The results of the heat transfer and pressure drop characteristics in horizontal double pipes with helical ribs are presented. Nine test sections with different characteristic parameters of. helical rib height to diameter, chi/d(i) = 0.12, 0.15, 0.19, and helical rib pitch to diameter, p/d(i) = 1.05, 0.78, 0.63, are tested. Cold and hot water are used as working fluids in the shell side and tube side, respectively. Experiments are performed under conditions of mass flow rates varying from 0.01 to 0.10 kg/s for cold water and from 0.01 to 0.10 kg/s for hot water, respectively. The inlet cold and hot water temperatures are between 15 and 20 degrees C and between 40 and 50 degrees C, respectively. The results obtained from the tubes with helical ribs are compared with those without helical ribs. It is found that the helical ribs have a significant effect on the heat transfer and pressure drop augmentations. Based on fitting the experimental data, non-isothermal correlations of the heat transfer coefficient and friction factor are proposed. The majority of the data falls within +/-15%, +/- 15% of the proposed correlations for heat transfer coefficient and friction factor, respectively. (c) 2006 Published by Elsevier Ltd.