_Form and dimensions of the Retorts originally employed for
manufacturing Coal Gas._
The proper mode of constructing the retorts in which the coal is
distilled, and the art of applying them form an object of primary
importance in every gas-light establishment. According as the
manufacture is conducted in these respects with a due regard to physical
principles, depends the quantity of gas which can be obtained in any
given time, from any given quantity of coal, the consumption of fuel
requisite for the production of that quantity of gas, the degree of
deterioration to which the distillatory vessel is subjected, the quality
in some measure, of the gas itself; and, as the ultimate result of all
these circumstances, the cheapness at which the gas light can be
furnished to the consumer.
The essential influence of these various particulars on the value of the
art of lighting with coal gas, has led to much assiduous enquiry to
ascertain that sort of construction and mode of operation in respect to
each of them which may be most advantageous. And in no branch of the new
art of procuring light, has a greater variety of plans of improvement
been submitted to the several directing boards of gas works, or more
labour and expence been incurred in experiments conducted on a large
scale, to ascertain the relative merits of these plans. Nor is there any
part of the gas-light process in which a greater number of material
alterations have been put in practice.
In the earlier periods of lighting with coal gas the retorts employed at
some of the gas-light establishments in the metropolis, were hollow
cast-iron cones from six to seven feet in length. The greatest diameter
of the cone which formed the mouth of the retort, measured from twelve
to fifteen inches, and its smallest diameter at the vertex from nine to
ten inches.
At other gas works the form of the retort was a parallelopiped from six
to seven feet long, the horizontal, and vertical sides were
respectively to each other, as 20 to 15 inches. The angles of these
retorts were slightly rounded. Fig. 16, plate V. exhibits a vertical
section of this retort.
Again at other establishments semi-cylindrical retorts, placed
horizontally upon their flat surfaces were employed; fig. 18. pl. V. The
length of these retorts was from five to six feet, and their vertical
and horizontal diameters were to each other as 6 inches, to 18 inches.
And at a few establishments, ellipsoidal retorts, fig. 17, plate V. were
used; these measured from five feet and a half, to six feet in length,
their major and minor axes bore different proportions to each other at
different establishments. At the first adoption of these retorts, the
proportions varied but little from the cylinder, but subsequently the
difference between the major and minor axes became gradually increased
till at last the major axis has become to the minor axis, as 20 to 10
inches, and at some gas works the proportions are as 25 to 10 inches.
With vessels of these forms the distillatory process was carried on for
some years, and the quantity of fuel employed to decompose a given
quantity of coal by means of them, amounted to from thirty to
thirty-six per cent.
When the dimensions of the retorts were increased, both the quantity of
fuel and time required for the decomposition of a given quantity of coal
was in a far greater ratio; and the operations of charging and
discharging the retorts, very troublesome.
Retorts of smaller dimensions have likewise been tried, but the more
frequent charging and discharging, which they require, occasioned such a
waste of time and labour, and such intermissions, in the temperature
necessary for the process of distillation, (besides being attended with
other disadvantages which will be afterwards explained), that they were
speedily discontinued at the gas works where they had been adopted.
The use of conical retorts, as well as of those of a semi-cylindrical
and parallelopipedal form, has of late been discontinued in most
establishments. The conical shape not only diminishes the capacity of
the vessel, but also renders it incapable of being heated economically.
From two comparative series of operations made on a large scale, and
continued for upwards of six months with conical and cylindrical
retorts, with a view to determine the comparative power of these
vessels, it has been proved that the same quantity of gas which can be
obtained by means of forty conical retorts, may be procured in the same
time and with the same quantity of coal and fuel, by means of
thirty-four cylindrical retorts.[10]
[10] These Experiments were made at the commencement of the new art of
lighting with gas, at the Westminster Chartered Gas Works, by Messrs.
Grant and Hargraves.
Similar experiments have been undertaken, to determine the comparative
action of semi-cylindrical and parallelopipedal retorts.[11] The latter,
when kept in action day and night, do not long retain their shape; their
sides collapse, their capacity becomes diminished, their angular form
causes the heat to act upon them unequally, in whatever manner it may be
applied, in consequence of which they suffer more deterioration in some
parts than in others. Besides, they require a much larger proportion of
fuel for decomposing a certain quantity of coal than the cylindrical
retorts.
[11] At the Birmingham Gas Works.
Semi-cylindrical retorts, with the base of the retort bent inwards, so
as to give the vessel a kidney-shaped form, have likewise been tried.
