The accessories to the pan are placed in a convenient position above the working floor and include a steam gauge for noting the pressure in the coils, a gauge for indicating the height of the liquor in the pan, vacuum gauge I, as shown in the drawing, air-cocks and a thermometer. The pan is also fitted with a small apparatus, by which portions of the boiling liquor can from time to time be drawn, without disarranging the vacuum, so that the progress of evaporation can be ascertained.
In working the pan, the storage tank F is first filled with the weak glue liquors to be evaporated; the valve on the supply pipe G is then closed, and the vacuum pump set in motion; a few strokes are sufficient to reduce the internal pressure, and the valve of the supply pipe is then opened, and the liquor allowed to fill up the pan to the desired mark on the gauge. The valve is then closed, the steam-inlet valve J, supplying the coils, opened. As the heat from the coils spreads through the liquor, the vacuum pump is kept steadily at work reducing the inside pressure to within 2 to 2½ inches of a perfect vacuum, as seen on the barometrical scale. In this vacuum the liquor will boil at 120° to 130° F., and the boiling is continued until the withdrawn samples, as tested by the glue-meter, show the desired strength. The pump is then stopped, the vacuum broken by opening the air-cocks, and the concentrated liquor is run through the valve H into suitably arranged receiving tanks, for supplying the trays or glasses for jellying.
For economical working with large quantities of weak liquors, a combination of two, three and even four vacuum pans, forming the double, triple and quadruple effect evaporators, have been designed for concentration purposes. The triple effect is, however, the system mostly in use, and consists of a grouping of three cylindrical pans, each connected by suitably arranged piping, by which the vapors of the first pan are conveyed to and made to heat the coils of a second pan, the resulting vapors from the second, passing on to the third pan, for a similar purpose. All the pans are connected with powerful pumps, producing a nearly absolute vacuum in each. The liquor is evaporated to a given density in the first pan, and then passed on to the second, and ultimately to the third, at which stage 80 per cent. of its water will have been driven off.
To obtain in all cases a product of equal concentration, it is advisable to have an instrument which will indicate the amount of dry glue in the solution. (Fig. 46.)
By immersing a glass aerometer in the glue-liquor, the percentage of glue is indicated by a scale registering from 0 to 70 per cent. with the jelly or glue solution at a temperature of 167° F.
To measure the temperature quickly, a thermometer is added, and for the execution of the entire test, a sheet-iron vessel consisting of a large and two small tubes, a, which when not in use, serve for the reception of the glass instruments contained in a special case. For testing, the small cylinder is placed in the large tube, a, and filled with jelly by means of the cap which serves as a cover. The large tube is filled with hot water to bring the jelly to the required temperature. The two instruments are then immersed in the tubes filled with glue-liquors to be tested, and temperature as well as percentage can be readily read off.
Fig. 46.
The evaporated and cooled glue-liquor is cut into cakes and dried in the same manner as previously described.