may be said that the reaction between calcium carbide and water is
exothermic, evolving either 14.0 or 29.1 calories according as the
byproduct is calcium oxide or solid calcium hydroxide; and therefore
either reaction proceeds without external assistance in the cold. The
reaction between carbide and slaked lime, however, is endothermic,
absorbing 1.1 calories; and therefore it requires external assistance
(presence of an elevated temperature) to start it, or continuous
introduction of heat (as from the reaction between the rest of the
carbide present and the water) to cause it to proceed. Of itself, and
were it not for the disadvantages attending the production of a
temperature remotely approaching 400° C. in an acetylene generator, which
disadvantages will be explained in the following paragraphs, there is no
particular reason why reaction (3) should not be permitted to occur, for
it involves (theoretically) no loss of acetylene, and no waste of calcium
carbide. Only one specific feature of the reaction has to be remembered,
and due practical allowance made for it. The reaction represented by
equation (2) proceeds almost instantaneously when the calcium carbide is
of ordinarily good quality, and the acetylene resulting therefrom is
wholly generated within a very few minutes. Equation (3), on the
contrary, consumes much time for its completion, and the gas
corresponding with it is evolved at a gradually diminishing speed which
may cause the reaction to continue for hours--a circumstance that may be
highly inconvenient or quite immaterial according to the design of the
apparatus. When, however, it is desired to construct an automatic
acetylene generator, _i.e._, an apparatus in which the quantity of
gas liberated has to be controlled to suit the requirements of any
indefinite number of burners in use on different occasions, equation (3)
becomes a very important factor in the case. To determine the normal
reaction (No. 2) of an acetylene generator, 64 parts by weight of calcium
carbide must react with 36 parts of water to yield 26 parts by weight of
acetylene, and apparently both carbide and water are entirely consumed;
but if opportunity is given for the occurrence of reaction (3), another
64 parts by weight of carbide may be attacked, without the addition of
any more water, producing, inevitably, another 26 parts of acetylene. If,
then, water is in chemical excess in the generator, all the calcium
carbide present will be decomposed according to equation (2), and the
action will take place without delay; after a few minutes' interval the
whole of the acetylene capable of liberation will have been evolved, and
nothing further can possibly happen until another charge of carbide is
inserted in the apparatus. If, on the other hand, calcium carbide is in
chemical excess in the generator, all the water run in will be consumed
according to equation (2), and this action will again take place without
delay; but unless the temperature of the residual carbide has been kept
well below 400° C., a further evolution of gas will occur which will not
cease for an indeterminate period of time, and which, by strict theory,
given the necessary conditions, might continue until a second volume of
acetylene equal to that liberated at first had been set free. In practice
this phenomenon of a secondary production of gas, which is known as
"after-generation," is regularly met with in all generators where the
carbide is in excess of the water added; but the amount of acetylene so
evolved rarely exceeds one-quarter or one-third of the main make. The
actual amount evolved and the rate of evolution depend, not merely upon
the quantity of undecomposed carbide still remaining in contact with the
damp lime, but also upon the rapidity with which carbide naturally
decomposes in presence of liquid water, and the size of the lumps. Where
"after-generation" is caused by the ascent of water vapour round lumps of
carbide, the volume of gas produced in a given interval of time is
largely governed by the temperature prevailing and the shape of the
apparatus. It is evident that even copious "after-generation" is a matter
of no consequence in any generator provided with a holder to store the
gas, assuming that by some trustworthy device the addition of water is
stopped by the time that the holder is two-thirds or three-quarters full.
In the absence of a holder, or if the holder fitted is too small to serve
its proper purpose, "aftergeneration" is extremely troublesome and
sometimes dangerous, but a full discussion of this subject must be
postponed to the next chapter.
EFFECT OF HEAT ON ACETYLENE.--The effect of excessive retention of heat
in an acetylene generator upon the gas itself is very marked, as
acetylene begins spontaneously to suffer change, and to be converted into
other compounds at elevated temperatures. Being a purely chemical
phenomenon, the behaviour of acetylene when exposed to heat will be fully
discussed in Chapter VI. when the properties of the gas are being
systematically dealt with. Here it will be sufficient to assume that the
character of the changes taking place is understood, and only the
practical results of those changes as affecting the various components of
an acetylene installation have to be studied. According to Lewes,
acetylene commences to "polymerise" at a temperature of about 600° C.,
when it is converted into other hydrocarbons having the same percentage
composition, but containing more atoms of carbon and hydrogen in their
molecules. The formula of acetylene is C_2H_2 which means that 2 atoms of
carbon and 2 atoms of hydrogen unite to form 1 molecule of acetylene, a
body evidently containing roughly 92.3 per cent. by weight of carbon and
7.7 per cent. by weight of hydrogen. One of the most noteworthy
substances produced by the polymerisation of acetylene is benzene, the
formula of which is C_6H_6, and this is formed in the manner indicated by
the equation--
(4) 3C_2H_2 = C_6H_6.
Now benzene also contains 92.3 per cent. of carbon and 7.7 per cent. by
weight of hydrogen in its composition, but its molecule contains 6 atoms
of each element. When the chemical formula representing a compound body
indicates a substance which is, or can be obtained as, a gas or vapour,
it convoys another idea over and above those mentioned on a previous
page. The formula "C_2H_2," for example, means 1 molecule, or 26 parts by
weight of acetylene, just as "H_2" means 1 molecule, or 2 parts by weight
of hydrogen; but both formulæ also mean equal parts by volume of the
respective substances, and since H_2 must mean 2 volumes, being twice H,
which is manifestly 1, C_2H_2 must mean 2 volumes of acetylene as well.
Thus equation (4) states that 6 volumes of acetylene, or 3 x 26 parts by
weight, unite to form 2 volumes of benzene, or 78 parts by weight. If
these hydrocarbons are burnt in air, both are indifferently converted
into carbon dioxide (carbonic acid gas) and water vapour; and, neglecting
for the sake of simplicity the nitrogen of the atmosphere, the processes
may be shown thus:
(5) 2C_2H_2 + 5O_2 = 4CO_2 + 2H_2O.
(6) 2C_6H_6 + 15O_2 = 12CO_2 + 6H_2O.
Equation (5) shows that 4 volumes of acetylene combine with 10 volumes of
oxygen to produce 8 volumes of carbon dioxide and 4 of water vapour;
while equation (6) indicates that 4 volumes of benzene combine with 30
volumes of oxygen to yield 24 volumes of carbon dioxide and 12 of water
vapour. Two parts by volume of acetylene therefore require 5 parts by
volume of oxygen for perfect combustion, whereas two parts by volume of
benzene need 15--_i.e._, exactly three times as much. In order to
work satisfactorily, and to develop the maximum of illuminating power
from any kind of gas consumed, a gas-burner has to be designed with
considerable skill so as to attract to the base of the flame precisely
that volume of air which contains the quantity of oxygen necessary to
insure complete combustion, for an excess of air in a flame is only less
objectionable than a deficiency thereof. If, then, an acetylene burner is
properly constructed, as most modern ones are, it draws into the flame
air corresponding with two and a half volumes of oxygen for every one
volume of acetylene passing from the jets; whereas if it were intended
for the combustion of benzene vapour it would have to attract three times
that quantity. Since any flame supplied with too little air tends to emit
free carbon or soot, it follows that any well-made acetylene burner
delivering a gas containing benzene vapour will yield a more or lens
smoky flame according to the proportion of benzene in the acetylene.
Moreover, at ordinary temperatures benzene is a liquid, for it boils at
81° C., and although, as was explained above in the case of water, it is
capable of remaining in the state of vapour far below its boiling-point
so long as it is suspended in a sufficiency of some permanent gas like
acetylene, if the proportion of vapour in the gas at any given
temperature exceeds a certain amount the excess will be precipitated in
the liquid form; while as the temperature falls the proportion of vapour
which can be retained in a given volume of gas also diminishes to a
noteworthy extent. Should any liquid, be it water or benzene, or any
other substance, separate from the acetylene under the influence of cold
while the gas is passing through pipes, the liquid will run downwards to
the lowest points in those pipes; and unless due precautions are taken,
by the insertion of draw-off cocks, collecting wells, or the like, to
withdraw the deposited water or other liquid, it will accumulate in all
bends, angles, and dips till the pipes are partly or completely sealed
against the passage of gas, and the lights will either "jump" or be
extinguished altogether. In the specific case of an acetylene generator
this trouble is very likely to arise, even when the gas is not heated
sufficiently during evolution for polymerisation to occur and benzene or
other liquid hydrocarbons to be formed, because any excess of water
present in the decomposing vessel is liable to be vaporised by the heat
of the reaction--as already stated it is desirable that water shall be so
vaporised--and will remain safely vaporised as long as the pipes are kept
warm inside or near the generator; but directly the pipes pass away from
the hot generator the cooling action of the air begins, and some liquid
water will be immediately produced. Like the phenomenon of after-
generation, this equally inevitable phenomenon of water condensation will
be either an inconvenience or source of positive danger, or will be a
matter of no consequence whatever, simply as the whole acetylene
installation, including the service-pipes, is ignorantly or intelligently
built.
As long as nothing but pure polymerisation happens to the acetylene, as
long, that is to say, as it is merely converted into other hydrocarbons
also having the general formula C_(2n)H_(2n), no harm will be done to the
gas as regards illuminating power, for benzene burns with a still more
luminous flame than acetylene itself; nor will any injury result to the
gas if it is required for combustion in heating or cooking stoves beyond
the fact that the burners, luminous or atmospheric, will be delivering a
material for the consumption of which they are not properly designed. But
if the temperature should rise much above the point at which benzene is
the most conspicuous product of polymerisation, other far more
complicated changes occur, and harmful effects may be produced in two
separate ways. Some of the new hydrocarbons formed may interact to yield
a mixture of one or more other hydrocarbons containing a higher
proportion of carbon than that which is present in acetylene and benzene,
together with a corresponding proportion of free hydrogen; the former
will probably be either liquids or solids, while the latter burns with a
perfectly non-luminous flame. Thus the quantity of gas evolved from the
carbide and passed into the holder is less than it should be, owing to
the condensation of its non-gaseous constituents. To quote an instance of
this, Haber has found 15 litres of acetylene to be reduced in volume to
10 litres when the gas was heated to 638° C. By other changes, some
"saturated hydrocarbons," _i.e._, bodies having the general formula
C_nH_(2n+2), of which methane or marsh-gas, CH_4 is the best known, may
be produced; and those all possess lower illuminating powers than
acetylene. In two of those experiments already described, where Lewes
observed maximum temperatures ranging from 703° to 807° C., samples of
the gas which issued when the heat was greatest were submitted to
chemical analysis, and their illuminating powers were determined. The
figures he gives are as follows:
I. II.
Per Cent. Per Cent.
Acetylene 70.0 69.7
Saturated hydrocarbons 11.3 11.4
Hydrogen 18.7 18.9
_____ _____
100.0 100.0
The average illuminating power of these mixed gases is about 126 candles
per 5 cubic feet, whereas that of pure acetylene burnt under good
laboratory conditions is 240 candles per 5 cubic feet. The product, it
will be seen, had lost almost exactly 50 per cent. of its value as an
illuminant, owing to the excessive heating to which it had been, exposed.
Some of the liquid hydrocarbons formed at the same time are not limpid
fluids like benzene, which is less viscous than water, but are thick oily
substances, or even tars. They therefore tend to block the tubes of the
apparatus with great persistence, while the tar adheres to the calcium
carbide and causes its further attack by water to be very irregular, or
even altogether impossible. In some of the very badly designed generators
of a few years back this tarry matter was distinctly visible when the
apparatus was disconnected for recharging, for the spent carbide was
exceptionally yellow, brown, or blackish in colour, [Footnote: As will be
pointed out later, the colour of the spent lime cannot always be employed
as a means for judging whether overheating has occurred in a generator.]
and the odour of tar was as noticeable as that of crude acetylene.
There is another effect of heat upon acetylene, more calculated to be
dangerous than any of those just mentioned, which must not be lost sight
of. Being an endothermic substance, acetylene is prone to decompose into
its elements--
(7) C_2H_2 -> C_2 + H_2
whenever it has the opportunity; and the opportunity arrives if the
temperature of the gas risen to 780° C., or if the pressure under which
the gas is stored exceeds two atmospheres absolute (roughly 30 lb. per
square inch). It decomposes, be it carefully understood, in the complete
absence of air, directly the smallest spark of red-hot material or of
electricity, or directly a gentle shock, such as that of a fall or blow
on the vessel holding it, is applied to any volume of acetylene existing
at a temperature exceeding 780° or at a gross pressure of 30 lb. per
square inch; and however large that volume may be, unless it is contained
in tubes of very small diameter, as will appear hereafter, the
decomposition or dissociation into its elements will extend throughout
the whole of the gas. Equation (7) states that 2 volumes of acetylene
yield 2 volumes of hydrogen and a quantity of carbon which would measure
2 volumes were it obtained in the state of gas, but which, being a solid,
occupies a space that may be neglected. Apparently, therefore, the
dissociation of acetylene involves no alteration in volume, and should
not exhibit explosive effects. This is erroneous, because 2 volumes of
acetylene only yield exactly 2 volumes of hydrogen when both gases are
measured at the same temperature, and all gases increase in volume as
their temperature rises. As acetylene is endothermic and evolves much
heat on decomposition, and as that heat must primarily be communicated to
the hydrogen, it follows that the latter must be much hotter than the
original acetylene; the hydrogen accordingly strives to fill a much
larger space than that occupied by the undecomposed gas, and if that gas
is contained in a closed vessel, considerable internal pressure will be
set up, which may or may not cause the vessel to burst.