But this shape is still less advantageous; they could not be made to
work uniform, they required more heat, and their deterioration was more
rapid than cylindrical retorts. They could not be kept fit for use when
worked day and night, more than about five months. And with regard to
ellipsoidal retorts, it must be confessed, that the experiments that
have as yet been made upon a large scale to ascertain their powers, are
not of a nature to enable us to decide on their merits. No experiments
have been carried on with retorts of this description in the metropolis
for a sufficient length of time, with that care and attention which the
subject demands, to ascertain their comparative power. From what however
has been done, there is reason to believe that ellipsoidal retorts,
might be found more advantageous, than those of a cylindrical form now
in use. An ellipsoidal retort, 20 inches by 10 in diameter, and six feet
long, weighs 14 Cwt.
The reader will thus observe, that of all the forms of retorts which
have been hitherto fairly tried, upon a large scale, it has been
satisfactorily ascertained, (excepting only as to the ellipsoidal
retorts), that the cylinder is the best form for decomposing coal in
masses, from five to eight or ten inches in thickness.
It is perhaps needless to state that in making experiments on the
comparative value of the best form of cast-iron retorts, it is obvious
that the operations should be continued for some months uninterruptedly;
no conclusion can be drawn that may become practically useful in the
large way, from processes carried on for a few weeks only. It is
absolutely essential that the comparative trials be continued for months
together, and that the inferences be taken from the total quantity of
coal used during that period, compared with the total quantity of gas
obtained, the deterioration of the retorts, and the time and labour
expended.
Proceeding on erroneous data, many have persuaded themselves of having
noticed that parallelopipedal and semi-cylindrical retorts last longer
fit for use than those of a cylindrical shape, an assertion of which
subsequent trials, conducted in the manner just stated, has clearly
shown the fallacy. Enough has been done at the different gas works in
the capital to settle this point, and there is now but one opinion
amongst those who are best qualified to judge of the subject. Every body
who has made the trial on a large scale, is convinced as already stated,
that the best form of the retort for manufacturing coal gas where the
process is conducted on the plan of decomposing coal in masses or layers
of from four to eight inches in thickness, is a cylinder six and a half
feet long, and one foot in diameter, and accordingly retorts of this
shape and dimensions are now used in all the best regulated gas
establishments in the metropolis.
A cylindrical retort of the description before named, weighs about nine
and a half to ten hundred weight. These and all other shaped retorts are
furnished with a moveable lid or cover having a conical edge to fit the
mouth-piece; the cover is rendered air-tight, not as formerly by
grinding, a mode which was costly, but by the interposition of a thin
coat of loom, between the lid and the mouth of the retort.
The mouth-piece forms a separate part of the retort. It is bolted and
screwed to a flanch which terminates the mouth of the retort, so that
when the retort is worn out, the mouth-piece may be detached and applied
to new retorts.
There are now in action 620 cylindrical retorts, at the two chartered
Gas Works[12] in the metropolis; and the total number of retorts at all
the London gas establishments amounts to 960.
[12]
{ Westminster Station 250 Retorts.
Westminster Gas Works, { Brick Lane ditto 190 ditto
{ Norton Falgate ditto 50 ditto
City of London Gas Works, Dorset Street, 130 ditto
---
620
_Application of heat.--Flue Plan originally adopted._
It must be obvious that the durability of the distillatory apparatus,
greatly depends on the manner in which the heat is applied, to effect
the decomposition of the coal contained within the retort. If the heat
be very intense the whole vessel is rapidly destroyed. If it be too
languid, the distillatory process is protracted, and much fuel, time,
and labour wasted to no purpose; and the retort is speedily
deteriorated, if the heat acts upon one part of it more than upon
another.
The different kind of retorts of which a description has been given in
the preceding pages, were originally heated by means of flues passing
under and over them. The retorts were placed horizontally and fixed in
brick-work. One fire-place at the extremity of the mouth of the retort
where the coals are introduced, and whence the coke is withdrawn, was
allotted to every two retorts in the series.
At the commencement of the new art of procuring light the quantity of
fuel as before stated, necessary to decompose a given quantity of coal,
amounted to from thirty to thirty-six per cent of the coal decomposed;
that is to say, it required from thirty to thirty-six parts of fuel to
decompose one hundred parts of coal. This quantity has been much
lessened by a better mode of setting the retorts, and it is now the
general opinion that the operation of decomposing coal, by means of
cylindrical, parallelopipedal, or semi-cylindrical retorts, must be
considered as well conducted when one hundred parts of coal are
decomposed by twenty or twenty-five parts of fuel. This appears to be
the minimum quantity of fuel, that can be employed for the complete
decomposition of coal by means of these retorts, and with the least
deterioration of the distillatory vessel.
The following statement will exhibit what has been done in this branch
of art.
