Saturated flow boiling of environmentally acceptable nonionic surfactant solutions of Alkyl (8-16) was compared to that of pure water. The concentration of surfactant solutions was in the range of 100-1000 ppm. The liquid flowed in an annular gap of 2.5 and 4.4 mm between two vertical tubes. The heat was transferred from the inner heated tube to two-phase flow in the range of mass flux from 5 to 18 kg/m(2) s and heat flux from 40 to 200 kW/m(2). Boiling curves of water were found to be heat flux and channel gap size dependent but essentially mass flux independent. An addition of surfactant to the water produced a large number of bubbles of small diameter, which, at high heat fluxes, tend to cover the entire heater surface with a vapor blanket. It was found that the heat transfer increased at low values of relative surfactant concentration C/C-0, reaches a maximum close to the value of C/C-0 = 1 (where CO = 300 ppm is the critical micelle concentration) and decreased with further increase in the amount of additive. The dependence of the maximal values of the relative heat transfer enhancement, obtained at the value of relative concentration of C/C-0 = 1, on the boiling number Bo may be presented as single curve for both gap sizes and the whole range of considered concentrations. (c) 2007 Elsevier Ltd. All rights reserved.