What has been said in the preceding paragraph about the temperature at
which acetylene decomposes is only true when the gas is free from any
notable quantity of air. In presence of air, acetylene inflames at a much
lower temperature, viz., 480° C. In a manner precisely similar to that of
all other combustible gases, if a stream of acetylene issues into the
atmosphere, as from the orifices of a burner, the gas catches fire and
burns quietly directly any substance having a temperature of 480° C. or
upwards is brought near it; but if acetylene in bulk is mixed with the
necessary quantity of air to support combustion, and any object exceeding
480° C. in temperature comes in contact with it, the oxidation of the
hydrocarbon proceeds at such a high rate of speed as to be termed an
explosion. The proportion of air needed to support combustion varies with
every combustible material within known limits (_cf._ Chapter VI.),
and according to Eitner the smallest quantity of air required to make
acetylene burn or explode, as the case may be, is 25 per cent. If, by
ignorant design or by careless manipulation, the first batches of
acetylene evolved from a freshly charged generator should contain more
than 25 per cent. of air; or if in the inauguration of a new installation
the air should not be swept out of the pipes, and the first makes of gas
should become diluted with 25 to 50 per cent. of air, any glowing body
whose temperature exceeds 480° C. will fire the gas; and, as in the
former instance, the flame will extend all through the mass of acetylene
with disastrous violence and at enormous speed unless the gas is stored
in narrow pipes of extremely small diameter. Three practical lessons are
to be learnt from this circumstance: first, tobacco-smoking must never be
permitted in any building where an escape of raw acetylene is possible,
because the temperature of a lighted cigar, &c., exceeds 480° C.;
secondly, a light must never be applied to a pipe delivering acetylene
until a proper acetylene burner has been screwed into the aperture;
thirdly, if any appreciable amount of acetylene is present in the air, no
operation should be performed upon any portion of an acetylene plant
which involves such processes as scraping or chipping with the aid of a
steel tool or shovel. If, for example, the iron or stoneware sludge-pipe
is choked, or the interior of the dismantled generator is blocked, and
attempts are made to remove the obstruction with a hard steel tool, a
spark is very likely to be formed which, granting the existence of
sufficient acetylene in the air, is perfectly able to fire the gas. For
all such purposes wooden implements only are best employed; but the
remark does not apply to the hand-charging of a carbide-to-water
generator through its proper shoot. Before passing to another subject, it
may be remarked that a quantity of air far less than that which causes
acetylene to become dangerous is objectionable, as its presence is apt to
reduce the illuminating power of the gas unduly.
EFFECT OF HEAT ON CARBIDE.--Chemically speaking, no amount of heat
possible of attainment in the worst acetylene generator can affect
calcium carbide in the slightest degree, because that substance may be
raised to almost any temperature short of those distinguishing the
electric furnace, without suffering any change or deterioration. In the
absence of water, calcium carbide is as inert a substance as can well be
imagined: it cannot be made to catch fire, for it is absolutely
incombustible, and it can be heated in any ordinary flame for reasonable
periods of time, or thrown into any non-electrical furnace without
suffering in the least. But in presence of water, or of any liquid
containing water, matters are different. If the temperature of an
acetylene generator rises to such an extent that part of the gas is
polymerised into tar, that tar naturally tends to coat the residual
carbide lumps, and, being greasy in character, more or less completely
protects the interior from further attack. Action of this nature not only
means that the acetylene is diminished in quantity and quality by partial
decomposition, but it also means that the make is smaller owing to
imperfect decomposition of the carbide: while over and above this is the
liability to nuisance or danger when a mass of solid residue containing
some unaltered calcium carbide is removed from the apparatus and thrown
away. In fact, whenever the residue of a generator is not so saturated
with excess of water as to be of a creamy consistency, it should be put
into an uncovered vessel in the open air, and treated with some ten times
its volume of water before being run into any drain or closed pipe where
an accumulation of acetylene may occur. Even at temperatures far below
those needed to determine a production of tar or an oily coating on the
carbide, if water attacks an excess of calcium carbide somewhat rapidly,
there is a marked tendency for the carbide to be "baked" by the heat
produced; the slaked lime adhering to the lumps as a hard skin which
greatly retards the penetration of more water to the interior.
COLOUR OF SPENT CARBIDE.--In the early days of the industry, it was
frequently taken for granted that any degradation in the colour of the
spent lime left in an acetylene generator was proof that overheating had
taken place during the decomposition of the carbide. Since both calcium
oxide and hydroxide are white substances, it was thought that a brownish,
greyish, or blackish residue must necessarily point to incipient
polymerisation of the gas. This view would be correct if calcium carbide
were prepared in a state of chemical purity, for it also is a white body.
Commercial carbide, however, is not pure; it usually contains some
foreign matter which tints the residue remaining after gasification. When
a manufacturer strives to give his carbide the highest gas-making power
possible he frequently increases the proportion of carbon in the charge
submitted to electric smelting, until a small excess is reached, which
remains in the free state amongst the finished carbide. After
decomposition the fine particles of carbon stain the moist lime a bluish
grey tint, the depth of shade manifestly depending upon the amount
present. If such a sludge is copiously diluted with water, particles of
carbon having the appearance and gritty or flaky nature of coke often
rise to the surface or fall to the bottom of the liquid; whence they can
easily be picked out and identified as pure or impure carbon by simple
tests. Similarly the lime or carbon put into the electric furnace may
contain small quantities of compounds which are naturally coloured; and
which, reappearing in the sludge either in their original or in a
different state of combination, confer upon the sludge their
characteristic tinge. Spent lime of a yellowish brown colour is
frequently to be met with in circumstances that are clearly no reproach
to the generator. Doubtless the tint is due to the presence of some
coloured metallic oxide or other compound which has escaped reduction in
the electric furnace. The colour which the residual lime afterwards
assumes may not be noticeable in the dry carbide before decomposition,
either because some change in the colour-giving impurity takes place
during the chemical reactions in the generator or because the tint is
simply masked by the greyish white of the carbide and its free carbon.
Hence it follows that a bad colour in the waste lime removed from a
generator only points to overheating and polymerisation of the acetylene
when corroborative evidence is obtained--such as a distinct tarry smell,
the actual discovery of oily or tarry matters elsewhere, or a grave
reduction in the illuminating power of the gas.
MAXIMUM ATTAINABLE TEMPERATURES.--In order to discover the maximum
temperature which can be reached in or about an acetylene generator when
an apparatus belonging to one of the best types is fed at a proper rate
with calcium carbide in lumps of the most suitable size, the following
calculation may be made. In the first place, it will be assumed that no
loss of heat by radiation occurs from the walls of the generator;
secondly, the small quantity of heat taken up by the calcium hydroxide
produced will be ignored; and, thirdly, the specific heat of acetylene
will be assumed to be 0.25, which is about its most probable value. Now,
a hand-fed carbide-to-water generator will work with half a gallon of
water for every 1 lb. of carbide decomposed, quantities which correspond
with 320 grammes of water per 64 grammes (1 molecular weight) of carbide.
Of those 320 grammes of water, 18 are chemically destroyed, leaving 302.
The decomposition of 64 grammes of commercial carbide evolves 28 large
calories of heat. Assuming all the heat to be absorbed by the water, 28
calories would raise 302 grammes through (28 X 1000 / 302) = 93° C.,
_i.e._, from 44.6° F. to the boiling-point. Assuming all the heat to
be communicated to the acetylene, those 28 calories would raise the 26
grammes of gas liberated through (28 X 1000 / 26 / 0.25) = 4308° C., if
that were possible. But if, as would actually be the case, the heat were
distributed uniformly amongst the 302 grammes of water and the 20 grammes
of acetylene, both gas and water would be raised through the same number
of degrees, viz., 90.8° C. [Footnote: Let x = the number of large
calories absorbed by the water; then 28 - x = those taken up by the gas.
Then--
1000x / 302 = 1000 (28 - x) / (26 X 0.25)
whence x = 27.41; and 28 - x = 0.59.
Therefore, for water, the rise in temperature is--
27.41 X 1000 / 302 = 90.8° C.;
and for acetylene the rise is--
0.59 X 1000 / 26 / 0.25 = 90.8° C.]
If the generator were designed on lines to satisfy the United States Fire
Underwriters, it would contain 8.33 lb. of water to every 1 lb. of
carbide attacked; identical calculations then showing that the original
temperature of the water and gas would be raised through 53.7° C.
Provided the carbide is not charged into such an apparatus in lumps of
too large a size, nor at too high a rate, there will be no appreciable
amount of local overheating developed; and nowhere, therefore, will the
rise in temperature exceed 91° in the first instance, or 54° C. in the
second. Indeed it will be considerably smaller than this, because a large
proportion of the heat evolved will be lost by radiation through the
generator walls, while another portion will be converted from sensible
into latent heat by causing part of the water to pass off as vapour with
the acetylene.
EFFECT OF HIGH TEMPERATURES ON GENERATORS.--As the temperature amongst
the carbide in any generator in which water is not present in large
excess may easily reach 200° C. or upwards, no material ought to be
employed in the construction of such generators which is not competent to
withstand a considerable amount of heat in perfect safety. The ordinary
varieties of soft solder applied with the bitt in all kinds of light
metal-work usually melt, according to their composition, at about 180°
C.; and therefore this method of making joints is only suitable for
objects that are never raised appreciably in temperature above the
boiling-point of water. No joint in an acetylene generator, the partial
or complete failure of which would radically affect the behaviour of the
apparatus, by permitting the charges of carbide and of water to come into
contact at an abnormal rate of speed, by allowing the acetylene to escape
directly through the crack into the atmosphere, or by enabling the water
to run out of the seal of any vessel containing gas so as to set up a
free communication between that vessel and the air, ought ever to be made
of soft solder--every joint of this character should be constructed
either by riveting, by bolting, or by doubly folding the metal sheets.
Apparently, a joint constantly immersed in water on one side cannot rise
in temperature above the boiling-point of the liquid, even when its other
side is heated strongly; but since, even if a generator is not charged
with naturally hard water, its fluid contents soon become "hard" by
dissolution of lime, there is always a liability to the deposition of
water scale over the joint. Such water scale is a very bad heat
conductor, as is seen in steam boilers, so that a seam coated with an
exceedingly thin layer of scale, and heated sharply on one side, will
rise above the boiling-point of water even if the liquid on its opposite
side is ice-cold. For a while the film of scale may be quite water-tight,
but after it has been heated by contact with the hot metal several times
it becomes brittle and cracks without warning. But there is a more
important reason for avoiding the use of plumbers' solder. It might seem
that as the natural hard, protective skin of the metal is liable to be
injured or removed by the bending or by the drilling or punching which
precedes the insertion of the rivets or studs, an application of soft
solder to such a joint should be advantageous. This is not true because
of the influence of galvanic action. As all soft solders consist largely
of lead, if a joint is soldered, a "galvanic couple" of lead and iron, or
of lead and zinc (when the apparatus is built of galvanised steel), is
exposed to the liquid bathing it; and since in both cases the lead is
highly electro-negative to the iron or zinc, it is the iron or zinc which
suffers attack, assuming the liquid to possess any corrosive properties
whatever. Galvanised iron which has been injured during the joint-making
presents a zinc-iron couple to the water, but the zinc protects the iron;
if a lead solder is present, the iron will begin to corrode immediately
the zinc has disappeared. In the absence of lead it is the less important
metal, but in the presence of lead it is the more important (the
foundation) metal which is the soluble element of the couple. Where
practicable, joints in an acetylene generator may safely be made by
welding or by autogenous soldering ("burning"), because no other metal is
introduced into the system; any other process, except that of riveting or
folding, only hastens destruction of the plant. The ideal method of
making joints about an acetylene generator is manifestly that of
autogenous soldering, because, as will appear in Chapter IX. of this
book, the most convenient and efficient apparatus for performing the
operation is the oxy-acetylene blow-pipe, which can be employed so as to
convert two separate pieces of similar metal into one homogeneous whole.
