_Horizontal Rotary Retorts, lately brought into use for manufacturing
coal gas._
The many disadvantages attendant on the plan of decomposing coal in
masses from five to ten inches in thickness, as already sufficiently
exposed in the preceding parts, had naturally the effect of developing a
principle of manufacturing coal gas, which practice has now fully
established, namely: that to decompose coal, in thin layers from two to
four inches in thickness, is to obtain the greatest quantity of gas from
a given quantity of coal at the least expence.
Mr. Clegg was the first person who pointed out to the public the
advantages that must accrue from this mode of operating, and to him we
are indebted for the construction of an apparatus, the great ingenuity
and superiority of which, entitles what is called the horizontal rotary
retort, to all the merit and praise that belongs to the character of an
original invention.
The numerous and great advantages of this distillatory apparatus, the
rapidly increasing adoption of it,[29] and the almost certain prospect
which exists of their ultimately superseding all former methods of
decomposing coal, make it proper that I should lay before the reader, as
full an account as my limits will permit, of the construction and
operation of this retort, and the mode of applying it; and this becomes
the more necessary on account of the many important improvements which
the apparatus has undergone since its first adoption,[30] and of which
no description has yet been laid before the public.
[29] Retorts of this description have been lately adopted, in the Gas
Works at Bristol, Birmingham, Chester, Kidderminster, and at many
other provincial Gas Establishments.
[30] An account of the original construction of the rotary retort may
be seen in the Repository of Arts, No. CLXXVI, 1816, page I. and also
in the Journal of Science, Vol. II. page 133.
The following account will render the construction of this retort
sufficiently obvious:
_Description of the Horizontal Rotary Retorts at the Royal Mint._[31]
[31] The retorts lately erected at the Gas Works, at Birmingham,
Chester, Bristol, &c. are similar to those at the mint.
The horizontal rotary Retorts at the Royal Mint, are hollow cylinders,
eight feet six inches in diameter and 15 inches high, arched a little at
the top. They are made of wrought-iron plates, half an inch thick,
rivetted together in the manner of a steam-engine boiler; A, A, A, fig.
2, plate III. exhibits a perpendicular section of the rotary retort. In
fig. 1, plate II. the retort is seen fixed in the brick-work; _a_, fig.
1, plate II. shews the mouth of the retort, through which the coals are
introduced, and from whence the coke is withdrawn. It is also shown in
perspective at B, B, B. fig. 2. plate VII. The mouth is closed with a
cast-iron door fitted on air-tight by grinding.
The door is connected at its upper and lower extremities, with a frame
and adjusting rod, see B, B, fig. 1, plate II., and also plate VII., by
means of which it may readily be slided down below the mouth of the
retort, when the coals are to be introduced, or coke is to be withdrawn.
To the upper extremity of the rod B, fig. 1, plate II., is fixed a
lever, loaded with a counterpoise weight C, to balance the door, and to
render the opening and closing of it easy and expeditious.
The mouth-piece and its door is three feet long, and nine inches wide;
it projects nine inches beyond the brick-work or furnace in which the
retort is fixed, as may be seen at fig. 1, plate II.
The fire-place, which is on the opposite side to that of the mouth of
the retort, heats only one-third part of the whole capacity of the
retort to that degree which is proper for the complete and rapid
decomposition of the coal, while the remaining parts, which are not over
the fire-place, and to which the fire flues do not extend, are kept at a
lower temperature.
The flues are directed under about one-third of the area of the bottom
of the retort, and after having passed over one-third part of the area
of the top of the retort, they pass into the chimney. Fig. 1, plate VI.,
exhibits the direction of the flues; A, A, the flues, and the
fire-place. The whole retort is guarded from the contact of the fire,
which would soon destroy it, by fire-bricks; it notwithstanding speedily
receives the full effect of the heat, and retains its temperature when
once heated for a long time. Fig. 1, plate II., exhibits one of the
retorts fixed in its furnace. A perspective view of three retorts may be
seen in fig. 2, plate VII.
