a fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and superheater coils. necessary safety valves are located at suitable points to relieve excessive boiler pressure. the air and flue gas path equipment include: forced draft (fd) fan, air preheater (ap), boiler furnace, induced draft (id) fan, fly ash collectors (electrostatic precipitator or baghouse), and the flue-gas stack.