212 



GREENHOUSES 



designed for soft coal will burn hard coal 

 equally well. If they are designed primarily 

 for hard coal they will not burn soft coal 

 efficiently. More grate surface is required 

 for soft coal than for hard coal, because it is 



Fig. 111. Wrought-iron boiler without flues 



more bulky weight for weight. Most mod- 

 ern greenhouse boilers will burn either hard 

 or soft coal, but a larger size will be required 

 for soft coal than for anthracite. 



Boiler Ratings. An approximate idea of 

 the size of boiler needed may be found by 

 figuring the amount of grate surface by the 

 method described on page 202. Boiler manu- 