Through the centre of the retort, passes perpendicularly, an iron shaft
D, as shown in the section of the retort, fig. 2, plate III., and also
in fig. 1, plate II. The lower extremity of the shaft revolves upon the
bottom of the retort, in a cup-shaped cavity, while its upper extremity
passes through the roof of the retort, where the latter is made
air-tight by means of a pipe E, fig. 1, plate II., and E, fig. 2, plate
III., closed at the top and surrounding the shaft, and hence the shaft
must always preserve its centre.
To the lower extremity of the shaft is keyed a box or centre piece,
(technically called a rose centre,) F, fig. 2, plate III. It is also
seen in the perpendicular section of the retort, fig. 1, plate II. From
this shaft radiate twelve wrought-iron arms, G, G, fig. 2, plate
III.,[32] fixed in sockets made in the box. These arms are elevated
three inches above the bottom of the retort, and extend to nearly within
its whole inner circumference. They are wedge-shaped, and their greatest
diameter is at right angles to the base of the retort, so that the
weight of the arms rests on the axis. They are intersected by two
concentric rings, as will be seen on inspecting fig. 5, plate III.,
which exhibits the plan of the retort, together with the iron arms, G,
G, and concentric rings. The centre of figure 5, shows also the plan of
the rose centre F, fig. 2, plate III., into which the arms are keyed.
[32] In the horizontal rotary Retorts at the Chester, Birmingham and
Bristol Gas Works, which are twelve feet six inches in diameter, there
are fifteen arms. At some Gas Works the arms are made of cast-iron.
Between the arms are placed twelve shallow iron trays or boxes, destined
to contain the coal from which the gas is to be obtained. They are
formed to the segment of a circle, hence the whole series of them when
arranged in the retort, exhibits a shallow circular tray, which, when
motion is given to the shaft, may be made to revolve within the retort.
Fig. 12, plate III. exhibits one of the shallow trays, or coal boxes in
perspective.
It will be obvious, that by the motion of the shaft, any number of the
trays or coal-boxes can readily be brought from the coldest, into the
hottest, and from the hottest into the coldest part of the retort.
H, fig. 1, plate II., and _a_, plate III., or H, plate VII., is a
perpendicular pipe situated at the margin of the retort, close behind
the mouth-piece, and consequently in the coldest part of the retort. It
serves to carry off the distillatory products evolved from the coal, and
causes part of the vaporous tar, which becomes condensed in it, to
trickle back again upon the coal in the retort, in order to become
converted into gas, when the coal on which it falls becomes situated
over the fire-place.
This pipe is furnished at its upper extremity with a _hydraulic valve_,
J, fig. 1, plate II. It consists simply of an inverted cup X, applied
over the upper open extremity of the perpendicular pipe H, and submersed
into a cup formed of a portion of larger pipe, surrounding the pipe H,
containing tar. The smaller, or inner cup X, is represented in the
design raised out of the liquid contained in the outer cup J, to show an
aperture Y, made in the smaller or inner cup; the use of which will be
mentioned hereafter. The inverted cup X, is furnished with a chain, one
extremity of which is fastened to the upper extremity of the cup, the
other passes over a small wheel, and descends through the roof of the
building, as shown in the design.
K, fig. 1, plate II., or K K, fig. 2, plate VII., is a branch pipe
proceeding laterally from the perpendicular pipe H; it communicates with
the hydraulic box L, fig. 1, plate II. N, is a pipe which proceeds from
the hydraulic box L; it serves to carry away the gaseous and liquid
products to their places of destination. The liquid products, namely,
the tar and ammoniacal fluid, become deposited in the tar cistern, fig.
3, plate II., into which the pipe N terminates. The tar cistern is
furnished with two floats Y Y; the one serves to indicate the quantity
of tar, and the other the quantity of aqueous ammoniacal fluid contained
in the vessel. These fluids may be drawn off without admitting air into
the vessel by the stop-cock and bent tube, exhibited in the figure.
The shorter pipe N, which proceeds from the tar cistern, fig. 3, plate
II., and communicates with the purifying apparatus or lime machine, fig.
2, plate II., serves to convey the gaseous fluid, which accompanied the
condensible liquids deposited in the tar cistern, back again into the
lime machine, or purifying apparatus, fig. 2, plate II., the
construction of which, together with the conveyance of the gas from this
vessel to its place of destination will be stated hereafter.
