12 
BULLETIN TIT, U. S. DEPARTMENT OF AGRICVLTX'RE. 
sufficient distance between the fuel bed and the boiler shell to allow 
for their complete expansion and ignition before coming into con- 
tact with the boiler shell. Soft coal usually contains a large pro- 
portion of volatile matter: consequently it needs considerably more 
space between the fuel bed and the boiler than anthracite. 
The radiation losses from' the sides- of the furnace and combus- 
tion chamber are comparatively small in any well-set boiler and as 
the combustible gases rise a considerable distance before losing any 
appreciable heat the distance between the grate bars and the boiler 
shell should be increased OTer that generally used in setting boilers. 
This dimension depends on the kind of coal burned, the rate of burn- 
ing, and tipon other factors, all of which should be known in order 
Tab] 
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