ON THE SETTING OF STEAM BOILERS. 



139 



by oxporiment to bo 14,600 thermal units.* (The thermal 

 unit is the quantity of heat that will raise 1 lb. of water 1° 

 at its greatest density, viz. a temperature of 39°.) When 

 the combustion is imperfect, because the supply of air is 

 insufficient, 1 lb. of carbon will combine with but 1.]- lb. of 

 oxygen, and form 2} lbs. carbonic oxido (CO); the quantity 

 of boat produced by this combination being 4,400 units only, 

 or equal to an evaporative power of but 4^ lbs. of water 

 (loss than one-third that of carbonic acid). When more air 

 is supplied after the formation of carbonic oxido, the 2} lbs. 

 of this gas combines with 1?,- lb. of oxygen and forms 3|lbs. 

 of carbonic acid, and produces heat units equal to the differ- 

 ence between those named— viz. 10,100, or 10} lbs. water. 



Now the actual process going on in a furnace is this (and 

 for convenience of illustration I take lib. of carbon as repre- 

 senting all the fuel, and exclude the hydrogen and hydro- 

 carbons as the quantity is but small). 



At first 1 lb. of solid carbon combines with its equivalent 

 2?, lbs. of oxygon, and makes 3J lbs. of carbonic acid gas, 

 producing 14,500 units of heat as stated. 



lC-f-2|0 = 3iC02 = 14,500 



When air can got freely to the carbon in sufficient 

 quantity, this is practically the whole of the process; but 

 when this is not the case, owing to the thickness of fuel on 

 the bars, the carbon becomes heated and expanded into gas 

 by the hot carbonic acid in the furnace, this latter gas 

 parting with one of its equivalents of oxygen to combine 

 with another pound of carbon. Thus wo have 3!|lbs. car- 

 bonic acid taking up an additional pound of carbon and 

 resolving the whole into 4,1 lbs. of carbonic oxide. 

 10 + 3| CO, = 4^ CO = 8800 units. 



• Eqnal to an evaporative power of 15 lbs. of water from 212=. 



