It is very desirable, however, that the fan should be driven by a small separate engine, the steam for which will only form a small proportion of that required for heating, and of which the whole of the heat will be recovered, since even that utilised in driving the fan will again be converted into heat by the friction of the air, and will therefore cost nothing. This arrangement will enable the drying to proceed so long as the necessary steam is maintained, which in bad weather can easily be done by the night watchman. It may also be pointed out that, during a great part of the year, the goods can be dried to a “sammied” condition without heat, or in the open air, or in the case of dressing leather, a considerable part of the water can be removed by pressing or squeezing, effecting a further economy.

Fig. 93.—Blackman Fan.

It must be left to the reader to apply the same calculation to other sorts of leather than sole, but it may be pointed out that the essential point, as regards heating and ventilation, is the weight of water to be evaporated in a given time, and that the actual size and shape of the drying room is unimportant, so long as adequate heating and circulation of the air between the leather is secured; and these remarks also apply to the particular form of fan or other ventilation employed, and to the means of heating. As the quantity of heat consumed is very considerable, it is well to look out for sources of waste heat which can be employed, or for means by which the heat of the fuel can be more directly and completely utilised than it is in raising steam. Thus a large amount of heat can sometimes be obtained by passing air through pipes or “economisers” fitted in a chimney-flue;[184] or gilled stoves or “calorifers” may be used in a separate chamber to directly heat the air which is drawn in by the fan.

[184] These pipes should be provided with scrapers to remove soot as in Green’s economiser, or their efficiency will be much diminished.

Fig. 94.—Section of Drying Rooms with Fan.

[Large section] (150 kB)

[Figs. 93] and [94], furnished by the James Keith and Blackman Co., Ltd., give a good idea of the construction of screw fans, and the general principle of arrangement of fan drying rooms, the air in this case being circulated in opposite directions on two floors, and the amount of change being regulated by the shutters at A, etc. The grouping of pipes at the ends of the two floors which it shows is in general a good arrangement, but the length between them should not be too great, or the drying will be unequal in different parts of the room. Sometimes this is convenient; thus if most of the heat be supplied to the air coming fresh from the inlet on the upper floor, the damper and colder air of the lower room can be continuously used for drying wet goods from the yard, and the upper reserved for drying off the finished leather. A disadvantage of this plan is that open air drying can seldom be utilised except in an elevated building; and even when it is adopted, means should be provided for heating the lower room in cold weather. In place of two floors, it is obvious that a single floor may be divided into two compartments by a longitudinal partition. Whatever pipes are grouped at the ends of the building, it is advisable to arrange sufficient to prevent frost, against the walls, or in the old-fashioned way on the floors beneath the leather, but not too close to it, and protected by a wooden lattice on which the workmen can stand, which removes the risk of accident from wet leather falling on the hot pipes. The latticed space should be open at the end facing the air current, so as to receive a portion of the draught, which will become heated and ascend, its place being taken by damp and cold air from the leather, to be re-warmed. Water-vapour in itself is lighter than air, but the contraction produced by the cooling of evaporation more than compensates this, and the damp air is therefore heavier than the dry. The arrangement of hot pipes near the ceiling of a drying room, which has been borrowed from some American tanneries, is wrong in principle, unless the air is forced in at the upper part of the room, or the upper floor is latticed, and only acts in other cases when the air is thoroughly mixed and circulated by mechanical ventilators; while pipes near the floor will continue to produce a certain amount of circulation of the air, even when the fan is not running. In protecting pipes by lattices care should be taken not to confine them too closely, or their heating effect will be seriously diminished. In fan-drying, leather should be hung edgeways to the current of air, so as to allow of its free and uniform passage between. In the case of sole leather the butts or bends are conveniently suspended by S-hooks of brass or iron wire, to hooks or nails fixed in the joists. If gangways between the leather must be left in the direction of the draught, they should be closed at intervals in the length of the room by curtains or shutters, so as to deflect the air-current into the leather.

Screw fans like the Blackman can be used either to suck or to blow the air, though the former is preferable where it can be arranged, because it produces a more uniform current in the room. On the blowing side the air issues with considerable velocity in a sort of cone, but little coming through the centre of the fan, while that near the edges spreads rapidly from its centrifugal motion. This is rather advantageous where the fan blows into an open room, but involves waste of power where it discharges into narrow and square air-ways. The ends of the vanes of the Blackman are turned in at the rim of the fan to prevent this tangential discharge, but it is probable that where a fan is to blow into a room, it would be more advantageous to put it on the inner side of the wall, and without curved ends to the vanes, so as to distribute the air as widely as possible. A somewhat similar result would be attained with a Blackman, by placing it in a position the reverse of that for which it is intended, and running it also the reverse way; but its “efficiency” might possibly be lessened.