By the above apparatus the drying can be perfected in a very much shorter time than can be done by the old alleys, and operations can be conducted in hot weather without hindrance from the condition of the atmosphere.
In cases where the atmosphere is dry enough to dispense with the heater and the condenser, the inventor can throw the doors F wide open, extend the partition C out to that end of the alley, and then preserve a continuous forced draft of sufficiently dry air in both alleys for the purpose. As there are many days during the year fine enough to give reasonably dry air, operations can be conducted with the blower alone in this way, and thus economize the steam and the refrigerating-brine.
Fig. 30.
Figs. 29 and 30 show a longitudinal section, with upper and ground floor plans of a modern drying-house, as given by Thomas Lambert. In the ground floor the whole of the liquors are jellied in coolers, and then cut into cakes by the two cutting machines in the centre. Here a hoist, E, Fig. 30, is placed which carries the cut cakes on “glasses” to the floor above. This forms the drying-floor, and is partitioned off in three divisions, running nearly the length of the building. The two outer divisions form the tunnels proper, and at the ends are fixed two powerful revolving fans, driven at high speed and drawing the air through the tunnels at a high rate. At the opposite ends of the fans are fixed a series of 6-inch pipes, heated up by waste steam, and the air passing between is warmed up to any desired temperature, of necessity below 78° F. In the center passage a number of girls are employed in transferring the cut cakes to the nets, which are built upon a carriage running on a small railway. The carriage with the complement of filled nets is run to the end of the division, transferred to the lower railway, C, by which it is carried either to the right or left hand tunnels as desired; the glue when dried on the nets is run to the other end and by another lower railway brought to the middle division, and ultimately by the hoist raised to a large store-room, seen in section, where the glue is sorted and bagged. At the end of the store-room is placed a grinding machine, and all the off-color and twisted cakes are ground to a powder and sold as powdered glue. The manufacturer by varying the size of the cake, its thickness, and its color, may make any number of grades from the same boiling.
Fleck proposes to accelerate the drying of glue by utilizing the water-absorbing power of some salts, such as Epsom and Glauber’s salts, ammonium sulphate, crystallized acid sodium sulphate, etc., for the purpose of withdrawing water from the glue cakes. For the practical application of this principal a shallow, water-tight, wooden box is required. The bottom of the box is sprinkled with a layer of the water-absorbing salt about half an inch deep, and covered with a moist linen cloth. Upon this is placed the jelly cut into sheets and also covered with a moist cloth, a layer of salt being finally scattered over it. After standing for a few hours, the box is slightly inclined and the salt solution allowed to drain off through a hole in the bottom, the dropping ceasing in from 12 to 18 hours. If now the upper cloth is taken off with its layer of salt, the glue beneath it will be found so far deprived of its moisture that when placed in the sun or exposed to other heat, it will become completely dry in a short time without either melting or spoiling, and in winter may be laid upon drying-floors with the same result. The salt-solution formed may be evaporated to crystallization and the salt thus obtained be again used.
After the treatment with the salt, the jelly contains 70 to 75 per cent. of anhydrous glue, while the content in jelly not thus treated varies between 7 and 28 per cent. according to the concentration of the liquor from which it has been derived. It is claimed that the adhesive power of the glue is not injured by this treatment.
Commercial glue must not only be thoroughly dry, but should also present a good appearance, showing especially lustre. However, after drying, the glue is dull, spotted, dusty and sometimes even mouldy. To give a good lustrous appearance the dry cakes are dipped in warm water and replaced upon the nets to dry.