| £ 187 | 2 | 7 | 1⁄2 | ||
| obtained at | 29 | 3 | 0 | less expence; | |
| consequently there is a balance of | £ 216 | 5 | 7 | 1⁄2 |
in favour of working the retorts, as stated in process C, over that method shewn in process A; and in such proportion, let the number of retorts worked be what it may, an advantage will always be gained in this mode of manufacturing coal gas, by working the retorts at eight hours charges, as the workmen call it, in preference to adopting the shorter process.
From a series of operations made[22] with twenty parallelopipedal and with twenty cylindrical retorts, worked for one month, it has been ascertained that the decomposition of coal is most economically conducted when each retort is charged with 100 pounds of coal, and the distillatory process be continued for eight hours. Two men, one by day and one by night, can attend nine or ten retorts.
[22] By H. Morrison, Esq. and Self; the coal used, was Newcastle (Bewick and Craister’s Walls End) coal.
PART VI.
Temperature best adapted for working Cylindrical Retorts.
There is perhaps no subject in the art of manufacturing coal gas, on which practical men are less agreed, than they are on the temperature most economically to be employed for the production of coal gas in the large way. It must be sufficiently evident, that cast-iron retorts, when worked at a low temperature, will last longer, than when exposed to higher degrees of heat.[23] Hence, according to some operators, the economy of the process consists in saving the retorts, at the expense of a diminution, even though considerable, in the quantity of gas obtained; whilst, according to others, it is more economical to obtain the largest possible quantity of gas at the expence of any consequent injury to the distillatory vessel.
[23] It is essential that the retorts should be kept in constant action night and day, or at least never allowed to go below a red heat. The first portion of oxide which forms upon the surface, when allowed to cool, cracks and falls off, leaving a new surface to be acted upon the next time it is heated. By thus being every day heated and cooled, a retort will be speedily destroyed.