Fouling of the multistage flash (MSF) system condenser tubes used for water desalination reduces the system overall heat transfer coefficient very rapidly. Consequently, hearts the system efficiency and increases the cost of distillate water. In this study, a comparison between enhanced and plain tube performance characteristics is investigated experimentally in the MSF system. The study is performed for aluminium-brass tubes with inside diameters of 19.05, 23, and 29.5 mm. A three different flow velocity of 0.1, 0.1645, and 0.2398 mrs are examined with these three tubes. The study is carried over for two different coolants fresh and brine water. In this paper, the results for fresh water are presented. The results indicated that the overall heat transfer coefficient (Li? varies with both coolant flow velocity and tube diameter. For both smooth and enhanced tubes, the higher the coolant speed or/and pipe diameter the larger the overall heat transfer coefficient. However, the effect of these two parameters is more significant with enhanced tubes. A gain in overall heat transfer coefficient by factor of 2.13 over smooth tubes can be achieved. Results also, showed that for certain tube diameter there is an optimum flow speed for maximum value of U. For a certain flow speed there is an optimum pipe diameter size. The results proved that the heat transfer coefficients of corrugated tubes are superior to plain tubes for most flow speeds and diameters investigated in the present study.