Chapter 7 – Thermal Design

This chapter, on heat exchanger design, builds on the side design and comparison arguments of Chapter 4 and the performance results of Chapter 6. The basic task dealt with in this chapter is to arrive at the most suitable combination of surfaces and sizes to meet the thermohydraulic specification. The main methods of heat exchanger design and analysis are those of the logarithmic mean temperature difference (LMTD) method and the effectiveness (ɛ) number of transfer units (e-NTU) method, with the latter being used for detailed application here. The calculation of ɛ for the exchanger, together with that of the capacity rate ratio C⁎ enables the estimation of the NTU, according to the flow arrangement chosen (counterflow, single or multipass crossflow). This, in turn, gives estimates of the side areas. Corresponding throughflow areas follow from the choice of surface and hydraulic diameter together with the pressure drop requirement, which determines the operating Reynolds number. The design is completed by an analysis and adjustment of geometric parameters, with corrections for surface effectiveness, longitudinal conduction, entry, exit and header losses, and fouling, as necessary. Design for boiling and condensation on one side is also described, and an outline of approach for reactor design is given, together with a summary of principles of pressure containment. An introduction to using computational fluid dynamics in various elements of compact heat exchanger design and optimisation is also given.

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