Leaches and liquors are generally heated by blowing in steam direct. In this case, the condensed water mixes with the liquor, and in heating a liquor to boiling point it may be taken that about 20 per cent. of water will be thus added. Where strong liquors are to be heated, it is therefore obviously much better to pass the steam into a closed copper coil in the liquor. Such a coil, with steam at 30 lb. pressure, will heat about 271/2 gal. per hour per square foot of surface from 46° F. to boiling, and evaporate about half that quantity of liquor already at boiling temperature. (See Box, 'Treatise on Heat,' p. 176.) Heating coils must of course be provided with steam traps to carry off condensed water; and in boiling by open steam it is very desirable to let the steam pass through such a trap before use, to stop water condensed in the pipes, which usually contains iron, and discolours the liquors.
[CHAPTER XXIII.]
DRYING-SHEDS FOR LEATHER.
The primitive way of drying leather was to hang it on poles in the open air, but this in our uncertain climate has become quite obsolete. The oldest plan now actually in use is to hang on poles in a shed generally raised some height above the ground, so as to catch the wind, and provided on all sides with louvre boards arranged so as to open and shut as required. These sheds, to give good results (especially on mixed tannages, which need much more care in drying than bark), demand very watchful management. In windy weather, and with wet leather at all times, the louvres must be kept nearly or quite closed, and on the sunny side of the shed the same precaution is generally necessary. Again, in very damp weather the leather does not dry at all, and in frosty seasons it is apt to freeze, by which sole leather is made soft and spongy, and dressing leather, though whitened, is said to be less capable of carrying grease. To prevent freezing, and to enable leather to be dried in damp or cold weather, it became customary to provide sheds with ranges of steam-pipes on the floor; this, though decidedly a valuable addition, has not proved by any means an entirely satisfactory solution of the problem of leather drying. No sufficient means are provided for controlling the ventilation, and the upward currents of hot air dry the leather irregularly, and produce bad colour. A much more satisfactory shed is the American turret drier.
This consists of a lofty building, 3 to 8 stories high, without louvres, but with latticed floors. J. S. Schultz recommends 5 stories, of 7 ft. clear between beams, as a convenient height, and the building should be divided by partitions from top to bottom into 4 or more series of chambers one above another, each of which is capable of having the heat and ventilation separately regulated. The Americans usually fill one of these series at once, and dry off the whole in about 10 days, so that as many will be required for a tannery as will hold a 10 days' production. For ventilation, each of these sets of chambers is provided with a lantern ventilator at the top for the exit, and shutters or dampers on the bottom floor for the admission of air. The bottom floor is also provided with steam-pipes, of which those for each set of compartments are controlled by a separate cock. When warmth is applied at the bottom, the tall building acts like a chimney, and a continuous current of air passes from the ventilators at the base up to those at the top. The usual American practice is, after filling one of these ranges of compartments, to apply no steam-heat for the first 3 or 4 days, and, if the weather be dry or windy to keep the ventilators also closed. After the third or fourth day, a moderate degree of heat is given, and this is increased so that at the end of about 10 days the stock is fully dry.
This is in accordance with a common American practice, in which the leather is fully dried before rolling, in order to fix the soluble colour, and prevent it striking out to the surface in the finishing. The wet leather is raised by an elevator, consisting of an endless chain provided with hooks, to which the leather is attached at the bottom, and from which it is taken at the top. Various ways are adopted to lower the leather from these tall turrets to the room where it is stored prior to damping down for rolling. In some cases, the lattice floors are made movable, and the whole contents of the room, including the sticks from which the leather is hung, are allowed to fall into the lowest room. This method is of very questionable advantage, if we take into account the labour of separating the sticks and carrying them back to their places. Another plan is to have shoots from each loft, down which the sides are slid to the rolling-room. The floors should have what light is necessary supplied through glass windows, so arranged as not to admit direct sunlight.
To adapt the turret drier for English requirements, some slight modification is needed, since we do not dry our leather right off, and then damp back, but, when it is suitably dry, lay it in a pile to "sammy" for striking; then, perhaps, after striking, hang up again for a short time to temper for rolling, possibly again between rollings, and finally to dry off at a temperature of, say, 68-77° F. (20-25° C.). Perhaps on this account, the writer has seen no complete turret-driers in use in England, though a portion of one of the large sheds at Dartford belonging to Messrs. Hepburn was converted by them some years since into a very good turret, which gave excellent results both for sole leather and kip butts in stuff. This turret is represented in section in [Fig. 66], and is about 56 ft. × 24 ft. in area, and 50 ft. high from the ground-line to the top of the roof, which is ventilated by a dormer, a, with fixed louvres at the top, while air is admitted at the bottom through ventilators with sliding flaps, b b. It is heated by 10 rows of 4-in. steam-pipe, c c, each 54 ft. long, making a total of 540 ft. run, or about 640 ft. superficial (a 4-in. pipe being about 45/8 in. diameter outside). I am informed by Mr. J. G. Hepburn that he considers 4-in. pipes inferior for the purpose to smaller ones, giving too much heat in one place, and without sufficiently distributing it, and were he constructing a new turret he would replace them by 11/2 in. wrought-iron, using about 3 of 11/2 in. to replace 2 of 4 in., small pipes being much more effective (as will be seen by table, [p. 250]) than larger ones, in proportion to their surface. He considers, however, that the best way of heating drying-sheds, though more expensive in first cost, is by means of hot water, which is much more constant in temperature than steam. Mr. Hepburn, to whom I am much indebted for the above information, informs me that the turret still acts very well, drying kip butts on the upper floor a good colour in all weathers in about a week. He finds, however, that the steam-pipes as described are hardly sufficient in very cold weather, and intends to increase them, or replace with 1300-1400 ft. of hot water pipe heated by a saddle boiler. At Lowlights tannery, a shed arranged on the turret principle (though much less completely carried out from want of height in the buildings) has been for many years in operation, principally for drying off sole-leather, with the most satisfactory results.