Abstract: A process for repairing heat exchanger tubes by partial tube replacement in the inside of closed tube apparatus with at least one cylindrical tube bottom from the tube side, wherein dents observed are first straightened, and a damaged partial tube section is separated and removed. A new partial tube section is then pushed in through a hole in the tube bottom. The joining of the new partial tube to the heat exchanger tube remaining inside the heat exchanger is performed by inner tube welding by means of an inner tube welding device, which is fastened to a bracket and a drive and is introduced through the new partial tube. Tests are performed after the welding together of the partial tube and the heat exchanger tube to demonstrate the quality.