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.

It is noted by Box ('Practical Treatise on Heat,' p. 166) that an exit for the moist air should not be placed at the top of a drying-chamber, but at the bottom, since in the first case, the hot dry air tends to rise at once to the opening, and pass away unsaturated with moisture, while that cooled by evaporating water from the goods, being heavier, tends to form downward currents and remain in the chamber. To this it may be objected that aqueous vapour is much lighter than air; this is true, other things being equal, but in practice the evaporation of a given quantity of water cools the air and makes it heavier in a materially greater degree than the admixture of aqueous vapour lightens it. This source of waste of heat exists in the turret drier, but is there, from its great height, reduced to a minimum. In lower sheds it becomes very material, and the air currents formed are productive of much harm by causing irregular drying. This difficulty has been met by Mr. Edward Wilson, of Exeter, to whom the leather trade owes several very useful inventions, by an ingenious drying-room constructed on the lines indicated by Box, though I do not know that he was in any way indebted to that writer for the idea. In this Mr. Wilson arranges the steam-pipes, instead of on the floor, in a vertical compartment partitioned from the chamber, through which air is admitted and heated. This hot air fills the top of the chamber and from its lightness floats in a horizontal layer, only descending and escaping by apertures in the floor as it becomes cooled by evaporating the moisture of the hides. Mr. Wilson states that the method answers well in practice, and it is certainly the most scientific in conception, but it might be feared that, as applied to a single floor, the upper parts of the butts, suspended near the ceiling, would dry more rapidly than those near the floor. If applied to a double-floored building, this disadvantage would, from the stronger draught, and consequent larger supply of air, be less likely to show itself, and the upper floor with its uniform warm air would be well adapted for drying off finished sole-leather, while the cooler and milder drying of the ground floor would be fitted both in character and situation for that wet out of the yard. Special precaution would be needed to prevent the heated air escaping by doors opening into the upper floor. There is little doubt that as regards heat this is the most economical system which has yet been invented.

A method has been introduced in the United States of drying wet and finished leather all together, in drying-rooms heated to a considerable temperature, and closely shut up. This is found to answer fairly on leather from sour liquors, but that from strong and sweet liquors is darkened, as might be expected. The drying is accomplished in much shorter time than by the turret drier. The mixture of wet and dry leather, and the lack of ventilation produce an atmosphere nearly saturated with moisture, and hence the drying is not nearly so harsh as might be supposed from the considerable temperatures made use of. There does not, however, seem anything in the principle to recommend its general adoption.

Another invention, of which we have as yet heard little definite in England, consists in drying at a low temperature by air artificially deprived of its moisture. This may be accomplished in several ways. Experiments have been made in drying in a closed chamber provided with trays of calcium chloride to absorb the moisture evaporated. Air when artificially cooled by compression and subsequent expansion, as in the case of ice-making machines, parts with a large portion of its moisture, which is condensed in the form of ice in the tubes of the machine. Such air, if subsequently warmed, would dry powerfully and rapidly.

Before leaving the subject of drying-sheds, a few words on the mechanics of drying in general may not be out of place. Air-drying is dependent on the condition that the air must be capable of taking up more moisture than it already contains. It is a matter of common experience that there are warm days when the air is so saturated with moisture in the form of invisible vapour, that scarcely any drying takes place; and similarly, cool dry days, when leather dries rapidly. The relative amount of moisture in the air is easily ascertained by the simple instrument known as the wet and dry bulb hygrometer; an instrument which ought to be in every drying-shed, especially where steam heat is used. It consists of two similar thermometers, side by side, of which one has the bulb covered with muslin and kept wet by a piece of lamp-cotton attached to it, and dipping in a cup or bottle of water. This water evaporates more or less rapidly, according to the dryness of the air; and as heat is consumed by it in passing into the gaseous condition, the wet thermometer falls more or less below the dry in proportion to the rapidity of the evaporation. On a summer's day, the difference may amount to 9°-12° F. (5°-7° C.), and this is about the extreme dryness permissible in a drying-room for finished leather. Wet leather should of course be dried much more slowly. The influence of heat on drying is two-fold. It increases the capacity of the air for moisture, and it replaces the heat consumed by evaporation. The following tables give the capacity of air for moisture at different temperatures, and the percentage of saturation as shown by the wet and dry thermometer. At Greenwich, the mean humidity for the year is 82 per cent.; or for the day-time only 76 per cent., varying from 62 in summer to 86 in winter:—

Table I.—Capacity of Air for Moisture.

Table II.—Hygrometer Table.

As regards the heat consumed in evaporation; it requires about 1000 times as much heat to convert 1 lb. of water into vapour, as it does to raise the temperature of the same quantity 1° F. At least as much heat as this must be supplied if the air which has been used in drying is to retain the same temperature it had at the outset, and therefore if a turret is to keep at a higher temperature than the air, which is necessary to create a draught, this is the minimum amount of heat which must be supplied per pound of water to be evaporated. In practice much more will be needed.

