THE HORIZONTAL RETORT BENCH 73
Where shovel or scoop charging is employed, it is inadvisable to liave more than three superimposed tiers of retorts, otherwise a movable stage has to be used.
CONSIDERATIONS IN DESIGN OF THE SETTING
The question of the capacity of various units has already been discussed in Chapter I (page 18). In the smaller works the unit is of more importance, owing to the necessity of provi-ding for comparatively small fluctuations in output. For this reason it is common to find arch.es of varying capacity, as is seen in tlie retort beuch shown in Figs. 21 and 22. In the medium and large-sized works the smaller fluctuations can best be coped with by the use of a water-gas plant.
While the design of the direct-fired setting is comparatively simple, the various portions of the gaseous setting require particular care and expe-rience if the many demands made upon it are to be satisfactorily met. In connection with the direct setting tlie follow-ing points in construction and working may be given separate notice, although. many of those hereafter classified under
, ,. . Fig. 21.—Retokt Bench with Varying Units.
gaseous settmgs also apply
(a) Where the longer retorts (9 feet and 10 feet) are employed, attention must be given to the method of heating the back-ends, otherwise the last foot or so will be duli in comparison with the remainder. This is best coped with by building a special double chamber at the back-end, th.us ensuring the thorough. circulation of tlie gases around the retort at this point.
(6) The air supply should be regulated, and split up into a primary and secondary supply (semi-gaseous) where possible.