FORCING AND FORCING STRUCTURES 67 



The heating surface of a boiler or hot-water heater is that part 

 which is exposed to the direct heat of the fire and of the heated gases. 



The grate surface is the number of square feet of grate in the 

 boiler or heater. 



In estimating the radiating surface required for greenhouses, 

 the area expressed, that is, the number of square feet of glass in 

 the roof, gables, and side walls, is taken as the basis of computation. 

 Certain rules of practice have been found to give fairly good re- 

 sults in proportioning radiating surface, grate surface, and heating 

 surface. The ratio of heating surface to grate surface will depend 

 upon the kind of coal to be burned and the economy desired. 

 The more heating surface, that is, the greater the area of boiler 

 exposed to fire and heated gases, provided per unit of grate sur- 

 face, the higher the efficiency and economy, but the greater the first 

 cost of the heater. The usual practice is to employ 4.0 square feet 

 of heating surface to I of grate surface for hard coal, and 80 feet 

 of heating surface to I of grate surface for soft coal. 1 



One foot of heating surface in a steam boiler or a hot-water 

 heater will supply heat for about 8 square feet of radiating surface, 

 under mean conditions. This will usually give a heater ample in 

 size for the work required ; but if more radiating surface is added, 

 the heater may in some instances prove to be too small. The table 

 on the following page gives more exact proportions. 



To maintain the temperature of the greenhouse at 70 F. above 

 that of the surrounding air, for hot-water heating in which the 

 maximum temperature of the water is maintained at 1 80 F. there 

 should be I square foot of radiating surface for every 4 square 

 feet of glass ; for low-pressure (under 5 pounds) steam heating 

 there should be I square foot of radiating surface for every 

 5 square feet of glass. Some authorities would give somewhat 

 higher figures, and there is no doubt that if the house is not 

 much exposed, the higher proportions will give satisfactory results. 



Table I gives more exact values for these quantities and will 

 be found to accord with the best practice in heating of greenhouses, 

 either by steam or hot water. The temperature for steam at 5 pounds 

 ^pressure is about 220 F, and for steam at 10 pounds pressure, 

 about 240 F. 



1 For small upright heaters, 25 per cent less. 