L, fig. 1, plate II., or fig. 2, plate VII., is an iron flap table,
placed level with the bottom of the mouth of the retort. It is
convenient to hold several coal trays ready charged with coal in a state
fit to be introduced into the retort.
The fire-place, flues, and ash-pit of the furnace, in which the retort
is fixed, are sufficiently obvious by mere inspection of fig. 1, plate
II. The front elevation of the retort is seen in fig. 2, plate VII.,
which exhibits three horizontal retorts; two of which have the door of
the mouth-piece slided down, and one with the door in its place, or
shut. The circular ring seen in this design, at the top of each retort,
which rests on iron-bearing bars, the extremities of which are let into
the end walls of the furnace, serves to support the roof of the retort
by means of bolts, proceeding from the inner side of the roof. This
arrangement is likewise shown in the section, fig. 1, plate II.[33] At
the bended part of the perpendicular pipe H, fig. 1, plate II., is seen
a bonnet, or cover, which closes an opening made into the pipe H,
through which, by means of an iron rod, the lower extremity of the pipe
H, may, from time to time, be examined, to guard against an incrustation
of decomposed tar or carbonaceous matter that might happen to accumulate
in that part of the pipe. The upper part of the pipe H, above the bonnet
at the bended part, requires no examination.
[33] A more economical method of supporting the roof of the retort has
lately been adopted by Mr. Clegg. It consists in giving the roof the
form of an inverted arch, supported on the Catenaria plan, by two
bolts only, placed at the most elevated extremity of the arch and
secured to an horizontal beam.
_b_, fig. 2, and _b_, fig. 5, plate III., is the flanch of the retort;
_c_, fig. 2, plate III., the flanch of the mouth-piece; _d_, the cutter,
or wedge, which draws the mouth-piece close; _e_, the cross bar, against
which the cutter _d_, bears, to render the mouth-piece air tight; _f_,
fig. 2, one of the eye-bolts or arms which support the cross bar _e_; it
is also seen at _e_, in the plan of the retort, fig. 5, plate II. In
this figure _b_ is the flanch of the retort, and _c_ the door.
These few particulars will be sufficient to enable the reader to
understand the construction of the retort; its action is as follows.
_Action and Management of the Horizontal Rotary Retort._
When the retort is heated to the proper temperature for the
decomposition of the coal, the door is slided down, and the coal boxes
charged with small coal are slided into the retort from the table, L,
fig. 1, plate II., one by one, so that each box rests firmly upon the
concentric rings placed between the arms of the retort; the door is then
slided up again into its place and rendered air-tight by means of
wedges.
When the whole circle fig. 5, plate III. is thus filled with
coal-boxes, (the coal should be spread in the boxes, in layers two or
three inches in depth,) it is obvious that of all the twelve boxes, four
only can be situated directly over the fire-place, while the remaining
eight are placed right and left towards the door of the retort. The coal
in the former boxes receives the full effect of the heat, (see the plan
of the fire flues of the retort, fig. 1, plate VI.,) while the remaining
eight boxes to which the fire does not extend, are less heated. The coal
in the four boxes which are in the hottest part of the retort becomes
rapidly decomposed, whilst the coal in all the other boxes is gradually
heated, and consequently deprived only of moisture, previous to being
subjected to the greatest heat. The box which is situated under the
condensing pipe H, plate II., near the entry door, receives the
condensed tar which trickles down the pipe H.
Now let us suppose that the coal in the four boxes over the fire place
is fully decomposed, which will be the case if 32¹⁄₂ pounds of coal are
in each box, in two hours, the workman then turns the shaft E, fig. 1,
plate II., one-third part of the circumference of a circle, by pulling
towards him by means of an iron hook the nearest iron arm that may
happen to be opposite to the door; this moves those boxes which at the
commencement of the operation were over the fire-place, towards the
coldest part of the retort, namely, towards the door which is opposite
to the fire-place, and a second series, or four of the adjacent boxes,
are brought in turn into the hottest part of the retort, or over the
fire-place, from whence the preceding boxes were removed.
