By evaporation is meant the vaporising o. a fluid by means of diminishing the atmospheric pressure, or exposing it to heat or a dry atmosphere; or the heat may be combined either with diminished pressure (boiling in vacuo), or with a dry atmosphere. It is resorted to for 2 distinct purposes: (a) for the sake of the material from which the vapour is liberated, (6) for the sake of the vapour itself. The former class only is to be dealt with here, the latter being chiefly represented by the evaporation of water to produce steam.
Evaporation is essentially a surface operation, hence a leading principle is the exposure of a maximum surface. Another point to be considered is facility for the escape of the vapour generated, preventing its impeding the progress of the operation by pressing on the surface. A third consideration is the avoidance of condensation of the liberated vapour by contact with a cold surrounding medium, either solid or vaporous, so that it can fall back into the mass undergoing evaporation.
The ordinary basis of calculation for evaporating surface is that 10 sq. ft. of heated surface will evaporate 1 lb. of water per minute; and that a thin copper tube exposing 10 ft. of surface will condense about 3 lb. of steam per minute, with a difference of temperature of about 90° F., in other words 30° F. per lb. Hence, steam employed for evaporating purposes should be at 212° + 30° = 242° F.
It has been established that evaporation is the only normal mode of vaporisation of liquids. Gernez has also shown that with all liquids evaporated at temperatures above the boiling point, there is a rate of evaporation which remains constant at every temperature, whatever may be the surrounding temperature, and that the rate of evaporation is sensibly independent of the nature of the medium into which the vapour is disengaged. The duration of evaporation of a column of liquid of determined height, measured when disengaged freely into the atmosphere, and when ignited at the extremity of a tube, proved this; a column of carbon bisulphide, 50 millimetres in height, heated to 90°, discharged itself into the atmosphere in 2 minutes 26 seconds, and in 2 minutes 27 seconds when the vapour was ignited at the end of a tube. In the same tube, containing the same quantity of liquid heated to 100°, the period of evaporation was 1 minute 46 seconds, whether the vapour was ignited or not. The rapidity of evaporation is inversely as the diameter of the tubes in which the evaporation is conducted, as the following numbers show : -
Diameters in millimetres. 15 5 3 2 1 0.35 0.2
Rate of evaporation. 1 2.2 2.7 3.6 10 21.90 30.0
In dealing with means of conducting artificial evaporation from a technical point of view, it will be convenient to adopt a classification based on the nature of the article treated and the conditions demanded by it. The chief kinds of liquids subjected to evaporation are: - (I) Saline solutions, for the purpose of recovering their solid contents in crystalline form; (2) syrups, necessitating precautions against organic changes; and (3) acids, with a view of rendering them more highly concentrated.