Figs. 2,745 and 2,746.—Comparison of chimney draft and mechanical draft. The illustrations show a plant of 2,400 H.P. of modern water tube boilers, 12 in number, set in pairs and equipped with economizers. Fig. 2,745 indicates the location of a chimney, 9 feet in internal diameter by 180 feet high, designed to furnish the necessary draft; fig. 2,746 represents the same plant with a complete duplex induced draught apparatus substituted for the chimney, and placed above the economizer connections. Each of the two fans is driven by a special engine, direct connected to the fan shaft, and each is capable of producing draft for the entire plant. A short steel plate stack unites the two fan outlets and discharges the gases just above the boiler house roof. All of the room necessary for the chimney is saved, and no valuable space is required for the fans.
Ques. How is mechanical draft secured?
Ans. In two ways, known respectively as induced draught and forced draught.
Ques. Describe the method of induced draft.
Ans. A fan is located in the smoke flue, and which in operation draws the gases through the furnace and discharges them into a short chimney.
Ques. Describe the method of forced draft.
Ans. In this method, air is forced into the furnace underneath the grate bars by means of a fan or a steam jet blower.
Fig. 2,747.—Forced draft plant with hollow bridge wall at the Crystal Water Co., Buffalo, N. Y. The air is delivered to the ash pit via the hollow bridge wall, being supplied under pressure by the blower seen at the side of the boiler setting. As shown, the blower is operated by a small reciprocating engine; however, compact blowing units with steam turbine drive can be had and which are designed to be placed in the boiler setting.