In Tube Fired Boilers the combustion gases travel within the tubes to heat the surrounding water. 

All steam boilers have four independent systems. The feed water system supplies water to the boiler, the fuel system transports the fuel, the draft system controls air flow for fuel combustion, and the steam system collects and controls the produced steam.

Most fire tube boilers are portable and are used in oil fields, saw mills, and at construction sites to generate heat or power. Water tube and cast iron boilers may be solid-fuel fired, or they may operate on standard liquid and gas fuels. The water tube boiler below operates on gas fuel and is part of a cogeneration plant where heat from the fuel is simultaneously converted into electrical and thermal energy.