The Principles of Leather Manufacture

by H. R. Procter

Screw-fans are good for moving large volumes of air at comparatively low velocities, and against little or no resistance, but they are quite unsuitable for forcing air against high resistance, or through narrow channels, and for this purpose centrifugal fans like the Capel ([Fig. 95]) are much more suitable, and mechanically more efficient. In any case there is much loss of power in forcing air through narrow airways, and if a screw fan must be employed for the purpose, the channel should be as large in section as the area of the fan, and all sharp angles in its course should be avoided. There is great loss of power where a current of air or water has to pass suddenly either from a wider to a narrower channel, or the reverse, and in both cases the resistance is diminished by making the enlargement or contraction gradual or “bell-mouthed.” Thus a pipe conveying water at a given head into or out of a cistern will discharge a much larger quantity, if the ends are bell-mouthed, than if it terminates abruptly. For the same reasons, air suffers considerable resistance if it has to pass suddenly into, or out of a larger space, such as a drying room; and unnecessary partitions, and other abrupt changes of dimension in the current should be avoided. Curves should also take the place of angles as much as possible.

Fig. 95.—Capel Centrifugal Fan.

Systems in which air is drawn or forced over systems of heating pipes by a centrifugal fan, and then distributed through comparatively small airways among the leather which is to be dried are in some cases convenient and advantageous. Among these may be mentioned the Sturtevant and the Seagrave-Bevington. There can be no valid patent on the general principle of heating by distributing air in this way, but only on the particular arrangement or appliances used in the special case. Centrifugal fans should be considerably larger in diameter than in axial length, those with long vanes of small radius being wasteful in power from the insufficient supply of air to the centre. There is also no reason why, in some cases, centrifugal fans should not be substituted for screw-fans in drying on the system which I first described, especially in cases where the air has to encounter considerable resistance, as for instance in traversing a filter to remove dust. One of the best filters for this purpose is a table of wire-gauze covered to a depth of 3 or 4 inches with loose wool. Hair or cheaper fibrous materials may be substituted for the wool, but are less efficient. The air must of course be sucked downwards through the gauze. When the wool becomes dirty, it may be washed, if possible in a wool- or hair-washing machine, and again spread on the table in a damp condition, as it will quickly be dried by the current of air. Flannel is also useful where the wool-filter is impracticable, but requires frequent washing.

Apart from wind, natural ventilation is seldom to be relied on for drying on any considerable scale. Heated air is, of course, lighter than cold, and this is the cause of chimney-draught, but to get a good circulation in this way, a high shaft, and high temperature is required. Nevertheless, in one of its best forms, the method has been a good deal used in America, in the so-called “turret-dryer,” a building of seven or eight stories in height, constructed of wood with latticed floors, and heated by steam-piping at the bottom, where the air is admitted. The method is not likely to be much used in this country, as apart from the questions of cost of building, fire-risk, and trouble of raising and lowering the leather, a good draught will only be obtained when the outer temperature is low in comparison to that inside, and in our milder and moister climate the conditions are not nearly so favourable as in the United States. As the air is rendered heavier by the cooling of evaporation to a larger extent than it is lightened by the water vapour, there is a tendency in drying by upward ventilation for the warm air to form local upward currents, while the cold and damp air falls back; and from this irregularity of flow, it is difficult to saturate the air equally. This may be avoided by downward ventilation, in which the warm air is admitted at the top of the drying room and the cold and damp air allowed to escape at the bottom. This fact suggests that in using systems of drying such as the Sturtevant, it would be better to place the distributing pipes at the top rather than the bottom of the room, but in this case care would have to be taken that there were no openings left by which the air could escape at the top of the room without descending through the leather. If this be avoided, the warm air will float on the top of the colder and damper, and press it uniformly down and out. I believe the merit of first having applied the principle of downward ventilation to leather-drying is due to Edward Wilson of Exeter. It is necessary that the hot air should be forced in at the top, or the cold air sucked out from the bottom; and the mere placing of hot pipes near the top of the room ([p. 436]) will not cause the required circulation. Wilson placed his heating pipes in a partitioned space at the side of the room, at the bottom of which cold air was admitted from the outside, which escaped into the room at the top. As the temperature of this side chamber was high and the air consequently light, an upward current was produced in it, though probably somewhat inefficiently, as the height of the column of heated air could only be small. Assisted by a fan, and circulating a part of the air, the method should give good results, especially over two (latticed) floors. As the air could not be satisfactorily heated in its downward course, the method would not be suited for more than about two floors, and the drying in the lower room would be cool and gentle.