In less critical situations in an acetylene plant, such as the partitions
of a carbide container, &c., where the collapse of the seam or joint
would not be followed by any of the effects previously suggested, there
is less cause for prohibiting the use of unfortified solder; but even
here, two or three rivets, just sufficient to hold the metal in position
if the solder should give way, are advisedly put into all apparatus. In
other portions of an acetylene installation where a merely soldered joint
is exposed to warm damp gas which is in process of cooling, instead of
being bathed in hard water, an equal, though totally dissimilar, danger
is courted. The main constituent of such solders that are capable of
being applied with the bitt is lead; lead is distinctly soluble in soft
or pure water; and the water which separates by condensation out of a
warm damp gas is absolutely soft, for it has been distilled. If
condensation takes place at or near a soldered joint in such a way that
water trickles over the solder, by slow degrees the metallic lead will be
dissolved and removed, and eventually a time will come when the joint is
no longer tight to gas. In fact, if an acetylene installation is of more
than very small dimensions, _e.g._, when it is intended to supply
any building as large as, or larger than, the average country residence,
if it is to give satisfaction to both constructor and purchaser by being
quite trustworthy and, possessed of a due lease of life, say ten or
fifteen years, it must be built of stouter materials than the light
sheets which alone are suitable for manipulation with the soldering-iron
or for bending in the ordinary type of metal press. Sound cast-iron,
heavy sheet-metal, or light boiler-plate is the proper substance of which
to construct all the important parts of a generator, and the joints in
wrought metal must be riveted and caulked or soldered autogeneously as
mentioned above. So built, the installation becomes much more costly to
lay down than an apparatus composed of tinplate, zinc, or thin galvanised
iron, but it will prove more economical in the long run. It is not too
much to say that if ignorant and short-sighted makers in the earliest
days of the acetylene industry had not recommended and supplied to their
customers lightly built apparatus which has in many instances already
begun to give trouble, to need repairs, and to fail by thorough
corrosion--apparatus which frequently had nothing but cheapness in its
favour--the use of the gas would have spread more rapidly than it has
done, and the public would not now be hearing of partial or complete
failures of acetylene installations. Each of these failures, whether
accompanied by explosions and injury to persons or not, acts more
powerfully to restrain a possible new customer from adopting the
acetylene light, than several wholly successful plants urge him to take
it up; for the average member of the public is not in a position to
distinguish properly between the collapse of a certain generator owing to
defective design or construction (which reflects no discredit upon the
gas itself), and the failure of acetylene to show in practice those
advantages that have been ascribed to it. One peculiar and noteworthy
feature of acetylene, often overlooked, is that the apparatus is
constructed by men who may have been accustomed to gas-making plant all
their lives, and who may understand by mere habit how to superintend a
chemical operation; but the same apparatus is used by persons who
generally have no special acquaintance with such subjects, and who, very
possibly, have not even burnt coal-gas at any period of their lives.
Hence it happens that when some thoughtless action on the part of the
country attendant of an acetylene apparatus is followed by an escape of
gas from the generator, and by an accumulation of that gas in the house
where the plant is situated, or when, in disregard of rules, he takes a
naked light into the house and an explosion follows, the builder
dismisses the episode as a piece of stupidity or wilful misbehaviour for
which he can in nowise be held morally responsible; whereas the builder
himself is to blame for designing an apparatus from which an escape of
gas can be accompanied by sensible risks to property or life. However
unpalatable this assertion may be, its truth cannot be controverted;
because, short of criminal intention or insanity on the part of the
attendant, it is in the first place a mere matter of knowledge and skill
so to construct an acetylene plant that an escape of gas is extremely
unlikely, even when the apparatus is opened for recharging, or when it is
manipulated wrongly; and in the second place, it is easy so to arrange
the plant that any disturbance of its functions which may occur shall be
followed by an immediate removal of the surplus gas into a place of
complete safety outside and above the generator-house.
GENERATION AT LOW TEMPERATURES.--In all that has been said hitherto about
the reaction between calcium carbide and water being instantaneous, it
has been assumed that the two substances are brought together at or about
the usual temperature of an occupied room, _i.e._, 15 degrees C. If,
however, the temperature is materially lower than this, the speed of the
reaction falls off, until at -5 degrees C., supposing the water still to
remain liquid, evolution of acetylene practically ceases. Even at the
freezing-point of pure water gas is produced but slowly; and if a lump of
carbide is thrown on to a block of ice, decomposition proceeds so gently
that the liberated acetylene may be ignited to form a kind of torch,
while heat is generated with insufficient rapidity to cause the carbide
to sink into the block. This fact has very important bearings upon the
manipulation of an acetylene generator in winter time. It is evident that
unless precautions are taken those portions of an apparatus which contain
water are liable to freeze on a cold night; because, even if the
generator has been at work producing gas (and consequently evolving heat)
till late in the evening, the surplus heat stored in the plant may escape
into the atmosphere long before more acetylene has to be made, and
obviously while frost is still reigning in the neighbourhood. If the
water freezes in the water store, in the pipes leading therefrom, in the
holder seal, or in the actual decomposing chamber, a fresh batch of gas
is either totally incapable of production, because the water cannot be
brought into contact with the calcium carbide in the apparatus, or it can
only be generated with excessive slowness because the carbide introduced
falls on to solid ice. Theoretically, too, there is a possibility that
some portion of the apparatus--a pipe in particular--may be burst by the
freezing, owing to the irresistible force with which water expands when
it changes into the solid condition. Probably this last contingency,
clearly accompanied as it would be by grave risk, is somewhat remote, all
the plant being constructed of elastic material; but in practice even a
simple interference with the functions of a generator by freezing,
ideally of no special moment, is highly dangerous, because of the great
likelihood that hurried and wholly improper attempts to thaw it will be
made by the attendant. As it has been well known for many years that the
solidifying point of water can be lowered to almost any degree below
normal freezing by dissolving in it certain salts in definite
proportions, one of the first methods suggested for preventing the
formation of ice in an acetylene generator was to employ such a salt,
using, in fact, for the decomposition of the carbide some saline solution
which remains liquid below the minimum night temperature of the winter
season. Such a process, however, has proved unsuitable for the purpose in
view; and the explanation of that fact is found in what has just been
stated: the "water" of the generator may admittedly be safely maintained
in the fluid state, but from so cold a liquid acetylene will not be
generated smoothly, if at all. Moreover, were it not so, a process of
this character is unnecessarily expensive, although suitable salts are
very cheap, for the water of the generator is constantly being consumed,
[Footnote: It has already been said that most generators "consume" a much
larger volume of water than the amount corresponding with the chemical
reaction involved: the excess of water passing into the sludge or by-
product. Thus a considerable quantity of any anti-freezing agent must be
thrown aside each time the apparatus is cleaned out or its fluid contents
are run off.] and as constantly needs renewal; which means that a fresh
batch of salt would be required every time the apparatus was recharged,
so long as frost existed or might be expected. A somewhat different
condition obtains in the holder of an acetylene installation. Here,
whenever the holder is a separate item in the plant, not constituting a
portion of the generating apparatus, the water which forms the seal of a
rising holder, or which fills half the space of a displacement holder,
lasts indefinitely; and it behaves equally well, whatever its temperature
may be, so long as it retains a fluid state. This matter will be
discussed with greater detail at the end of Chapter III. At present the
point to be insisted on is that the temperature in any constituent of an
acetylene installation which contains water must not be permitted to fall
to the freezing-point; while the water actually used for decomposition
must be kept well above that temperature.
GENERATION AT HIGH TEMPERATURES.--At temperatures largely exceeding those
of the atmosphere, the reaction between calcium carbide and water tends
to become irregular; while at a red heat steam acts very slowly upon
carbide, evolving a mixture of acetylene and hydrogen in place of pure
acetylene. But since at pressures which do not materially exceed that of
the atmosphere, water changes into vapour at 100° C., above that
temperature there can be no question of a reaction between carbide and
liquid water. Moreover, as has been pointed out, steam or water vapour
will continue to exist as such at temperatures even as low as the
freezing-point so long as the vapour is suspended among the particles of
a permanent gas. Between calcium carbide and water vapour a double
decomposition occurs chemically identical with that between carbide and
liquid water; but the physical effect of the reaction and its practical
bearings are considerably modified. The quantity of heat liberated when
30 parts by weight of steam react with 64 parts of calcium carbide should
be essentially unaltered from that evolved when the reagent is in the
liquid state; but the temperature likely to be attained when the speed of
reaction remains the same as before will be considerably higher for two
conspicuous reasons. In the first place, the specific heat of steam in is
only 0.48, while that of liquid water is 1.0. Hence, the quantity of heat
which is sufficient to raise the temperature of a given weight of liquid
water through _n_ thermometric degrees, will raise the temperature
of the same weight of water vapour through rather more than 2 _n_
degrees. In the second place, that relatively large quantity of heat
which in the case of liquid water merely changes the liquid into a
vapour, becoming "latent" or otherwise unrecognisable, and which, as
already shown, forms roughly five-sixths of the total heat needed to
convert cold water into steam, has no analogue if the water has
previously been vaporised by other means; and therefore the whole of the
heat supplied to water vapour raises its sensible temperature, as
indicated by the thermometer. Thus it appears that, except for the
sufficient amount of cooling that can be applied to a large vessel
containing carbide by surrounding it with a water jacket, there is no way
of governing its temperature satisfactorily if water vapour is allowed to
act upon a mass of carbide--assuming, of course, that the reaction
proceeds at any moderate speed, _e.g._, at a rate much above that
required to supply one or two burners with gas.
The decomposition which with perfect chemical accuracy has been stated to
occur quantitatively between 36 parts by weight, of water and 64 parts of
calcium carbide scarcely ever takes place in so simple a fashion in an
actual generator. Owing to the heat developed when carbide is in excess,
about half the water is converted into vapour; and so the reaction
proceeds in two stages: half the water added reacting with the carbide as
a liquid, the other half, in a state of vapour, afterwards reacting
similarly, [Footnote: This secondary reaction is manifestly only another
variety of the phenomenon known as "after-generation" (cf. _ante_).
After-generation is possible between calcium carbide and mechanically
damp slaked lime, between carbide and damp gas, or between carbide and
calcium hydroxide, as opportunity shall serve. In all cases the carbide
must be in excess.] or hardly reacting at all, as the case may be.
Suppose a vessel, A B, somewhat cylindrical in shape, is charged with
carbide, and that water is admitted at the end called A. Suppose now (1)
that the exit for gas is at the opposite end, B. As the lumps near A are
attacked by half the liquid introduced, while the other half is changed
into steam, a current, of acetylene and water vapour travels over the
charge lying between the decomposing spot and the end B. During its
passage the second half of the water, as vapour, reacts with the excess
of carbide, the first make of acetylene being dried, and more gas being
produced. Thus a second quantity of heat is developed, equal by theory to
that previously evolved; but a second elevation in temperature, far more
serious, and far less under control, than the former also occurs; and
this is easily sufficient to determine some of those undesirable effects
already described. Digressing for a moment, it may be admitted that the
desiccation of the acetylene produced in this manner is beneficial, even
necessary; but the advantages of drying the gas at this period of its
treatment are outweighed by the concomitant disadvantages and by the
later inevitable remoistening thereof. Suppose now (2) that both the
water inlet and the gas exit of the carbide cylinder are at the same end,
A. Again half the added water, as liquid, reacts with the carbide it
first encounters, but the hot stream of damp gas is not permitted to
travel over the rest of the lumps extending towards B: it is forced to
return upon its steps, leaving B practically untouched. The gas
accordingly escapes from the cylinder at A still loaded with water
vapour, and for a given weight of water introduced much less acetylene is
evolved than in the former case. The gas, too, needs drying somewhere
else in the plant; but these defects are preferable to the apparent
superiority of the first process because overheating is, or can be, more
thoroughly guarded against.
PRESSURE IN GENERATORS.--Inasmuch as acetylene is prone to dissociate or
decompose into its elements spontaneously whenever its pressure reaches 2
atmospheres or 30 lb. per square inch, as well as when its temperature at
atmospheric pressure attains 780° C., no pressure approaching that of 2
atmospheres is permissible in the generator. A due observance of this
rule, however, unlike a proper maintenance of a low temperature in an
acetylene apparatus, is perfectly easy to arrange for. The only reason
for having an appreciable positive pressure in any form of generating
plant is that the gas may be compelled to travel through the pipes and to
escape from the burner orifices; and since the plant is only installed to
serve the burners, that pressure which best suits the burners must be
thrown by the generator or its holder. Therefore the highest pressure it
is ever requisite to employ in a generator is a pressure sufficient
(_a_) to lift the gasholder bell, or to raise the water in a
displacement holder, (_b_) to drive the gas through the various
subsidiary items in the plant, such as washers and purifiers, (_c_)
to overcome the friction in the service-pipes, [Footnote: This friction
manifestly causes a loss of pressure, _i.e._, a fall in pressure, as
a gas travels along a pipe; and, as will be shown in Chapter VII., it is
the fall in pressure in a pipe rather than the initial pressure at which
a gas enters a pipe that governs the volume of gas passing through that
pipe. The proper behaviour and economic working of a burner (acetylene or
other, luminous or incandescent) naturally depend upon the pressure in
the pipe to which the burner is immediately attached being exactly suited
to the design of that burner, and have nothing to do with the fall in
pressure occurring in the delivery pipes. It is therefore necessary to
keep entirely separate the ideas of proper burner pressure and of maximum
desirable fall in pressure within the service due to friction.] and
(d) to give at the points of combustion a pressure which is
required by the particular burners adopted. In all except village or
district installations, (_c_) may be virtually neglected. When the
holder has a rising bell, (_a_) represents only an inch or so of
water; but if a displacement holder is employed the pressure needed to
work it is entirely indeterminate, being governed by the size and shape
of the said holder. It will be argued in Chapter III. that a rising
holder is always preferable to one constructed on the displacement
principle. The pressure (d) at the burners may be taken at 4
inches of water as a maximum, the precise figure being dependent upon the
kind of burners--luminous, incandescent, boiling, &c.--attached to the
main. The pressure (_b_) also varies according to circumstances, but
averages 2 or 3 inches. Thus a pressure in the generator exceeding that
of the atmosphere by some 12 inches of water--_i.e._, by about 7
oz., or less than half a pound per square inch--is amply sufficient for
every kind of installation, the less meritorious generators with
displacement holders only excepted. This pressure, it should be noted, is
the net or effective pressure, the pressure with which the gas raises the
liquid in a water-gauge glass out of the level while the opposite end of
the water column is exposed to the atmosphere. The absolute pressure in a
vessel containing gas at an effective pressure of 12 inches of water is 7
oz. plus the normal, insensible pressure of the atmosphere itself--say
15-1/4 lb. per square inch. The liquid in a barometer which measures the
pressure of the atmosphere stands at a height of 30 inches only, because
that liquid is mercury, 13.6 times as heavy as water. Were it filled with
water the barometer would stand at (30 X 13.6) = 408 inches, or 34 feet,
approximately. Gas pressures are always measured in inches of water
column, because expressed either as pounds per square inch or as inches
of mercury, the figures would be so small as to give decimals of unwieldy
length.
