This paper is the first of two papers that present the result of a study of turbulent flow and pressure drop in a horizontal tube with strip type inserts. Experimental data taken for air for a class of strip type inserts (longitudinal, LS and cross, CS inserts) used as a tube side heat transfer augmentative device for a single-phase cooling mode operation are presented. To broaden the understanding of the underlying physical phenomena responsible for the heat transfer enhancement, flow mechanisms through velocity measurements are combined with pressure drop measurements to develop friction factor correlations for 6500less than or equal toReless than or equal to19500 where Re is the Reynolds number. Friction factor increases were typically between 1.1 and 1.5 from low Re(congruent to6500) to high Re(congruent to19 500) with respect to bare tubes.