U-tube bundle for installation in existing vessels (e.g. double-circuit steam boiler, steam drum or storage tank). The tube bundles can be used for heating or cooling and can be operated with condensing steams or fluids as heating medium. The bundle tubes are attached by expanding or shrink-welding or both (depending on the operating conditions). The flow can be double- or multi-flow depending on the fluid throughput. If condensing steam is employed as heating medium on the tube side, the heating tubes are installed at an angle in order to ensure that the condensate is properly drained. The sliding guide is only suitable for light tube bundles.