A new device of inserting an insulation sheet or an impermeable sheet with negligible thermal resistance to divide an open duct into two channels with uniform wall temperature and with external refluxes at the ends, resulting in substantially improving the heat transfer, has been studied and investigated by an orthogonal expansion technique. The analytical results are represented graphically and compared with that in an open conduit of the same size without recycle. Considerable improvement in heat transfer is obtainable by employing double-pass operations with an impermeable sheet of negligible thermal resistance instead of using double-pass operations with an insulation sheet inserted and single-pass operations. The effect of sheet location on the enhancement of heat transfer efficiency as well as on the increment of power consumption, has been also discussed.