PRINCIPLES OF COMBUSTION. 



155 



then supposing you wish to erect a hot water apparatus the 

 relative proportions between the area of the bars and the length 

 of pipe would be as follows : 



Area of Bars. 



75 square inches will supply 

 100 " " 



150 " " 



200 " " 



250 " " 



300 " " 



400 " " 



500 " " 



4 inch pipe. 3 inch pipe. 2 inch pipe. 

 150 feet, or 200 feet, or 300 feet. 



1000 



1333 " 2000 " 



Thus, suppose there are six hundred feet of pipe, four inches 

 in diameter, in an apparatus, then the area of the bars should 

 be three hundred square inches, so that thirteen inches in 

 breadth, and twenty-three inches in length, will give the re- 

 quired quantity of surface. When it is required to obtain the 

 greatest heat in the shortest time, the area of the bars may be a 

 little increased. 



In order to make the fire burn for a long time without atten- 

 tion, the furnace should extend beyond the bars, both in length 

 and breadth ; and the coals, which are placed on this blank part 

 of the furnace, in consequence of receiving no air from below, 

 will burn slowly, and will only enter into complete combustion 

 when the rest of the coal, on the bars, has been consumed. 



It may be observed, that as the maximum effect of the furnace 

 is seldom required, the register on the ash-pit door, and the 

 damper in the flue, must be used to regulate the draught, and 

 thus limit the consumption of fuel. 

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