One or two points in the practical arrangement of steam-pipes may be mentioned, as they are often overlooked even by professional engineers. The steam must always be admitted at the highest point in the system, and there must be a steady descent, without hollow places where condensed water can accumulate, to the steam-trap by which it is removed. In horizontal pipes, about 1 inch descent in 100 is sufficient. If water accumulates, there is not merely serious danger in case of frost, but during use, by the sudden condensation of the steam, a vacuum is frequently formed, into which the water is shot like the liquid in a “water hammer,” producing violent and noisy concussions, and in some cases even fracture of the pipes, or loosening of their joints. If high-pressure steam is used, a very small supply-pipe will feed a considerable system of heating pipes or radiators, but with exhaust steam, great pains should be taken to have pipes of ample size, to avoid back-pressure on the engines. In both cases it is often convenient to arrange the pipes, not as a continuous line, in which drainage is generally difficult, but in parallels like the bars of a gridiron. With high-pressure steam, there need be no fear, if the pipes are kept clear of air by allowing a little escape through small air-taps, of the steam failing to find its way to all parts of the pipe, as a vacuum is produced by condensation in proportion to the heat given off. With exhaust-steam, no steam-trap is desirable, but any steam not condensed should escape freely into the open air or a chimney (after separating condensed water), and it is well to render the resistance in all the pipes of a gridiron approximately equal, which may be done by admitting steam at one corner, and allowing it to escape at the opposite (diagonal) one. In the arrangement of steam-pipes in parallels, the practicability of repair to one pipe or joint without interfering with the others must always be considered. If screwed wrought-iron pipes are used, each parallel must be provided with a bolted flange, or “running socket,” to permit of unscrewing. The difficulty of accurately adjusting the lengths of the several parallels must be considered, especially with flanged metal pipes, and also their motion by expansion when hot, which amounts to 1 or 2 parts per 1000 of length according to the temperatures of steam and air. Expansion-joints with stuffing boxes are costly and troublesome, and apt to leak, and may in many cases be avoided by suitable arrangement of the pipes. Thus instead of having the pipes rigidly fixed at both ends, one end of the system may be left free to move, each pipe being separately returned to an exit pipe at the same end but lower in level than the supply; or a single exit pipe may be thus returned, its expansion and contraction being practically the same as that of the heating pipes. In moderate lengths of wrought-iron pipe, sufficient relief may often be obtained from the flexure of the pipe, if in some part of its course it is carried at right angles to its general direction, which is often necessary for other reasons. If pipes are laid in long lengths, the loose end should be supported on rollers or short pieces of pipe, so as to avoid moving the supports or straining the pipe in expansion.

It is useless to attempt to regulate the temperature of low pressure steam-pipes by turning down the steam, since, so long as the pipe is supplied with sufficient steam to fill it, its temperature cannot be less than 100°, and even with high-pressure pipes, the power of regulation by altering the steam-pressure is very limited. It is far better to arrange the pipes or radiators in groups, from some of which the steam can be turned off entirely when less heat is needed. It must not be forgotten that if these discharge into a common steam-trap, it will be necessary to turn off their exits as well as their steam supply, or steam will come back into them from the other pipes, and probably prevent the escape of condensed water. In some cases it is more convenient to give the several sections independent exits or steam-traps.

Many good steam-traps are now on the market, depending either on the expansion and contraction of metals, or on floats in a closed box, which open a valve as the water accumulates. Traps of the latter class with closed copper balls are to be avoided, as the ball is sure eventually to become filled with water. Several traps have been devised in which an open vessel is used as a float, which is always kept empty by the discharge of the water through a pipe dipping into it.

The condensed water from steam-pipes is rarely suitable for use in the tannery, from the dissolved and suspended iron-oxide which it contains, from which it can only be freed by boiling and filtering, or treatment with precipitants ([p. 95]). Its most appropriate use is generally return to the boiler. Systems were formerly in vogue by which it was allowed to run back to the boiler as it condensed, but these could only answer when the pressure in the pipes was equal to that in the boiler, which is rarely the case. It must generally be forced in by the feed-pump or injector.

Hot water has often been advocated in preference to steam for heating, but is more costly, as it requires a separate boiler, and much larger pipe-surface for the same effect. Its only important advantage is that the pipes maintain their heat for some time, even when the fire has gone down, while steam-pipes cool at once if steam is allowed to go down in the boiler. In any considerable tannery, however, this will seldom or never be the case, since if a good pressure of steam is up at night, when the fires are banked up, the boiler will in itself contain a large reserve of heat, and, of course, working pressure will be required before the engines can start in the morning. Hot water systems require careful planning to obtain reliable and uniform circulation.