A. Introduction

Extended (fumed) surfaces are used widely in heat transfer applications. Heat transfer in such systems is by conduction from the base surface along the fin with subsequent convective heat transfer from the fin into the surrounding fluid. If the material of the fin has a very high thermal conductivity, then its surface will be at nearly uniform temperature and the heat release from the finned surface will be governed only by the heat transport phenomena between the surface and the surrounding fluid (i.e. the convective heat transfer). However, for practical systems, the conduction in the fins is of significance and the fins will be less efficient in transferring heat than would those with very high conductivity. Here we will define a fin efficiency which is the ratio of the heat-transfer rate from a given fin system to that which would occur if the fin had infinite thermal conductivity.

In this present Section, the question of fin efficiency is dealt with on the assumption that the coefficient of heat transfer from the finned surface is known. Methods for the estimation of such coefficients are given in the relevant convective heat transfer sections (for instance Section 170 for heat transfer from finned tube banks in single phase flow).

In what follows, Section B describes the operating principles of extended surfaces and Section C describes examples of the various types of extended surface which have been employed. Sections D, E and F deal with the classical, simplified and weighted fin efficiencies for single phase convection from the fins. Section G deals with fin efficiency under radiation, condensation and boiling conditions and, finally, Section H gives a brief discussion of heat transfer enhancement of the finned surfaces themselves.

B. Operational principles of extended surfaces