EVAPORATION

(Eng. and Fr.; Abdampfen ; Abdunsten , Germ.) is the proce by which any substance is converted into, and carried off in, vapour. Though ice, camphor, and many other solids evaporate readily in dry air, I shall consider, at present, merely the vaporization of water by heat artificially applied. The vapour of water is an elastic fluid, whose tension and density depend upon the temperature of the water with which it is in contact. Thus the vapour rising from water heated to 165° F. po e es an elastic force capable of supporting a column of mercury 10·8 high; and its density is such that 80 cubic feet of such vapour contain one pound weight of water; whereas 32 1 ⁄ 2 cubic feet of steam of the density corresponding to a temperature of 212° and a pre ure of 30 inches of mercury, weigh one pound. When the temperature of the water is given, the elasticity and specific gravity of the vapour emitted by it, may be found. Since the vapour rises from the water only in virtue of the elasticity due to its gaseous nature, it is obvious that no more can be produced, unle what is already incumbent upon the liquid have its tension abated, or be withdrawn by some means. Suppose the temperature of the water to be midway between freezing and boiling, viz. 122° Fahr., as also that of the air in contact with it, to be the same but replete with moisture, so that its interstitial spaces are filled with vapour of corresponding elasticity and specific gravity with that given off by the water, it is certain that no fresh formation of vapour can take place in these circumstances. But the moment a portion of vapour is allowed to escape, or is drawn off by condensation to another ve el, an equivalent portion of vapour will be immediately exhaled from the water. The pre ure of the air and of other vapours upon the surface of water in an open ve el, does not prevent evaporation of the liquid; it merely retards its progre . Experience shows that the space filled with an elastic fluid, as air or other gaseous body, is capable of receiving as much aqueous vapour as if it were vacuous, only the repletion of that space with the vapour proceeds more slowly in the former predicament than in the latter, but in both cases it arrives eventually at the same pitch. Dr. Dalton has very ingeniously proved, that the particles of aeriform bodies present no permanent obstacle to the introduction of a gaseous atmosphere of another kind among them, but merely obstruct its diffusion momentarily, as if by a species of friction. Hence, exhalation at atmospheric temperatures is promoted by the mechanical diffusion of the vapours through the air with ventilating fans or chimney draughts; though under brisk ebullition, the force of the steam readily overcomes that mechanical obstruction. The quantities of water evaporated under different temperatures in like times, are proportional to the elasticities of the steam corresponding to these temperatures. A ve el of boiling water exposing a square foot of surface to the fire, evaporates 725 grains in the minute; the elasticity of the vapour is equivalent to 30 inches of mercury. To find the quantity that would be evaporated from the same surface per minute at a heat of 88° F. At this temperature the steam incumbent upon water is capable of supporting 1·28 inch of mercury; whence the rule of proportion is 30: 1·28 ∷ 725: 30·93; showing that about 31 grains of water would be evaporated in the minute. If the air contains already some aqueous vapour, as it commonly does, then the quantity of evaporation will be proportional to the difference between the elastic force of that vapour, and what rises from the water. Suppose the air to be in the hygrometric state denoted by 0·38 of an inch of mercury, then the above formula will become: 30: 1·28 - 0·38 ∷ 725: 21·41; showing that not more than 21 1 ⁄ 2 grains would be evaporated per minute under these circumstances. The elastic tension of the atmospheric vapour is readily ascertained by the old experiment of Le Roi, which consists in filling a gla cylinder (a narrow tumbler for example) with cool spring water, and noting its temperature at the instant it becomes so warm that dew ceases to be deposited upon it. This temperature is that which corresponds to the elastic tension of the atmospheric vapour. See Vapour , Table of. Whenever the elasticity of the vapour, corresponding to the temperature of the water, is greater than the atmospheric pre ure, the evaporation will take place not only from its surface, but from every point in its interior; the liquid particles throughout the ma a uming the gaseous form, as rapidly as they are actuated by the caloric, which subverts the hydrostatic equilibrium among them, to constitute the phenomena of ebullition. This turbulent vaporization takes place at any temperature, even down to the freezing point, provided the pneumatic pre ure be removed from the liquid by the air pump, or any other means. Ebullition always accelerates evaporation, as it serves to carry off the aqueous particles not simply from the surface, but from the whole body of the water. The vapours, exhaled from a liquid at any temperature, contain more heat than the fluid from which they spring; and they cease to form whenever the supply of heat into the liquid is stopped. Any volume of