This study experimentally examines the hydrothermal performance of horizontal Double Pipe Heat Exchangers (DPHEs) with and without continuous Helical Tape Insert (HTI) conducted on the outer surface of the internal pipe. Ten DPHEs of counter-flow configurations were constructed; nine of them were with HTI fabricated with different ratios of HTI height to the clearance between the two pipes (δ), and different ratios of HTI pitch to HTI diameter (). The experiments were performed with pure water in both sides with 2050 ≤ Rean ≤ 15925, and Ret ≈ 26700. Compared with plain annulus-case, the results showed that using the HTI increases both annulus average Nusselt number (Nuan) and Fanning friction factor ( fan), with average increases of 69.4–164.4% and 48.6–113.1%, respectively, when δ increases from 0.275 to 1, and with average increases of 78.1–183.2% and 67.6–99.2%, respectively, when decreases from 1 to 0.333. The hydrothermal performance index (HTPI) was determined to compare the performance of DPHEs with HTI to that of plain annulus case. The results demonstrated that HTI of δ = 0.667 provides the highest HTPI, while HTPI increases with decreasing λ. Finally, correlations for Nuan, fan, and HTPI for DPHEs with HTI in the annulus as a function of the investigated parameters were proposed.