This paper analyzes the optimal Re for the steady, laminar, fully developed forced convection in a helical coiled tube with constant wall heat flux based on minimal entropy generation principle. Two working fluids, water and air, are considered. It is found that the entropy generation distributions are relatively insensitive to coil pitch, A. Through the entropy generation analysis for cases of coil curvature ratio, delta ranging from 0.01 to 0.3, and two dimensionless duty parameters, eta(1) from 0.1 to 3.0, and eta(2)/1020 from 0.01 to 1.0, the optimal Re for cases with various combinations of the design parameters is reported. In addition, a correlation equation for the optimal Re, delta, eta(1), and eta(2) is proposed after a least-square-error analysis. The optimal Re should be adopted as the operating condition according to the relevant design parameters of the helical coils so that the thermal system can have the best exergy utilization with the least irreversibility. (c) 2005 Elsevier Ltd. All rights reserved.