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.