Experiments are performed to understand the critical beat flux (CHF) mechanism in a concentric-tube open thermosyphon working with R22. An electrically heated bottom-closed outer tube with an unheated inner tube inserted concentrically serves as an evaporator of the thermosyphon. CHF data are collected by using saturated R22 at 1.5, 2.0 and 2.5 MPa by varying different geometrical parameters of the thermosyphon. For a particular outer tube, there is an optimum diameter of the inner tube for which the CHF is maximum. The optimum diameter of the inner tube is about half of the diameter of the outer tube and divides the CHF into two characteristic regions. Available correlations for predicting the CHF in each region are compared with the present data and more generalized correlations have been proposed. (c) 2005 Elsevier Ltd. All rights reserved.