Mance: Quarry Industry of Southern Indiana 



85 



draft saves coal and labor cost, but when the life of the installa- 

 tion is compared with the life of a good stack or chimney, the 

 saving is not so apparent. In every form of mechanical draft, 

 power is necessary to drive the machinery, and the machinery 

 works under conditions that cause it to wear out rapidly. 

 Mechanical draft is of two classes: forced draft and induced 

 draft. Both forms have their advantages. In forced draft the 

 air is driven in under the fire to cause the draft, and in this case 

 the gases in the firebox are under higher pressure than the air 

 of the boiler-room and the doors of the furnace must be kept 

 shut or the draft turned off. If the setting of the boiiers becomes 

 defective, the boiler-room is filled with smoke which has escaped 

 from the furnace. In induced draft the fan is placed in the 

 stack or the breechings, and sucks the hot gases out of the furnace 

 and tubes. In this case the pressure is lower, and there can be 

 no escaping gases while the fan is running, but the disadvantage 

 of this type of draft lies in the fact that the fan is handling very 

 hot gases which tend in a short time to destroy it. 



Another consideration is the fact that in induced draft a larger 

 fan is needed to handle the gases after they have been heated in 

 the furnace. Hot gases occupy much more space than the same 

 weight of cold gases. Altho it will require a much larger fan, 

 it will not require increased power in the same ratio, on account 

 of the fact that hot gases are lighter and consequently less power 

 per volume is necessary to handle them. 



The power consumption of mechanical draft is usually figured 

 at from 1 to 5 per cent of the total capacity. The cost of in- 

 stallation is from one-fifth to one-third the cost of an equivalent 

 brick stack. In considering this low figure, the life of the equip- 

 ment must be kept in mind because the cost of upkeep and the 

 life of the apparatus are important factors. 



The largest item in the waste heat from boiler and furnace 

 equipment is the heat lost in the hot gases passing up the stack. 

 The following table shows the percentage of the heat of combus- 

 tion present in the flue gases at various temperatures, under 

 average conditions: 



Heat Wasted 



Flue Gas Temperature (per cent) 



200 degrees Fahrenheit 6.0 



300 degrees Fahrenheit 10.6 



400 degrees Fahrenheit 15.3 



500^ degrees Fahrenheit 19.9 



600^degrees Fahrenheit 24 . 5 



700^degrees Fahrenheit 29 . 2 



800;;degrees Fahrenheit 33 . 8 



