[180] A gram-calorie of one-thousandth part of the above is also in use for some scientific purposes, but the kilogram-calorie only is used in the following pages.

[181] This is equal to 76·04 kilogrammeters per sec., but the metrical horse-power is only taken at 75 kilogrammeters in France and Germany.

In evaporating liquids in the open pan 536 calories is required to evaporate 1 kilo of water already raised to boiling temperature, and a larger amount for salt-solutions, and it makes comparatively little difference whether this is done at 100° or at a lower temperature. Where, however, evaporation is done in vacuo, considerable economy can be effected by what are known as multiple “effects,” in which the steam from one vacuum-pan is employed to boil a second under a reduced pressure, and consequently boiling at a lower temperature. This principle can be practically applied to as many as five or six successive “effects,” the weaker liquor being usually evaporated at the highest temperature and lowest vacuum in the first “effect,” by the exhaust steam of the engine used for the vacuum pumps, while the steam from the first effect heats that of the next higher concentration, and so on. In the Yaryan evaporator ([p. 339]), the boiling liquid is sprayed through coil-tubes, thus exposing an enormous surface to evaporation, and the whole concentration of any given portion of liquid takes place as it passes through the apparatus, which does not, even in multiple effects, occupy more than 4 or 5 minutes; and without the temperature of the liquid ever rising above 60° or 70° C. In the case of liquids, like sugar- and tannin-solutions which are liable to chemical change from continued heating, the shortness of the time is a very great advantage. The number of effects which it is desirable to use depends greatly on the cost of fuel as compared to the largely increased cost of the apparatus. 1 lb. of coal employed in raising steam will evaporate 81⁄2 lb. in a single-effect Yaryan, 16 lb. in a double-effect, 231⁄2 lb. in a triple, 301⁄2 lb. in a quadruple, and 37 lb. in a quintuple-effect apparatus.

Where liquids are evaporated in the open air at temperatures below boiling, it is advisable by some means to spread the liquid in a thin film, so as to expose a large surface, which must be continuously removed by agitation, so as to prevent the formation of a skin. A good apparatus for this purpose is the Chenalier evaporator ([Fig. 92]), which consists of steam-heated copper discs rotating in a trough containing the liquid, which is taken up by buckets attached to the rims of the discs, and poured over their heated surfaces. In other forms, the liquid is allowed to trickle over steam-heated pipes or corrugated plates. Such evaporators should be placed in a current of air so as to rapidly carry off the vapour formed. Their use is very objectionable for liquids, like tannin-liquors, which are injured by oxidation, and they are not nearly so economical as vacuum-pans.

The drying of leather depends on the same laws as the evaporation of liquids, but demands special consideration from its very different conditions of temperature and supply of heat. It is important to remember that evaporation cannot go on unless the vapour-pressure of the liquid to be evaporated is higher than that of the vapour in contact with it, and that air-pressure does not prevent evaporation, so that if we sweep away the stagnant vapour with dry air, evaporation will go on as quickly as in vacuo, except that the liquid cannot boil. We must also bear in mind that evaporation consumes quite as much heat at low temperatures as in a steam boiler, and that this heat must generally come from the surrounding air, the temperature of which it reduces.

Fig. 92.—Chenalier Evaporator and Glue Coolers.

The rapidity of evaporation, and the quantity of moisture which can be taken up by a given volume of air depends on the vapour-pressure, which increases with temperature. The relation between the two, and the weight of water in grams per cubic meter which can be dissolved in dry air is given in the following table. (Grams per cubic meter is practically equivalent to ounces per 1000 cubic feet. Vapour-pressure is given in millimeters of mercury of the barometer, [p. 422].)

Vapour Pressure of Water.

Temperature,°C-10-50510152025303540
„°F14233241505968778695104
Pressure, mm.2·23·24·66·59·112·717·423·531·541·954·9
Grams per cb. m.2·43·44·96·89·312·817·222·830·139·2..