An experimental study was made in the first of two papers to determine the effect of liquid sprays used to cool a hot surface. Both pure water and R-134a were served as a working medium sprayed from a single circular nozzle onto a Cu (oxygen free) metal of an electrically heated surface which was heated to an initial temperature with a range of wall superheat for steady-state nucleate boiling experiments using thermocouples for heat transfer measurements. Cooling characteristics (boiling curves) were obtained over a range of spray mass flux, Weber number, wall superheat and degree of subcooling. Boiling visualization was also conducted with varied heat flux levels at a specified We for R-134a and water. (C) 2004 Elsevier Ltd. All rights reserved.