Heating Surface. While the points thus far considered are more or less important in their bearing in the generation of steam, yet the amount of heating surface is, as a rule, the most important. As previously stated, the lower the rate of combustion per square foot of heating surface, the higher will be the rate of evaporation per pound of coal. The ratio of the heating surface of the flues to that of the fire-box varies greatly, in some cases being only 9 to 1 while in others it is found as great as 18 to 1. There is perhaps a correct value for this ratio, but at the present time it is unknown. The relation existing between the total heating surface and the grate area varies between wide limits for different cases. Table III, taken from the Proceedings of the Master Mechanics' Association for 1902, gives the ratio of heating surface to grate area in passenger and freight locomotives burning various kinds of fuel.
Ratio of Heating Surface to Grate Area
Fuel Passenger Locomotive Simple Passenger Locomotive Compound Freight Locomotive Simple Freight Locomotive Compound Free Burning Bituminous 65 to 90 75 to 95 70 to 85 65 to 85 Average Bituminous 50 to 65 60 to 75 45 to 70 50 to 65 Slow Burning Bituminous 40 to 50 35 to 60 35 to 45 45 to 50 Bituminous, Slack, and Free Burning Anthracite 35 to 40 30 to 35 30 to 35 40 to 45 Low Grade Bituminous, Lignite, and Slack 28 to 35 24 to 30 25 to 30 30 to 40
From the foregoing, it is evident that it is exceedingly difficult to determine just how much heating surface a locomotive boiler should have to give the best results. As a rule, they are made as large as possible so long as the total allowable weight of the locomotive is not exceeded. This is not, however, a scientific rule to follow but it is safe to say that the value of no locomotive has ever been impaired by having too much heating surface. The greater the boiler power, the higher will be the speed which can be maintained. It is important that the boiler be covered with a good lagging in order to prevent loss of heat due to radiation.
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