596
MODERN GASWORKS PRACTICE
charges are lower, owing to the absence of heavy girder-work and stanchions. In cases of treacherous subsoils it may not be possible to adopt the overhead type, owing to the impossibility of concentrating the wliole load on small areas (i.e. the bases of the standards). In spite of tlieir good features, the overhead purifiers are comparatively rare. The explanation probably lies in the faet that the interest accruing from the additional Capital expenditure involved, combined with increased maintenance costs, would probably be a greater item than. the saving which. would be effected in labour.
Concbete Purifiers
The recently constructed purifiers at the Tottenham gasworks provide the most notable example of the application of concrete for this purpose. The capacity cd the installation is 5 million cubic feet of coal gas per diem, the nurnber of purifiers
Fig. 359.
being five, each 56 ft. 1 in. by 42 ft. 6 in. inside, with an internal depth of 6 ft. The main set is formed by four of the boxes, the remaining box acting as a catch vessel. The general construction of the installation is shown in Fig. 359. To minimize the effect of temperature the main series of four boxes was divided into two pairs with an intervening space wide enough for an elevator. The catch vessel was constructed as an entirely separate structure.
The main beams, 33inch.es deep, are divided into four spåns and are carried on columns 15 inches square for the interior and 13 inches square for the exterior. These columns have reinforced feet resting on mass concrete foundations with a steel