. Science of railways . les of coal from the grates through the tubesunburned, thus permitting rapid inflows of cold air when andwhere it is least desired. It is evident then that sufficientair for complete combustion cannot be admitted through thegrates alone, and that an additional amount should be ad-mitted above the fire to complete combustion. When air is admitted above the fire it must be introducedin such a manner that it will at once mix with the gasesin the fire-box very completely, otherwise it will do moreharm than good. If it is admitted in a large volume, as whenthe fire-box door
. Science of railways . les of coal from the grates through the tubesunburned, thus permitting rapid inflows of cold air when andwhere it is least desired. It is evident then that sufficientair for complete combustion cannot be admitted through thegrates alone, and that an additional amount should be ad-mitted above the fire to complete combustion. When air is admitted above the fire it must be introducedin such a manner that it will at once mix with the gasesin the fire-box very completely, otherwise it will do moreharm than good. If it is admitted in a large volume, as whenthe fire-box door is held open, the air will not mix with thegases, but will form a distinct current of its own. As thegases can only come in contact with the outer surfaces of thiscold draft of air, they will not mix and, being cooled belowtheir burning temperature, will pass away unconsumed. It is of real necessity then that air must be admitted overthe fire, and that it must be heated in order to mix readily LOCOMOTIVE APPLIANCES. 6#5. 696 LOCOMOTIVE APPLIAXCES. with the gases. In the Franklin fire door this necessity isprovided for in the hollow frame and hollow door plates. Thecold air passes in over the deck and is heated as it passes upthrough the hollow frame and plates. Near the top of theupper door plates the heated air passes in over the fire througha number of small openings provided for the purpose. Thisprovides the circulation of free heated air necessary to burnthe gases. In this it can also be considered as a smoke con-sumer. We now arrive at the conclusion that additional coal canbe saved through the exclusion of cold air, since the moreuniform the temperature in the fire-box the more perfect iscombustion, and, consequently, a lighter fire and less coal. Itis possible then to reduce the size of the scoop one or twonumbers. By doing this if only 30 scoops are saved in 100miles, based on a No. 3 scoop, averaging 15 pounds each, itwould amount to 450 pounds per trip, or 67% tons, f
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