6 Thermal and Mechanical Design of Compact Heat Exchanger Available to Purchase

This chapter gives detailed information about compact heat exchanger design, and in particular how the size and shape of the exchanger are affected by the thermal and allowable pressure drop requirements. Most compact heat exchangers are formed of layers of plates or finned channels of fixed length and width. The surface shape of the plate or its fin determines the surface performance, which is described by non-dimensional heat transfer coefficient and friction factor relationships. Thus a feature of design is to determine a channel size and then find the number of layers to meet the duty. For the specialized cases of plate and frame exchangers and other welded plate products, the plate sizes are already specified in ranges by the manufacturers. The design problem is then confined to selecting the most appropriate size and the number of plates. This selection process is readily achieved by adaptation of the procedure given here. In general, it is assumed for the sake of brevity that the fluid properties are constant throughout the exchanger, and are evaluated for each stream at an appropriate mean temperature. In the case of the numerical method designs described in Chapter 4, local fluid properties can be applied for a more accurate design process. The main methods of heat exchanger design and analysis are those of the logarithmic mean temperature difference (LMTD) method, the effectiveness-number of transfer units (ϵ-NTU) method and the P-NTU method. The methods can be shown to be mathematically equivalent to each other. In the description here for single-phase flows attention is mainly confined to the ϵ-NTU method as it has distinct advantages in certain aspects of design and analysis, and is also physically easier to understand and interpret. An outline of the LMTD method is also given.