A double-flow laminar counterflow heat exchanger device is an open circular tube divided into two subchannels with uniform wall temperature, inserted in parallel into an impermeable, resistless tube. Heat transfer performance was considerably improved, compared with an open circular tube (without an impermeable, resistless tube inserted), using an orthogonal expansion technique with eigenfunction power series expansion. The impermeable tube location can effectively enhance the heat transfer efficiency, leading to improved performance. The dimensionless outlet temperature and average Nusselt number decrease with increasing Graetz number (Gz) as the subchannel thickness ratio (kappa) moves away from 0.5, especially for kappa>0.5, for flow pattern A. Heat transfer rate improvement occurs for flow pattern B for any Graetz number with kappa<0.6. The power consumption increment is also delineated.