It would of course be perfectly safe so to arrange an acetylene plant
that the pressure in the generating chamber should reach the 100 inches
of water first laid down by the Home Office authorities as the maximum
allowable. There is, however, no appreciable advantage to be gained by so
doing, or by exceeding that pressure which feeds the burners best. Any
higher original pressure involves the use of a governor at the exit of
the plant, and a governor is a costly and somewhat troublesome piece of
apparatus that can be dispensed with in most single installations by a
proper employment of a well-balanced rising holder.
CHAPTER III
THE GENERAL PRINCIPLES OF ACETYLENE GENERATION--ACETYLENE GENERATING
APPARATUS
Inasmuch as acetylene is produced by the mere interaction of calcium
carbide and water, that is to say, by simply bringing those two
substances in the cold into mutual contact within a suitable closed
space, and inasmuch as calcium carbide can always be purchased by the
consumer in a condition perfectly fit for immediate decomposition, the
preparation of the gas, at least from the theoretical aspect, is
characterised by extreme simplicity. A cylinder of glass or metal, closed
at one end and open at the other, filled with water, and inverted in a
larger vessel containing the same liquid, may be charged almost
instantaneously with acetylene by dropping into the basin a lump of
carbide, which sinks to the bottom, begins to decompose, and evolves a
rapid current of gas, displacing the water originally held in the
inverted cylinder or "bell." If a very minute hole is drilled in the top
of the floating bell, acetylene at once escapes in a steady stream, being
driven out by the pressure of the cylinder, the surplus weight of which
causes it to descend into the water of the basin as rapidly as gas issues
from the orifice. As a laboratory experiment, and provided the bell has
been most carefully freed from atmospheric air in the first instance,
this escaping gas may be set light to with a match, and will burn with a
more or loss satisfactory flame of high illuminating power. Such is an
acetylene generator stripped of all desirable or undesirable adjuncts,
and reduced to its most elementary form; but it is needless to say that
so simple an apparatus would not in any way fulfil the requirements of
everyday practice.
Owing to the inequality of the seasons, and to the irregular nature of
the demand for artificial light and heat in all households, the capacity
of the plant installed for the service of any institution or district
must be amply sufficient to meet the consumption of the longest winter
evening--for, as will be shown in the proper place, attempts to make an
acetylene generator evolve gas more quickly than it is designed to do are
fraught with many objections--while the operation of the plant, must be
under such thorough control that not only can a sudden and unexpected
demand for gas be met without delay, but also that a sudden and
unexpected interruption or cessation of the demand shall not be followed
by any disturbance in the working of the apparatus. Since, on the one
hand, acetylene is produced in large volumes immediately calcium carbide
is wetted with water, so that the gas may be burnt within a minute or two
of its first evolution; and, on the other, that acetylene once prepared
can be stored without trouble or appreciable waste for reasonable periods
of time in a water-sealed gasholder closely resembling, in everything but
size, the holders employed on coal-gas works; it follows that there are
two ways of bringing the output of the plant into accord with the
consumption of the burners. It is possible to make the gas only as and
when it is required, or it is possible in the space of an hour or so,
during the most convenient part of the day, to prepare sufficient to last
an entire evening, storing it in a gasholder till the moment arrives for
its combustion. It is clear that an apparatus needing human attention
throughout the whole period of activity would be intolerable in the case
of small installations, and would only be permissible in the case of
larger ones if the district supplied with gas was populous enough to
justify the regular employment of two men at least in or about the
generating station. But with the conditions obtaining in such a country
as Great Britain, and in other lands where coal is equally cheap and
accessible, if a neighbourhood was as thickly populated as has been
suggested, it would be preferable on various grounds to lay down a coal-
gas or electricity works; for, as has been shown in the first chapter,
unless a very material fall in the price of calcium carbide should take
place--a fall which at present is not to be expected--acetylene can only
be considered a suitable and economical illuminant and heating agent for
such places as cannot be provided cheaply with coal-gas or electric
current. To meet this objection, acetylene generators have been invented
in which, broadly speaking, gas is only produced when it is required,
control of the chemical reaction devolving upon some mechanical
arrangement. There are, therefore, two radically different types of
acetylene apparatus to be met with, known respectively as "automatic" and
"non-automatic" generators. In a non-automatic generator the whole of the
calcium carbide put into the apparatus is more or less rapidly
decomposed, and the entire volume of gas evolved from it is collected in
a holder, there to await the moment of consumption. In an automatic
apparatus, by means of certain devices which will be discussed in their
proper place, the act of turning on a burner-tap causes some acetylene to
be produced, and the act of turning it off brings the reaction to an end,
thus obviating the necessity for storage. That, at any rate, is the
logical definition of the two fundamentally different kinds of generator:
in automatic apparatus the decomposition of the carbide is periodically
interrupted in such fashion as more or less accurately to synchronise
with the consumption of gas; in the non-automatic variety decomposition
proceeds without a break until the carbide vessels are empty.
Unfortunately a somewhat different interpretation of these two words has
found frequent acceptance, a generator being denominated non-automatic or
automatic according as the holder attached to it is or is not large
enough to store the whole of the acetylene which the charge of carbide is
capable of producing if it is decomposed all at once. Apart from the fact
that a holder, though desirable, is not an absolutely indispensable part
of an acetylene plant, the definition just quoted was sufficiently free
from objection in the earliest days of the industry; but now efficient
commercial generators are to be met with which become either automatic or
non-automatic according to the manner of working them, while some would
be termed non-automatic which comprise mechanism of a conspicuously self-
acting kind.
AUTOMATIC AND NON-AUTOMATIC GENERATORS.--Before proceeding to a detailed
description of the various devices which may be adopted to render an
acetylene generator automatic in action, the relative advantages of
automatic and non-automatic apparatus, irrespective of type, from the
consumer's point of view may be discussed. The fundamental idea
underlying the employment of a non-automatic generator is that the whole
of the calcium carbide put into the apparatus shall be decomposed into
acetylene as soon after the charge is inserted as is natural in the
circumstances; so that after a very brief interval of time the generating
chambers shall contain nothing but spent lime and water, and the holder
be as full of gas as is ever desirable. In an automatic apparatus, the
fundamental idea is that the generating chamber, or one at least of
several generating chambers, shall always contain a considerable quantity
of undecomposed carbide, and some receptacle always contain a store of
water ready to attack that carbide, so that whenever a demand for gas
shall arise everything may be ready to meet it. Inasmuch as acetylene is
an inflammable gas, it possesses all the properties characteristic of
inflammable gases in general; one of which is that it is always liable to
take fire in presence of a spark or naked light, and another of which is
that it is always liable to become highly explosive in presence of a
naked light or spark if, accidentally or otherwise, it becomes mixed with
more than a certain proportion of air. On the contrary, in the complete
absence of liquid or vaporised water, calcium carbide is almost as inert
a body as it is possible to imagine: for it will not take fire, and
cannot in any circumstances be made to explode. Hence it may be urged
that a non-automatic generator, with its holder always containing a large
volume of the actually inflammable and potentially explosive acetylene,
must invariably be more dangerous than an automatic apparatus which has
less or practically no ready-made gas in it, and which simply contains
water in one chamber and unaltered calcium carbide in another. But when
the generating vessels and the holder of a non-automatic apparatus are
properly designed and constructed, the gas in the latter is acetylene
practically free from air, and therefore while being, as acetylene
inevitably is, inflammable, is devoid of explosive properties, always
assuming, as must be the case in a water-sealed holder, that the
temperature of the gas is below 780° C.; and also assuming, as must
always be the case in good plant, that the pressure under which the gas
is stored remains less than two atmospheres absolute. It is perfectly
true that calcium carbide is non-inflammable and non-explosive, that it
is absolutely inert and incapable of change; but so comprehensive an
assertion only applies to carbide in its original drum, or in some
impervious vessel to which moisture and water have no access. Until it is
exhausted, an automatic acetylene generator contains carbide in one place
and water in another, dependence being put upon some mechanical
arrangement to prevent the two substances coming into contact
prematurely. Many of the devices adopted by builders of acetylene
apparatus for keeping the carbide and water separate, and for mixing them
in the requisite quantities when the proper time arrives, are as
trustworthy, perhaps, as it is possible for any automatic gear to be; but
some are objectionably complicated, and a few are positively inefficient.
There are two difficulties which the designer of automatic mechanism has
to contend with, and it is doubtful whether he always makes a sufficient
allowance for them. The first is that not only must calcium carbide and
liquid water be kept out of premature contact, but that moisture, or
vapour of water, must not be allowed to reach the carbide; or
alternatively, that if water vapour reaches the carbide too soon, the
undesired reaction shall not determine overheating, and the liberated gas
be not wasted or permitted to become a source of danger. The second
difficulty encountered by the designer of automata is so to construct his
apparatus that it shall behave well when attended to by completely
unskilled labour, that it shall withstand gross neglect and resist
positive ill-treatment or mismanagement. If the automatic principle is
adopted in any part of an acetylene apparatus it must be adopted
throughout, so that as far as possible--and with due knowledge and skill
it is completely possible--nothing shall be left dependent upon the
memory and common sense of the gasmaker. For instance, it must not be
necessary to shut a certain tap, or to manipulate several cocks before
opening the carbide vessel to recharge it; it must not be possible for
gas to escape backwards out of the holder; and either the carbide-feed
gear or the water-supply mechanism (as the case may be) must be
automatically locked by the mere act of taking the cover off the carbide
store, or of opening the sludge-cock at the bottom. It would be an
advantage, even, if the purifiers and other subsidiary items of the plant
were treated similarly, arranging them in such fashion that gas should be
automatically prevented from escaping out of the rest of the apparatus
when any lid was removed. In fact, the general notion of interlocking,
which has proved so successful in railway signal-cabins and in
carburetted water gas-plant for the prevention of accidents duo to
carelessness or overnight, might be copied in principle throughout an
acetylene installation whenever the automatic system is employed.
It is no part of the present argument, to allege that automatic
generators are, and must always be, inherently dangerous. Automatic
devices of a suitable kind may be found in plenty which are remarkably
simple and highly trustworthy; but it would be too bold a statement to
say that any such arrangement is incapable of failure, especially when
put into the hands of a person untrained in the superintendence of
machinery. The more reliable a piece of automatic mechanism proves itself
to be, the more likely is it to give trouble and inconvenience and
utterly to destroy confidence when it does break down; because the better
it has behaved in the past, and the longer it has lasted without
requiring adjustment, the less likely is it that the attendant will be at
hand when failure occurs. By suitable design and by an intelligent
employment of safety-valves and blow-off pipes (which will be discussed
in their proper place) it is quite easy to avoid the faintest possibility
of danger arising from an increase of pressure or an improper
accumulation of gas inside the plant or inside the building containing
the plant; but every time such a safety-valve or blow-off pipe comes into
action a waste of gas occurs, which means a sacrifice of economy, and
shows that the generator is not working as it should.
As glass is a fragile and brittle substance, and as it is not capable of
bearing large, rapid, and oft-repeated alterations of temperature in
perfect safety, it is not a suitable material for the construction of
acetylene apparatus or of portions thereof. Hence it follows that a
generator must be built of some non-transparent material which prevents
the interior being visible when the apparatus is at work. Although it is
comparatively easy, by the aid of a lamp placed outside the generator-
shed in such a position as to throw its beams of light through a window
upon the plant inside, to charge a generator after dark; and although it
is possible, without such assistance, by methodical habits and a
systematic arrangement of utensils inside the building to charge a
generator even in perfect darkness, such an operation is to be
deprecated, for it is apt to lead to mistakes, it prevents any slight
derangement in the installation from being instantly noticed, and it
offers a temptation to the attendant to break rules and to take a naked
light with him. On all those grounds, therefore, it is highly desirable
that every manipulation connected with a generator shall be effected
during the daytime, and that the apparatus-house shall be locked up
before nightfall. But owing to the irregular habits engendered by modern
life it is often difficult to know, during any given day, how much gas
will be required in the ensuing evening; and it therefore becomes
necessary always to have, as ready-made acetylene, or as carbide in a
proper position for instant decomposition, a patent or latent store of
gas more than sufficient in quantity to meet all possible requirements.
