138 



ON THE SETTINO OF STEAM BOILEIIS. 



will be pulled or driven tlirougli them too quickly to allow 

 of tlioir parting with their heat, which will thus be wasted 

 up the chimney. 



On the other hand, a slow movement of the gases through 

 the flues may bo obtained by partially closing the damper ; 

 but the result of this will only be to check the admission of 

 air to the furnace, and so spoil the combustion. This chok- 

 ing or snuffing-out process actually figured some years ago 

 in a report addressed to the Liverpool Corporation, as one of 

 the recommendations for the prevention of smoke ! 



Now, seeing we all recognise and. insist upon the necessity 

 of a good draught to ensure good combustion, it may help us 

 to realize this necessity bettor, if wo consider for a moment 

 what the process of combustion is, and how the draught 

 affects it. 



Perfect combustion in a furnace (I use the word " perfect " 

 in a comparative, not absolute sense) requires a supply of 

 air amounting to aboiit oightoou times the volume of the 

 gases evolved, or, in other words, about 230 cubic feet of air 

 is required for every lb. of coal consumed ; and according to 

 the law of the expansion of gases, the products of com- 

 bustion are increased in the furnace to about three times 

 their original bulk. Hence the necessity for removing every 

 obstacle to the passing away of the products, in order that 

 the requisite quantity of air may be admitted. 



Then, as to the process. According to Rankine, the carbon 

 of the fuel, which forms at least 80 per cent, of its weight, 

 passes during combustion into the gaseous state, combining 

 with the oxygen in the air to form two distinct gases, accord- 

 ing to the proportion in which the combination is effected. 

 When the combustion is perfect, 1 lb. of carbon will combine 

 with 2|- lbs. of oxygen, and form carbonic acid gas (CO2) ; the 

 quantity of heat produced by this combination being found 



