Insert tube and a system of insert tubes

A method of repairing and improving a pre-heating tube arranged in an exhaust gas pathway of a fuel burner, comprising:

• providing an insert tube, having a first and a second sealing ring, the insert tube having grooves defined therein at each end on an outer periphery surface of the insert tube;
• providing a heated pre-heating tube having an inlet and an outlet;
• passing air, having a first temperature, from the inlet through the heated pre-heating tube and out through the outlet;
• leading air at the first lower temperature from the inlet channel at one side of the exhaust gas pathway and heating the air passing through the pre-heating tube by hot exhaust gases flowing externally to the pre-heating tube;
• the pre-heating tube heating the air for the fuel burner to a second temperature being higher than the first temperature;
• when the preheating tube is eroded, sliding the insert tube into the pre-heating tube while compressing the sealing rings during insertion;
• and
• the inserted first and second sealing rings of the insert tube sealing air disposed in an insulation layer formed between the insert tube and the pre-heating tube;
• the sealed air in the insulation layer raising the first temperature of the air at the inlet to a raised temperature.