INCANDESCENT BURNERS.
A review of gas-burners would scarcely be complete without some
reference to the incandescent burners of M. Clamond and Mr. Lewis.
Although their dependence upon an artificially produced blast or
current of air removes them from the list of appliances applicable to
ordinary conditions, the remarkable results which they afford, not
less than their originality, demand for them at least a passing
notice. The production of light by the agency of these burners is
brought about in a manner altogether different, and is due to quite
other causes than those which are concerned in the production of an
ordinary illuminating gas flame. In the latter case, the illuminating
power developed is solely due to the hydrocarbons contained in the
gas, which are decomposed by the heat of the flame, the separated
carbon being raised to a white heat. In the former, the illuminating
power is not obtained directly from the gas; but advantage is taken of
the heat of the flame, enhanced by the application of a blast of air,
to raise to incandescence some refractory foreign material, which
latter is thus made to give out light. In the Clamond burner this
refractory substance is a basket composed of magnesia, spun into
threads; in the Lewis burner it is a cage of platinum wire.
To the unthinking reader it may perhaps appear somewhat surprising
that results so remarkable as are yielded by these burners should be
obtained, while disregarding, as a source of light, the hydrocarbons
contained in gas, and employing them, in common with the other
constituents, solely as a source of heat. An explanation, however, is
readily forthcoming. As was shown in a former part of this
treatise,[16] the great bulk of ordinary coal gas consists of
constituents which, in the act of combustion, produce considerable
heat, but scarcely any light; the illuminating power developed in an
ordinary gas flame being almost wholly dependent upon the very small
proportion of heavy hydrocarbons which the gas contains. Thus, the
quantity of heat-producing elements contained in the gas being quite
disproportionate to the light-yielding hydrocarbons, there is always
produced, in an ordinary gas flame, more heat than is necessary for
effectively consuming the free carbon, which is liberated in the flame
by the decomposition of the heavy hydrocarbons. This is shown by the
fact that coal gas can usually be naphthalized--that is, impregnated
with the vapour of naphtha--to a considerable extent before the limit
of effective combustion is reached. The object aimed at in the
incandescent burners about to be described is to utilize, in the
development of illuminating power, the combined heat produced by the
combustion of all the constituents of the gas. To this end the heat
of combustion is brought to bear upon, and caused to raise to
incandescence, some refractory material, extraneous to, but brought
within the operation of the flame.
[16] See Chap. II., p. 21.
[Sidenote: Effect of injecting a blast of air into a gas flame.]
A further explanation of the superior results yielded by these burners
may be found in the employment of an artificial blast or current of
air. Indeed, without some such arrangement the desired end could not
be attained. The heat developed by the unaided flame is diffused over
too wide an area to raise the temperature of the heated substance to
the necessary degree of incandescence to enable it to give out
sufficient light. By injecting a current of air into its midst, the
flame is condensed into a smaller compass; and is brought to bear more
directly upon the precise locality where its heat may be most
effectively employed. Thus, although the total quantity of heat
developed remains exactly the same as before, it is concentrated upon
a smaller surface of the refractory substance; and the latter is
consequently more intensely heated, or, in other words, raised to a
more exalted temperature. The very superior illuminating power which
is thereby obtained is due to the circumstance that the quantity of
light yielded by an incandescent body increases in a higher ratio than
the temperature to which it is raised.
[Sidenote: Lewis's incandescent gas-burner.]
Proceeding now to describe the burners. The one invented by Mr. Lewis
(various forms of which are illustrated on the next page) consists of
an upright tube, connected at its base to the gas supply, and
surmounted by a cap or cage of platinum wire gauze; which latter
constitutes a combustion chamber, as it is there that the mixture of
gas and air is consumed. Into the lower part of the upright tube the
nozzle of an air-pipe is inserted, through which a supply of air can
be injected, under pressure, into the burner, after the manner of a
blowpipe. There are also small branch tubes leading into the upright
gas-tube, and open to the atmosphere. Through these an additional
quantity of air enters the burner; being drawn or sucked in by the
agency of the main current, which flows through the upright tube. The
resemblance to an ordinary Bunsen burner is, therefore, very close.
