PHILIPPINE COALS AS FUEL. 349 



the loss to the stack by the gases escaping at too high a temperature as 

 compared with that of the steam in the boiler. 



■ Breckenridge et aP^ from results of boiler trials made to determine 

 the effect of soot deposits on the eyaporation in a horizontal tubular 

 boiler conclude that it is not very marked. They found that the soot 

 burned upon reaching a certain thickness, leaving but a very thin layer. 

 Even with frequent and perfect sweeping of the tubes, no boiler cools 

 the furnace gases to the temperature of the steam, but a certain amount 

 of this heat waste may be recovered and the efficiency somewhat raised 

 by the use of an economizer in the stack. 



The effect of scale on the transmission of heat through boiler tubes is 

 very variable, the mechanical structure of the scale being at least as 

 important a factor as the mere thickness. Schmidt and Snodgrass^^ 

 have investigated this effect on locomotive boiler tubes and feel warranted 

 in summing up the results of their tests in the following conclusions : 



"1. Considering scale of ordinary tliickness, say of thicknesses varying up 

 to one-eiglith inch, the loss in heat transmission due to scale may vary in 

 individual cases from insignificant amounts to as much as 10 or 12 per cent. 



"2. The loss increases somewhat with the thickness of the scale. 



"3. The mechanical structure of the scale is of as much or more importance 

 than the thickness in producing this loss. 



"4. Chemical composition, except in so far as it affects the structure of the 

 scale, has no direct influence on its heat transmitting qualities." 



Boiler pressure. — The true boiler efficiency is the ratio of the heat 

 absorbed to the heat which is available to the boiler; that is, that 

 portion of the heat in the furnace gases which is above the temperature 

 of the steam. Prom this it is evident that the higher the working 

 pressure^ — that is, the higher the steam temperature — the less difference 

 between a fixed temperature of the furnace gas and that of the steam 

 and therefore the less heat available to the boiler. In order to obviate 

 this difference in efficiency I have tried to maintain approximately the 

 same steam |)ressure in the various tests. In those cases where there is 

 a deviation, the efficiency attained is greater or less than the average 

 accordingly as the steam temperature is greater or less. The facts have 

 not been established giving the exact value of the effect for all changes 

 in steam pressure upon the evaporative efficiency of a boiler. Goss^" 

 has shown that "changes in steam pressure between the limits 130 pounds 

 and 240 pounds will produce an effect upon the efficiency of the 

 boiler which will be less than 0.5 pounds of water per pound of coal." 

 The difference is not large for the small ranges of pressure common in 

 stationary practice; and although slightly more heat is available and 



^^ Loe. cit. 



''Univ. of 111. Bull. (1907), 4, No. 15, 1. 



-"High steam pressure in locomotive service (1907), 10. Published by the 

 Carnegie Institute of Washington. 

 75034 4 



