An analytical method is proposed to predict the temperature distribution and local Nusselt number for laminar How in a double-pass countercurrent heat exchanger with sinusoidal heat flux distribution. A design of inserting in parallel an impermeable barrier to divide an open conduit into two subchannels for conducting double-pass operations, resulting in substantially improved the heat transfer rate, has been evaluated theoretically in the fully developed region. Comparison with the theoretical results shows that the heat-transfer efficiency improvement for double-pass concentric circular heat exchangers is generally higher than those in the single-pass operations without an impermeable barrier inserted. The influences of the impermeable-barrier location on the heat-transfer efficiency improvement and power consumption increment, as indicated from theoretical predictions, can be used to determine the economical feasibility in operating double-pass devices. (C) 2008 Elsevier Ltd. All rights reserved.