Air is practically never dry, and in damp weather is frequently saturated with moisture to the full extent corresponding to its temperature. In England the average quantity of moisture contained in the air throughout the year is 82 per cent. of the total possible, and even in the driest summer weather it is never less than 58 per cent. So long as the water is in the form of vapour, the air remains quite clear and does not feel damp; in fogs, the air is not only saturated with moisture, but contains small liquid particles floating in it. Of course when the air is really saturated with moisture, it has no drying power whatever.
As is evident from the table, the amount of water which can be dissolved in a given volume of air rapidly increases with temperature. Air at 0° C. is only capable of containing 4·9 grams per cubic meter, or not much more than 20 per cent. of what it can contain at 25° C. It hence rapidly increases in drying power as it is warmed, and consequently the air in a warm well-ventilated drying room in winter is generally much drier, and has greater capacity for absorbing moisture than the open air in the driest summer weather. This is the principal cause of the tendency to harsh and irregular drying by the use of artificial heat; and may be remedied by a proper circulation of the air by a fan without too frequent change with the colder air outside. On the other hand the use of a little artificial heat in damp summer weather, when the air is saturated with moisture, may be quite as necessary as in winter. The amount of moisture in the air is most easily ascertained by a device known as the “wet and dry bulb thermometers.” This consists of two thermometers mounted on a board; one of which has the bulb covered with muslin, and kept moist by a lamp-wick attached to it, and dipping in a vessel of water. The temperature of the wet bulb is lowered by the heat consumed in evaporation, and the difference of its temperature from that of the dry bulb is proportionate to the drying power of the air. This may be approximately calculated in grams per cubic meter by multiplying the difference by 0·64 for Centigrade or 0·35 for Fahrenheit degrees; and if deducted from the total capacity for moisture corresponding to the temperature of the wet bulb as given in [table], [p. 426], will give the actual moisture in grams contained in a cubic meter of air; but for practical purposes, all that is necessary is to find by experience the temperature and difference between the wet and dry bulbs, which gives the best result for the drying required, and to maintain it as nearly as possible by regulation of the heating and ventilation. Cheap forms of the instrument are made for use in cotton-mills, where it is necessary to maintain a certain degree of moisture; or it may be improvised from two chemical thermometers which agree well together. Distilled (rain or steam) water should be used to moisten the bulb, or it will quickly become coated with lime salts, and it should be placed in a draught, or its indications will not be accurate.
It is of course obvious that not only the wet thermometer, but the wet hides or skins are cooled by evaporation, and they, in their turn, cool the air with which they are in contact, which not only becomes moistened, but is lessened in its capacity for moisture by cooling, and thus rapidly reaches a condition when it can absorb no more moisture. It is thus necessary to maintain its temperature by artificial heat, or to replace it constantly by fresh air from the outside, and which of these expedients is most economical will depend on the temperature of the air outside as compared with that which it is required to maintain. If the outside air is sufficiently warm, and not saturated with moisture, it is generally best to use it in large quantities without artificial heat, wind usually supplying the necessary motive power for its circulation. Wet goods from the pits may thus be dried to a “sammed” condition by any air which is not saturated, and above freezing point; though the drying will often be slow. For drying “off,” artificial heat is generally necessary, since the attraction of the fibre for the last traces of moisture is very considerable, and to remove it the drying power of the air must be considerably higher than that required for the evaporation of free water.[182] In drying stuffed leather a temperature must generally be maintained sufficient to keep the fats employed in partial fusion, and so permit their absorption by the leather, while at the same time the drying must be gradual, or the water may be dried out before the fats have time to take its place. This is generally best attained by the use of artificial heat, and ventilation by circulating the air by a fan without its too frequent renewal, especially in cold weather. Frequently air which has been heated and used for drying off finished goods, and so partially saturated with moisture, may be used with advantage for wet goods, or for other purposes where a more gentle drying is required. If the temperature is low outside, the amount of heat consumed in heating cold air to the temperature required may be very considerable. The weight of a cubic meter of air at 0° C. and atmospheric pressure is 1·293 kilos, and its specific heat at constant pressure is 0·2375 of that of water. Therefore to heat a cubic meter of air at ordinary pressure and temperature 1° C. will require the same amount of heat as that used to heat 0·3 kilo of water to the same extent, or in other words 0·3 of a k.-calorie. If steam-heating is used, 1 kilo of good coal burnt under the boiler should heat about 1800 cubic meters 10° C., or 1 lb. should heat 52,000 cubic feet 10° F., assuming that the condensed water is not cooled below 100° C. These seem large volumes, but if we reflect that a 48-inch Blackman fan may move 30,000 cubic feet per minute, we shall realise that the cost of coal in heating air is not inconsiderable.
