b are heated by steam passing through them ; and the whole cylinder c is caused to revolve by the engine d. The large heating surface enables steam at very low pressure to be used, exhaust-steam from the cane-mill engine:being sometimes utilized for the purpose. By fitting the pipes diagonally (instead of horizontally) between the discs, the churning is modified, but not altogether prevented. The greater exposure to the air also causes increased oxidation of the juice and inversion of the sugar.