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.