A two-part experimental study is conducted on pool boiling heat transfer over enhanced cylindrical microchannel test surfaces with water at atmospheric pressure. The objective of this work is to investigate the heat transfer enhancement and study the effects of geometric parameters on the pool boiling performance of the open microchannel surfaces over circular tubes. The effects of the horizontal and vertical orientation on heat transfer enhancement are also studied. In this part of the study, the results for the circumferential rectangular microchannels are presented. A maximum heat transfer coefficient of 129 kW/m(2) K was achieved with test surface CRM3 in the horizontal orientation at a heat flux of 1095 kW/m(2). The corresponding values for the vertical orientation are 109 kW/m(2) K and 1093 kW/m(2), respectively. The critical heat flux limit was also extended by a factor of 1.6 or more over a plain tube. (C) 2013 Elsevier Ltd. All rights reserved.