When the coal in the second series of boxes has been two hours in the
hottest part of the retort, its decomposition will be completed; the
workman therefore turns the shaft again one-third part of a circle, and
a third series advances in their place, while at the same time the first
series becomes situated opposite the entry door of the retort, from
whence they may be withdrawn and exchanged for an extra set of trays,
ready charged with coal and placed on the iron table for that purpose.
In this manner the operation proceeds. One-third part of the whole
charge of coal within the retort is always in the act of becoming
decomposed; another third part is gradually heated, and totally deprived
of moisture, previous to its being exposed to the temperature necessary
for its decomposition; and the remaining third part placed in the
coldest part of the retort, receives that portion of tar, which escapes
decomposition, and trickles down the perpendicular pipe, in order to be
decomposed when the coal upon which it falls becomes situated over the
fire-place. Hence the quantity of tar obtained from one chaldron of
Newcastle coal, when decomposed by means of an horizontal rotary retort,
seldom amounts to more than sixty or seventy pounds, whereas the same
quantity of coal when decomposed by means of cylindrical or
parallelopipedal retorts, yields never less than from one hundred and
fifty, to one hundred and eighty pounds. An horizontal rotary retort,
twelve feet six inches in diameter, and fifteen inches high, furnishes
in the ordinary way of working every twenty-four hours, fifteen thousand
cubic feet of gas, when five trays of the retort are charged with three
bushels of Newcastle coal. The weight of the retort is three tons; its
capacity, one hundred and fifty cubic feet.
The hydraulic valve described page 116, serves merely to restore the
equilibrium, between the gas within the retort, and the atmospheric air
without, previous to the opening of the door of the mouth of the retort.
To effect this the workman raises the cup X, by means of the chain, so
that the small hole Y, in the cup X, becomes raised out of the tar in
the cup L, and he closes it again when the retort is charged: this
operation requires two minutes. We have stated already, that the door of
the retort is ground air-tight, and hence it requires no luting.
_Advantages of the method of manufacturing Coal Gas by means of
Horizontal Rotary Retorts._
The advantages of the mode of manufacturing coal gas by means of
horizontal rotary retorts, consist in a saving of fuel, time, labour,
and machinery, a gain in the quantity of gas, and increase in the
quantity of coke.
_Saving of fuel._--The mass of coal subjected to decomposition being
reduced from the dimension required in the old plan (by means of
cylindrical retorts) to the narrowest available limits, there being no
outward crust of coke to be kept red hot for hours to no purpose, while
the decomposition of the interior mass of coal is going on;--the coke
itself being as soon as formed removed from the source of heat, and
applied while cooling, to warm up a fresh supply of coal next in order
of becoming decomposed, instead of being discharged in a red hot state,
into the open air, as requires to be done in the practice before
detailed--the whole fuel in short being necessarily and beneficially
expended--the saving of coal employed as fuel in this respect, is
exactly the gaining of all that is lost on the plan of employing
cylindrical or any of the retorts before described. Hence one chaldron
of coal is decomposed at the gas establishments where horizontal rotary
retorts are in action by means of twenty per cent of fuel, and at some
establishments an expert stoker will work the retorts with fifteen per
cent of fuel.
_Saving of time._--The saving of time does not merely amount to what is
consequent on the speedier decomposition of the coal, and the saving of
that heat which formerly required to be kept up a length of time to no
adequate purpose; it also includes all that is gained in consequence of
the revolving motion to which the coal is submitted, superseding, as has
been already mentioned, the necessity of discharging the coke in an
ignited state from the retort.
When the coke is removed, as previously explained, page 72, red hot from
the cylindrical, parallelopipedal, semi-cylindrical or ellipsoidal
retorts, the charging of the distillatory vessel with fresh coal
produces such a sudden reduction of temperature, that from three to four
hours inevitably elapse before the retort is again in a full working
state, and to this circumstance the workmen (perhaps very justly)
attribute the frequent sudden injury which the distillatory cast-iron
vessel sustains.
