In the present study, the pressure drop of the nanofluid flow of carbon-water nanotubes (CNT/water) in a helical three-tube heat exchanger with constant fluid physical properties has been experimentally evaluated. For this purpose, first, the experimental device was designed and manufactured and then the carbon-water nanotube nanofluid with volume percentages of 0.01%, 0.1%, and 0.5% was prepared and stabilized. For the experiment, two triple-tube helical heat exchangers with different geometries are considered, in which the diameter of the middle pipe varies in two geometries. The pitch of the helical coil is 100mm and the helix radius is 9.235mm. The experiment was performed on Dean numbers between 1000 and 5000. The measured and calculated data were according to the available correlation in the literature with an error of less than 4%. It is found that at low volumetric percentages of CNT, the pressure drop is almost equal to that of the base fluid, and with increasing volumetric percentage of nanoparticles, the pressure drop also increases. By changing the geometry of the tube (decreasing the middle diameter of the tube), the pressure drop decreases.