This paper provides long-term cooling tower water fouling data in seven 15.54 mm I.D. copper. helically ribbed tubes taken at low water velocity (1.07 m/s). The ranges of geometric parameters were number of rib starts (18-45), helix angle (25-45degrees), and height (0.33-0.55 mm). These geometries provide a new class of internal enhancement that is typical of commercially enhanced tubes presently used in water chillers. There are two ranges of fouling characteristics based on internal dimensions: linear range and non-linear range. The ratio of the enhanced-to-plain tube fouling resistance linearly increases with increasing the product of area indexes and efficiency indexes in linear range and the linear relationship fails in non-linear range. A series of semi-theoretical fouling correlations as a function of the product of area indexes and efficiency indexes were developed. They were applicable to different internally ribbed geometries within the above dimensional range. A series of linear multiple regression correlations as a function of geometric variables and Reynold numbers were also developed. The average deviation of the two series of correlations was 4.4%, and 4.8%, respectively. The correlations can be directly used to assess the fouling potential of enhanced tubes in actual cooling water situations.