Although numerous experiments and numerical calculations have been carried out in order to study, with different parameters, the relationship between the flow structure and energy separation performance in a vortex tube, there has been less progress on its design optimization, which is due to the lack of a systematic analysis of the flow structure inside the vortex tube. According to the evidence, the large-scale vortex structures resulting from the vortex breakdown and vortex core precession determine the heat and mass transfer processes and flow structure in the vortex tube, and they are sensitive to the operation conditions. To study the vortex structure, attention can be paid to the shape of the reverse flow boundary and the location of the stagnation point, because they are the bond between the hot and cold streams. Based on the understanding of the energy separation mechanism and different flow structures inside a vortex tube, this study proposed a flow structure inside the main tube that bears the best energy separation performance as the optimization criteria for the main tube design. Based on a basic description of the counter flow structure in a confined slender tube, a preliminary quantitative tube design procedure can be realized to achieve the key parameters of the main tube, such as length, diameter, and diameter of the cold exit. (C) 2018 Elsevier Ltd. All rights reserved.