Now, as already stated, a non-automatic apparatus has its store of
material in the form of gas in a holder; and since this is preferably
constructed on the rising or telescopic principle, a mere inspection of
the height of the bell--on which, if preferred, a scale indicating its
contents in cubic feet or in burner-hours may be marked--suffices to show
how near the plant is to the point of exhaustion. In many types of
automatic apparatus the amount of carbide remaining undecomposed at any
moment is quite unknown, or at best can only be deduced by a tedious and
inexact calculation; although in some generators, where the store of
carbide is subdivided into small quantities, or placed in several
different receptacles, an inspection of certain levers or indicators
gives an approximate idea as to the capacity of the apparatus for further
gas production. In any case the position of a rising holder is the most
obvious sign of the degree of exhaustion of a generator; and therefore,
to render absolutely impossible a failure of the light during an evening,
a non-automatic generator fitted with a rising holder is best.
Since calcium carbide is a solid body having a specific gravity of 2.2,
water being unity, and since 1 cubic foot of water weighs 62.4 lb., in
round numbers 137 lb. of _compact_ carbide only occupy 1 cubic foot
of space. Again, since acetylene is a gas having a specific gravity of
0.91, air being unity, and since the specific gravity of air, water being
unity, is 0.0013, the specific gravity of acetylene, water being unity,
is roughly O.00116. Hence 1 cubic foot of acetylene weighs roughly 0.07
lb. Furthermore, since 1 lb. of good carbide evolves 5 cubic feet of gas
on decomposition with water, acetylene stored at atmospheric pressure
occupies roundly 680 times as much space as the carbide from which it has
been evolved. This figure by no means represents the actual state of
affairs in a generator, because, as was explained in the previous
chapter, a carbide vessel cannot be filled completely with solid; and,
indeed, were it so "filled," in ordinary language, much of its space
would be still occupied with air. Nevertheless it is incontrovertible
that an acetylene plant calculated to supply so many burners for so long
a period of time must be very much larger if it is constructed on the
non-automatic principle, when the carbide is decomposed all at once, than
if the automatic system is adopted, when the solid remains unattacked
until a corresponding quantity of gas is required for combustion. Clearly
it is the storage part of a non-automatic plant alone which must be so
much larger; the actual decomposing chambers may be of the same size or
even smaller, according to the system of generation to which the
apparatus belongs. In practice this extra size of the non-automatic plant
causes it to exhibit two disadvantages in comparison with automatic
apparatus, disadvantages which are less serious than they appear, or than
they may easily be represented to be. In the first place, the non-
automatic generator requires more space for its erection. If acetylene
were an illuminating agent suitable for adoption by dwellers in city or
suburb, where the back premises and open-air part of the messuage are
reduced to minute proportions or are even non-existent, this objection
might well be fatal. But acetylene is for the inhabitant of a country
village or the occupier of an isolated country house; and he has usually
plenty of space behind his residence which he can readily spare. In the
second place, the extra size of the non-automatic apparatus makes it more
expensive to construct and more costly to instal. It is more cosily to
construct and purchase because of its holder, which must be well built on
a firm foundation and accurately balanced; it is more costly to instal
because a situation must be found for the erection of the holder, and the
apparatus-house may have to be made large enough to contain the holder as
well as the generator itself. As regards the last point, it may be said
at once that there is no necessity to place the holder under cover: it
may stand out of doors, as coal-gas holders do in England, for the seal
of the tank can easily be rendered frost-proof, and the gas itself is not
affected by changes of atmospheric temperature beyond altering somewhat
in volume. In respect of the other objections, it must be remembered that
the extra expense is one of capital outlay alone, and therefore only
increases the cost of the light by an inappreciable amount, representing
interest and depreciation charges on the additional capital expenditure.
The increased cost of a year's lighting due to these charges will amount
to only 10 or 15 per cent, on the additional capital sunk. The extra
capital sunk does not in any way increase the maintenance charges; and
if, by having a large holder, additional security and trustworthiness are
obtained, or if the holder leads to a definite, albeit illusive, sense of
extra security and trustworthiness, the additional expenditure may well
be permissible or even advantageous.
The argument is sometimes advanced that inasmuch as for the same, or a
smaller, capital outlay as is required to instal a non-automatic
apparatus large enough to supply at one charging the maximum amount of
light and heat that can ever be needed on the longest winter's night, an
automatic plant adequate to make gas for two or three evenings can be
laid down, the latter must be preferable, because the attendant, in the
latter case, will only need to enter the generator-house two or three
times a week. Such an argument is defective because it ignores the
influence of habit upon the human being. A watch which must be wound
every day, or a clock which must be wound every week, on a certain day of
the week, is seldom permitted to run down; but a watch requiring to be
re-wound every other day, or a fourteen-day clock (used as such), would
rarely be kept going. Similarly, an acetylene generator might be charged
once a week or once a day without likelihood of being forgotten; but the
operation of charging at irregular intervals would certainly prove a
nuisance. With a non-automatic apparatus containing all its gas in the
holder, the attendant would note the position of the bell each morning,
and would introduce sufficient carbide to fill the holder full, or partly
full, as the case might be; with an automatic apparatus he would be
tempted to trust that the carbide holders still contained sufficient
material to last another night.
The automatic system of generating acetylene has undoubtedly one
advantage in those climates where frost tends to occur frequently, but
only to prevail for a short period. As the apparatus is in operation
during the evening hours, the heat evolved will, or can be made to,
suffice to protect the apparatus from freezing until the danger has
passed; whereas if the gas is generated of a morning in a non-automatic
apparatus the temperature of the plant may fall to that of the atmosphere
before evening, and some portion may freeze unless special precautions
are taken to protect it.
It was shown in Chapter II that overheating is one of the chief troubles
to be guarded against in acetylene generators, and that the temperature
attained is a function of the speed at which generation proceeds. Seeing
that in an automatic apparatus the rate of decomposition depends on the
rate at which gas is being burnt, while in a non-automatic generator it
is, or may be, under no control, the critic may urge that the reaction
must take place more slowly and regularly, and the maximum temperature
therefore be lower, when the plant works automatically. This may be true
if the non-automatic generator is unskilfully designed or improperly
manipulated; but it is quite feasible to arrange an apparatus, especially
one of the carbide-to-water or of the flooded-compartment type, in such
fashion that overheating to an objectionable extent is rendered wholly
impossible. In a non-automatic apparatus the holder is nothing but a
holder and may be placed wherever convenient, even at a distance from the
generating plant; in an automatic apparatus the holder, or a small
similarly constructed holder placed before the main storage vessel, has
to act as a water-supply governor, as the releasing gear for certain
carbide-food mechanism, or indeed as the motive power of such mechanism;
and accordingly it must be close to the water or carbide store, and more
or less intimately connected by means of levers, or the like, with the
receptacle in which decomposition occurs. Sometimes the holder surrounds,
or is otherwise an integral part of, the decomposing chamber, the whole
apparatus being made self-contained or a single structure with the object
of gaining compactness. But it is evident that such methods of
construction render additionally awkward, or even hazardous, any repair
or petty operation to the generating portion of the plant; while the more
completely the holder is isolated from the decomposing vessels the more
easily can they be cleaned, recharged, or mended, without blowing off the
stored gas and without interfering with the action of any burners that
may be alight at the time. Owing to the ingenuity of inventors, and the
experience they have acquired in the construction of automatic acetylene
apparatus during the years that the gas has been in actual employment, it
is going too far boldly to assert that non-automatic generators are
invariably to be preferred before their rivals. Still in view of the
nature of the labour which is likely to be bestowed on any domestic
plant, of the difficulty in having repairs or adjustments done quickly in
outlying country districts, and of the inconvenience, if not risk,
attending upon any failure of the apparatus, the greater capital outlay,
and the larger space required by non-automatic generators are in most
instances less important than the economy in space and prime cost
characteristic of automatic machines when the defects of each are weighed
fairly in the balance. Indeed, prolonged experience tends to show that a
selection between non-automatic and automatic apparatus may frequently be
made on the basis of capacity. A small plant is undoubtedly much more
convenient if automatic; a very large plant, such as that intended for a
public supply, is certainly better if non-automatic, but between these
two extremes choice may be exercised according to local conditions.
CONTROL OF THE CHEMICAL REACTION.--Coming now to study the principles
underlying the construction of an acetylene generator more closely it
will be seen that as acetylene is produced by bringing calcium carbide
into contact with water, the chemical reaction may be started either by
adding the carbide to the water, or by adding the water to the carbide.
Similarly, at least from the theoretical aspect, the reaction, may be
caused to stop by ceasing to add carbide to water, or by ceasing to add
water to carbide. Apparently if water is added by degrees to carbide,
until the carbide is exhausted, the carbide must always be in excess; and
manifestly, if carbide is added in small portions to water, the water
must always be in excess, which, as was argued in Chapter II., is
emphatically the more desirable position of affairs. But it in quite
simple to have carbide present in large excess of the water introduced
when the whole generator is contemplated, and yet to have the water
always in chemical excess in the desired manner; because to realise the
advantages of having water in excess, it is only necessary to subdivide
the total charge of carbide into a number of separate charges which are
each so small that more than sufficient water to decompose and flood one
of them is permitted to enter every time the feed mechanism comes into
play, or (in a non-automatic apparatus) every time the water-cock is
opened; so arranging the charges that each one is protected from the
water till its predecessor, or its predecessor, have been wholly
decomposed. Thus it is possible to regard either the carbide or the water
as the substance which has to be brought into contact with the other in
specified quantity. It is perhaps permissible to repeat that in the
construction of an automatic generator there is no advantage to be gained
from regulating the supply of both carbide and water, because just as the
mutual decomposition will begin immediately any quantity of the one meets
any quantity of the other, so the reaction will cease (except in one case
owing to "after-generation") directly the whole of that material which is
not in chemical excess has been consumed-quite independently of the
amount of the other material left unattacked. Being a liquid, and
possessing as such no definite shape or form of its own irrespective of
the vessel in which it is held, water is by far the more convenient of
the two substances to move about or to deliver in predetermined volume to
the decomposing chamber. A supply of water can be started instantaneously
or cut oil as promptly by the movement of a cock or valve of the usual
description; or it may be allowed to run down a depending pipe in
obedience to the law of gravitation, and stopped from running down such a
pipe by opposing to its passage a gas pressure superior to that
gravitational force. In any one of several obvious ways the supply of
water to a mass of carbide may be controlled with absolute certainty, and
therefore it should apparently follow that the make of acetylene should
be under perfect control by controlling the water current. On the other
hand, unless made up into balls or cartridges of some symmetrical form,
calcium carbide exists in angular masses of highly irregular shape and
size. Its lumps alter in shape and size directly liquid water or moisture
reaches them; a loose more or loss gritty powder, or a damp cohesive mud,
being produced which is well calculated to choke any narrow aperture or
to jam any moving valve. It is more difficult, therefore, by mechanical
agency to add a supply of carbide to a mass of water than to introduce a
supply of water to a stationary mass of carbide; and far more difficult
still to bring the supply of carbide under perfect control with the
certainty that the movement shall begin and stop immediately the proper
time arrives.
But assuming the mechanical difficulties to be satisfactorily overcome,
the plan of adding carbide to a stationary mass of water has several
chemical advantages, first, because, however the generator be
constructed, water will be in excess throughout the whole time of gas
production; and secondly, because the evolution of acetylene will
actually cease completely at the moment when the supply of carbide is
interrupted. There is, however, one particular type of generator in which
as a matter of fact the carbide is the moving constituent, viz., the
"dipping" apparatus (cf. _infra_), to which these remarks do not
apply; but this machine, as will be seen directly, is, illogically
perhaps, but for certain good reasons, classed among the water-to-carbide
apparatus. All the mechanical advantages are in favour, as just
indicated, of making water the moving substance; and accordingly, when
classified in the present manner, a great majority of the generators now
on the markets are termed water-to-carbide apparatus. Their disadvantages
are twofold, though these may be avoided or circumvented: in all types
save one the carbide is in excess at the immediate place and time of
decomposition; and in all types without exception the carbide in the
whole of the generator is in excess, so that the phenomenon of "after-
generation" occurs with more or less severity. As explained in the last
chapter, after-generation is the secondary production of acetylene which
takes place more or less slowly after the primary reaction is finished,
proceeding either between calcium hydroxide, merely damp lime, or damp
gas and calcium carbide, with an evolution of more acetylene. As it is
possible, and indeed usual, to fit a holder of some capacity even to an
automatic generator, the simple fact that more acetylene is liberated
after the main reaction is over does not matter, for the gas can be
safely stored without waste and entirely without trouble or danger. The
real objection to after-generation is the difficulty of controlling the
temperature and of dissipating the heat with which the reaction is
accompanied. It will be evident that the balance of advantage, weighing
mechanical simplicity against chemical superiority, is somewhat even
between carbide-to-water and water-to-carbide generators of the proper
type; but the balance inclines towards the former distinctly in the ease
of non-automatic apparatus, and points rather to the latter when
automatism is desired. In the early days of the industry it would have
been impossible to speak so favourably of automatic carbide-to-water
generators, for they were at first constructed with absurdly complicated
and unreliable mechanism; but now various carbide-feed gears have been
devised which seem to be trustworthy even when carbide not in cartridge
form is employed.
