Heat exchanger in horizontal design for two fluids with non-exchangeable straight-tube bundle. The bundle tubes can be cleaned easily. The floating head heat exchanger allows a good length extension in case of temperature changes. For better medium flow on the shell side the floating head heat exchanger is equipped with an enlarged cover. To maintain the floating head side the tubes need not be disassembled. The tubes can also be cleaned from the floating head side. The interior of the shell can be inspected with the tube bundle pulled out. The flow can be double- or multi-flow depending on the fluid throughput.