In this work the effect of propellers, which were able to rotate freely due to the force exerted by the fluid flowing through the inner pipe of the double-pipe heat exchangers, on the heat transfer and pressure was investigated expermentally. The experiments were undertaken for the Reynolds number range of 2500-15000 and for several propellers. In the system, heat-transfer rates were about 250% better than those found with the system without propellers. Thus, because of rotating the fluid, propellers and their number become influential upon heat transfer. The increase in pressure-drop changed between 500 and 1000% in comparison with that for the empty tube, depending on Reynolds number and number of propellers.