Another striking advantage of the new mode of decomposing coal is, that
besides saving the time which is wasted in keeping up an intense
temperature unnecessarily the revolving apparatus prevents entirely the
loss occasioned by these three or four hours of unnecessary cooling of
the distillatory vessel. For each series of trays, or coal boxes,
containing the ignited coke, of the horizontal rotary retort, being
suffered to cool within the retort before the coke is discharged, and
being placed in contact with a fresh supply of coal, the temperature of
the retort is kept up uniformly the same from beginning to end.
_Saving of Labour._--In consequence of the superior facility with which
the mode of decomposing coal in thin layers and removing the coke as
fast as it is formed is effected, the saving in point of labour is very
great. The charging and discharging of the retort is performed in two
minutes. Hence one chaldron of coal may be decomposed by means of three
horizontal rotary retorts, each twelve feet six inches in diameter, and
with the attendance of two men, in eight hours, and produces from
fifteen thousand, to eighteen thousand cubic feet of gas, whilst ten
thousand cubic feet of gas can only be obtained from the same quantity
of coal in eight hours, by means of twenty cylindrical retorts, attended
by the same number of workmen.
_Saving of machinery._--When we compare the original cost and wear and
tear of the horizontal rotary retorts, with the cost and deterioration
of a set of cylindrical, parallelopipedal, ellipsoidal, or
semi-cylindrical retorts of an equal power, (that is to say to produce a
like quantity of gas, in a given time,) a difference not less striking
presents itself in favour of the horizontal retort.
We have stated already, that cylindrical, ellipsoidal, parallelopipedal,
or semi-cylindrical retorts, when constantly kept in action, and worked
to the greatest advantage, cannot be made to last longer than six
months.[34]
[34] They are frequently rendered unfit for use in three months, and
at some works in two months, owing not less to the irregularity of the
temperature at which they are worked, than to the carelessness of the
workmen.
Only one-third part of the top and bottom plates of the rotary retort
being exposed to the action of heat, are alone liable to deterioration.
It is only necessary therefore that these parts of the vessel be
renewed, while the other parts remain uninjured for years. The new top
and bottom plates being rivetted to the old and undecayed part, without
deranging the rest, the retort is rendered as good as new.
_Gain in the quantity of gas._--A large increase in the quantity of gas
obtained, is a natural consequence of the mode in which the
decomposition of coal is effected by means of the horizontal rotary
retort.
Every body knows that coal, when decomposed slowly, affords a larger
quantity of tar and ammoniacal liquor, but a less quantity of gas than
when decomposed rapidly.
In the former case, the formation of the proximate products which coal
is capable of furnishing is effected properly; the bituminous part of
the coal is developed under the most favourable circumstances.
But when coal, after being previously deprived of moisture, is very
suddenly heated to a high temperature, in thin strata, and small
portions at a time, so that the vaporous products instead of becoming
condensed, are made to come into contact with a substance (which in this
case is the roof of the retort,) kept constantly at a temperature rather
higher than that at which gold, silver, and copper melts, (32°,
Wedgwood, or 5237°, Fahrenheit,) a very different arrangement of
principles takes place.
The greatest portion of tar which the coal is capable of furnishing,
instead of being produced in a liquid form, becomes then decomposed into
carburetted hydrogen, and olifiant gas. That portion of tar which
escapes decomposition, is condensed in the perpendicular pipe _H_, fig.
2, plate II., or _H_, fig. 2, plate VII., and falls back again into the
retort, where it is also decomposed when the coal upon which it falls
comes under the process of decomposition.
Hence the quantity of tar obtained by means of horizontal rotary
retorts, is very small; it seldom exceeds the proportion mentioned page
123, when the retort is worked to the greatest advantage. This quantity
is considerably diminished, when Newcastle coal, broken into pieces of
the size of split pease is decomposed in strata, not exceeding two
inches in thickness. The quantity of tar afforded by a chaldron of coal
then amounts to thirty pounds, whilst at the same time the quality of
the gas is improved; because coal tar furnishes olifiant gas, which the
coal alone, when distilled by means of cylindrical or other shaped
cast-iron retorts of the usual form, cannot produce, or at least but in
a small quantity. One gallon of coal tar yields 15 cubic feet of
olifiant gas, which greatly increases the illuminating power of the
carburetted hydrogen.
