Thermal performances of two-phase loop thermosyphons (TPLT) with smooth and scale-roughened boiling surfaces are compared by examining their boiling flow structures, boiling heat transfer properties with the associated instability phenomena, thermodynamic cycles, boiling flow maps and networks of thermal resistances along the flow pathway. With cooling applications to electronic chipsets which limit the range of operating temperatures for these TPLTs, the working fluid of treated water has to circulate at sub-atmospheric pressures. With each tested TPLT, the flow visualization and thermal performance tests are individually performed at the identical test conditions controlled by boiling heater power (Q) and condenser thermal resistance (R-th,R-con) to image the boiling flow structures and to measure the boiling heat transfer rates, phase-change pressures in evaporator and condenser and the loop-wise temperature distributions, respectively. The differential Q and R-th,R-con impacts on boiling flow structures and thermal performances between the TPLTs with smooth and scaled boiling surfaces are illustrated using a set of comparative results collected from these TPLTs. With the scaled boiling surface over evaporator at the filling ratio (FR) of 50%, the Q-driven transition of boiling flow structures still follows the smooth-walled TPLT route; but the increase of nucleation sites and the improved bubble departure from the scaled surface elevate the boiling heat transfer rates. Particularly, the segmentation of Taylor bubbles by the scale imprints considerably suppresses the oscillatory amplitudes of liquid level and temperatures in evaporator; and thereof enhances thermal stabilities to reduce the constituent thermal resistance triggered by boiling instabilities. Acting together by the suppression of boiling instabilities and the improved boiling heat transfer properties, the cooling powers required for the scaled TPLT are noticeably reduced from the smooth-walled TPLT counterparts at the conditions with similar overall thermal resistances. Heat transfer correlations determining the boiling heat transfer coefficients over pool-boiling, intermittent and vapor regions for the scale-roughened evaporator along with the empirical correlations that permit the evaluation of individual and interdependent Q and R-th,R-con effects on the overall thermal resistances and evaporator pressures of the scaled TPLT are generated to assist design activities. (C) 2012 Elsevier Ltd. All rights reserved.