Fig. 59.
Fig. 59.
Fig. 60.
Fig. 60.
The dry leather is rapidly passed through tepid water, and after being hung for a very short time, to allow the water to drain off, is trodden tightly into chests, and allowed to remain in them for about 12 hours, so that the moisture may be uniformly distributed. It is then trodden on hurdles (Horden), composed of square bars of wood, joined corner to corner, so as to make a floor of sharply angular ridges,Fig. 59. The next operation is stretching over a circular knife, called theStollmond(stollen, Eng. "staking"), shown inFig. 60; then the leather is dried nearly completely, and staked again.
Dyeing.
The dyeing of glove-kids is done in 2 ways:—a.The skins are plunged into the dye-bath (Tunkfarben). In this way, all light colours are ordinarily produced, such asgris-perle(pearl-grey),paillé(straw-yellow),chamois(reddish yellow), silver-grey, aquamarine, &c.b.The skins are spread on an inclined or rounded table of stone or metal, and brushed over, on the grain side, first with a mordant (Beize), then with the dye-liquor, and lastly, with a solution of a mineral salt (Plate 7). The mordant serves to fix the colour on the surface of the skin, to prevent its striking through, to produce certain modifications of colour, and to enable any parts of the skin which yet contain fat to take the colour evenly with the rest. To satisfy these conditions, the composition of the mordants is very varied. Potash bichromate, ammonia, potash, soda, and stale urine are among the most frequently employed, seldom separately, but usually in a mixture containing 2 or more.
Dye-stuffs of vegetable origin have always held the first place. Those most in use are logwood (Blauholz), Brazilwood (Rothholz), the two fustics—Cuba Gelbholz(Morus tinctoria) andUngarisches Gelbholz(Rhus cotinus), several species of willow-bark and of berries, indigo-carmine, and indigo dissolved in sulphuric acid.
Aniline colours used alone remained in fashion for a short time only, but are now usefully employed as top-colours (Ueberfarben), viz. brushed in very dilute solution over vegetable colours. In this way, particularly tasteful shades of green, violet, and marine-blue may be produced.
After the mordant has been applied once or twice, and the colour 3-6 times, a wash (Ueberstrich) containing some metallic salt is generally applied, with the object either of bringing out the special tone required, or of making the colour more lively and permanent. The so-called "vitriols" are mostly employed: "white vitriol" (zinc sulphate),"blue vitriol" (copper sulphate), "green vitriol" (iron sulphate), and occasionally other salts.
Before dyeing, the greater part of the flour, salt, and alum must be removed from the skins by washing with tepid water; and they therefore require a second feeding (Nahrung) of egg-yolk and salt. In the case of the skins which are dyed by plunging into the dye-vat (Tunkfarben), this is done after the dyeing is completed; in that of brush-dyeing, before the dyeing process.
After the dyeing, the skins, if dipped, are wrung out; if brush-dyed, sleeked out with a brass plate, to get rid of superfluous water. They are then dried in an airy room. Before staking (stretching), the skins are laid or hung in a damp cellar, or in moist saw-dust. They are staked twice: once damp, and once nearly dry.
Skins which are much damaged on the grain, or otherwise faulty, are smoothed with lump pumice on the flesh-side, either by hand or machine. They are then dyed on this side, mostly by dipping, but occasionally with the brush, in which case, the method described is slightly modified.
Indebtedness is acknowledged to F. Kathreiner, of Worms, and David Richardson, of Newcastle, for much information on the production of light leathers. The Plates 1 to 8 represent the works of Messrs. Tréfousse et Cie., at Chaumont (Haute-Marne).
CONSTRUCTION AND MAINTENANCE OF TANNERIES.
Asfew architects have specially studied the construction of tanneries, and in most cases much of the arrangement depends on the knowledge of the tanner himself, a short chapter on the subject will not be out of place.
In the selection of a site, a clay or loamy soil is to be preferred to a gravelly or sandy one, as lessening the liability to leakage, and waste of liquor. Perhaps, however, the first consideration of all is the water supply, since for manufacturing purposes town water is generally very expensive. With regard to quality and impurities of water, information may be found onp. 83; but, as a general rule, the softer and purer the supply the better. It is also of great advantage when the source is at such a level as to flow into the tan-yard, or at least into the beam-house, without pumping. Filtration too, when needed, is much facilitated by a sufficient head of water.
