WATER GAS
755
cent, is effected. so that it-would appear to be somewhat problematical whether the initial cost and maintenance of economizers would be justified when coke or breeze is used as the fuel.
It is well known that uncovered steam mains and unlagged boilers lose a large amount of heat by radiation, but few realize that an.unprotected surface of a steam pipe of 1,000 square feet with steam at, say, 100 1b. per square inch involves a loss of something like 300 tons of coal per annum. Trials recently made in Pennsylvania with two similar locomotive type boilers of 80 and 60 h.p. have produced some interesting figures in. this connection. The boilers bad an exposed external surface of about 630 square feet. The tests continued for twenty-four hours. One boiler was unlagged and the exposed surfaces of the other were covered with a non-conducting composition (manganese cement) to a depth of about 2 in. Düring the first trial 5-39 1b. of water were evaporated per 1b. of coal; in the second, 6-35 1b. per 1b., and, moreover, more water was evaporated for less coal fired. The figures proved that the lagging had resulted in a saving of 15 per cent, of fuel, although the boiler only had been so treated and the steam pipes were uncovered.
This proved economy represents throughout a working year of 300 days a saving of 37 tons of coal for eacli 100 square feet of protected surface.
Trials conducted in America to determine the efiect of air leakages into the furnace and flues of boilers by cracks in brickwork indicated that the cooling effect brought about by the introduction of air in the flues in this maimer is from 1-75 to 2’0 per cent., in terms of evaporative efliciency. A test upon a water-tube boiler showed that, while the excess of air in the furnace was 45 per cent., analysis showed an excess of air in the flue at the base of the chimney of 96 per cent. ; producing a loss of efficiency of 4-63 per cent, when burning about 22 lb. of coal per hour per square foot of grate surface.