Heat transfer enhancement using the electrohydrodynamic (EHD) technique during evaporation inside horizontal smooth and micro-fin tubes is experimentally investigated. The test section is a counter-flow concentric tube-in-tube heat exchanger with R-134a flowing inside inner tube and heating water flowing in the annulus. Smooth and micro-fin inner tubes are used as test section. The outer diameter and length of inner tubes are 9.52 mm and 2.50 m, respectively. The outer tube is a smooth copper tube having inner diameter of 27 mm. The electrode is made from stainless steel rod of 1.47 mm diameter. Experiments are performed at average saturation temperatures ranging between 10 and 20 degrees C, heat fluxes ranging between 10 and 20 kW/m(2) at mass flux of 400 kg/m(2) s and applied voltage of 2.5 kV. The maximum heat transfer enhancement ratios are 1.25 and 1.15 for smooth tube and micro-fin tube, respectively. New correlations based on the data gathered during this work for predicting the evaporation heat transfer coefficient in smooth tube and micro-fin tube are proposed for practical applications. (C) 2009 Elsevier Ltd. All rights reserved.