Modern Gasworks Practice

THE HORIZONTAL RETORT BENCH 103 demand by setting down units of varying capacity. Although. in rare instances it is done, it is not advisable to work a portion of an arch. during slack periods, as fuel consumption is unavoidably increased. If, however, there is no other means of regulating the output, it will be found preferable to work the retorts in one vertical. row. The damper on this side of the setting is ihen opened and that on the opposite side closed, so that the furnace gases are pulled around those retorts in operation. The greater the number of retorts to a bed, the lower (pro rata) will be the fuel con-sumption. Hence settings of twelve have come into fairly extended use. One of the first considerations—of particular moment in cases where land is expensive and the quantity limited—is the productive capacity per square foot of area. The highest yield in this respect is given by the settings of ten retorts in two vertical rows—commonly known as the Bishop of London’s bench. The output necessarily varies with. the size of retorts, but may be taken, on the average, as 200 to 260 cubic feet per diem per square foot of ground area. This figure is based on the outside dimensions of the retort house and 24 inches x 16 inches through retorts. For a setting of eights in three rows the capacity would be reduced to 150 to 180 cubic feet per diem per square foot. For vertical retorts the figure (according to the system) may be as much as from 350 to 475 cubic feet per square foot, which, again, may be considerably increased by steaming. The disposition of the retorts in the setting must be such that the heating of each one is as uniform as possible. In settings of two vertical rows this is more easily assured than with three vertical’rows, as with. the latter, the retort immedi-ately above the combustion chamber has to face the fiercest portions of the flame. In a scientifically constructed setting there is no reason why the average temperature of any of the retorts in that setting should diller by more than 65° C. Under all circumstances “ local heating ” must be avoided. This is said to be taking place when certain defined portions of the setting are under considerably greater heat than the remainder. The term really applies to the difEering sections of the bench ; for instance, we do not speak of a combustion chamber being sub-jected to local heating merely because its temperature is greater than that of the remainder of the setting. If, however, one section of this chamber were considerably hotter than the remainder, the hptter section would be giving rise to local heating. The effects of this derangement are very often seen when retort doors are opened—rings of duli red colour encircle the retort in contrast to the white heat of the remainder. This is indicative of bad spacing of the “ nostrils,” or of stoppage of one or more of these; but, beyond choking up by dust or by “ drip-pings ” from the upper portion of the bench, the cause of local heating can usually be attributed to constructional defects or to inferior design. The following considerations may be set down as being among the more important items to be kept in mind when designing the upper portions of a setting:— (1) The combustion chamber should be of ampie size, so that sufficient space is allowed for the mixing of the air and gas. (2) The “ nostril holes ” must be so spaced out that they come midway between the cross-walls of the setting and the combustion chamber.