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.