From what has been so far stated, it will be understood why one chaldron
of Newcastle coal, when decomposed by the new process, may readily be
made to produce from 15,000 to 18,000 cubic feet of gas and upwards,
whereas the same quantity of coal, if decomposed by the old method,
yields only upon an average 10,000 cubic feet of gas.[35]
[35] The experiments exhibiting the maximum quantity of gas obtainable
from coal, see page 44, were made with the horizontal rotary retorts
at the Royal Mint. Similar results have also been obtained at the
Westminster Gas-Works.
In the former case, the greater part of the essential oil and tar which
the coal would have afforded is decomposed, as stated already by virtue
of the high temperature to which the vapourous tar is suddenly exposed
in the horizontal rotary retort, which is not the case when coal is
decomposed in the retorts of the old construction.
_Gain in the quantity of coke._--With the cylindrical or cast-iron
retorts of the old shapes, the quantity of coke obtained from a given
quantity of coal is upon an average 25 per cent. increase by measure
from the best kind of Newcastle and Sunderland coal, but taking into
account the waste incurred in breaking out and removing the red hot coke
from the retort, which requires the application of rakers and crow bars,
a considerable portion of it becomes reduced to dust or breeze, and
hence no more than bulk for bulk of the coal decomposed can seldom be
depended upon as the ultimate saleable quantity of coke.[36]
[36] There is a vast difference with regard to the quality as well as
quantity of coke obtained from different kinds of coal. Some kinds of
coal produce a species of coke which is so friable that it will hardly
bear being moved from place to place without crumbling into dust,
others produce coke in pieces of the size of small pebbles, while a
third sort affords coke of a stony hardness.
In the new mode of carbonizing coal by means of the horizontal rotary
retorts, the increase of coke is 150 per cent. by measure, so that one
chaldron of Newcastle coal produces two and a half chaldron of
coke--this is the quantity produced upon an average. But when the retort
is worked at a temperature to produce at the rate of 18,000 cubic feet
of gas from the chaldron of coal, the increase of coke by measure is 175
per cent.; in that case, the layers of coal in the coal boxes must not
exceed two inches in thickness, so that the volume of coke is in the
ratio of the quantity of gas produced and the rapidity and elevation of
temperature at which the decomposition of the coal is effected.
The coke being withdrawn from the place where it is formed by merely
turning the boxes containing it, upside down, all waste is avoided.
With respect, again, to the quality of the coke, it will be observed
that when the coal is rapidly carbonized in thin layers, and has full
liberty to expand freely, as in the case of the horizontal rotary
retort, it affords a light and porous coke, whereas in the cylindrical,
paralellopipedal, semi-cylindrical, or ellipsoidal retorts, the coke
being compressed, the intense heat to which it is so long and
superfluously exposed, renders it extremely dense, and of a stony
hardness.
The latter sort of coke is unquestionably preferable for the smelter,
and all furnace operations, standing the blast of the bellows well. But
the coke produced in the new mode of operating, is better suited for
the great majority of domestic purposes, kindling more readily, and
making a more cheerful fire. The combustion of the dense, or as it is
now called, cylinder coke, can be only kept up when used in a common
grate, by a strong draft of air, and it is therefore not so well suited
for fuel for domestic purposes, to make a small fire; but the coke
obtained by the horizontal rotary retort, readily maintains its own
combustion, even when in small masses; hence it may be used without any
trouble, either in the fire-place of the cottager, or of the prince, and
accordingly it bears a higher price in the market.
_Directions to workmen, with regard to the management of Horizontal
Rotary Retorts._
The circumstance most essential to the economical application of the
horizontal rotary retort, is, as has been repeatedly stated, that the
coal shall be spread in thin layers in the boxes of the retort, not
exceeding from two to four inches in thickness; and it may be laid down
as a general rule, that the thinner the layers, and the higher the
temperature, the greater will be the proportion of gas, the greater the
bulk of coke, and the smaller the quantity of tar.
