Abstract: An apparatus according to the invention utilizes an elongated combustion chamber at least partially surrounded by a cooling air chamber receiving cooling air, directing a portion of the heated cooling air into the combustion chamber, and discharging the remaining heated cooling air into the atmosphere. A number of separated ducts discharge afterburned exhaust gas and are connected to a combustion chamber discharge pipe. Each duct has a wall forming a joint heat exchange surface with the cooling air chamber and the openings of the ducts into the atmosphere are separate from the air chamber opening. The ducts may each be formed of a corrugated metal shell with the corrugation peaks connected with the exterior wall of the cooling air chamber.