NON-AUTOMATIC CARBIDE-TO-WATER GENERATORS.--There is little to be said in
the present place about the principles underlying the construction of
non-automatic generators. Such apparatus may either be of the carbide-to-
water or the water-to-carbide type. In the former, lumps of carbide are
dropped by hand down a vertical or sloping pipe or shoot, which opens at
its lower end below the water-level of the generating chamber, and which
is fitted below its mouth with a deflector to prevent the carbide from
lodging immediately underneath that mouth. The carbide falls through the
water which stands in the shoot itself almost instantaneously, but during
its momentary descent a small quantity of gas is evolved, which produces
an unpleasant odour unless a ventilating hood is fixed above the upper
end of the tube. As the ratio of cubical contents to superficial area of
a lump is greater as the lump itself is larger, and as only the outer
surface of the lump can be attacked by the water in the shoot during its
descent, carbide for a hand-fed carbide-to-water generator should be in
fairly large masses--granulated material being wholly unsuitable--and
this quite apart from the fact that large carbide is superior to small in
gas-making capacity, inasmuch as it has not suffered the inevitable
slight deterioration while being crushed and graded to size. If carbide
is dropped too rapidly into such a generator which is not provided with a
false bottom or grid for the lumps to rest upon, the solid is apt to
descend among a mass of thick lime sludge produced at a former operation,
which lies at the bottom of the decomposing chamber; and here it may be
protected from the cooling action of fresh water to such an extent that
its surface is baked or coated with a hard layer of lime, while
overheating to a degree far exceeding the boiling-point of water may
occur locally. When, however, it falls upon a grid placed some distance
above the bottom of the water vessel, the various convection currents set
up as parts of the liquid become warm, and the mechanical agitations
produced by the upward current of gas rinse the spent lime from the
carbide, and entirely prevent overheating, unless the lumps are
excessively large in size. If the carbide charged into a hand-fed
generator is in very large lumps there is always a possibility that
overheating may occur in the centre of the masses, due to the baking of
the exterior, even if the generator is fitted with a reaction grid.
Manifestly, when carbide in lumps of reasonable size is dropped into
excess of water which is not merely a thick viscid cream of lime, the
temperature cannot possibly exceed the boiling-point--_i.e._, 100°
C.--provided always the natural convection currents of the water are
properly made use of.
The defect which is, or rather which may be, characteristic of a hand-fed
carbide-to-water generator is a deficiency of gas yield due to
solubility. At atmospheric temperatures and pressure 10 volumes of water
dissolve 11 volumes of acetylene, and were the whole of the water in a
large generator run to waste often, a sensible loss of gas would ensue.
If the carbide falls nearly to the bottom of the water column, the rising
gas is forced to bubble through practically the whole of the liquid, so
that every opportunity is given it to dissolve in the manner indicated
till the liquid is completely saturated. The loss, however, is not nearly
so serious as is sometimes alleged, because (1) the water becomes heated
and so loses much of its solvent power; and (2) the generator is worked
intermittently, with sufficiently long intervals to allow the spent lime
to settle into a thick cream, and only that thick cream is run off, which
represents but a small proportion of the total water present. Moreover, a
hand-fed carbide-to-water generator will work satisfactorily with only
half a gallon [Footnote: The United States National Board of Fire
Underwriters stipulates for the presence of 1 (American) gallon of water
for every 1 lb. of carbide before such an apparatus is "permitted." This
quantity of liquid might retain nearly 4 per cent. of the total acetylene
evolved. Even this is an exaggeration; for neither her, nor in the
corresponding figure given in the text, is any allowance made for the
diminution in solvent power of the water as it becomes heated by the
reaction.] of liquid present for every 1 lb. of carbide decomposed, and
were all this water run off and a fresh quantity admitted before each
fresh introduction of carbide, the loss of acetylene by dissolution could
not exceed 2 per cent. of the total make, assuming the carbide to be
capable of yielding 5 cubic feet of gas per lb. Admitting, however, that
some loss of gas does occur in this manner, the defect is partly, if not
wholly, neutralised by the concomitant advantages of the system: (1)
granted that the generator is efficiently constructed, decomposition of
the carbide is absolutely complete, so that no loss of gas occurs in this
fashion; (2) the gas is evolved at a low temperature, so that it is
unaccompanied, by products of polymerisation, which may block the leading
pipes and must reduce the illuminating power; (3) the acetylene is not
mixed with air (as always happens at the first charging of a water-to-
carbide apparatus), which also lowers the illuminating power; and (4) the
gas is freed from two of its three chief impurities, viz., ammonia and
sulphuretted hydrogen, in the generating chamber itself. To prevent the
loss of acetylene by dissolution, carbide-to-water generators are
occasionally fitted with a reaction grid placed only just below the
water-level, so that the acetylene has no more than 1 inch or so of
liquid to bubble through. The principle is wrong, because hot water being
lighter than cold, the upper layers may be raised to the boiling-point,
and even converted into steam, while the bulk of the liquid still remains
cold; and if the water actually surrounding the carbide is changed into
vapour, nearly all control over the temperature attending the reaction is
lost.
The hand-fed carbide-to-water generator is very simple and, as already
indicated, has proved itself perhaps the best type of all for the
construction of very large installations; but the very simplicity of the
generator has caused it more than once to be built in a manner that has
not given entire satisfaction. As shown at L in Fig. 6, p. 84, the
generator essentially consists of a closed cylindrical vessel
communicating at its top with a separate rising holder. At one side as
drawn, or disposed concentrically if so preferred, is an open-mouthed
pipe or shoot (American "shute") having its lower open extremity below
the water-level. Into this shoot are dropped by hand or shovel lumps of
carbide, which fall into the water and there suffer decomposition. As the
bottom of the shoot is covered with water, which, owing to the small
effective gas pressure in the generator given by the holder, stands a few
inches higher in the shoot than in the generator, gas cannot escape from
the shoot; because before it could do so the water in the generator would
have to fall below the level of the point _a_, being either driven
out through the shoot or otherwise. Since the point _b_ of the shoot
extends further into the generator than _a_, the carbide drops
centrally, and as the bubbles of gas rise vertically, they have no
opportunity of ascending into the shoot. In practice, the generator is
fitted with a conical bottom for the collection of the lime sludge and
with a cock or other aperture at the apex of the cone for the removal of
the waste product. As it is not desirable that the carbide should be
allowed to fall directly from the shoot into the thicker portion of the
sludge within the conical part of the generator, one or more grids is
usually placed in the apparatus as shown by the dotted lines in the
sketch. It does not seem that there is any particular reason for the
employment of more than one grid, provided the size of the carbide
decomposed is suited to the generator, and provided the mesh of the grid
is suited to the size of the carbide. A great improvement, however, is
made if the grid is carried on a horizontal spindle in such a way that it
can be rocked periodically in order to assist in freeing the lumps of
carbide from the adhering particles of lime. As an alternative to the
movable grid, or even as an adjunct thereto, an agitator scraping the
conical sides of the generator may be fitted which also assists in
ensuring a reasonably complete absence of undecomposed carbide from the
sludge drawn off at intervals. A further point deserves attention. If
constructed in the ideal manner shown in Fig. 6 removal of some of the
sludge in the generator would cause the level of the liquid to descend
and, by carelessness, the level might fall below the point _a_ at
the base of the shoot. In these circumstances, if gas were unable to
return from the holder, a pressure below that of the atmosphere would be
established in the gas space of the generator and air would be drawn in
through the shoot. This air might well prove a source of danger when
generation was started again. Any one of three plans may be adopted to
prevent the introduction of air. A free path may be left on the gas-main
passing from the generator to the holder so that gas may be free to
return and so to maintain the usual positive pressure in the decomposing
vessel; the sludge may be withdrawn into some vessel so small in capacity
that the shoot cannot accidentally become unsealed; or the waterspace of
the generator may be connected with a water-tank containing a ball-valve
attached to a constant service of water be that liquid runs in as quickly
as sludge is removed, and the level remains always at the same height.
The first plan is only a palliative and has two defects. In the first
place, the omission of any non-return valve between, the generator and
the next item in the train of apparatus is objectionable of itself; in
the second place, should a very careless attendant withdraw too much
liquid, the shoot might become unsealed and the whole contents of the
holder be passed into the air of the building containing the apparatus
through the open mouth of the shoot. The second plan is perfectly sound,
but has the practical defect of increasing the labour of cleaning the
generator. The third plan is obviously the best. It can indeed be adopted
where no real constant service of water is at hand by connecting the
generator to a water reservoir of relatively large size and by making the
latter of comparatively large transverse area, in proportion to its
depth; so that the escape of even a largo volume of water from the
reservoir may not involve a large reduction in the level at which it
stands there.
The dust that always clings to lumps of carbide naturally decomposes with
extreme rapidity when the material is thrown into the shoot of a carbide-
to-water generator, and the sudden evolution of gas so produced has on
more than one occasion seriously alarmed the attendant on the plant.
Moreover, to a trifling extent the actual superficial layers of the
carbide suffer attack before the lumps reach the true interior of the
generator, and a small loss of gas thereby occurs through the open mouth
of the shoot. To remove these objections to the hand-fed generator it has
become a common practice in large installations to cause the lower end of
the shoot to dip under the level of some oil contained in an appropriate
receptacle, the carbide falling into a basket carried upon a horizontal
spindle. The basket and its support are so arranged that when a suitable
charge of carbide has been dropped into it, a partial rotation of an
external hand-wheel lifts the basket and carbide out of the oil into an
air-tight portion of the generator where the surplus oil can drain away
from the lumps. A further rotation of the hand-wheel then tips the basket
over a partition inside the apparatus, allowing the carbide to fall into
the actual decomposing chamber. This method of using oil has the
advantage of making the evolution of acetylene on a large scale appear to
proceed more quietly than usual, and also of removing the dust from the
carbide before it reaches the water of the generator. The oil itself
obviously does not enter the decomposing chamber to any appreciable
extent and therefore does not contaminate the final sludge. The whole
process accordingly lies to be favourably distinguished from those other
methods of employing oil in generators or in the treatment of carbide
which are referred to elsewhere in this book.
NON-AUTOMATIC WATER-TO-CARBIDE GENERATORS.--The only principle underlying
the satisfactory design of a non-automatic water-to-carbide generator is
to ensure the presence of water in excess at the spot where decomposition
is taking place. This may be effected by employing what is known as the
"flooded-compartment" system of construction, _i.e._, by subdividing
the total carbide charge into numerous compartments arranged either
vertically or horizontally, and admitting the water in interrupted
quantities, each more than sufficient thoroughly to decompose and
saturate the contents of one compartment, rather than in a slow, steady
stream. It would be quite easy to manage this without adopting any
mechanism of a moving kind, for the water might be stored in a tank kept
full by means of a ball-valve, and admitted to an intermediate reservoir
in a slow, continuous current, the reservoir being fitted with an
inverted syphon, on the "Tantalus-cup" principle, so that it should first
fill itself up, and then suddenly empty into the pipe leading to the
carbide container. Without this refinement, however, a water-to-carbide
generator, with subdivided charge, behaves satisfactorily as long as each
separate charge of carbide is so small that the heat evolved on its
decomposition can be conducted away from the solid through the water-
jacketed walls of the vessel, or as the latent heat of steam, with
sufficient rapidity. Still it must be remembered that a water-to-carbide
generator, with subdivided charge, does not belong to the flooded-
compartment type if the water runs in slowly and continuously: it is then
simply a "contact" apparatus, and may or may not exhibit overheating, as
well as the inevitable after-generation. All generators of the water-to-
carbide type, too, must yield a gas containing some air in the earlier
portions of their make, because the carbide containers can only be filled
one-third or one-half full of solid. Although the proportion of air so
passed into the holder may be, and usually is, far too small in amount to
render the gas explosive or dangerous in the least degree, it may well be
sufficient to reduce the illuminating power appreciably until it is swept
out of the service by the purer gas subsequently generated. Moreover, all
water-to-carbide generators are liable, as just mentioned, to produce
sufficient overheating to lower the illuminating power of the gas
whenever they are wilfully driven too fast, or when they are reputed by
their makers to be of a higher productive capacity than they actually
should be; and all water-to-carbide generators, excepting those where the
carbide is thoroughly soaked in water at some period of their operation,
are liable to waste gas by imperfect decomposition.
