EVAPORATION. A term generally used to ify the dissipation of the volatile parts of a compound body, whether caused by the action of the sun and atmosphere, or by artificial means ; although some authors restrict the use of the word to the former case, and employ the term vaporization in the latter. A distinction is likewise drawn in the case where the volatile parts are the objects of the process, which is then termed distillation ; but the fixed parts, or the residuum, are the products sought by evaporation. The vessels are accordingly different, evaporation being commonly carried on in open shallow vessels, and distillation in vessels nearly closed from the external air. The degree of heat must be duly regulated in evaporation. When the fixed and more volatile matters do not differ greatly in their tendency to fly off, the heat must be very carefully adjusted ; but in other cases this is less necessary. As evaporation consists in the assumption of the elastic form, its rapidity will be in proportion to the degree of heat and the diminution of the pressure of the atmosphere ; a current of air is likewise of service in this process. Dr. Ike, in his Chemical Dictionary, mentions the following method of evaporating liquors, as being practised in some large alum manufactories. A water-tight stone cistern, about three or four feet broad, two feet deep, and from twenty to forty feet long, is covered over by a low brick arch. At one extremity of this tunnel a grate is built, and at the other a lofty chimney. When the cistern is filled, and a strong fire kindled in the reverberatory grate, the flame and hot air sweep along the surface of the liquor, raise the temperature of the uppermost stratum almost instantly to near the boiling point, and draw it off in vapour. The Doctor observes, that the great extent, rapidity, and economy of this process recommend it to general adoption on a large scale.
More recently, Mr. Jacob Perkins has obtained a patent for a novel mode of forming steam of very high pressure, by confining the water under mechanical pressure, in a boiler properly constructed for the purpose and intensely heated; and subsequently he obtained another patent for an adaptation of that appa ratus for the evaporating of water and other fluids. For this purpose the
high pressure steam is projected from the generator through pipes which circulate through the evaporating pans or boilers; and such an arrangement is made, that the water produced by the condensation of the steam is returned into the generator by means of valves and a force pump, the steam or water being always under mechanical pressure. The preceding engravings give a sectional elevation and a plan, and the same letters of reference apply to similar parts in each figure. a is the generator ; b the forcing pump ; c a pipe opening into, and projecting from the upper part of the generator at d, and opening to, and projected from the lower part of it, opposite to f is a pipe leading from the pipe c to the forcing pump; g is a valve ; h a vessel, containing the liquid to be boiled or evaporated ; and j a safety valve. At 1, in the plan, is a valve opening into the generator ; it is not seen in Fig.1, but is in a line with the part marked e in that figure. When steam is admitted from the generator into the pipe c c, it becomes condensed in heating the surrounding fluid in the vessel A, and collects in the form of water at the valve g. Upon raising the handle of the forcing pump, the valve g opens, and the water fills that portion of the pipe marked f, between the pump and the valve g, the pressure in the generator keeping the valve at the opening 1 shut. When the handle of the force pump is depressed, the valve g shuts, and the water being prevented, in consequence, from returning into the pipe c, necessarily forces open the valve at 1, and is returned into the generator; and this operation is of course successively repeated at every stroke of the piston rod of the pump.