Modern Gasworks Practice

FOUNDATIONS, GASWORKS BUILDINGS, ETC. 57 A point which has accounted for a good deal of discussion is the effect of the type of house on the air supply to the producers. Some authorities have objection to the stage type owing to its tendency to set up draughts that may influence the composition of the furnace gases. On the other hånd, it is said that a freer air supply is obtained, therefore more regulär working of the producers. The author can point to cases where the subway, on the side of the retort bench opposite to that which contains the producers, has been made only some 3 feet in width with. no provision whatever for ventilation. An analysis of the air in such a subway shows only some 15 per cent, of oxygen, with the result that it is almost impossible to obtain a sufiiciency of secondary air. As a general rule it will be found that water is the deciding factor, and if this is found near the surface, a stage house should forthwith be decided upon. Concerning relative costs, it may be taken that the stage house costs anything from £5 to £15 more per mouthpiece than the subway type costs. In smaller works the subway may be left eutirely uncovered, and the retorts operated from a travelUng platform running on ball-bearings, and readily operated by hand. In such cases the precaution must be taken of railing ofE the subway, unless this is provided for by a running cable as shown in Fig. 15. RETORT-HOUSE ROOFS The use of timber for the construction of principals must be avoided in retort houses. The building for horizontal benches is usually provided with an all-steel truss ; the well-known “ English ” or “ French ” types made up of all-rolled sections are effective and economical. Slates are the most general covering, although. many forms of patent compressed asbestos sheets and tiles, costing about 25 per cent, less than slates, have recently been introduced with success. Owing to som.ew.hat heavy deterioration due to corrosion, steel used in retort-house roofs and other work should not be highly stressed, and a factor of safety of 7 on the breaking strength should be allowed for, i.e. a stress not exceeding 4-J tons per square inch in tension or compression. All the members of roof trusses should preferably be made from angles or tees instead of flats or rounds, as these sections are better able to withstand the slight compressive stresses wliich may come on members which are normally in tension as a result of reversal of wind. RETORT-HOUSE FLOORS Except in connection with. the smaller works the stage house will usually be decided upon, as the many contingent advantages will be found to neutralize the additional first cost entailed. For small works, where stoking apparatus (such as the manual type) is in use, the floor, whether at ground-level or above it, is frequently composed of f-inch or J-inch chequer plates, bolted down to cross joists. For the larger works, where heavy machines giving rise to high rolling loads have to be allowed for, additional strength. is necessary, and wear and tear on a chequer-plate floor is considerable. Several types of retort-house floors are shown in Fig. 16,