202 GREENHOUSES 



from six to ten pounds per square foot per 

 hour. 



A pound of best coal has a heating value 

 of about 14,000 B. T. U. per pound, of which 

 only about 60 per cent, or 8,400 B. T. U. are 

 utilized in heating water or producing 

 steam. It is the usual practice to estimate 

 that each pound of coal will impart about 

 8,000 B. T. U. to the heating medium, and 

 that each square foot of grate surface will 

 burn about six pounds of coal per hour. This 

 gives 48,000 B. T. U. per square foot of grate 

 surface per hour. 



To find the approximate number of square 

 feet of grate surface required to heat a given 

 house, find the number of heat units re- 

 quired, by the method described in Chapter 

 XI, and divide by 48,000. 



In general, a square foot of grate surface 

 is sufficient to. supply 250 square feet of 

 radiating surface. 



Fire Surface. Fire surface (sometimes 

 known as heating surface or water surface) 

 is of two kinds; direct and indirect. The 

 direct fire surface is that immediately above 

 or around the fire, against which the light 

 of the burning fuel shines. Indirect fire sur- 

 face is that which receives the heat from the 



