a firebox consisting of steam or hot-water boilers or furnaces, in which the fuel (coal dust, atomized mazut, or a gas) burns in a spray that occupies a large portion of the furnace chamber, in contrast to a laminar combustion firebox. Depending on the arrangement of the burners, the fuel sprays may be straight (with the burners at the bottom or top), bent at an angle of 90° (with horizontal arrangement of the burners), or bent 180° (U-shaped fuel sprays). The temperature of the fuel spray in the center of combustion reaches 2000°C and gradually decreases to approximately 1000°C at the furnace exit.
Air-jet furnaces are characterized by intense thermal radiation of the flame at the furnace walls. The walls are usually covered by a lattice of water-cooled pipes or, in large, modern steam boilers, of finned tubes welded together. The heavy exterior of firebrick used in old boilers has been replaced by lightweight insulation suspended from the outside onto the finned tubes. A large percentage of current fireboxes are air-jet furnaces.