Use of Heat Transfer Enhancement Techniques in the Design of Heat Exchangers

Abstract

Heat transfer enhancement refers to application of basic concepts of heat transfer pro-cesses to improve the rate of heat removal or deposition on a surface. In the flow ofa clean fluid through the tube of a heat exchanger, the boundary layer theorem estab-lishes that a laminar sublayer exists where the fluid velocity is minimal. Heat transferthrough this stagnant layer is mainly dominated by thermal conduction, becoming themajor resistance to heat transfer. From an engineering point of view, heat transfer can beenhanced if this stagnant layer is partially removed or eliminated. In single-phase heattransfer processes, three options are available to increase the heat transfer rate. One ofthem is the choice of smaller free flow sectional area for increased fluid velocity bring-ing about a reduction of the thickness of the laminar sublayer. A second option is theengineering of new surfaces which cause increased local turbulence, and the third optionconsists in the use of mechanical inserts that promote local turbulence. The applicationof these alternatives is limited by the pressure drop. This chapter describes the conceptof heat transfer enhancement and the ways it is applied to the development of new heatexchanger technology