Of scarcely less importance than the water supply is the drainage of the yard. It not unfrequently happens that tanneries are prohibited from discharging their refuse liquors, limes, and soaks into rivers and watercourses, and it is sometimes a matter of extreme difficulty to find any other way of getting rid of them. In default of an outlet, recourse must be had to precipitation and filtration, but this is a costly expedient, and in fixing a site for a new yard it is far better to provide against such a possible contingency. Should, however, such means become necessary, it may be borne in mind that limes and liquors in great measure mutually precipitate each other, and that if all the variousrefuse is run into one tank, mixed, and settled, much is accomplished in the direction of purification. The further treatment of the effluent water must be determined by its nature and composition.
The site chosen, the next question is the arrangement of the buildings. It is very doubtful, where ground is not inordinately expensive, whether it is wise to erect drying-sheds over the pits. In case of fire, very serious damage is done to liquor and leather by the heat and burning timber. If the turret form of drier be decided on, strong foundations are required, and the ground-floor or basement is occupied with heating apparatus; and, on the other hand, the tan-house may be easily and cheaply covered with slated roofs, with sections of glass, to the north, if possible, like a weaving-shed, through which sufficient light for convenient work and cleanliness is admitted. The direct rays of the sun should be avoided, but in the writer's opinion the balance of advantage is largely in favour of a liberal supply of light. Iron roofs are unsuitable, since the moisture condenses on, and rusts them; and particles of oxide fall into the liquors, and cause iron-stains.
Good ventilation along the ridge of the roof should be provided, wherever there is any steam or hot liquor used; or the condensed moisture soon leads to decay.
As regards the general plan of the buildings, much depends on local circumstances; but as far as possible, they must be so arranged that the hides and leather work straight forward from one department to another with as little wheeling or carrying as possible; that the buildings where power is used be near to the engine, so as to avoid long transmissions, which are very wasteful of power; and that the different buildings be so isolated as to diminish the risk of the whole being destroyed in case of fire.
As regards the first of these conditions, if the various soaks, limes, bates, and handlers are well arranged, it is hardly necessary to do more than draw the goods from one pit into the next throughout the whole of the process. To, and fromthe layers, the goods must generally be carried or wheeled. In the sheds, if it be a sole-leather tannery, the butts should first come into turrets or open sheds for the rough drying; then into a room sheltered from draughts to temper for striking. The striking machines or beams should be in an adjoining room, or immediately below; then a small shed-space for drying before rolling; next the roller room; and then the warm stove for drying off. If two of these can be provided to be used alternately, it will allow the goods to be aired off without taking down, and they may then be immediately handed or lowered into the warehouse, without fear of over-drying, which is sometimes difficult to avoid where leather must be taken direct out of the hot drying-room.
To fulfil the second condition named, the engine should be at the centre of the main range of buildings, with perhaps the grinding machinery on one side, and the leather-finishing on the other; but this would be rather contrary to the third requirement. A very good plan would be to have the engine-house in the centre as suggested, but separated from the buildings on each side by brick gables; and with the boiler-house behind it, and under a separate roof, say of corrugated iron. Figs.61,62, from Eitner's book on American Tanning, show the arrangement of a sole-leather tannery in the United States. If it be impossible to have the engine near its work, it is in most cases better to employ a separate high-pressure engine, which may be within a glass partition, and will work all day with scarcely any attention. The loss of power in carrying steam for moderate distances through sufficiently large and well-clothed pipes is much smaller than that of long lines of shafting. The writer has known cases where fully half the indicated power of the engine was consumed in friction of the engine, shafting, and belts. High-pressure engines are as a rule to be preferred to condensing for tannery use, since the waste steam can generally be employed for heating, and both the first cost and that of maintenance are smaller. Where much fuel is used, it is quite worthwhile to have the cylinders indicated occasionally, both running light, and driving the machinery; much information is gained in this way as to the power spent on the various machines, and very frequently large economy is effected by proper adjustment of the valves. To work economically, an engine should be of ample power for all it has to do; and adjusted to its work, not by lowering the pressure of steam, or by checking it at the throttle-valve, but by setting the slide-valves to cut off as early in the stroke as may be. As to how early this is possible, an indicator-diagram will at once give information. In arranging shafting, moderate speeds, say 100-150 rev. per min., should be chosen for main lines, and when higher speeds are necessary, they should be got up by light and well-balanced counter-shafts, with wrought-iron pulleys. In calculating speeds, it must be remembered that they vary inversely as the size of the pulleys. Thus a 3 ft. pulley running at 100 rev., will drive a 2 ft. one at 150 rev., and a 12 in. one at 300. Of course the higher its speed, the more power any shaft will transmit, but increased friction and wear and tear soon limit this advantage. The velocity of a belt in feet per min. is obtained by multiplying the number of revolutions perminute by the girth of the pulley in feet or by its diameter multiplied by 31/7, or more accurately, 3·1416.
