The means by which beat is applied to the evaporation of saccharine juices and syrups may be described under 4 separate heads, according to their principles: - (a) Pans heated by fire, (b) pans heated by steam, (c) film evaporators, (d) vacuum-pans.
The earliest and crudest system of evaporation was the "copper wall," or "battery" of open pans called "teaches" (laches, tayches, etc.) The first 2 pans of the series are the clarifiers; thence the juice flows into the teaches, sheet-copper pans set in masonry on a descending plane. As the juice concentrates, each lower pan fills up with liquor from the one immediately above it, until the density of the liquor in the "striker-teach" permite granulation, when the mass is ladled into shallow wooden vessels, and conveyed away to be "cured." By the oldest method,the liquor was ladled throughout the aeries. More recently an improvement was introduced, consisting of a copper dipper, fitting inside the striking-teach, and having at the bottom a large valve, opening upwards and worked by a lever. The dipper is attached to a crane, which commands the striking-teach and the gutter leading to the coolers. This greatly economises time. The furnace for heating the series is set under the striking-teach; the heat passes by flues to the chimney or to the boiler-flue.
In working a battery, the difficulty is determining the exact moment when the boiling of the "sling" in the striking-teach must cease, i.e. when to make a "skip"; great skill and experience are required to suit each kind of juice.
The drawbacks of the copper-wall are : - (1) Waste of fuel; (2) the amount of labour required and length of time M 2 occupied; (3) considerable waste of liquor in the sloppy manipulation; (4) the proportion of molasses produced is intensified by the churning up of the liquor and consequent admixture of air, and by the irregular and uncontrollable action of the heat upon the surface of the metal with which the liquor is in contact. The temperature prevailing in the striking-teach is not less than 230°-235° F. (110°-113° C.) in any part, and much greater at the bottom of the mass. It is therefore not surprising that liquor showing 10 per cent. of inverted (uncrystallisable) sugar in the first pan, should have 22-23 per cent. by the time it is finished in the striking-teach.
The simplest form of steam evaporating-pan consists of a rectangular wrought-iron 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 supports. The tank is fitted at the side with a steam valve at one end of the steam-pipe range; at the other side is a cast iron-box, fitted with a wrought-iron pipe, for the escape of the condenser water to a condense box. This form of evaporator presents a large heating surface, with facility for cleaning. By passing the ends of the steam-pipe range through stuffing boxes, the pipes can be turned up, and all parts of the interior of the tank be readily cleaned, a matter of great importance.
The 5 steam concen-trating-pans erected at Aba-el-Wakf receive the juice when it has fallen to a temperature of about 160° F. (71° C). Each consists 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 steam evolved from the juice. The steam-boilers work under 60 lb. pressure. The heating surface of each tray is increased by 495 vertical nozzles screwed into it; these are of brass, cast very thin, and slightly tapered. Their mean external diameter is 2 1/8 in., and they project 4 1/2 in. above the plate. If the juice is in good order, it makes very little foam; if not properly tempered, a thick froth soon forms, 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 steam on its surface, the syrup gains hardly more colour than would be due to the increased density.
The steam 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 concentrating syrup under pressure in this manner has been the subject of much discussion. It is usually held that any temperature above 140° F. (60° C.) is prejudicial to sugar solutions, and that above 165°-170° F. (74°-77° C.) the proportion of sugar inverted to the uncrystallisable condition is very large. A perfectly white refined sugar exposed to a temperature of 224° F. (107° C.) for 3 hours becomes quite yellow. The normal boiling point of syrup at 15° Tw. (10° B.) is about 214° F. (101° C). 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 mischievous as high temperature. It is easy to avoid one 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 290° F. (143° C.) on the under side, and the juice being at 222° F. (105 1/2° C), actually made less molasses (i. e. inverted and charred sugar) than some more generallyrecognised plans. Stilt the system cannot be recommended for adoption where there is no necessity for using the water evaporated from the juice.