M, COMBUSTION AND FUELS 



the air and the oil is not perfect. The excess of air is required 

 in order that the combustible elements may be surrounded 

 by sufficient oxygen during the subsequent mixing in the com- 

 bustion chamber. The percentage of this excess should be as 

 small as can be obtained without forming smoke. In some 

 boiler tests the excess has amounted to only 10%, which is 

 extremely low; but under average conditions the excess of air 

 is usually over 15%. When coal is used as fuel, the excess of 

 air is from 50 to 100%, or more; therefore, it is not surprising 

 that a fireman accustomed to burning coal should admit too 

 much air when burning oil fuel. To serve as a guide to the 

 fireman in the regulation of combustion, it is a good plan to 

 install a COz recorder, which will give a continuous record of 

 the percentage of COz in the flue gases. The amount of COt 

 formed bears a known relation to the amount of air admitted, 

 and by instructing the fireman to obtain as high a percentage 

 of COz as possible, the economy of operation may be increased. 

 In practice, about 15% of COz indicates the best performance 

 obtainable, and an average of from 12 to 13% may be consid- 

 ered very satisfactory. 



Evaporative Power of Oil Fuel. Owing to its higher calo- 

 rific value, oil fuel has a greater evaporative capacity, per lb., 

 than is possessed by coal. Moreover, the conditions under 

 which oil fuel is burned enable a greater proportion of the 

 heating value to be obtained for evaporation of the water than 

 is the case with coal. As a consequence, the number of pounds 

 of water evaporated per lb. of oil is greater than the evapora- 

 tion per lb. of coal. Tests of boilers using a good grade of 

 coal have shown an evaporation of slightly more than 1 1 lb. 

 of water per lb. of coal and, under particularly favorable condi- 

 tions, even better results have been obtained. On the other 

 hand, an average evaporation of from 12 to 13 lb. of water per 

 lb. of oil has frequently been obtained with oil fuel, and in some 

 cases an evaporation of 16 lb. has been reached. 



Effect of Sulphur in Oil. The presence of sulphur in oil 

 fuels, particularly in crude oils, has caused some engineers to 

 fear that the plates and tubes of oil-burning boilers would be 

 pitted and corroded by the sulphurous gases generated. Their 

 fears, however, seem to have been groundless, inasmuch as the 





