&6 BOILER TESTS. 



which had 3-inch tubes and one 3J-inch, though the two boilers 

 are of somewhat different type. A practical objection to the 

 use of too small tubes must be kept in mind in those cases where 

 a very smoky grade of bituminous coal is used, and frequent 

 opportunity cannot be had to clean the tubes, so as to prevent 

 a serious accumulation of soot. 



There appears to be no reason why the relations of heating 

 surface to grate surface , found desirable in the horizontal boiler, 

 should not apply with equal force to the vertical boiler, and this 

 view of the matter is justified by the results of the test on 

 Boiler No. 60. In this boiler there are 44.5 square feet of 

 water-heating surface to 1 of grate, and 7.1 square feet of grate 

 to 1 of tube area, which agree practically with the proportions 

 named for the best work in horizontal boilers using bituminous 

 coal. If we allow the equivalent evaporation for the effect of 

 the 18 degrees of superheating in the same manner as in the 

 tests on the vertical boilers, considered under a previous head- 

 ing, the evaporation into saturated steam containing one per 

 cent, of moisture from and at 212 degrees per pound of com- 

 bustible in this case is 12.75 pounds. 



The discussion of the general conditions which secure economy 

 applies to medium rates of combustion of say 10 to 12 pounds 

 per square foot of grate per hour, such as will secure the rated 

 capacity of the boiler when the power is based on 12 square 

 feet of water-heating surface per horse-power. 



3. COMPARISON OF DIFFERENT KINDS OF BOILERS. 



It seems to the author that the general principles of economy, 

 to which attention has just been devoted, though deduced mainly 

 from a study of horizontal tubular boilers, are based upon such 

 reasonable grounds that they may be applied to all forms 

 of steam boilers. The proportion most to be desired between 

 tube opening and grate surface cannot be applied to a boiler 

 which has no tubes ; but the relations established between heat- 

 ing surface and grate surface, and the best condition as to the 

 temperature of the escaping gases, are principles which can be 

 applied in all cases. Before making a comparison of the 



