A hydrodynamic study of a new graphite evaporator has been performed. In this exchanger the fluid flows as a thin film on a spirally wound fin which forms a spiral staircase inside a tube. The slope theta(m) of the spirally wound Rn is modifiable. The residence time distribution (RTD) of the film was measured for different fin slopes theta(m) and several film flowrates m(f). The Row pattern was then modelled thanks to the DTS software as a cascade of perfect mixed cells and the mean residence time tau(m) was calculated. The evolution of tau(m) as a function of theta(m) and m(f) is accurately represented by a power law relation.