Heat transfer characteristics and performances of a multi tubes in tube helically coiled ( MTTHC ) heat exchanger are numerically investigated. The effects of operating and geometrical parameters on the thermal and hydro- dynamic performances of the coil are studied and presented for turbulent flow regimes. The current simulation model is compared and validated with available experimental results and a good agreement is obtained. The nu- merical results showed that, the overall heat transfer coefficient ( U o ) enhanced by 23% and 28% with increasing Reynolds numbers of hot and cold water ( Re hw and Re cw ) from 9000 to17000 and 14,000 to 22,000, respectively. Coil effectiveness (