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MODERN GASWORKS PRACTICE
exceed from 2,000 to 2,500 cubic feet per 1,000 cubic feet of gas made. Thus, with a medium-sized plant, making 8,000 cubic feet during a five-minutes run, the total air passed should lie between 4,000 and 5,000 cubic feet per minute. It has been previously pointed out that as the “blow"’ proceeds, so does the proportion of carbon monoxide in the producer gas increase. For this reason it has been suggested that the fan should be gradually speeded up so that a greater volume of air is passed towards the end of the “ blow.” An arrangement whicli is occasionally found is that of gradually increasing the secondary air to the car-burettor and superheater during the “ blow.” This can be conveniently done in. a liydraulically operated plant such, as that at the Beckton works. Air meters fitted to the biast main are more or less a luxury, but are occasionally found. Fig. 449 shows an air meter of the Venturi type.
The satisfactory regula-tion of the biast is a con-siderable factor in water-gas operation, for it must be remembered. that very much. more fuel may be consumed during the “ blow ” than. is actually made use of for gas-making. With extravagantly operated plants, in which. the biast may be excessive, the fuelburnt during the “ blow ” may amount to three times the quantity disposed of during the “ run.” A. G. Glasgow in a test with a plant consuming hard coal found that, working
Fig. 449.—Air Metbb attached to Gesep.atop. Blast under faVOUiable conditions, the consumption of carbon during “ blow ” and “ run ” was as 13-88 to 9-62, ot, about 60 per cent, was accounted for during the “ blow,” and 40 per cent, was used for gasmaking.
For effective results it is essential that local blowing, or imperfect distribution of the air biast andthesteam supply, should be avoided. For this reason the fuel supplied to the generator should be as uniform in size as possible. If clinkering is imperfectly earried out and large lumps are allowed to form and remain lodged