water requires for its conversion into vapour five and a half times as much heat as is sufficient to heat it from the freezing to the boiling temperature. The heat, in the former case, seems to be absorbed, being inappreciable by the thermometer; for steam is no hotter than the boiling water from which it rises. It has been therefore called latent heat ; in contradistinction to that perceived by the touch and measured by the thermometer, which is called sensible heat . The quantity of heat absorbed by one volume of water in its conversion into steam, is about 1000° Fahr.; it would be adequate to heat 1000 volumes of water, one degree of the same scale; or to raise one volume of boiling water, confined in a non-conducting ve el, to 1180°. Were the ve el charged with water so heated, opened, it would be instantaneously emptied by vaporization, since the whole caloric equivalent to its constitution as steam, is present. When, upon the other hand, steam is condensed by contact with cold substances, so much heat is set free as is capable of heating five and a half times its weight of water, from 32° to 212° F. If the supply of heat to a copper be uniform, five hours and a half will be required to drive off its water in steam, provided one hour was taken in heating the water, from the freezing to the boiling pitch, under the atmospherical pre ure. Equal weights of vapour of any temperature contain equal quantities of heat; for example, the vapour exhaled from one pound of water, at 77° F., absorbs during its one pound of water, at 212° F. The first portion of vapour with a tension = 30 inches, occupies a space of 27·31 cubic feet; the second, with a tension of 0·92 inch, occupies a space of 890 cubic feet. [29] Suppose that these 890 volumes were to be compre ed into 27·31 in a cylinder capable of confining the heat, the temperature of the vapour would rise from 77° to 212°, in virtue of the condensation, as air becomes so hot by compre ion in a syringe, as to ignite amadou . The latent heat of steam at 212° F. is 1180° - 180 = 1000; that of vapour, at 77°, is 1180 - 45 = 1135°; so that, in fact, the lower the temperature at which the vapour is exhaled, the greater is its latent heat, as Joseph Black and James Watt long ago proved by experiments upon distillation and the steam engine. [29] One pound avoirdupois of water contains 27·72 cubic inches; one cubic inch of water forms 1696 cubic inches of steam at 212° F.: therefore one pound of water will form 27·31 cubic feet of such steam: and 0·92: 30 ∷ 27·31: 890 cubic feet. From the preceding researches it follows, that evaporation may be effected upon two different plans :— 1. Under the ordinary pre ure of the atmosphere; and that either, A , by external application of heat to boilers, with a , an open fire; b , steam; c , hot liquid media . B , by evaporation with air; a , at the ordinary temperature of the atmosphere; b , by currents of warm air. 2. Under progre ively lower degrees of pre ure than the atmospheric, down to evaporation in as perfect a vacuum as can be made. It is generally affirmed, that a thick metallic boiler obstructs the pa age of the heat through it so much more than a thin one, as to make a considerable difference in their relative powers of evaporating liquids. Many years ago, I made a series of experiments upon this subject. Two cylindrical copper pans, of equal dimensions, were provided; but the metal of the one was twelve times thicker than that of the other. Each being charged with an equal volume of water, and placed either upon the same hot plate of iron, or immersed, to a certain depth, in a hot solution of muriate of lime, I found that the ebullition was greatly more vigorous in the thick than in the thin ve el, which I ascribed to the conducting substance up the sides, above the contact of the source of heat, being 12 times greater in the former case than in the latter. If the bottom of a pan, and the portions of the sides, immersed in a hot fluid medium, solution of caustic potash or muriate of lime, for example, be corrugated, so as to contain a double expanse of metallic surface, that pan will evaporate exactly double the quantity of water, in a given time, which a like pan, with smooth bottom and sides, will do immersed equally deep in the same bath. If the corrugations contain three times the quantity of metallic surface, the evaporation will be threefold in the above circumstances. But if the pan, with the same corrugated bottom and sides, be set over a fire, or in an oblong flue, so that the current of flame may sweep along the corrugations, it will evaporate no more water from its interior than a smooth pan of like shape and dimensions placed alongside in the same flue, or over the same fire. This curious fact I have verified upon models constructed with many modifications. Among others, I caused a cylindrical pan, 10 inches diameter, and 6 inches deep, to be made of tin-plate, with a vertical plate soldered acro its diameter; dividing it into two equal semi-cylindrical compartments. One of these was smooth at the bottom, the other corrugated; the former afforded as rapid an evaporation over the naked fire as the latter, but it was far outstripped by its neighbour when plunged into the heated liquid medium. If a shallow pan of extensive surface be