_Report on a course of Operations, made with sets of 66, of 30, of 116,
and of 64 retorts, worked on the Flue Plan._
In order to determine the relative value of the best method of setting
cast-iron retorts, it was deemed necessary to ascertain whether three
retorts might not be heated, instead of two, as before stated, by one
fire-place and branching flues. To determine this the following
processes were carried into effect.
_Process I._
Sixty-six cast-iron cylindrical retorts, of the usual size, namely, six
and a half feet long, (exclusive of the mouth-piece) and one foot in
diameter, internal dimensions, where set on the plan of three retorts to
one fire-place, at the Westminster gas-work station, and a series of 30
similar retorts were erected at another station belonging to the same
company, at the East end of London.
The experiments were pursued with every degree of justice in the detail,
the retorts were kept in action day and night for upwards of four
months, and the results noted down with exactness. The final reports
from the two establishments were found to concur in showing that nothing
was to be gained by this method over that previously in use.
The time occupied for the distillatory process was not abridged. The
consumption of fuel was greater--no larger quantity of gas was obtained
from the quantity of coal carbonized. The produce with regard to coke
was in the usual ratio, and the retorts were destroyed in about one
third less time than when only two were heated by one fire-place.
_Process II._
The apparently conclusive results of these experiments did not, however,
prevent another set of experiments from being made on the same
principle, extended even a degree farther. The problem now proposed to
be solved, was, whether four retorts might not be heated with economy,
in a manner which had been found already wasteful with respect to three,
that is, whether four instead of two retorts might not be heated
economically by means of one fire-place.
On this plan one hundred and sixteen cylindrical retorts of the usual
dimensions were again erected at the Westminster gas establishment, and
sixty-four at another station belonging to the same chartered company.
These retorts were kept in action in the best possible manner night and
day, and the results, as might have been anticipated, only served to
confirm the facts already established by experiment with three retorts.
Nothing was found to be gained; and so far from their being any saving
in respect of fuel and wear and tear of the retorts, the waste beyond
that which takes place on the plan of two retorts to one fire-place,
was increased to nearly twenty-five per cent., accompanied by a
corresponding acceleration of injury to the retorts.
It was still imagined, however, that the great waste of fuel and the
ultimate unfavourable result of these proceedings, which were repeated
with as little success at several other gas-works in the metropolis with
parallelopipedal retorts, and at other works with retorts of a
semi-cylindrical form, set in a way different from that pursued at the
Westminster station, might probably have been owing to the unavoidable
circumstance, that the heat was not made to act upon all the retorts
employed uniformly in each series of four retorts, but in a manner so
variable that one, or even two of the series would become destroyed and
rendered useless, while the others continued uninjured in a sound and
working state.
The excessive waste of fuel was occasioned, we are told, by the number
of injured retorts, which became useless, and were nevertheless required
to be kept red hot to no purpose; for it was actually found that when
one retort of a series of four became injured, the same fire which had
heated the whole four, still required to be kept up to maintain in
action the remaining three of the series, and so on with respect to the
whole range, till ultimately when there might remain only eighty retorts
actually in use, as many fire-places were required to be in full action
as would have been sufficient to serve for one hundred retorts.
Attempts accordingly were now made to get over this supposed cause of
the losing results, already obtained from the plan of four retorts to
one fire-place, by a new series of similar operations, in which the
retorts were fixed in such a manner, that those which happened to become
injured during the process, might easily and immediately be withdrawn
without materially disturbing the rest, and replaced by new ones. The
waste of fuel was, it is clear, greatly lessened by the expedient; yet
still upon the whole there was no such variation from the general
results obtained by the preceding experiments, as to justify the
adoption of this plan of increasing the number of retorts worked by one
fire-place, on any principle of sound economy.
The great obstacle, as the reader will at once perceive, to working more
than two retorts, no matter whether cylindrical, or of any of the other
forms before named, with economy, by means of one fire-place, evidently
arose from the difficulty of conducting the heat by means of flues
around the series of retorts, in such a manner that the heat shall act
with equal force on all the retorts.
It is almost needless to state, that the construction of the
fire-places, and the direction of the flues for applying the heat to the
retorts, were varied by different workmen, who prided themselves on
being able to aid the object in view, but the result always showed even
that when the draft of the fire-place was well obtained, the action of
the heat upon the series of retorts could not be distributed equally and
kept up uniformly, except at a great expence of fuel and vast
deterioration of the distillatory vessels. The retorts always became
injured more in some parts than in others. The concentration and
rapidity of the draught of the fire, beyond a certain velocity was
always found highly injurious to the retort, and this observation has
been since amply confirmed.
