In the present study, experiments were conducted for the first time ill tube ill tube helical heat exchanger at the pilot plant scale to investigate the fluid flow and heat transfer under turbulent flow conditions. The experiments were carried out with hot compressed air in the inner tube and cooling water in the outer tube in the countercurrent mode of operation. The flow rate of compressed air flowing in the inner tube was varied for Reynolds numbers from 14 000 to 86 000. The pressure of compressed air was varied from 10 to 30 kgf/cm(2). The friction factor and Nusselt number values for compressed air flowing ill the inner tube were calculated for elevated pressure and compared with the experimental data reported in the literature at ambient conditions. Most of the works reported in the literature oil heat transfer in coiled tubes have been carried out considering wall boundary conditions of either constant heat flux or constant wall temperature. In the present work, heat transfer calculations were based oil physically realistic condition of fluid to fluid heat transfer. The inner tube Nusselt number of compressed air in the present study was found to be slightly higher than the data reported in the literature for ambient conditions while the friction factor values are within the range reported in the literature for ambient conditions. The friction factor as well as Nusselt number calculated for cooling water flowing in the outer tube was higher than the existing literature data. Oil the basis of the experimental results, new correlations for friction factor and Nusselt. number in the inner as well as Outer tubes have been developed.