An influence of plate spacing on heavy water enrichment in continuous concentric-tube Frazier-scheme thermal-diffusion columns at fixed operational expense has been investigated, both theoretically and experimentally, under flow-rate fraction variations. The analytical predictions are represented graphically and compared with those of a classical Frazier-scheme having the same column length and flow-rate fraction at both the top and the bottom of the column. Considerable improvement in the separation efficiency of the heavy water in thermal-diffusion columns of the same working dimensions is achieved by employing a Frazier-scheme with optimal plate spacing (2w)* and flow-rate fraction variations. Experimental work was conducted to confirm the quantitative agreement with the theoretical prediction of the separation efficiency and the proposed separation theory. The effects of the plate spacing and flow-rate fraction on the separation efficiency of heavy water enhancement in the Frazier-scheme thermal-diffusion column with the column number, feed fraction concentration and feed mass flow rate have also been discussed. (c) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.