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 progressively lower degrees of pressure 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
passage 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 vessel, 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 across 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 less 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 issues with concentrated
force and velocity from the contracted orifice, the air must offer less
proportional resistance, upon the known hydrostatic principle of the
pressure being as the areas of the respective bases, in communicating
vessels.
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 vessel 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 vessel 8
connected tubes, each 5 inches in diameter and 5 feet long, possessing
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.
[Illustration: 381]
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 glass
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 glass 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 issues 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
vessel 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 necessity
will be less 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 vessel 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 essential 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 brightness 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 brightness, 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.
[Illustration: 382]
_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 vessel. 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_.
[Illustration: 383]
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.
[Illustration: 384]
_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 passes off into the chimney. A is a rectangular
vessel, 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.
EUDIOMETER, is the name of any apparatus subservient to the chemical
examination of the atmospheric air. It means a _measure of purity_, but
it is employed merely to determine the proportion of oxygen which it may
contain. The explosive eudiometer, in which about two measures of
hydrogen are introduced into a graduated glass tube, containing five
measures of atmospheric air, and an electric spark is passed across the
mixture, is the best of all eudiometers; and of these the syphon form,
proposed by me in a paper published by the Royal Society of Edinburgh in
1819, is probably the surest and most convenient. Volta’s explosive
eudiometer as made in Paris, costs 3 guineas; mine may be had nicely
graduated for 6 or 8 shillings.
EXPANSION (Eng. and Fr.; _Ausdehnung_, Germ.), is the increase of bulk
experienced by all bodies when heated, unless a change of chemical
texture takes place, as in the case of clays in the potter’s kiln. Table
I. exhibits the linear expansion of several solids by an increase of
temperature from 32° to 212° Fahr.; Table II. exhibits the expansion in
bulk of certain liquids.