Heat transfer correlations are given in terms of Nusselt, Reynolds and Rossby numbers, the flow mode (parallel or countercurrent) and the inner rotating-pipe surface area. The effects of these parameters on the pressure drop have also been evaluated. The results are compared with values for conventional systems. Rotation of the helically threaded inner pipe leads to Nu numbers which may be several times higher than those found for a smooth stationary pipe. Although the rotation of the inner pipe caused some increase in pressure drop, this effect was unimportant compared with the improvement in heat-transfer capacity.