Heat tranfer coefficient calculations (outside and inside areas)

Q: I work with concentric heat exchangers, specifically a double-pipe unit with hot water flow in the inner tube and the cold water flow in the annulus. I calculate the flow rates and temperatures for the hot and the cold water. There is a plastic element inside the inner tube to enhance the heat transfer and I want to compare the heat transfer in the heat exchanger both with and without the element. What is the equation to calculate the overall heat transfer coefficient and what is different between based on outside area or inside area? How can I calculate heat transfer coefficient inside the tube and also outside the tube (in the annulus)?

A: 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.