Fig. 2,738.—Curves showing comparative costs of chimney and mechanical draft. In certain of these, the cost of the existing chimney is known, and that of the complete mechanical draft plant is estimated, while in others, the cost of mechanical draft installation is determined from the contract price, and the expense of a chimney to produce equivalent results is calculated. Costs are shown for both single, forced and induced engine driven fans and for duplex engine driven plants, in which either fan may serve as a relay. An apparatus of the latter type is the most expensive, and finds its greatest use where economizers are employed.
Both brick and iron chimneys, require an inner wall or lining of brick, which forms the flue proper, and in order that this wall be not cracked by sudden cooling an air space is left between it and the outer wall. In a brick chimney the inner wall need not extend much beyond half the height of the chimney, but when iron is used it should reach to the top.
Ques. Upon what does the force of natural draught in a chimney depend?
Ans. It depends upon the difference between the weight of the column of hot gases inside the chimney and the weight of a like column of the cold external air.
Figs. 2,739 and 2,740.—Substituting mechanical draught in place of chimney. The relative proportions of a brick chimney, and of the smoke pipe required when mechanical draft is introduced are forcibly shown in the illustrations, which show the works of the B.F. Sturtevant Co., at Jamaica Plain, Mass. The removal of the boilers to a position too far distant from the existing chimney to permit of its longer fulfilling its office, led to the substitution of an induced draft fan and the subsequent removal of the chimney. The present stack or smoke pipe, barely visible in fig. 2,740, extends only 31 feet above the ground, and no trouble is experienced from smoke.
Ques. How is the intensity of the draught expressed?
Ans. In terms of the number of inches of a water column sustained by the pressure produced.