The following table shows the heat given out by different sizes of pipes at different temperatures, and steam pressures, in units equal to the heat required to raise 1 lb. of water 1° F., and the cubic feet of air which they will heat.[V]

Table III.—Heating Effect of Pipes freely exposed to Air at 60° F.

It may be taken that ¹/₂₀ of the above volumes may be heated 20°, from 50° F. to 70° F., and so on; but if the average temperature is higher than 60° F., the duty will be less, and to obtain the same effect the pipe must be heated so much hotter as to keep the same difference as before between the pipe and air. Thus a pipe at 300° F. will only heat as much air at 80° F. as one of 280° F. will of air at 60° F.

It will be noted that the efficiency of small pipes is much greater than that of larger ones, and in these days of high-pressure steam, much may be said in favour of the use of comparatively small wrought-iron steam-pipes instead of the larger cast metal ones. The first cost is small, the pipes are easily obtained ready screwed, and in the lengths required, and may be put together by any intelligent workman. The risk of fracture by the concussion of condensed water is very trifling, as compared to that of metal, and much lighter pipes are safe for high pressures. Steam-pipes must always be laid with an incline of say 1 in. in 10 ft. from the end where the steam is admitted, so that the condensed water may get away, and at the lowest point a steam-trap must be provided for its escape. In the writer's experience, the best form is that of Holman, made by Tangye of Birmingham, of which the principle will readily be understood from Fig. 67. The cup-shaped vessel a floats on the water in the outer casing, and so closes the valve b until a gets full, when it sinks and allows the water to escape until it floats up again. It is important that this trap should be set level, or the valve will not close properly. Each pound of condensed water is equivalent to about 1000 units of heat given off (see Table III.). In planning steam-pipes, it is not necessary that they should be arranged in a single line. Even if in gridiron form the steam will still reach every part, in proportion to the condensation which takes place. A series of large pipes may be supplied by small pipes from a common main, and discharge their condensed water into a common waste-pipe with branch from each. A ¹/₂-in. pipe from a high-pressure boiler will supply a considerable range, say 100 ft. of 4-in. pipe, though a larger size is advisable. At the farther end of a range of steam-pipes a small tap must be provided to let out the air which accumulates in them. In employing the exhaust steam of an engine for heating purposes, the pipes must be of ample size and freely open at the ends to avoid back-pressure. For this purpose the gridiron form is a very good one.

The planning of hot-water pipes is much more difficult than that of steam-pipes, but the general principle is that the pipes must rise all the way from the boiler to the farther end, where there must be an expansion-box or supply-cistern to allow the water to rise and fall and dissolved air to escape. From this the pipes must fall more or less, throughout the distance, back to the boiler, entering it at the bottom. If at any point the pipe has to fall, leaving an upward bend, a tap must be provided for the escape of air, but such upward bends are a fertile source of difficulty and failure of action. With long runs of either steam or water pipes, arrangements must be made to allow of expansion and contraction, which will amount to 1-2 in. per 100 ft., according to the temperature employed. If one end of the system can be left free, all that is needed is to support the pipes on rollers (pieces of old pipe may be used); if not, stuffing-boxes must be provided.

The air heated by boilers, and other sources of waste heat, may often be utilised for heating purposes, but generally requires to be driven by a fan, unless the drying-room can be arranged directly above the source of heat. If air has to be conveyed, the air-ways must be of ample size, and if the ascending force of heated air be relied on, passages less than 2 ft. sq. are seldom of much use. This ascending force is generally much overrated where the differences of temperature are so small as those employed in a drying-room. In a boiler chimney, where the temperature of the escaping gases is 552° F. (289° C.), the specific gravity of the air is about half that outside, and a chimney of 50 ft. in height gives a draught equal to the pressure of a column of about ¹/₃ in. of water, and the hot gases theoretically have a velocity of about 80 ft. per second; whereas the same chimney with a difference of temperature of 30° F. would have a draught equal to ¹/₃₀₀ in. of water only, and a velocity of 8 ft. per second.

The following table will enable the reader to calculate approximately loss in friction in air-passages and the pressure required to pass a given volume of air. The pressure needed increases in proportion to the length of the pipe and the square of the velocity of the current of air to be passed. Thus if we double the length of the pipe we must double the pressure to pass the same quantity; and in order to double the quantity of air through a given pipe, the pressure must be quadrupled.

Table IV.

Head, or Difference of Pressure at the two ends of a Circular Pipe 1 yd. long in inches of water required to pass 1000 cub. ft. of air per minute.

To calculate the head required for a long pipe, multiply the head given by the table by the length in yards. The air passed by square pipes of the same diameters will be 1·273 times greater with the same heads.

To be added to the pressure required to overcome friction is that needed to force the air out at the end of the pipe. This varies with the shape of the tube, &c., but for our purpose may be taken as given in the following table:—

Table V.

Approximate Pressure needed to force Air out of a Pipe with a Velocity of—

Air-passages should be, as far as practicable, of uniform area throughout their length, as much velocity is lost in passing even from a smaller to a larger tube. Of course sharp bends must be avoided.