To prevent the workroom from being filled with steam arising from the pan, the latter is covered with a hood of wood which terminates in the pipe S projecting above the roof, and a narrow pipe R branching off from the steam pipe D passes into S.
When vapors commence to arise from the liquor, the cock on the pipe R is slightly opened whereby a jet of steam is blown into the pipe S, the latter then acting as an exhauster, and the vapors in the hood C are carried along by the jet of steam. By this arrangement no vapor passes into the workroom and steam is also very rapidly evolved from the surface of the liquor.
Sufficient steam should be admitted to the pan for the liquor to give out an abundance of vapor without, however, being brought to the boiling-point, as in that case foam would be formed and the liquor in cooling yield a product full of blisters. When the liquor has acquired the proper degree of concentration, the admission of steam to D and R is interrupted and the valve V having been raised the liquor is run into the cooling-boxes. The latter are of wood lined with zinc, or better of stout zinc or heavily galvanized iron. They hold about ½ cwt. and are of two shapes: one deep and nearly square, another long and shallow, for quick cooling of clear liquors. Iron should not be used, as it readily rusts and causes discoloration of the glue.
Cooling is effected by cold water where it is available, but often merely by cold air, aided by fans or blowers, in a room protected from heat or frost. According to S. Rideal, refrigerating machines are now also employed, which, by the evaporation of liquid gases, such as ammonia, sulphurous or carbonic acid, reduce a tank of brine to near freezing-point. The temperature should not be allowed below 33° or 34° F., for if frozen the jelly is hard and difficult to cut. The brine circulates in iron pipes placed near the ceiling of the room; they must be kept as clear as possible of ice and dirt, and the cooling house should be scrupulously clean and sweet.
Spiral evaporators are recommended by Thomas Lambert as forming a ready and economical means of evaporation. The evaporator consists of a spiral steam coil, made of copper, and 2 inches in diameter, revolving on a centre shaft; the lower half of the coil is covered with the glue-liquor in the trough. The shaft rests on two plummer-blocks, one receiving the steam, and the other discharging the spent steam and condensed water. The shaft is hollow to the first coil, and the steam is thus conveyed to the spiral. From the last coil to the end of the plummer-block the shaft is also hollow, and in that portion resting on the block two openings are made. In the inside of the plummer-block, two openings are bored to the outside, each forming a covered channel; as the shaft revolves, all the holes directly face each other at intervals, and thus allow any condensed water in the coils to be blown through. From 25 to 28 coils are generally used in each spiral. The glue-liquors are fed into the trough at one end, and have a temperature of 75° F.; the temperature of the evaporated liquor is 85° F. In the rather slow passage through the trough, the liquors receiving the heat of the revolving coils are raised in strength from 20 per cent. to 32 per cent. dry glue, at which point they are ready for jellying.
Vacuum pans are much used in this country for evaporating glue-liquors, though there are some complaints made of the great waste owing to spray and froth being carried off in the steam. As is well known the boiling-point is lowered by increasing the pressure on the surface of a fluid. By enclosing water in a vessel connected with a constantly working air-pump, it is brought to the boiling-point by heating to between 95° and 104° F. The construction of vacuum pans is based upon this principle, and such apparatus is largely used in many manufacturing processes for evaporating to a certain degree fluids, for instance, sugar solutions, which readily become decomposed at a higher temperature. Vacuum pans are also very suitable for the evaporation of glue-solution, especially in plants working on a large scale.
Fig. 45 represents an elevation of a vacuum pan for evaporating glue and gelatine liquors as described by Thomas Lambert. The pan is built of steel plates, and lined outside with wood work, and rests on a floor constructed of rolled steel plates, supported on four columns, with a stairway leading to the working platform. One half of the lower part is shown in section, giving a view of the coils by which the pan is heated. The various parts are as follows: A, the body of the pan; B, the dome; C, exhaust pipe leading from the dome to the condenser; D, condenser; E, air or vacuum pump; F, storage tank for glue or gelatine liquors, warmed with steam coil; G, supply pipe leading from storage tank to vacuum pan; H, discharge valve; I, barometer gauge for indicating vacuum; J, inlet steam pipe for supplying the coils; K, exhaust end of vacuum coils; L, iron staircase; M, steel floor.
Fig. 45.
