Heat exchanger in horizontal design for two fluids or two gases with nonexchangeable straight-tube bundle. The bundle tubes can be easily cleaned after the disassembly of the heads. The interior of the shell is not accessible. 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 throughput of fluids or gases. Depending on the operating conditions a compensator may be required to compensate for expansion.