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]