Heat exchanger in horizontal design for two fluids with non-exchangeable straighttube bundle. The bundle tubes can be cleaned easyily. The quick locks are used if the heat exchanger must be cleaned frequently and quickly. The tubes need not be disassembled. The interior of the shell is not accessible and can only be inspected through the connection nozzles. The bundle tubes are attached by shrink-welding. The flow can be single- or multiflow depending on the fluid throughput. Depending on the operating conditions a compensator may be required to compensate for expansion.