DEVICES TO SECURE AUTOMATIC ACTION,--The devices which are commonly
employed to render a generator automatic in action, that is to say, to
control the supply of one of the two substances required in the
intermittent evolution of gas, may be divided into two broad classes: (A)
those dependent upon the position of a rising-holder bell, and (B) those
dependent upon the gas pressure inside the apparatus. As the bell of a
rising holder descends in proportion as its gaseous contents are
exhausted, it may (A^1) be fitted with some laterally projecting pin
which, arrived at a certain position, actuates a series of rods or
levers, and either opens a cock on the water-supply pipe or releases a
mechanical carbide-feed gear, the said cock being closed again or the
feed-gear thrown out of action when the pin, rising with the bell, once
more passes a certain position, this time in its upward path. Secondly
(A^2), the bell may be made to carry a perforated receptacle containing
carbide, which is dipped into the water of the holder tank each time the
bell falls, and is lifted out of the water when it rises again. Thirdly
(A^3), by fitting inside the upper part of the bell a false interior,
conical in shape, the descent of the bell may cause the level of the
water in the holder tank to rise until it is above some lateral aperture
through which the liquid may escape into a carbide container placed
elsewhere. These three methods are represented in the annexed diagram
(Fig. 1). In Al the water-levels in the tank and bell remain always at
_l_, being higher in the tank than in the bell by a distance
corresponding with the pressure produced by the bell itself. As the bell
falls a pin _X_ moves the lever attached to the cock on the water-
pipe, and starts, or shuts off, a current passing from a store-tank or
reservoir to a decomposing vessel full of carbide. It is also possible to
make _X_ work some releasing gear which permits carbide to fall into
water--details of this arrangement are given later on. In A^1 the water
in the tank serves as a holder seal only, a separate quantity being
employed for the purposes of the chemical reaction. This arrangement has
the advantage that the holder water lasts indefinitely, except for
evaporation in hot weather, and therefore it may be prevented from
freezing by dissolving in it some suitable saline body, or by mixing with
it some suitable liquid which lowers its point of solidification. It will
be observed, too, that in A^1 the pin _X_, which derives its motive
power from the surplus weight of the falling bell, has always precisely
the same amount of work to do, viz., to overcome the friction of the plug
of the water-cock in its barrel. Hence at all times the pressure
obtaining in the service-pipe is uniform, except for a slight jerk
momentarily given each time the cock is opened or closed. When _X_
actuates a carbide-feed arrangement, the work it does may or may not vary
on different occasions, as will appear hereafter. In A^2 the bell itself
carries a perforated basket of carbide, which is submerged in the water
when the bell falls, and lifted out again when it rises. As the carbide
is thus wetted from below, the lower portion of the mass soon becomes a
layer of damp slaked lime, for although the basket is raised completely
above the water-level, much liquid adheres to the spent carbide by
capillary attraction. Hence, even when the basket is out of the water,
acetylene is being produced, and it is produced in circumstances which
prevent any control over the temperature attained. The water clinging to
the lower part of the basket is vaporised by the hot, half-spent carbide,
and the steam attacks the upper part, so that polymerisation of the gas
and baking of the carbide are inevitable. In the second place, the
pressure in the service-pipe attached to A^2 depends as before upon the
net weight of the holder bell; but here that net weight is made up of the
weight of the bell itself, that of the basket, and that of the carbide it
contains. Since the carbide is being gradually converted into damp slaked
lime, it increases in weight to an indeterminate extent as the generator
in exhausted; but since, on the other hand, some lime may be washed out
of the basket each time it is submerged, and some of the smaller
fragments of carbide may fall through the perforations, the basket tends
to decrease in weight as the generator is exhausted. Thus it happens in
A^2 that the combined weight of bell plus basket plus contents is wholly
indefinite, and the pressure in the service becomes so irregular that a
separate governor must be added to the installation before the burners
can be expected to behave properly. In the third place, the water in the
tank serves both for generation and for decomposition, and this involves
the employment of some arrangement to keep its level fairly constant lest
the bell should become unsealed, while protection from frost by saline or
liquid additions is impossible. A^2 is known popularly as a "dipping"
generator, and it will be seen to be defective mechanically and bad
chemically. In both A^1 and A^2 the bell is constructed of thin sheet-
metal, and it is cylindrical in shape; the mass of metal in it is
therefore negligible in comparison with the mass of water in the tank,
and so the level of the liquid is sensibly the same whether the bell be
high or low. In A^3 the interior of the bell is fitted with a circular
plate which cuts off its upper corners and leaves a circumferential space
_S_ triangular in vertical section. This space is always full of
air, or air and water, and has to be deducted from the available storage
capacity of the bell. Supposing the bell transparent, and viewing it from
above, its effective clear or internal diameter will be observed to be
smaller towards the top than near the bottom; or since the space _S_
is closed both against the water and against the gas, the walls of the
bell may be said to be thicker near its top. Thus it happens that as the
bell descends into the water past the lower angle of _S_, it begins
to require more space for itself in the tank, and so it displaces the
water until the levels rise. When high, as shown in the sketch marked
A^3(a), the water-level is at _l_, below the mouth of a pipe
_P_; but when low, as in A^3(b), the water is raised to the point
_l'_, which is above _P_. Water therefore flows into _P_,
whence it reaches the carbide in an attached decomposing chamber. Here
also the water in the tank is used for decomposition as well as for
sealing purposes, and its normal level must be maintained exactly at
_l_, lest the mouth of _P_ should not be covered whenever the
bell falls.
[Illustration: FIG. 1.--TYPICAL METHODS OF AUTOMATIC GENERATION
CONTROLLED BY BELL GASHOLDER.]
The devices employed to render a generator automatic which depend upon
pressure (B) are of three main varieties: (B^1) the water-level in the
decomposing chamber may be depressed by the pressure therein until its
surface falls below a stationary mass of carbide; (B^2) the level in a
water-store tank may be depressed until it falls below the mouth of a
pipe leading to the carbide vessel; (B^3) the current of water passing
down a pipe to the decomposing chamber may be interrupted by the action
of a pressure superior to the force of gravitation. These arrangements
are indicated roughly in Fig. 2. In B^1, D is a hollow cylinder closed at
all points except at the cock G and the hole E, which are always below
the level of the water in the annulus F, the latter being open to the air
at its top. D is rigidly fastened to the outer vessel F so that it cannot
move vertically, and the carbide cage is rigidly fastened to D. Normally
the water-levels are at _l_, and the liquid has access to the
carbide through perforations in the basket. Acetylene is thus produced;
but if G is shut, the gas is unable to escape, and so it presses
downwards upon the water until the liquid falls in D to the dotted line
_l"_, rising in F to the dotted line _l'_. The carbide is then
out of water, and except for after-generation, evolution of gas ceases.
On opening G more or less fully, the water more or less quickly reaches
its original position at _l_, and acetylene is again produced.
Manifestly this arrangement is identical with that of A^2 as regards the
periodical immersion of the carbide holder in the liquid; but it is even
worse than the former mechanically because there is no rising holder in
B^1, and the pressure in the service is never constant. B^2 represents
the water store of an unshown generator which works by pressure. It
consists of a vessel divided vertically by means of a partition having a
submerged hole N. One-half, H, is cloned against the atmosphere, but
communicates with the gas space of the generator through L; the other
half, K, is open to the air. M is a pipe leading water to the carbide.
When gas is being burnt as fast as, or faster than, it is being evolved,
the pressure in the generator is small, the level of the water stands at
_l_, and the mouth of M is below it. When the pressure rises by
cessation of consumption, that pressure acts through L upon the water in
H, driving it down in H and up in K till it takes the positions
_l"_, and _l'_, the mouth of M being then above the surface. It
should be observed that in the diagrams B^1 and B^3, the amount of
pressure, and the consequent alteration in level, is grossly exaggerated
to gain clearness; one inch or less in both cases may be sufficient to
start or retard evolution of acetylene. Fig. B^3 is somewhat ideal, but
indicates the principle of opposing gas pressure to a supply of water
depending upon gravitation; a method often adopted in the construction of
portable acetylene apparatus. The arrangement consists of an upper tank
containing water open to the air, and a lower vessel holding carbide
closed everywhere except at the pipe P, which leads to the burners, and
at the pipe S, which introduces water from the store-tank. If the cock at
T is closed, pressure begins to rise in the carbide holder until it is
sufficient to counterbalance the weight of the column of water in the
pipe S, when a further supply is prevented until the pressure sinks
again. This idea is simply an application of the displacement-holder
principle, and as such is defective (except for vehicular lamps) by
reason of lack of uniformity in pressure.
[Illustration: FIG. 2.--TYPICAL METHODS OF AUTOMATIC GENERATION
CONTROLLED BY INTERNAL GAS PRESSURE.]
DISPLACEMENT GASHOLDERS.--An excursion may here be made for the purpose
of studying the action of a displacement holder, which in its most
elementary form is shown at C. It consists of an upright vessel open at
the top, and divided horizontally into two equal portions by a partition,
through which a pipe descends to the bottom of the lower half. At the top
of the closed lower compartment a tube is fixed, by means of which gas
can be introduced below the partition. While the cock is open to the air,
water is poured in at the open top till the lower compartment is
completely full, and the level of the liquid is at _l_. If now, gas
is driven in through the side tube, the water is forced downwards in the
lower half, up through the depending pipe till it begins to fill the
upper half of the holder, and finally the upper half is full of water and
the lower half of gas an shown by the levels _l'_ and _l"_. But
the force necessary to introduce gas into such an apparatus, which
conversely is equal to the force with which the apparatus strives to
expel its gaseous contents, measured in inches of water, is the distance
at any moment between the levels _l'_ and _l"_; and as these
are always varying, the effective pressure needed to fill the apparatus,
or the effective pressure given by the apparatus, may range from zero to
a few inches less than the total height of the whole holder. A
displacement holder, accordingly, may be used either to store a varying
quantity of gas, or to give a steady pressure just above or just below a
certain desired figure; but it will not serve both purposes. If it is
employed as a holder, it in useless as a governor or pressure regulator;
if it is used as a pressure regulator, it can only hold a certain fixed
volume of gas. The rising holder, which is shown at A^1 in Fig. 1
(neglecting the pin X, &c.) serves both purposes simultaneously; whether
nearly full or nearly empty, it gives a constant pressure--a pressure
solely dependent upon its effective weight, which may be increased by
loading its crown or decreased by supporting it on counterpoises to any
extent that may be required. As the bell of a rising holder moves, it
must be provided with suitable guides to keep its path vertical; these
guides being arranged symmetrically around its circumference and carried
by the tank walls. A fixed control rod attached to the tank over which a
tube fastened to the bell slides telescope-fashion is sometimes adopted;
but such an arrangement is in many respects less admirable than the
former.
Two other devices intended to give automatic working, which are scarcely
capable of classification among their peers, may be diagrammatically
shown in Fig. 3. The first of these (D) depends upon the movements of a
flexible diaphragm. A vessel (_a_) of any convenient size and shape
is divided into two portions by a thin sheet of metal, leather,
caoutchouc, or the like. At its centre the diaphragm is attached by some
air-tight joint to the rod _c_, which, held in position by suitable
guides, is free to move longitudinally in sympathy with the diaphragm,
and is connected at its lower extremity with a water-supply cock or a
carbide-feed gear. The tube _e_ opens at its base into the gas space
of the generator, so that the pressure below the diaphragm in _a_ is
the same as that elsewhere in the apparatus, while the pressure in
_a_ above the diaphragm is that of the atmosphere. Being flexible
and but slightly stretched, the diaphragm is normally depressed by the
weight of _c_ until it occupies the position _b_; but if the
pressure in the generator (_i.e._, in _e_) rises, it lifts the
diaphragm to somewhat about the position _b'_--the extent of
movement being, as usual, exaggerated in the sketch. The movement of the
diaphragm is accompanied by a movement of the rod _c_, which can be
employed in any desirable way. In E the bell of a rising holder of the
ordinary typo is provided with a horizontal striker which, when the bell
descends, presses against the top of a bag _g_ made of any flexible
material, such as india-rubber, and previously filled with water. Liquid
is thus ejected, and may be caused to act upon calcium carbide in some
adjacent vessel. The sketch is given because such a method of obtaining
an intermittent water-supply has at one time been seriously proposed; but
it is clearly one which cannot be recommended.
[Illustration: FIG. 3.--TYPICAL METHODS OF AUTOMATIC GENERATION
CONTROLLED BY A FLEXIBLE DIAPHRAM OR BAG.]
ACTION OF WATER-TO-CARBIDE GENERATORS.--Having by one or other of the
means described obtained a supply of water intermittent in character, it
remains to be considered how that supply may be made to approach the
carbide in the generator. Actual acetylene apparatus are so various in
kind, and merge from one type to another by such small differences, that
it is somewhat difficult to classify them in a simple and intelligible
fashion. However, it may be said that water-to-carbide generators,
_i.e._, such as employ water as the moving material, may be divided
into four categories: (F^1) water is allowed to fall as single drops or
as a fine stream upon a mass of carbide--this being the "drip" generator;
(F^2) a mass of water is made to rise round and then recede from a
stationary vessel containing carbide--this being essentially identical in
all respects save the mechanical one with the "dip" or "dipping"
generator shown in A^2, Fig. 1; (F^3) a supply of water is permitted to
rise round, or to flow upon, a stationary mass of carbide without ever
receding from the position it has once assumed--this being the "contact"
generator; and (F^4) a supply of water is admitted to a subdivided charge
of carbide in such proportion that each quantity admitted is in chemical
excess of the carbide it attacks. With the exception of F^2, which has
already been illustrated as A^2 Fig. 1, or as B^1 in Fig. 2, these
methods of decomposing carbide are represented in Figs. 4 and 5. It will
be observed that whereas in both F^1 and F^3 the liberated acetylene
passes off at the top of the apparatus, or rather from the top of the
non-subdivided charge of carbide, in F^1 the water enters at the top, and
in F^3 it enters at the bottom. Thus it happens that the mixture of
acetylene and steam, which is produced at the spot where the primary
chemical reaction is taking place, has to travel through the entire mass
of carbide present in a generator belonging to type F^3, while in F^1 the
damp gas flows directly to the exit pipe without having to penetrate the
lumps of solid. Both F^1 and F^3 exhibit after-generation caused by a
reaction between the liquid water mechanically clinging to the mass of
spent lime and the excess of carbide to an approximately equal extent;
but for the reason just mentioned, after-generation due to a reaction
between the vaporised water accompanying the acetylene first evolved and
the excess of carbide is more noticeable in F^3 than in F^1; and it is
precisely this latter description of after-generation which leads to
overheating of the most ungovernable kind. Naturally both F^1 and F^3 can
be fitted with water jackets, as is indicated by the dotted lines in the
second sketch; but unless the generating chamber in quite small and the
evolution of gas quite slow, the cooling action of the jacket will not
prove sufficient. As the water in F^1 and F^3 is not capable of backward
motion, the decomposing chambers cannot be employed as displacement
holders, as is the case in the dipping generator pictured at B^1, Fig. 2.