heated by a subjacent fire, by a liquid medium, or a series of steam pipes upon its bottom; it will give off le vapour in the same time when it is left open, than when partially covered. In the former case, the cool incumbent air precipitates by condensation a portion of the steam, and also opposes considerable mechanical resistance to the diffusion of the vaporous particles. In the latter case, as the steam i ues with concentrated force and velocity from the contracted orifice, the air must offer le proportional resistance, upon the known hydrostatic principle of the pre ure being as the areas of the respective bases, in communicating ve els. In evaporating by surfaces heated with ordinary steam, it must be borne in mind that a surface of 10 square feet will evaporate fully one pound of water per minute, or 725 × 10 = 7250 gr., the same as over a naked fire; consequently the condensing surface must be equally extensive. Suppose that the ve el is to receive of water 2500 libs, which corresponds to a boiler 5 feet long, 4 broad, and 2 deep, being 40 cubic feet by measure, and let there be laid over the bottom of this ve el 8 connected tubes, each 5 inches in diameter and 5 feet long, po e ing therefore a surface of 5 feet square. If charged with steam, they will cause the evaporation of half a pound of water per minute. The boiler to supply the steam for this purpose must expose a surface of 5 square feet to the fire. It has been proved experimentally that 10 square feet surface of thin copper can condense 3 libs of steam per minute, with a difference of temperature of 90 degrees Fahr. In the above example, 10 square feet evaporate 1 lib. of water per minute; the temperature of the evaporating fluid being 212° F., consequently 3: 1 ∷ 90: 90 3 . During this evaporation the difference of the temperature is therefore = 30°. Consequently the heat of the steam placed in connection with the interior of the boiler, to produce the calculated evaporation should be, 212 + 30 = 242°, corresponding to an elastic force of 53·6 inches of mercury. Were the temperature of the steam only 224, the same boiler in the same time would produce a diminished quantity of steam, in the proportion of 12 to 30; or to produce the same quantity the boiler or tubular surface should be enlarged in the proportion of 30 to 12. In general, however, steam boilers employed for this mode of evaporation are of such capacity as to give an unfailing supply of steam. IMG:4147767755307473660_illo0446.png:Evaporation in vacuo I shall now illustrate by some peculiar forms of apparatus, different systems of evaporation. Fig. 381. explains the principles of evaporating in vacuo. A B represents a pan or kettle charged with the liquor to be evaporated. The somewhat wide orifice c , secured with a screw-plug, serves to admit the hand for the purpose of cleaning it thoroughly out when the operation is finished; h is the pipe of communication with the steam boiler; b is a tube prolonged and then bent down with its end plunged into the liquor to be evaporated, contained in the charging back, (not shown in the figure). H is a gla tube communicating with the vacuum pan at the top and bottom, to shew by the height of the column the quantity of liquid within. The eduction evaporating pipe c is provided with a stop-cock to cut off the communication when required. i is a tube for the discharge of the air and the water from the steam-case or jacket; the refrigerator E is best formed of thin copper tubes about 1 inch in diameter, arranged zig-zag or spirally like the worm of a still in a cylinder. The small air-tight condenser F , connected with the efflux pipe f of the refrigerator, is furnished below with a discharge cock g , and surrounded by a cooling case, for the collection of the water condensed by the refrigerator. In its upper part there is a tube k , also furnished with a cock, which communicates with the steam boiler, and through which the pan A B is heated. The operation of this apparatus is as follows: after opening the cocks C , f , g , and before admitting the cold water into the condenser E , the cock of the pipe k is opened, in order that by injecting steam it may expel the included air; after which the cocks k and g are to be shut. The water must now be introduced into the condenser, and the cock b opened, whereon the liquid to be evaporated rises from the charging back, through the tube b , and replenishes the vacuum pan to the proper height, as shown by the register gla tube H . Whenever the desired evaporation or concentration is effected, the cock C must be closed, the pipe k opened, so as to fill the pan with steam, and then the efflux cock a is opened to discharge the residuary liquor. By shutting the cocks a and k , and opening the cock b , the pan will charge itself afresh with liquor, and the operation will be begun anew, after b has been shut and C opened. The contents of the close water cistern F , may be drawn off during each operation. For this purpose, the cock f must first be shut, the cold water is to be then run out of the condenser G , and k and g are to be opened. The steam entering by k makes the water flow, but whenever the steam itself i ues from the cock g , this orifice must be immediately shut, the cock f opened, and the cold water again introduced, whereupon