Marice: Quairy Industry of Southern Indiana 71 



pressure. These figures of course depend upon the kind of 

 engine in which the steam is to be used. The amount of steam 

 mentioned above will develop about 3 horse-power in the best 

 compound condensing engine of today, but only about one-half 

 a horse-power in a small non-condensing engine. A very com- 

 mon method of rating the horse-power of boilers is on the area of 

 the heating surface. Ten to 12 square feet of heating surface is 

 considered as equivalent to a horse-power. The power of a boiler 

 to evaporate water depends on the rate at which coal can be 

 burned on the grate and the area of the grate. With natural 

 draft, from 20 to 45 pounds of coal per hour can be burned per 

 square foot of grate area. When forced draft is used these 

 figures can be increased. 



The efficiency of a boiler and its grate is expressed as follows: 

 Efficiency of boiler Heat absorbed by boiler per pound of coal as fired 

 and grate Calorific value of one pound of coal as fired 



The fact that different firms use different heating area ratings 

 is likely to confuse purchasers, but a careful study of the size and 

 position of these surfaces will avoid this difficulty. 



Cost of Boilers. Figures on the cost of boilers are hard to 

 give with accuracy since the great variation in types and arrange- 

 ment makes a single statement based on the horse-power rating 

 misleading. In fact, the cost does not increase in the same 

 ratio; and so many other considerations enter into the calculation 

 that only approximate prices can be given. The wide variation 

 in the cost of boilers is partly accounted for by the fact that the 

 horse-power basis differs with different manufacturers. Geb- 

 hardt in Steam Power Plant Engineering (first edition, p. 96), 

 quotes a price of $1 per square foot of heating surface for all 

 boilers of over 100 horse-power. He also quotes the following 

 method from C. H. Benjamin {Ejigineer [V . S.], Nov. 15, 1902): 



(A) Cost in dollars = 500 + 9.2 X rated horse -power. 



(B) Cost in dollars = 500 + 8.5 X rated horse-power. 



(C) Cost in dollars = 100 + 6.5 X rated horse-power. 



(D) Cost in dollars = 100 +5 X rated horse-power. 



(A) Horizontal water-tube boilers, 125 pounds pressure, 10 square feet 

 heating surface per horse-power. 



(5) Vertical water-tube boilers, other conditions same as in (A). 



(C) Horizontal return tubular boilers, 12 square feet heating surface 

 per horse-poAver, 



(D) Small vertical fire-tube boilers. 



The cost of Scotch marine boilers rated on a basis of 8 square feet per 

 horse-power may be estimated by means of formula (A) 