Fig. 61.
Fig. 61.
Fig. 62.
Fig. 62.
Pulleys should always be of ample breadth for the power they have to transmit; and it is more economical both in power and cost, to use broad single belting than the samestrength in double. If the pulley will not take a belt broad enough for the work it has to do, a second belt may be made to run on the top of the first, and will do its share of the work. Belts should be washed occasionally with soap and tepid water, and oiled with cod-oil; but if of sufficient breadth, should not require the use of rosin, or adhesive materials, to make them grip the pulley. Makers of machines often err in constructing their driving pulleys too small both in breadth and diameter.
The horse-power which a belt is capable of transmitting obviously varies extremely with circumstances, but may be approximately calculated by the formula
whereais the area of contact of the belt with the smallest pulley, andvits velocity in feet per minute. Another rule is, that at a velocity of 1000 ft. per min. each inch of breadth of belt should transmit 21/2horse-power on metal pulleys, or 5 on wooden ones, on which the adhesion is greater. Adhesion may also be increased by covering the pulleys with leather or india-rubber. Both rules assume that the belt is of ample strength. One horse-power would be transmitted by a belt running 1000 ft. per min. with a pull of 33 lb. A good single belt should not break with a much less strain than 1000 lb. per inch of breadth, and should stand about1/10as much as a working strain.
Countershafting and high-speed machinery, such as disintegrators, striking machines of the Priestman type, &c., should run without material jar or vibration. If this occurs, it is generally a sign that the running part is not equally balanced. In this case, the shaft must be taken out of its bearings, and supported on two exactly horizontal straight-edges, when it will roll till the heaviest part is downwards; and weight must be taken off or added till it will lie in any position. In this way, the writer had recently to add fully 2 lb. of iron to the drum of a striking machine before equilibrium was secured, and a most troublesome vibration prevented. Of course all machinery should be supported as solidly as possible; and if circumstances permit, mostmachines are better on a ground-floor. In placing bark mills, however, it is frequently convenient to fix them in the top of a building, so that the ground material may be sent down shoots by its own weight to the required places. An alternative plan is to set the mill on the ground-floor, and to raise the ground material with a bucket-elevator. This may be done successfully by letting the material fall directly from the mill into the buckets; but otherwise it must be thrown in with a shovel, as buckets will not pick up ground bark, even from a hopper; and in any case such elevators are often troublesome. In a grinding plant designed by the writer, the unground material is filled on the basement floor into an iron barrow, which may be wheeled into an iron bow working between upright guide-rails. On pulling a brake-line, the barrow is raised to the top of the building, and its contents are tipped into a large hopper, after which the barrow rights itself, and descends for another load. In the bottom of the hopper is a sliding shover, which forces the material on to vibrating screens, by which it is guided either into a disintegrator, or crusher rolls, at pleasure. Both these discharge through iron spouts into large hoppers on the outside of a brick gable, from which, powdery materials like myrabolanes and valonia, can be run direct into barrows or trucks. It is very desirable that such hoppers should be separated from the main building by a fireproof partition. The writer is glad to say, he does not know of a case of fire from disintegrators grinding tanning materials, but he is informed that a Carter's disintegrator employed in grinding bones in a manure works has repeatedly set fire to the flannel bag into which the dust was allowed to escape. If this were to occur with a dry and dusty tanning material, it is not unlikely that it might result in an explosion such as sometimes happens in flour-mills from a similar cause. On the whole, however, mills of the coffee-mill type are probably more dangerous than disintegrators; since if they become partially choked, the heat caused by friction is very great.
For lubricating purposes, mineral oils of high density arenot more dangerous than animal or vegetable, but rather the reverse; as, though they are possibly more inflammable, their mixture with cotton-waste and other porous vegetable materials is not spontaneously combustible, while vegetable and animal oils occasionally are. Heavy mineral oils should always be used as cylinder oils in high-pressure engines, in preference to other oils or tallow, since they are not decomposed by steam, and do no harm if blown into the feed-water, but serve to loosen and prevent scale and deposit. Ordinary oils and tallow, on the other hand, when submitted to the action of high-pressure steam, are separated into glycerin and fatty acids (seep. 60), and the latter corrode the valve faces and seatings, and in combination with temporary hardness in the boilers form a very dangerous porous deposit, which often leads to overheating of the tubes.
