In the present study, heat transfer enhancement with the use of water based PANI (polyaniline) nanofluid was investigated in vertical helically coiled tube heat exchanger. Initially, PANI nanofibers were prepared with the use of ultrasound assisted emulsion polymerization method. Then the prepared PANI nanofibers were dispersed in base fluid in varying concentration (0.1–0.5 vol%) in order to get uniformly dispersed PANI nanofluid in the presence of sonication. The effect of PANI nanofibers concentration in nanofluid and Reynolds number on heat transfer coefficient have been investigated in helical coiled heat exchanger. It was found that the average heat transfer coefficient increases with an increase in the volume% of PANI nanofibers in nanofluid and Reynolds number. The heat transfer coefficient of base fluid i.e. distilled water was found to be 304 W/m2 C (at xi/D = 692.3) whereas it was found to be increased to 515.8 W/m2 C (at xi/D = 692.3) for 0.5 vol% PANI concentration in nanofluid. The percentage enhancement in the heat transfer coefficient was found to be 10.52% at 0.1 vol% of PANI nanofibers in nanofluid and was found to be increased to 69.62% for 0.5 vol% of PANI nanofibers.