A. Introduction

This Section considers techniques for augmenting single-phase heat transfer in heat exchangers and miscellaneous heat transfer equipment. The previous version of this section was issued as part of the first Edition of HEDH in 1983. Though much of the theoretical basis of the topic remains unchanged since the previous version, there have been extensive further studies of heat transfer enhancement reflecting the growth of interest in two areas which rely essentially upon enhancement methods. These are process intensification and microfluidics. Some aspects of both of these are considered below, as they might be applied to heat exchanger enhancement. Another difference compared to earlier documentation of enhancement is the introduction of nano-particles into fluids — and some claim that this enhances convective heat transfer in liquid flows.

The scope covers single phase heat transfer in gases and liquids. It also includes one aspect that some may consider broaches the boundary with two-phase heat transfer, but is one in which it is believed that enhancement in both phases occurs and is critical to successful plant operation — namely enhancement of heat transfer in solid-liquid phase-change media. If we a generous in our definitions, it might be included in the same category as a fluidised bed, which was considered in earlier Editions.

Heat transfer augmentation, synonymous with "enhancement" (which will be the term principally used here) and "intensification" — now, when linked with processes, becoming a major player in the search for more acceptable industrial unit operations, means an increase in the heat transfer coefficient. There are many ways of doing this, covering both single- and two-phase heat transfer and impinging in many instances on mass transfer — most of course involve moving mass, if not changing the phase of the mass! The main types of enhancement are briefly described below.

B. Classification of enhancement techniques