In a well constructed furnace, the deterioration of all the retorts in
the series is uniform over the whole vessel; no part of the retort is
_burnt out_, as the workmen call it, sooner than another part; and
whenever the contrary happens, we may pronounce the fire to be badly
applied. When there is such misapplication of the heat, the manufacturer
cannot depend upon the duration of the distillatory vessel; he is always
in a state of uncertainty with regard to their wear and tear, and it not
unfrequently has happened, under such circumstances, that a whole series
of retorts have become suddenly deteriorated.
_Oven plan lately adopted._
The results before detailed, with regard to the mode of setting
cylindrical retorts suggested the propriety of an entire change in the
mode of applying the heat, and this was at length fully carried into
effect by the adoption of ovens, or air furnaces, in which the retorts
are equally exposed to the action of heat on all sides. Mr. Rackhouse
has the merit of having first carried into effect this method, since
generally known by the name of the _oven plan_.
The first experiments with these ovens were made on only one retort,
exposed in an oven to air intensely heated; but they were afterwards
repeated on two, three, four, and five retorts, successively. The
retorts suffered the action of heat thus applied, exceedingly well;
their deterioration was uniform, and the quantity of fuel required to
work them, was found always to be in a direct ratio to the number of
retorts employed. These experiments were carried on for upwards of nine
months, and it was found, that with five retorts in one oven, so that
the heated air could act upon all of them equally, without the flame
being directed forcibly upon them, this plan had a decided advantage, in
point of economy, over every other method previously adopted. Each oven,
containing five retorts, is heated by means of three fire-places, and
although it is true that the number of retorts is less by one, than what
could have been heated by three fire-places, on the original plan of two
retorts to one fire, yet still this method has been found to be far more
productive. The front wall of the oven may be readily taken down so that
a retort, when damaged, may be withdrawn, and replaced without
materially disturbing the rest.
The oven plan of applying heat has been found equally advantageous for
parallelopipedal and semi-cylindrical retorts.[13]
[13] The only gas-light establishment of great extent in the
metropolis, at which parallelopipedal retorts are still in use, is the
_South London Gas Works_. But it is solely owing to the very peculiar
care and economy with which all the details of this establishment are
conducted, under the immediate superintendence of a few active,
skilful and scientific proprietors, that they are able to compensate
for the loss, which in all ordinary cases is inseparable from the
employment of vessels of that description.
_Description of the Retort Oven._
Fig. 1, plate IV., represents a transverse section of one of the retort
ovens now in action at the Westminster Chartered Gas-Light Company’s
Works; similar ovens are likewise in use at the City of London Chartered
Gas-Light Works, and in many other provincial gas establishments.
Fig. 2, plate IV., exhibits a longitudinal section, and fig. 1, plate V.
shows the front elevation of the oven, built about ten feet above the
ground, upon piers or arches, which saves brick-work and allows a stage
or platform to be erected in front of the fire-places of the ovens. See
fig. 2, plate IV.
Between the back part of the ovens and the wall of the building in which
they are erected, is left an empty space of a few inches to prevent the
heat of the oven being communicated to the wall, as is seen at Y in fig.
2, plate IV.
The whole interior of the oven, as well as the horizontal flue which
pass underneath the crown of it, near the upper tier of retorts, is
lined with fire bricks. The uppermost part or crown of the arch is
constructed of large fire bricks of such a shape as will allow to
flatten the upper part of the arch as much as possible, in order to
contract the space between the two upper retorts and the crown of the
arch of the oven.
R. R. fig. 1, and 2, plate IV. and fig. 1, plate V. are cylindrical
retorts, placed horizontally in the oven, the lower series are either
supported by a large fire-brick, placed edgeways underneath the retort,
or by means of a stout wrought-iron pillar, as shown in the design. The
two upper retorts are supported by wrought iron straps, T, T, T, fig.
1, and T, fig. 2, plate IV. The straps pass through the brick-work of
the upper part of the oven, as shown in the designs, and they are
secured with screws and nuts to an iron bearing bar, the extremities of
which are supported by the outer walls of the oven. Each retort is
furnished at the extremity opposite to the mouth-piece, with a short
projecting piece or tail let into the brick-work of the oven, as seen in
the design, fig. 2, plate IV.
M. Fig. 2, plate IV. shows the mouth-piece of the retort with its cross
bar and hand-screw; and fig. 6, plate V. shows the mouth-piece drawn to
a larger scale. E. is the hand-screw, with its cross or bearing bar D,
which passes through the projecting arms C. C. The lid of the
mouth-piece has a conical edge, so that it fits close when pushed into
its place by means of the hand-screw E. Fig. 7, plate V. is the lid
which closes the mouth-piece; the handscrew E, fig. 6, presses the lid
close, to render it air-tight, a thin stratum of loom luting being first
applied to the orifice of the mouth-piece.