[182] Commercially-dry leather generally, if unstuffed, contains about 15 per cent. of residual moisture, which varies in amount with the weather, and can be more or less completely removed by drying at high temperatures. If leather has been over-dried, it only slowly regains its weight on exposure to cold air. Commercial disputes not unfrequently arise on the dryness of leather. In the opinion of the writer, a customer can only claim that the leather should be sufficiently dry not to lose weight when exposed to dry air at the ordinary temperature and degree of dryness of a warehouse or factory, and claims based on re-drying in hot drying rooms are distinctly fraudulent.
We must now consider the heat consumed by the actual evaporation of the water in the leather. The actual evaporation of water already raised to 100° C. consumes 536 k.-calories, but the evaporation of water which has not previously been heated so far consumes more heat, and we may take that required at ordinary temperatures as in round numbers 600 k.-calories per kilo, or 1080 lb. × F. units per lb. Disregarding small fractions, this is equivalent to the cooling to the same temperature of an equal weight of steam in the heating pipes, and this, as we have seen, demands about 1⁄10 of its weight of coal for its production from water already heated to 100° C.
The cooling takes place, in the first instance, in the leather, the temperature of which is reduced like that of the wet-bulb thermometer; and this in its turn cools the air in contact with it. Thus in air-drying without artificial heat, the whole heat must be supplied by the air and the loss reduces its capacity for moisture, greatly increasing the volume required. This is not of much consequence in open-air drying, since even a light wind will supply air in enormous volume. A moderate breeze of ten miles an hour moves about 15 feet or 41⁄2 meters per second. When, however, the air must be moved by fans, the power required becomes important. The evaporation of 1 kilo of water at summer temperature will cool about 2000 cubic meters, and that of 1 lb. 32,000 cubic feet of air 1° C.
In calculating the ventilating and heating power required in fitting up drying rooms, it is usually necessary to ascertain that required under the most unfavourable circumstances, and then add a liberal margin to cover errors and accidents. As the calculations are, in consequence of the many varying conditions, somewhat complex, it may be convenient to give as examples the quantities of air and heat required to evaporate 1 kilo (2·205 lb.) of water under different ordinary conditions, and these may serve as a basis of calculation of the drying power which must be provided for different tanneries.
1. Indifferent Open-Air Drying.—Air at 10° C. (50° F.), wet-bulb thermometer 7° C. (44·3° F.), indicating a total capacity for moisture of about 2 grm. per cubic meter; air not to be cooled beyond 7·75° C. (46° F.), leaving a residual capacity for moisture of 0·5 grm. per cubic meter. Each cubic meter will therefore take up 1·5 grm. of moisture, and as 1 kilo contains 1000 grm. we have 10001·5 = 666 cubic meters per kilo required to absorb moisture; and 6002·25° × 0·3 = 888 cubic meters reduced 2·25° to furnish the 600 cal. required for evaporation. Total air used 1554 cubic meters or 54,900 cubic feet.
2. Drying with Heat.—Outside-air at 10° saturated with moisture, heated to 20° C. (68° F.) acquires a capacity for 7·9 grm. per cubic meter. If we assume that a drying capacity of 2 grm. per meter is required to complete the drying, we have an effective capacity of 5·9 grm.
10005·9 = 170 cubic meters or 6000 cubic feet, and to heat this 10° C. will require 510 cal. Evaporation of 1 kilo will consume 600 cal. Total heat 1110 cal.