Measurements of the pressure drops for water flowing in small diameter tubes having U-type wavy configuration are presented. The inner diameters of the test copper tubes $D$ are 3.43, 5.07, and 8.29 mm, whereas the curvature ratios $2R/D$ and spacer length $L/D$ span from 3.75 to 7.87 and 1.93 to 7.0, respectively. The test range of the Reynolds number for water is about 200<Re<18000. The measured pressure loss in U-type wavy tube includes the loss in U-bends and the loss caused by the distorted flow in the downstream straight tube. Thus, an equivalent friction factor, $fB,$ is then defined. For both laminar and turbulent flow the bend friction factor increases with the decrease of dimensionless curvature ratio and spacer length. The test results indicate that the recent reported correlations by Popiel and Wojkowiak (2000) and Wojkowiak (2000) do not accurately predict the data. A simple friction factor equation is developed based on the experimental data with characterizing parameters like curvature ratio and spacer length, new Dean number, and the Reynolds number. A good agreement with a mean standard deviation of 5.6% is observed between the proposed correlation and the existing data, which includes the test results of this study and those from Popiel and Wojtkowiak (2000).

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