F. fig. 2, plate IV., is the fire-place, with the ash-pit E of the
oven. The door of the ash-pit is provided with three slits covered
within by a register slide, to regulate the admission of air as occasion
may require.
The fire passes freely and uniformly round all the retorts, and the
whole cavity of the oven acquires an equable temperature, which it
retains, if the workman takes care to admit as little air as possible,
through the register door of the ash pit, when the upper part of the
arch, or crown of the oven has acquired a bright cherry red heat.
We have stated already that in front of the oven, is a platform, as
represented in the sketch, fig. 2, plate IV. In the floor of this
platform, and directly underneath the mouth-piece of the retorts, all of
which project beyond the brick-work of the oven, is an opening covered
with an iron trap door; through this door the red hot coke, discharged
from the retorts, is suffered to fall below the stage or platform into a
cellar, or other fire-proof place, that it may not annoy the workmen. O,
O, fig. 1, plate V. denotes this opening through which the coke falls.
P, fig. 2, plate IV., and P. P. fig. 1, plate V. is a pipe proceeding
perpendicularly from the upper part of the mouth-piece of each retort,
the other extremity of which descends into the horizontal hydraulic main
H, which is shown in fig. 2, plate IV. and plate V., supported upon iron
columns. This pipe serves to convey away the liquid and gaseous products
which become disengaged from the coal in the retort during the
distillatory process.
The liquid substances, namely the tar and ammoniacal fluid, collect in
the hydraulic main H, plate IV. and V., which is furnished with a
perpendicular diaphragm or partition plate to cause a certain quantity
of the liquid deposited in it to accumulate to a certain height, and
thus to seal the perpendicular pipe P. The liquid cannot flow out of the
horizontal pipe H, till it rises to the level of the diaphragm. This
arrangement is distinctly shewn at H. fig. 2, plate IV., where the
diaphragm or partition plate is seen in the section of the hydraulic
main, together with the extremity of the perpendicular pipe P.,
descending into the fluid contained in the hydraulic main.
K, Fig. 1, plate V. is the discharging pipe, connected with the upper
part of the horizontal main H: it serves to convey away the gaseous and
liquid products from the hydraulic main H. By means of this pipe the tar
and ammoniacal fluids are conveyed into any convenient reservoir, called
the tar cistern, which is perfectly air-tight, and from this vessel the
liquid may be drawn of by means of a pipe or stop-cock. The extremity of
the pipe which communicates with the liquid, is bent downwards, so that
no air can enter the vessel: this arrangement is shown at fig. 3, plate
II.
It is essential that the condensation of the vaporous fluids should be
fully completed before they reach the tar cistern. To effect this, there
is usually allowed a considerable distance to intervene between the
discharging pipe K, fig. 1, plate V., and the reservoir destined to
receive the condensible products; or the pipe is made to pass through a
vessel containing water, called the condenser, which acts in a similar
manner as the refrigeratory of a common still. It is obvious that it is
immaterial how the condensation of the vaporous fluid is effected; it is
essential, however, that the condensation should be complete before the
liquid tar and ammoniacal fluid reach the reservoir destined to receive
these products.
The gaseous fluid which accompanies the condensible products, are then
made to pass into the lime machine, of which we shall speak hereafter,
in order to be deprived by means of quick-lime and water, from the
portion of sulphuretted hydrogen and carbonic acid gas which was
combined with the gas. And when this has been accomplished, the purified
gas is conveyed into the gas-holder, where it is stored up for use. This
part of the operation will be rendered more obvious hereafter. In some
establishments, the hydraulic main is furnished with two discharging
pipes, the one carries away the condensible fluid, into which the
perpendicular pipes P, fig. 2, plate IV. dip, whilst the other serves to
convey away the gaseous fluids to a condensor, in order to deposit the
vaporous portion of condensible liquid it may contain, and from thence
the gas passes into the purifying apparatus, or lime machine. X, fig. 2,
plate IV., is a small screw plug, which, when opened, restores the
equilibrium of the air within and without the retort previous to the lid
being taken off, to prevent the loud report, which otherwise happens
when the lid or cover of the retort is suddenly removed. To avoid these
explosive reports which had become a nuisance to the neighbourhood of
gas works, the practice of gradually withdrawing the lid of the retort,
and at the same time presenting a lighted torch has been adopted at some
works, which fully remedies the evil.
The number of retort ovens at the Westminster Chartered Gas Works’
Stations, amounts to four hundred and ninety.