DESIGN OF OPEN-HEARTH FURNACES TABLE 16—Continued 263 Gas on Gas Air on Air Gas on Waste Gas Air on Waste Gas Weight of checker brick required to store up heat in waste gas (100° change in brick temperature 70% effective): 15-minute cycle: Gas [1 791 000 ~(100XO 25)] -7-0 7 = 102,350kg Air: [3,164,000 -r (100X0.25)] -v-0.7 = 180,800kg 20-minute cycle: Gas: [2,388,000 -5- (100X 0.25)] -s-0.7 = 136,460kg Air: [4 219 000 -r (100X0.25)] -^0.7 = 241,100kg 25-minute cycle: Gas [2 985 000 — (100X0 25)] -5-07 = 170,600kg Air: [4 974 000 -r (100X 0 25)] -5-0.7 = 284,300kg Any change in the base assumptions made will alter these values in a corresponding manner. The change in temperature of the gas, air and waste gases will vary the quantity of heat available. The cyclic change in temperature may be varied; this will change the volume of brick required to store up the heat, and the surface required for heat interchange. The heat-storage capacity must be sufficient to supply the desired preheat, as well as the loss through the chamber walls, when on air or gas. Too much checker adds to the first cost; too small a checker increases the operating cost. The conditions will be changed by the use of other fuels, as well as by changes in the ratio of the air supply.