A new device of inserting an impermeable sheet or a permeable barrier to divide the thermal-diffusion columns into two subchannels with external refluxes at the ends, resulting in substantial improvement of the separation efficiency of heavy water, has been developed and investigated using orthogonal expansion technique. The analytical results are represented graphically and compared with that in a Clusius-Dickel column of the same size with recycle. Considerable improvement on enrichment of heavy water is obtained by employing such devices with an impermeable sheet or a permeable barrier instead of using the Clusius-Dickel thermal-diffusion column. The effect of sheet or barrier location on the enhancement of the separation efficiency of heavy water has also been discussed.