A design of inserting in parallel an impermeable sheet to divide an open conduit into two subchannels for conducting double-pass laminar countercurrent operations under uniform heat fluxes, resulting in substantially improved the heat-transfer rate, has been designed and investigated theoretically by using an eigenfunction expansion in terms of power series for the homogeneous part and an asymptotic solution for the non-homogeneous part. The theoretical results of heat-transfer efficiency enhancement in double-pass parallel-plate heat exchangers are represented graphically and compared with those in the single-pass operations without an impermeable sheet inserted. The influence of the impermeable-sheet location on the heat-transfer efficiency enhancement as well as on the power consumption increment in double-pass operations has also been delineated. (c) 2006 Elsevier Ltd. All rights reserved.