This study presents single-phase and two-phase frictional data for R-410A in four U-type return bends with tube diameter D = 3.3 and 5.07 mm and curvature ratio (2R/D) ranged from 3.91 to 8.15, and their frictional performances are investigated. The friction factor fc and two-phase pressure gradient in the return bend considerably increase with the decrease of curvature ratio. For D = 5.07 turn and G greater than or equal to 200 kg M-2 S-1, the ratio of (dp/dz)(c)/(dp/dz)(s) is relatively independent of the vapor quality x. However, a considerable rise of this ratio is encountered at x < 0.5 and G = 100 kg m(-2) s(-1). The significant increase of this ratio may be attributed to the change of flow pattern from stratified flow to annular flow. In contrast, for D = 3.3 rum and G greater than or equal to 300 kg m(-2) s(-1), the higher pressure gradient ratio at lower x was not observed. The flow pattern at low vapor quality is slug flow and no dramatic change of flow pattern across the return bend is seen. For the single-phase results, existing correlations give fair agreements with the present fc data. For two-phase results, the Geary correlation shows a better agreement with the data. A modified two-phase friction factor based on the Geary correlation is then proposed. The proposed correlation gives a good agreement to the present R-410A data and Geary's R-22 data with a mean deviation of 19.1%. (C) 2003 Elsevier Ltd. All rights reserved.