Boilers

A boiler operating process including the steps of operating a boiler at a furnace temperature of about 900.degree. C., the boiler comprising a firebox with a central axis, a gaseous or liquid fuel burner arranged at one axial end of the firebox on the central axis thereof, first wall means surrounding the firebox to define an annular water jacket therearound, a circular array of fire tubes surrounding the firebox and extending axially through the water jacket, means defining an annular gas collector at the burner end of the firebox, the fire tubes communicating at one end with the axial end of the firebox remote from the burner and at their other ends with the gas collector, and second wall means defining a hot water boiler, the firebox being formed by first and second cylindrical wall portions respectively defining a primary and a secondary furnace having a larger diameter than the diameter of the primary furnace, and a stepped wall portion joining the first and second wall portions, the hot water boiler being annular in form and surrounding the water jacket, the first wall means separating the water jacket and hot water boiler and lying against the circular array of fire tubes, the operating step being carried out at a slight excess pressure of about 1 to 2 mm water column in the primary furnace, forcing gases from the secondary furnace at a temperature of 250.degree. to 450.degree. C. into the fire tubes, and exhausting the forced gases from the boiler at a gas temperature of 100.degree. to 120.degree. C.