Next to the machinery, the pits demand special consideration. The chapter on the subject in Mr. Schultz's book on 'Leather Manufacture,' is well worth attentive study as giving American practice on the subject.
The old-fashioned method of sinking pits is to make them of wood, and carefully puddle them round with clay, which should be well worked up before use. Such pits, if made of good pine and kept in constant use, are very durable, some of the original pits at Lowlights Tannery, constructed in 1765, being still in use. Loam mixed with water to the consistence of thin mortar may also be employed, the pits being filled up with water, to keep them steady, at the same rate as the loam is run in. Probably the best materials for pit-sides are the large Yorkshire flagstones. Where these are not attainable, very durable pits may be made of brick, either built with Lias lime, and pointed with Portland cement, or built entirely with the latter. Common lime cannot be used, as it spoils both liquors and leather; and even cements with too large a percentage of lime are unsatisfactory. Brick and common mortar are, however, suitable for lime-pits.
The writer has constructed wooden pits in two ways. Inthe one case, after making the excavation, beams were laid in a well-puddled bed of clay; on these a floor of strong tongued and grooved deals was laid, and on this the pits were constructed of similar wood to the floor, and puddled round with clay. In the second case the pits were built like large boxes above ground, and when finished, lowered on to a bed of clay prepared for them, and then puddled both around and between. It may have been from defective workmanship in the first case, but those made on the last-named plan, which is that adopted from very early times, have certainly proved the tightest and most satisfactory. Mr. Schultz describes a plan as the Buffalo method, in which a floor is laid as just described, and grooves cut with a plane for the reception of the sides, which are formed of perpendicular planks, each end and side being finally tightened up by the insertion of a "wedge plank."
If bricks be used, great care must be taken that the cement is not merely laid so as to fill the joints towards the two surfaces of the wall, as is the habit of modern bricklayers, but actually floated into all the joints so as to make the wall a solid mass; or leaks can hardly be avoided. Cement pits are very good, and, though not particularly cheap in material, which must be of the best, are readily made by intelligent labourers under good supervision. The first step is to lay a level floor of good concrete, in which glazed pipes for emptying the pits may be embedded; care being also taken that all joints in these are thoroughly tight, since future repairs are impossible. The next step is to make frames, the exact length and breadth of the pits required, and perhaps 15 in. deep. These are arranged on the floor where the pits are to be, and the intervening spaces are filled with concrete of perhaps 1 of cement to 3 or 4 of crushed stone or brick. Rough stones and bricks may also be bedded in the concrete as the work goes on, to help to fill up. After the first layer has set, the frames may be raised and a second added, and so on. The work is generally finished by floating over it, while still damp, a little pure cement, to give a smooth surface.Before using, the cement should be tried on a small scale, to be sure that it does not discolour leather or liquors, and the pits should always be seasoned with old or cheap liquor before actual use.
Fig. 63.
Fig. 63.
If possible, both latches and handler-pits should be provided with plugs and underground pipes, communicating with a liquor-well some feet below their levels. Glazed fire-clay is very suitable both for pipes and plug-holes, which should be in the pit corners. Some means should also be provided for the ready clearing of the pipes when choked with tanning materials. A good plan is to let each line of pipes end in a liquor-well large enough for a man to go down. As it is almost impossible to make plugs fit without occasional leakage, it is not well to run pits with very different strengths of liquors to one well, but the layers, handlers, and different sets of leaches should each have their own, so as to avoid mixture. A good means of clearing pipes consists in a series of iron rods 3-4 ft. long, connected by hooks fitting into double eyes, as shown inFig. 63. It is obvious that in a narrow pipe or drain, these cannot become disconnected.
It is, as Schultz points out, of questionable advantage to lay wooden troughs for supplying liquor to each pit under the alleys, since it is almost impossible to preserve them from decay; but the same objection would not apply to glazed pipes, well clayed or cemented. A very good and cheap plan in practice, is to let the liquor-pump, or a raised liquor-cistern, discharge into a large and quite horizontal trough raised 5 or 6 feet above the level of the yard, and provided with plug-holes at intervals, under which short troughs may be set to run the liquor into the various pits.
