An experimental study on heat transfer enhancement in mini scale coiled tubing for constant wall temperature conditions is conducted. Copper coils with three different radii of curvature (1 cm, 2 cm, and 4 cm), and in four lengths (1, 2, 3 and 4 coil turns) were used. The tube length is long enough to consider the flow to be hydro-dynamically fully developed. Hence, the effects of varying curvatures and lengths on heat transfer are studied. The pitch of the coil is restricted to diameter of the tube to minimize the effect of coiling. Dean number is used instead of Coiled number (modified Dean number), and hence, the results can be expanded to spiral and curved tubing. Water and two different silicone oils (0.65 cSt, 1 cSt) were used in the experiments to examine the effect of Prandtl number on coiled tubing heat transfer augmentation. Prandtl number from 5 to 15 is covered in this paper. A new asymptotic correlation is proposed to calculate Nusselt number in fully developed coiled tubing based on the current results. In addition, dimensionless mean wall flux and dimensionless thermal length are also considered besides Nusselt, Reynolds, and Dean numbers. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.