the condensed water that had meanwhile collected in the under part of the refrigerator, flows off into the condenser ve el F . Since some air always enters with the liquor sucked into the pan, it must be removed at the time of drawing off the water from the two condensers, by driving steam through the apparatus. This nece ity will be le urgent if the liquor be made to boil before being introduced into the vacuum pan. Such an apparatus may be modified in size and arrangement to suit the peculiar object in view, when it will be perfectly adapted for the concentration of extracts of every kind, as well as saline solutions containing vegetable acids or alkalis. The interior ve el of A B should be made of tinned or plated copper. For an account of Howard’s vacuum pan, made upon the same principle, see Sugar . When a boiler is set over a fire, its bottom should not be placed too near the grate, lest it refrigerate the flame, and prevent that vivid combustion of the fuel e ential to the maximum production of heat by its means. The evil influence of leaving too little room between the grate and the copper may be illustrated by a very simple experiment. If a small copper or porcelain capsule containing water be held over the flame of a candle a little way above its apex, the flame will suffer no abatement of brightne or size, but will continue to keep the water briskly boiling. If the capsule be now lowered into the middle of the flame, this will immediately lose its brightne , becoming dull and smoky covering the bottom of the capsule with soot; and, owing to the imperfect combustion, though the water is now surrounded by the flame, its ebullition will cease. IMG:4147767755307473660_illo0447a.png:Fuel-efficient evaporating coppers Fig. 382. is a section of two evaporating coppers en suite , so mounted as to favour the full combustion of the fuel. A is the hearth, in which wood or coal may be burned. For coal, the grate should be set higher and be somewhat smaller, a is the door for feeding the fire; d , an arch of fire-bricks over the hearth; c , a grate through which the ashes fall into the pit beneath, capable of being closed in front to any extent by a sliding door b . B and C are two coppers encased in brickwork; f the flue. At the end of the hearth near m , where the fire plays first upon the copper, the sole is made somewhat lower and wider, to promote the spreading of the flame under the ve el. The second copper, C , receives the benefit of the waste heat; it may be placed upon a higher level, so as to discharge its concentrated liquor by a stop-cock or syphon into the first. When coals are burned for heating such boilers, the grate should be constructed as shown in the figure of the brewing copper , page 116 . Fig. 383. represents a pan for evaporating liquids, which are apt, during concentration, to let fall crystals or other sediment. These would be injured either by the fire playing upon the bottom of the pan, or, by adhesion to it, they would allow the metal to get red hot, and in that state run every risk of being burnt or rent on the sudden intrusion of a little liquor through the incrustation. When large coppers have their bottoms planted in loam, so that the flame circulates in flues round their sides, they are said to be cold-set . IMG:4147767755307473660_illo0447b.png:Evaporating pan A is a pear-shaped pan, charged with the liquid to be evaporated; it is furnished with a dome cover, in which there is an opening with a flange f , for attaching a tube, to conduct the steam wherever it may be required. a is the fire-place; b , the ash-pit. The conical part terminates below in the tube g , furnished with a stop-cock at its nozzle h . Through the tube c d c′ , furnished above and below with the stop-cocks c and c′ , the liquid is run from the charging back or reservoir. During the operation, the upper cock c is kept partially open, to replace the fluid as it evaporates; but the under cock c′ is shut. The flame from the fire-place plays round the kettle in the space e , and the smoke escapes downwards through the flue i into the chimney. The lower cylindrical part g , remains thus comparatively cool, and collects the crystalline or other solid matter. After some time, the under stop-cock c′ , upon the supply-pipe, is to be opened to admit some of the cold liquor into the cylindrical neck. That cock being again shut, the sediment settled, and the large stop-cock (a horizontal slide-valve would be preferable) h opened, the crystals are suffered to descend into the subjacent receiver; after which the stop-cock h is shut, and the operation is continued. A construction upon this principle is well adapted for heating dyeing coppers, in which the sediment should not be disturbed, or exposed to the action of the fire. The fire-place should be built as for the brewing copper . IMG:4147767755307473660_illo0448.png:Another evaporating pan Fig. 384. represents an oblong evaporating pan, in which the flame, after beating along its bottom, turns up at its further end, plays back along its surface, and pa es off into the chimney. A is a rectangular ve el, from 10 to 15 feet long, 4 to 6 feet broad, and 1 or 1 1 ⁄ 2 feet deep. The fire-bricks, upon which the pan rests, are so arranged as to distribute the flame equably along its bottom.