54
MODERN GAS WORKS PRACTICE
of the wall, and from the triangle of forces the position of the resultant may be found, and, accordingly, the thickness of the wall at the base. If the same treat-ment is applied to find the pressure at various depths, the allowable decrease in thickness as the wall rises is obtained.
In cases where the wall simply serves as a retaining wall, so that coal can be surcliarged above it, the formula must be used in the following somewhat compli-cated form, viz. :—
_ WA2 „ /Cos y — Cos 2 y — Cos 2 <p
P = ------ . Cos y ( ---------------=— ....' ..-■.-JL
2 VCos y + V Cos 2 y — Cos 2 <p
where y is the angle of surcharge.
This, however, reduces itself to a simple and easily workable form when the angle of surcharge is equal to the angle of repose of the coal, as would nearly always be th.e case. The formula then becomes:—
t, WA2 „
P — -----■ Cos (p
2
The above treatment, of course, applies to the stacking of broken or small coal. When the coal itself is formed into a wall (as commonly seen in railway yards), a. reduced wall-thickness may be taken.
Wh.en the erection of coal stores is contemplated, the question of cliief importance is as to whether the projected floor-level is of sufficient height to escape ground water after a heavy rainfall, also its relation to the level of the retort-house working stage. The floor should preferably be paved with slabs of stone or concrete, and designed so that moisture can drain away. In this respect the design in Fig. 12 is to be preferred to that shown in Fig. 11. The practice is often foliowed of dividing
Fig. 11.
the store into separate bins by means of vertical cross-walls, and this is certainly of advantage in preventing a fire from spreading. Moreover, each of these bins can be made of a definite capacity (so many tons per foot of depth), which enables a good idea of the stock to be obtained at ,a glance. Iron columns and ties passing through bins should be avoided as far as possible, as they are said to favour the spread of fire—in any case, they should be coated with. concrete. Plenty of fresh air in the covered store is an important item, for men s uff er from the ill-effects of a poorly ventilated building. This is supposed to be due to the vapours of acetaldehyde, which are known to interfere with. respiration. Ventilation is best provided for by a continuous louvre running along the roof of the store, or by large openings made above the level at which it is proposed to stack coal.