16 BULLETIN 741, U. S. DEPARTMENT OF' AGRICULTURE. 



The functions of the fire box are to provide' means for burning the 

 fixed carbon in the fuel, for the distillation of the gases, and for the 

 thorough mixing of air at a high temperature with the unburned 



gases. 



COMBUSTION CHAMBER. 



The combustion chamber is really an extension of the fire box in 

 which the burning of the volatile gases takes place and where the 

 heat thus produced is absorbed by the water through the heating sur- 

 faces of the boiler. With horizontal return-tubular-boiler settings 

 the term " combustion chamber " is applied to the space between the 

 bridge wall and the ends of the boiler tubes. The horizontal dimen- 

 sions of the combustion chamber are fixed to a great extent by the 

 type of boiler setting. The depth seems to have little effect on the 

 efficiency of the setting ; consequently, the depth of' the combustion 

 chamber may be as great as the form of setting will permit. The 

 common practice of sloping the floor of the combustion chamber 

 downward from the bridge wall to the clean-out door seems to have 

 little value other than facilitating the removal of ashes. 



ASH PIT. 



The ash pit beneath the grates is to catch the ashes and refuse from 

 the fire above and to provide an air reservoir for supplying air to- the 

 burning fuel. The depth of the ash pit should be sufficient to provide 

 plenty of air for the burning of the fuel and to hold a considerable 

 accumulation of ashes and clinkers without choking the air supply and 

 burning the grate bars. The bottom of the ash pit should be made 

 wedge-shaped, of cement, to facilitate cleaning and to allow water to 

 be placed under the grates to prevent them from being burned. It 

 should not be necessary to keep water in the ash pit except when the 

 boiler is being forced. 



ASH-PIT AND FLUE DOORS. 



The ash-pit door, usually called the " ash-pit damper," is an open- 

 ing to the ash pit for the removal of ashes, and it also serves to con- 

 trol the air supply through the fuel bed. It is very important to 

 have this door- tight fitting in order to control effectually the air 

 supply through the grates. 



The flue doors provide an opening to the smoke box at the front 

 ends of the flues. These doors, unless they fit tightly, are a source of 

 air leaks. 



CHIMNEY. 



The function of a chimney is to provide a draft to effect combus- 

 tion of fuel on the grate and to carry off the resulting obnoxious 

 gases. The chimney should be built carefulty to prevent cracks 



