How to Generate Power from Garbage
(Page 8 of 12)
May/June 1970
By the Mother Earth News editors
It is also necessary to guide the floating cover in its vertical movements. This is done by the installation of a system of rollers and U-shaped iron guides, fastened to the cover by welding or other means. Cylindrical floating covers require three guides, while rectangular covers need a minimum of four. It is also desirable to provide one or more weep-holes 5-8 cm from the bottom of the bell cover through which excess gas may escape when the gas-holder is full and has reached the top of its run.
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It is the weight of the floating cover which imparts pressure to the gas. If the cover is too heavy, it may be necessary to install a counter-weight system to reduce excessive gas pressure at points of use. If, on the contrary, the cover is too light, additional weight is necessary to provide the recommended gas pressure of 10-20 g per cm 2 (4-8 inches of water) at the outlet of the gas-holder, assuming that the latter is situated at a reasonable distance from the house and the apparatus which it serves.
A heavy concrete cover might be used if counter-weights on pulleys were provided to reduce the pressure. The following is an example of determination of gas pressure in the circular gasholder illustrated in Figure 6. It is assumed that the cover is built of sheet iron 2 mm ( 0.079") thick, weighing 16 kg per m 2 ( 0.023 lb. per sq. in.)
Area of top of floating cover = 2.54 m 2 (3945 sq. in.)
Total weight of floating cover= 300 kg (661.5 lb.)
Pressure of gas
= total weight divided by surface area of top of cover
= 300 kg/2.54 m 2
= 118 kg per m 2 or 11.8 g per cm 2
= 661.5 lb./3945 sq. in. = 0.17 lb./sq. in.
The piping of the gas-holder can be conveniently arranged in the manner shown in figures 7 and 8. A single gas pipe enters at the bottom and serves both to bring in the gas from the digester and to convey it to the points of use. The trap mounted at the lowest point on the gas piping in the adjoining pit collects and evacuates water condensation from the gas piping.
HEATING OF DIGESTERS AND GASHOLDERS
Various systems have been designed for insulating or heating digesters and gas-holders in cold climates. For insulation purposes, double walls are sometimes built, the space being filled with straw, sawdust, or fibre-glass. The simplest and most economical method of heating tanks consists in surrounding them with a manure pile of one meter, or more, thickness, as shown in Figure 10. The pile is built in layers leaning against the tank's outside surfaces, but sloping away from them. In this way, the heat generated in the pile is deflected upwards and sideways towards the tank walls. The top of the tank may also be covered with such a manure pile. The latter should be renewed every 2-3 months, in order to make use of its maximum heating potential.
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