120 DRAUGHT IN MARINE BOILERS. 



be taken to reduce the other losses from radiation, in- 

 complete combustion, etc., to their lowest practicable 

 amount (5%), these losses cannot be eliminated altogether, 

 and under the best conditions will generally be found to 

 exceed 25% and 5% respectively. So that the maximum 

 evaporative e'ficiency of this system of draught cannot 

 exceed 70%, and will in general be found to be nearer 

 60% ; and however the ratio of these losses are varied, by 

 reducing the temperature of the outgoing gases, by extend- 

 ing the heating surface, or otherwise, their sum will 

 remain fairly constant, the other losses being increased by 

 the sluggishness of the draught and the cooling of the 

 furnace gases. 



With " Forced Draught " the losses vary within much 

 wider limits, averaging 32% and 17% respectively, in 

 sympathy with the intensity of the draught; so that with a 

 pressure of two inches of water or above, the maximum 

 evaporative efiBciency is found not to exceed 59%, and will 

 in general be found to be nearer 50%. And notwithstanding 

 the greater advantage that this system secures in regulating 

 the minimum supply of air, the gain is more than counter- 

 balanced by the loss from the very high temperature of 

 the escaping gases and the higher rate of radiation, etc., 

 which can only be reduced by very greatly extending 

 the heating surface, not without sacrificing one of the 

 claims upon which its advocates rely, — namely, economy 

 in weight and space, — and it is difficult to see how the 

 claim for a large increase in evaporative efficiency can 

 be substantiated, in the absence of more precise data; for 

 granted that an economy of 4% can be secured by heating 

 the draught by the outgoing gases, the total efficiency only 

 amounts to 63%, or about equal to that of the worst of the 

 natural draught boilers, and is still lower than case B by 4%. 



