88 BOILER TESTS. 



forced draught, and 105.6 horse-power with natural draught. 

 Taking the results, however, as they stand, there is a gain 

 amounting to 2 per cent, in favor of forced draught, figured on 

 coal, and 0.5 per cent, figured on combustible. The small 

 advantage produced by the forced draught may be attributed 

 wholly to the heated air which was used. It is to be borne in 

 mind that with forced draught there is a chance for loss from 

 excessive supply of air at times when the bed of coal is allowed 

 to burn through to the grate. The loss from this source is 

 liable to be greater with forced draught than with natural 

 draught, on account of the greater quantity and force of the 

 air supplied by the blower. Little, if any benefit, for this 

 reason, can be expected under ordinary conditions of firing 

 from a forced draught, and the results of the tests bear out 

 these expectations. 



The evaporative results, taken by themselves, show a favor- 

 able performance of the boiler, for the class of fuel that was 

 used. Compared with the results obtained from boilers set in 

 the ordinary way, with the fire box beneath the shell, the 

 figures do not show any special improvement, and the conclu- 

 sion may be drawn that the use of a detached furnace is not 

 especially advantageous. In this form of furnace the fire 

 is surrounded by highly heated brick work, which keeps 

 the furnace at a high temperature. In the ordinary setting the 

 temperature is much reduced by the close proximity of the 

 heating surfaces of the boiler to the furnace. The use of a 

 detached furnace, on account of the higher temperature, would 

 be expected to secure the best result. The same cause, how- 

 ever, which produces the high temperature, leads to a consid- 

 erable amount of loss of heat, on account of radiation from the 

 top and sides of the furnace, and this stands in the way of 

 realizing the whole benefit, which would otherwise be expected 

 from this system. 



Another test was made on Boiler No. 8, using screenings 

 alone with forced draught ( the screenings being wet 7 per 

 cent, before firing ) . The percentage of ash was 15 per cent. ; 

 the horse-power developed was the same as in test No. 14 ; the 