The mixture of gas and air thus produced, when ignited, burns at the
platinum cap; the heat which is developed causing the latter to become
highly incandescent, and so to give out a brilliant light. To prevent
the conduction of heat from the incandescent platinum, through the
upright tube, a non-conducting material--such, for instance, as
steatite or porcelain--is interposed between the gauze cap and the
metal tube.
[Illustration: FIG. 26.--LEWIS'S INCANDESCENT GAS-BURNER.]
The light produced by this burner is said to approximate more closely
to daylight than that yielded by an ordinary gas flame (the colours of
textile fabrics, for instance, being shown as well by its aid as by
daylight); while, on account of its resulting from the incandescence
of a fixed body, instead of being emitted from a flame, it is
unaffected by a gust of wind, and maintains perfect steadiness under
every condition of weather. The illuminating power developed is stated
to be equal to 5 standard candles per cubic foot of gas consumed.
[Sidenote: Clamond's incandescent gas-burner.]
M. Clamond's burner, which is shown in fig. 27, is a much more
complicated apparatus than the preceding one, and not so easily
described; but its main features may be briefly enumerated as
follows:--The air (which, as in Mr. Lewis's burner, is supplied under
pressure) is divided, as it enters the apparatus, into two portions.
One portion is at once mixed with the gas; the remainder being
conveyed, through a peculiarly constructed tube composed of small
pieces of refractory material, to the combustion chamber, or "wick,"
as it is termed, of the burner. This "wick" is a small conical basket,
made of a kind of lacework of spun magnesia, which, when raised to
incandescence by the heat produced by the combustion of the gas,
furnishes the desired illumination. The mixture of gas and air is
subdivided, by a "distributor," into two portions, one of which goes
direct to the magnesia "wick," there to be burnt, while the other is
distributed among a number of tubes, forming so-called "auxiliary
burners," the flames of which are utilized to heat the chief air
supply; being directed upon the sides of the before-mentioned tube of
refractory material, through which it is conveyed. By this means the
air is raised to a very high temperature (1000 deg. C., or 1800 deg. Fahr., it
is said) before it impinges upon the flame. The result is the
production of a most intense heat within the magnesia basket; the
latter being raised to brilliant incandescence, and so developing a
high illuminating power.
[Illustration: FIG. 27.--CLAMOND'S INCANDESCENT GAS-BURNER.]
The magnesia basket must be renewed after being in use a period of
from 40 to 60 hours, as it gradually deteriorates by the action of the
intense heat to which it is subjected; but as the cost is said to be
insignificant, this should not be a great drawback. The basket is
placed at the base of the burner, in order to obviate the shadow which
would otherwise be cast by the apparatus; and it is attached to the
main body of the apparatus by platinum wires. As to illuminating
power, the only particulars which have been made public refer to the
first two models constructed; one of which was said to develop a light
equal to that from 6.208 candles, and the other to 9.72 candles per
cubic foot of gas consumed.
[Illustration: FIG. 28.--CLAMOND'S IMPROVED INCANDESCENT BURNER.]
[Sidenote: Clamond's new burner.]
In a recently designed modification of the burner (which is shown in
the accompanying illustration) M. Clamond dispenses with an artificial
supply of air under pressure, and endeavours to obtain similar results
by other and simpler means. To this end the position of the magnesia
"wick" is reversed (it being placed at the top of the apparatus); the
current of gas is allowed to draw in upon itself a quantity of air by
a precisely similar arrangement to that adopted in the Bunsen burner;
while an additional supply of air is drawn upon the flame by the
accelerated draught produced by the aid of a glass chimney. As in the
more complicated and complete burner, the air supply is heated by
means of auxiliary burners in the interior of the apparatus. It has
been stated, on the authority of M. Clamond, that this modified burner
develops, from the gas consumed, a duty of about 6 candles per cubic
foot; being equal to the results yielded by the more complicated
apparatus. Should this be borne out in practice, M. Clamond will
have achieved a noteworthy success. It is, however, advisable to
reserve expressing any definite opinion of its merits until further
information is received, or until the burner has been tried in this
country.