220 MODERN GASWORKS PRACTICE
Fig. 116.—Types of Built-up Retokts.
116. In all cases where the latter types are used, the modern tendency is to employ a highly refractory material of a silica nature ; but experience has shown that this clay is not altogetlier capable of with-standing the erosion caused by friction of the coke. Accord-ingly, while the arch of the retort is composed of silica material, the shoulders and base are made from an alumi-nous clay of a Stourbridge nature, as shown in Fig. 117. With. occasional repairs, retorts of this description have, in some instances, recorded a working life of 3,000 days. Some idea of the longevity of the segmental chamber retort may be gathered from the faet that the ovens employed in the Munich chamber installations have, in many cases, been in use for nine consecu-
tive years.
To overcome the inability of the silica wäre to withstand friction, A. Cliff has suggested the adoption of a composite retort as shown. in Fig. 118.
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Fig. 117.—Segmental Retort with Silica Akch and Aluminous Base.
Fig. 118.—Composite Retort with Aluminous Bottom.
In some instances the segments composiug a rebated retort are carried right up ito both ends, and cast-iron sheaths (similar to that shown in< Fig. 147, p. 245) are