They must be coupled, accordingly, to a separate holder of the
displacement or, preferably, of the rising type; and, in order that the
gas evolved by after-generation may not be wasted, the automatic
mechanism must cut off the supply of water to the generator by the time
that holder is two-thirds or three-quarters full.
[Illustration: FIG. 4.--TYPICAL METHODS OF DECOMPOSING CARBIDE (WATER TO
CARBIDE).]
[Illustration: FIG. 5.--TYPICAL METHODS OF DECOMPOSING CARBIDE (WATER TO
CARBIDE).]
The diagrams G, H, and K in Figs. 4 and 5 represent three different
methods of constructing a generator which belongs either to the contact
type (F^3) if the supply of water is essentially continuous, _i.e._,
if less is admitted at each movement of the feeding mechanism than is
sufficient to submerge the carbide in each receptacle; or to the flooded-
compartment type (F') if the water enters in large quantities at a time.
In H the main carbide vessel is arranged horizontally, or nearly so, and
each partition dividing it into compartments is taller than its
predecessor, so that the whole of the solid in (1) must be decomposed,
and the compartment entirely filled with liquid before it can overflow
into (2), and so on. Since the carbide in all the later receptacles is
exposed to the water vapour produced in that one in which decomposition
is proceeding at any given moment, at least at its upper surface, some
after-generation between vapour and carbide occurs in H; but a partial
control over the temperature may be obtained by water-jacketing the
container. In G the water enters at the base and gas escapes at the top,
the carbide vessels being disposed vertically; hero, perhaps, more after-
generation of the same description occurs, as the moist gas streams round
and over the higher baskets. In K, the water enters at the top and must
completely fill basket (1) before it can run down the depending pipe into
(2); but since the gas also leaves the generator at the top, the later
carbide receptacles do not come in contact with water vapour, but are
left practically unattacked until their time arrives for decomposition by
means of liquid water. K, therefore, is the best arrangement of parts to
avoid after-generation, overheating, and polymerisation of the acetylene
whether the generator be worked as a contact or as a flooded-compartment
apparatus; but it may be freely admitted that the extent of the
overheating due to reaction between water vapour and carbide may be kept
almost negligible in either K, H, or G, provided the partitions in the
carbide container be sufficient in number--provided, that is to say, that
each compartment holds a sufficiently small quantity of carbide; and
provided that the quantity of water ultimately required to fill each
compartment is relatively so large that the temperature of the liquid
never approaches the boiling-point where vaporisation is rapid. The type
of generator indicated by K has not become very popular, but G is fairly
common, whilst H undoubtedly represents the apparatus which is most
generally adopted for use in domestic and other private installations in
the United Kingdom and the Continent of Europe. The actual generators
made according to the design shown by H usually have a carbide receptacle
designed in the form of a semi-cylindrical or rectangular vessel of steel
sliding fairly closely into an outside container, the latter being either
built within the main water space of the entire apparatus or placed
within a separate water-jacketed casing. Owing to its shape and the
sliding motion with which the carbide receptacle is put into the
container these generators are usually termed "drawer" generators. In
comparison with type G, the drawer generator H certainly exhibits a lower
rise in temperature when gas is evolved in it at a given speed and when
the carbide receptacles are constructed of similar dimensions. It is very
desirable that the whole receptacle should be subdivided into a
sufficient number of compartments and that it should be effectively
water-cooled from outside. It would also be advantageous if the water-
supply were so arranged that the generator should be a true flooded-
compartment apparatus, but experience has nevertheless shown that
generators of type H do work very well when the water admitted to the
carbide receptacle, each time the feed comes into action, is not enough
to flood the carbide in one of the compartments. Above a certain size
drawer generators are usually constructed with two or even more complete
decomposing vessels, arrangements being such that one drawer can be taken
out for cleaning, whilst the other is in operation. When this is the case
a third carbide receptacle should always be employed so that it may be
dry, lit to receive a charge of carbide, and ready to insert in the
apparatus when one of the others is withdrawn. The water-feed should
always be so disposed that the attendant can see at a glance which of the
two (or more) carbide receptacles is in action at any moment, and it
should be also so designed that the supply is automatically diverted to
the second receptacle when the first is wholly exhausted and back again
to the first (unless there are more than two) when the carbide in the
second is entirely gasified. In the sketches G, H, and K, the total space
occupied by the various carbide receptacles is represented as being
considerably smaller than the capacity of the decomposing chamber. Were
this method of construction copied in actual acetylene apparatus, the
first makes of gas would be seriously (perhaps dangerously) contaminated
with air. In practice the receptacles should fit so tightly into the
outer vessel and into one another that when loaded to the utmost extent
permissible--space being left for the swelling of the charge and for the
passage of water and gas--but little room should be left for the
retention of air in the chamber.
ACTION OF CARBIDE-TO-WATER GENERATORS.--The methods which may be adopted
to render a generator automatic when carbide is employed as the moving
material are shown at M, N, and P, in Fig. 6; but the precise devices
used in many actual apparatus are so various that it is difficult to
portray them generically. Moreover it is desirable to subdivide automatic
carbide-to-water generators, according to the size of the carbide they
are constructed to take, into two or three classes, which are termed
respectively "large carbide-feed," "small carbide-feed," and "granulated
carbide-feed" apparatus. (The generator represented at L does not really
belong to the present class, being non-automatic and fed by hand; but the
sketch is given for completeness.) M is an automatic carbide-feed
generator having its store of carbide in a hopper carried by the rising-
holder bell. The hopper is narrowed at its mouth, where it is closed by a
conical or mushroom valve _d_ supported on a rod held in suitable
guides. When the bell falls by consumption of gas, it carries the valve
and rod with it; but eventually the button at the base of _c_
strikes the bottom of the generator, or some fixed distributing plate,
and the rod can descend no further. Then, when the bell falls lower, the
mushroom _d_ rises from its seat, and carbide drops from the hopper
into the water. This type of apparatus has the defect characteristic of
A^2, Fig. 1; for the pressure in the service steadily diminishes as the
effective weight of bell plus hopper decreases by consumption of carbide.
But it has also two other defects--(1) that ordinary carbide is too
irregular in shape to fall smoothly through the narrow annular space
between the valve and its seat; (2) that water vapour penetrates into the
hopper, and liberates some gas there, while it attacks the lumps of
carbide at the orifice, producing dust or causing them to stick together,
and thus rendering the action of the feed worse than ever. Most of these
defects can be avoided by using granulated carbide, which is more uniform
in size and shape, or by employing a granulated and "treated" carbide
which has been dipped in some non-aqueous liquid to make it less
susceptible to the action of moisture. Both these plans, however, are
expensive to adopt; first, because of the actual cost of granulating or
"treating" the carbide; secondly, because the carbide deteriorates in
gas-making capacity by its inevitable exposure to air during the
granulating or "treating" process. The defects of irregularity of
pressure and possible waste of gas by evolution in the hopper may be
overcome by disposing the parts somewhat differently; making the holder
an annulus round the hopper, or making it cylindrical with the hopper
inside. In this case the hopper is supported by the main portion of the
apparatus, and does not move with the bell: the rod and valve being given
their motion in some fashion similar to that figured. Apparatus designed
in accordance with the sketch M, or with the modification just described,
are usually referred to under the name of "hopper" generators. On several
occasions trouble has arisen during their employment owing to the jamming
of the valve, a fragment of carbide rather larger than the rest of the
material lodging between the lips of the hopper and the edges of the
mushroom valve. This has been followed by a sudden descent of all the
carbide in the store into the water beneath, and the evolution of gas has
sometimes been too rapid to pass away at the necessary speed into the
holder. The trouble is rendered even more serious should the whole charge
of carbide fall at a time when, by neglect or otherwise, the body of the
generator contains much lime sludge, the decomposition then proceeding
under exceptionally bad circumstances, which lead to the production of an
excessively high temperature. Hopper generators are undoubtedly very
convenient for certain purposes, chiefly, perhaps, for the construction
of table-lamps and other small installations. Experience tends to show
that they may be employed, first, provided they are designed to take
granulated carbide--which in comparison with larger grades is much more
uniform and cylindrical in shape--and secondly, provided the quantity of
carbide in the hopper does not exceed a few pounds. The phenomenon of the
sudden unexpected descent of the carbide, popularly known as "dumping,"
can hardly be avoided with carbide larger in size than the granulated
variety; and since the results of such an accident must increase in
severity with the size of the apparatus, a limit in their capacity is
desirable.
[Illustration: FIG. 6.--TYPICAL METHODS OF DECOMPOSING CARBIDE (CARBIDE
TO WATER).]
When it is required to construct a carbide-feed generator of large size
or one belonging to the large carbide-feed pattern, it is preferable to
arrange the store in a different manner. In N the carbide is held in a
considerable number of small receptacles, two only of which are shown in
the drawing, provided with detachable lids and hinged bottoms kept shut
by suitable catches. At proper intervals of time those catches in
succession are knocked on one side by a pin, and the contents of the
vessel fall into the water. There are several methods available for
operating the pins. The rising-holder bell may be made to actuate a train
of wheels which terminate in a disc revolving horizontally on a vertical
axis somewhere just below the catches; and this wheel may bear an
eccentric pin which hits each catch as it rotates. Alternatively the
carbide boxes may be made to revolve horizontally on a vertical axis by
the movements of the bell communicated through a clutch; and thus each
box in succession may arrive at a certain position where the catch is
knocked aside by a fixed pin. The boxes, again, may revolve vertically on
a horizontal axis somewhat like a water-wheel, each box having its bottom
opened, or, by a different system of construction, being bodily upset,
when it arrives at the bottom of its circular path. In no case, however,
are the carbide receptacles carried by the bell, which is a totally
distinct part of the apparatus; and therefore in comparison with M, the
pressure given by the bell is much more uniform. Nevertheless, if the
system of carbide boxes moves at all, it becomes easier to move by
decrease in weight and consequent diminution in friction as the total
charge is exhausted; and accordingly the bell has less work to do during
the later stages of its operation. For this reason the plan actually
shown at N is preferable, since the work done by the moving pin,
_i.e._, by the descending bell, is always the same. P represents a
carbide-feed effected by a spiral screw or conveyor, which, revolved
periodically by a moving bell, draws carbide out of a hopper of any
desired size and finally drops it into a shoot communicating with a
generating chamber such as that shown in L. Here the work done by the
bell is large, as the friction against the blades of the screw and the
walls of the horizontal tube is heavy; but that amount of work must
always be essentially identical. The carbide-feed may similarly be
effected by means of some other type of conveyor instead of the spiral
screw, such as an endless band, and the friction in these cases may be
somewhat less than with the screw, but the work to be done by the bell
will always remain large, whatever type of conveyor may be adopted. A
further plan for securing a carbide-feed consists in employing some
extraneous driving power to propel a charge of carbide out of a reservoir
into the generator. Sometimes the propulsive effort is obtained from a
train of clockwork, sometimes from a separate supply of water under high
pressure. The clockwork or the water power is used either to drive a
piston travelling through the vessel containing the carbide so that the
proper quantity of material is dropped over the open mouth of a shoot, or
to upset one after another a series of carbide receptacles, or to perform
some analogous operation. In these cases the pin or other device fitted
to the acetylene apparatus itself has nothing to do beyond releasing the
mechanism in question, and therefore the work required from the bell is
but small. The propriety of employing a generator belonging to these
latter types must depend upon local conditions, _e.g._, whether the
owner of the installation has hydraulic power on a small scale (a
constant supply of water under sufficient pressure) at disposal, or
whether he does not object to the extra labour involved in the periodical
winding up of a train of clockwork.
It must be clear that all these carbide-feed arrangements have the defect
in a more or less serious degree of leaving the carbide in the main
storage vessel exposed to the attack of water vapour rising from the
decomposing chamber, for none of the valves or operating mechanism can be
made quite air-tight. Evolution of gas produced in this way does not
matter in the least, because it is easy to return the gas so liberated
into the generator or into the holder; while the extent of the action,
and the consequent production of overheating, will tend to be less than
in generators such as those shown in G and H of Figs. 4 and 5, inasmuch
as the large excess of water in the carbide-feed apparatus prevents the
liquid arriving at a temperature at which it volatilises rapidly. The
main objection to the evolution of gas in the carbide vessel of a
carbide-to-water generator depends on the danger that the smooth working
of the feed-gear may be interfered with by the formation of dust or by
the aggregation of the carbide lumps.