The coal before it is submitted to the distillatory process, should be
as dry as possible, and the more it is comminuted the better. The very
refuse of the coal called _slack_, provided it is perfectly free from
foreign matter, answers best. It should also be spread in the trays, in
even layers.
When the retort is in a good working state, the temperature should be
kept up by the application of small quantities of fuel at a time. A
prodigious saving of fuel may be effected by attending the fire
properly, and it is this which distinguishes a careful stoker from a
bungler. For in the working of this retort particularly, it is a
wasteful process to clog up the fire-place with a large quantity of fuel
injudiciously applied. The difference in this respect, with regard to
the economy of fuel is so great, that an expert stoker will work the
retort with one-third less of fuel and half the labour that would be
employed by a negligent workman.
The quantity of gas produced from a chaldron of coal may be ascertained
by the gas metre, or by the gas holder, if the outlet valve of the
latter be shut during the distillatory process.
The heat at the same time employed for working the retort, will be best
defined for the stoker’s guide, by copying carefully on paper the red
tint of the retort, as seen through the sight hole, made for that
purpose in the brick-work directly over the fire-place.
The first six feet of the perpendicular pipe H, fig. 1, plate II., which
conveys the distillatory products from the retort, should be well
cleaned out once a month, the bonnet at the bended part of the pipe H,
fig. 1, is provided for that purpose, as already stated, page 119.
When the retort remains uncharged, the fire must be kept low in order to
prevent its getting beyond the usual temperature, and the arms and
moveable axis should be turned occasionally, and the door kept close.
The fire tiles which cover the flues under the retort should be examined
about once a fortnight, and if a tile is melted or broken, it must be
replaced by a new one, because the preservation of the retort greatly
depends upon this precaution.
All the parts of the arms composing the moveable disc within the retort,
may be taken out of the door of the retort, if they should require a
repair, first taking off the cap from the perpendicular pipe E, fig. 1,
plate II., surrounding the shaft of the retort, then the centre piece,
or rose centre, F, fig. 2, plate II., the shaft D, fig. 2, plate III.,
may be drawn up through the pipe which surrounds it.
When the retort requires cleaning, which should be done once every six
or eight months, a screw may be attached to the upper extremity of the
shaft D, which passes through the retort; by this means, the arms and
rose centre within the retort can easily be raised, to leave the bottom
of the retort quite clear, in order that the lumps of coke, that may be
scattered about, may be easily removed. And if an incrustation of coke
should happen to be attached to the bottom of the retort, it may be
readily detached by a crow bar, or other suitable instrument.
The trays or coal boxes, fig. 12, plate II., may be made by the stoker,
of sheet iron, (called in commerce No. 16,) framed upon a wooden mould
made for the purpose.
The temperature best suited for the decomposition of coal by means of
the horizontal rotary retort depends, as has been already stated in the
case of cylindrical cast-iron retorts, altogether on the price of coal,
and the price which can be obtained for the coke.
In all places where the average price of coal, equal in quality to
(Bewick and Craister’s Walls End) Newcastle coal, or any other species
of coal, capable of producing from fifteen to eighteen thousand feet of
gas from one chaldron, is not less than £ 2. 8s. the chaldron (27 Cwt.)
or upwards, and where coke can be sold at the average price of £. 1 the
chaldron, the horizontal rotary retort should be worked at such a
temperature, that when viewed through the sight hole, it shall appear of
a bright cherry redness, and at which it produces from 15,000 to 16,000
cubic feet of gas, from a chaldron of coal.
But in all other places where coal of the same quality to (Bewick and
Craister’s Walls End) Newcastle coal, may be purchased at £. 1 8s. the
chaldron, or at a less price, it will be more advantageous to the
manufacturer, to work the horizontal rotary retort, at a lower
temperature, so as to produce only at the rate of thirteen or fourteen
thousand cubic feet of gas from the chaldron of coal. In the latter case
the manufacturer expends coal in order to save his retort, whereas in
the former case he economizes the fuel, as productive of a gain more
than commensurate for the waste of the retort.
When the supply of gas required, happens at any time to be less than the
retort when in action is capable of furnishing, the fire must then be
kept low, but the retort should never be suffered to acquire a lower
temperature, than that of a dull red heat visible by day-light.