Abstract: In a swirl-flow internal thermosyphon boiling device, besides the swirl-flow, a bypass stream may exist between the outer edge of the helix and the inside wall of the heated tube. The flow distribution of these two streames may be solved with the basic concepts of the streams Analysis Method and the boiling heat transfer coefficients can be calculated by means of the weighted mean method. Various published correlations of single-phase and two-phase flow in helical channels are discussed. A calculation method for engineering design is proposed. The predicted values agree with experimental results quite well.