Richard Law

Compact Heat Exchangers in Practice

This chapter discusses a number of factors ultimately affecting the long-term satisfactory operation of compact heat exchangers. Following the selection of the type of heat exchanger, guidance is given to those involved in the operation and maintenance of the unit. Fouling and, to a lesser extent, corrosion, and their minimisation, remain key priorities during the life of many heat exchangers, particularly those in arduous process industry duties. Specific types of fouling are described and design approaches to reduce fouling are presented. As process plants become more compact—process intensification—the need to handle solids becomes more demanding, and new efforts to address this are briefly presented.

Industrial Compact Exchangers

Abstract In this Chapter, the basic physical features and construction of the principal industrial compact heat exchanger types are described. The definition of ‘compact’ in this respect is consciously chosen as a wide one, implying surface area densities upwards of about 200xa0m2/m3, representing hydraulic diameters lower than about 14xa0mm. Several new developments are described in the later descriptive sections. A distinction is made between heat exchangers and heat exchanger reactors, the latter being the subject of rapidly increasing industrial interest. An area that will have a major impact on heat exchanger design and manufacture is 3D printing (additive manufacturing), and examples are given in this Chapter.

Surface Types and Correlations

This chapter presents up-to-date theoretical results and correlations for the most important surfaces used in compact heat exchangers. Full-developed Nusselt numbers and friction factors are given for many continuous duct cross-sections, followed by a summary of entrance effects (developing flow), which give rise to increased Nusselt numbers and friction factors. Turbulent and transitional flow data for ducts are then given. Plate-fin surfaces, in a variety of forms, are by far the commonest of all compact types, being used for applications from oil refining to air conditioning; and some of the many semiempirical correlations are assessed. In particular, the offset strip fin (OSF) data and correlations are given critical attention, based on the theoretical results developed in previous. After a summary of the performance of perforated fins, a more in-depth assessment of louvered and offset convex louvered fins is presented. Pressed-plate surface performance correlations are then given, followed by considerations of the performance of printed circuit channels and microchannels. The chapter ends with a brief discussion of porous and sintered surfaces for which, it is argued that no consistent methodology is yet in place for performance assessment.

Surface Comparisons, Size, Shape and Weight Relationships

The purpose of this chapter is to build on and amplify the elements of compactness outlined in the Introduction (chapter 1) and to explore its implications for the size and shape of compact exchangers. For grasping the physical implications of compactness on size and shape, it is only necessary to consider one side. This can be justified on the grounds that there is usually one stream which is critical from, for example, the pressure drop requirement. The design requirement is specified by Number of Thermal Units (N), pressure drop and mass flow rate for the side considered. The latter two parameters also provide an equivalent specification of pumping power. The consequences for cross- sectional area, volume and weight are examined for different surfaces, including type and scale (represented by hydraulic diameter), and are developed in terms of comparison ratios for two surfaces. The analysis is given for two regimes, firstly the conventional regime based on the core velocity equation, for which turbulent or high transitional Reynolds numbers apply 1 , and secondly for fully- developed laminar flow. Both approaches are developed for pure counterflow, which does not invalidate the general significance of the results for single or multi-pass cross- flow operation. Comparisons of some typical surfaces are then made, and indications are given for criteria for selection. Optimisation approaches are briefly discussed. More complete thermal design and analysis procedures are given in Chapter 6, whilst detailed performance correlations for different surfaces are discussed in Chapter 5.

Aspects of Flow and Convective Heat Transfer Fundamentals for Compact Surfaces

Chapter 5 introduces a new approach for books on heat exchangers, in the form of a detailed development of the boundary layer equations in laminar flow. It is argued that an understanding of the fundamental processes is essential for both the sensible utilisation and the future development of compact surfaces. The traditional Blasius and approximate power law solutions for velocity and thermal boundary layers are given, with the corresponding Prandtl number dependence for the latter. The resultant relationships for strip surfaces, in terms of length-dependent Nusselt numbers and skin friction, are then compared with the results from both direct numerical simulations (DNS) and experiments (Kays and London data). An interpretation of the latter is given, taking into account the number of ‘rows’ of offset strip fins. Analytical results are also developed for normal flow over wedges and cylinders. Finally, an outline of possible approaches to designed-in, three-dimensional flow surfaces is given.

The Heat Exchanger as Part of a System: Exergetic (Second Law) Analysis

Abstract In this chapter, the basic principles of exergy analysis using the first and second laws of thermodynamics are outlined. The application of these principles to heat exchangers is then developed, first for a zero pressure drop, for which a resolution of the so-called ‘entropy generation rate paradox’ is given by an appropriate choice of a dimensionless entropy generation rate parameter. The effect of the finite pressure drop is then analysed, for which an entropy generation minimum criterion is derived. Its implications for design choices are discussed. The effect of flow imbalance is discussed, including the limiting conditions of evaporation and condensation. Finally, a brief discussion is given on the application of the principles for heat exchanger networks.