The calculation of the inside and outside area on a double pipe heat exchanger is simply the inside and outside surface areas of the inner pipe that carries the tube side fluid(i.e. pi x Do x L and pi x Di x L). If the fluid used is water, it will be very difficult to determine enhancement with plastic inserts unless the flow is near laminar. The normal methods for calculating heat transfer coefficients in the tube and annuli are via equations like Dittus-Boelter. Please refer to Perry’s Chemical Engineering Handbook or Process Heat Transfer by Hewitt, Shires & Bott.