Modern Gasworks Practice

RETORT-BENCH APPURTENANCES 271 and six-tenths “ draw,” and when the gas quality under such conditions is found to vary either above or below the usual standard, the “ draw ” is, respectively, increased or lo wer ed. Control of Quality With the Reflux Pressure System, however, the process of adjustment is reversed, the “ draw ” on the foul main is kept constant and the seal is deepened or lightened at will. For instance, we may assume that normal conditions of quality are maintained by working with a |-inch seal and a six-tenths “ draw.” Then, when gas quality is below normal, the same “ draw ” is maintained, but by a touch. of the special controller the seal may be increased by one-tenth, or as much as is found ■desirable. On the other hånd, it has recently been shown by oiie of the Metropolitan gas Companies that the yield of thermal units per ton of coal and per retort may be measurably increased if the dip pipes are unsealed and a draw of some five-tenths is worked when the retort doors are closed. When working in this way, the retort-liouse governor has to be adjusted so that the “ draw ” is rechiced to about one-tenth when the doors are open. Working on this principle, however, is very much simplified by the Reflux Pressure System, for, by a turn of the controller, the dips may be slightly sealed during the operation of charging and discharging, and no alteration of the retort-liouse governor is necessary. The metliods at present employed for removing tar and liquor from the hydraulics may be broadly classified under two headings, namely:— (a) Continuous systems, i.e., overflow by weir valve, tar-pot systems, etc. (&) Intermittent systems, i.e., tar towers. The former systems possess the disadvantage of causing the tar to travel up hill before it leaves the hydraulic. Thus, apart from the drawback of causing a viscous liquid to travel upwards over a weir or into a standpipe, there is always, with tar-pot ■systems, some unavoidable deepening or lightening of the seal owing to the difference in specific gravity of tlie tar obtained from different varieties of coal. The tar-tower system possesses the great merit of constantly drawing-off the tar by way of a pipe having a decided fall, thus there is no question of causing the tar to ascend at any period in its travel. The inherent defect of the system is, however, its intermittent nature, necessitating running-off at fairly frequent periods. General experien.ee lias shown that it is atter the procedure of running-off the tower that inerts in the gas make their appearance. This is due to the faet that it is extremely difficult to run liquor into tlie tower or mains as quickly as the tar is run out. The result is that unsealing of the dips takes place, and it may be so long as half an hour after running-off before the normal seal is made up. The Reflux Pressure System combines the advantages of the tar-pot and tower, but eliminates their defeets. For instance, in this system the tar is continually rlinning away in a down-hill direction; but the system is continuous and demands no periodical running-off. The seal, therefore, is always maintained at the level at which it is adjusted. In many gasworks it is customary to stop gasmaking on Sundays for a period of