Pl. VIII.E. & F. N. Spon, London & New York."INK-PHOTO." SPRAGUE & CO. LONDON.RUBBING-DOWN DYED SKINS.
Pl. VIII.
E. & F. N. Spon, London & New York.
"INK-PHOTO." SPRAGUE & CO. LONDON.
RUBBING-DOWN DYED SKINS.
In tan-yard construction, iron should, as far as possible, be avoided wherever it can come into contact with liquor, as it discolours the leather. In default of underground pipes, india-rubber suction hose may be employed. Direct-acting steam pumps without fly-wheels are not suitable for tanneries, as they "hammer" when the pit is nearly sucked up. Steam-jet elevators and the pulsometer are very useful for some purposes, but slightly warm, and dilute the liquors with condensed steam.
Fig. 64.
Fig. 64.
Much that has been said about pits applies also to leaches. They may be constructed either of wood, or brick and cement, and where heat is employed the latter is the better. They are also to be provided with plugs and pipes leading to a liquor-well. About 6 in. from the bottom of the pit is a false bottom B made of boards, perforated with holes or set a little distance apart; and in the corner is an "eye" C (Ger.Pfaff) consisting of 2 boards set at right angles, so as to preserve a vertical channel communicating with the space under the false bottom. This serves, in pits provided with pipes, for the insertion of the plug; and where this is absent, for that of a suction hose to pump off the liquor. In the American Press-leck System, the eye of one pit communicates by a horizontal spout with the top of the next (see D,Fig. 64). The Allen and Warren Sprinkler Leck (Fig. 65) has very much superseded this arrangement in America, though it is doubtful if it spends the bark so completely. The round tubs, however, have several advantages andmay well be used for many purposes in English yards. Their construction is described in some detail in Mr. Schultz's book above cited. Some details will also be found onp. 209of the 'Manufacture of Leather' by Davis. The rule for finding the capacity of a round tub with perpendicular sides in cubic feet is to square the diameter and multiply by ·7854, and by the depth in feet; or roughly, to square half the diameter and multiply by the depth and by 31/7.
Fig. 65.
Fig. 65.
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 271/2gal. 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.
DRYING-SHEDS FOR LEATHER.
Theprimitive 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 partitionsfrom 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 suppliedthrough 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 inFig. 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 45/8in. 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 11/2in. wrought-iron, using about 3 of 11/2in. 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.
Fig. 66.
Fig. 66.
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 lighterthan 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 isconsumed 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]
[V]To illustrate the use of such tables, the following example may be given. To dry 100 butts in a turret, each containing 20 lb. of moisture, at least 20 × 1000 × 100 = 2,000,000 units of heat will be required to replace the loss by evaporation alone. As a 4-in. pipe at 300° gives off 669 units per foot per hour (see Table III.), about 125 ft. would give off 2,000,000 units per day. If we compare this with Mr. Hepburn's practical experience, supposing the 4 working floors of his turret to hold 100 butts each (a low estimate), and to dry in 10 days; we have 540 ft. for 40 butts or 1350 ft. for 100 butts a day; showing that more than 10 times the minimum is required in practice. Of course this allows for weather in which the air must be heated considerably before it will dry at all, for heat that escapes uselessly at the top and sides of the building, and for the fact that the pipes are not heated the whole time, and probably, on the average, to a much lower temperature.
[V]To illustrate the use of such tables, the following example may be given. To dry 100 butts in a turret, each containing 20 lb. of moisture, at least 20 × 1000 × 100 = 2,000,000 units of heat will be required to replace the loss by evaporation alone. As a 4-in. pipe at 300° gives off 669 units per foot per hour (see Table III.), about 125 ft. would give off 2,000,000 units per day. If we compare this with Mr. Hepburn's practical experience, supposing the 4 working floors of his turret to hold 100 butts each (a low estimate), and to dry in 10 days; we have 540 ft. for 40 butts or 1350 ft. for 100 butts a day; showing that more than 10 times the minimum is required in practice. Of course this allows for weather in which the air must be heated considerably before it will dry at all, for heat that escapes uselessly at the top and sides of the building, and for the fact that the pipes are not heated the whole time, and probably, on the average, to a much lower temperature.
Table III.—Heating Effect of Pipes freely exposed to Air at 60° F.
It may be taken that1/20of 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.