Thermal Design, 2nd Edition

4Compact Heat Exchangers

4.1 INTRODUCTION

A heat exchanger is a device to transfer thermal energy between two or more fluids, one comparatively hot and the other comparatively cold. Heat exchangers are typically classified according to flow arrangement. When the hot and cold fluids move in the same direction, it is called parallel‐flow arrangement. When they are in opposite direction as shown in Figure 4.1a, it is called counterflow arrangement. There is also crossflow arrangement, where the two fluids move in crossflow perpendicular each other. Double‐pipe heat exchangers, shell‐and‐tube heat exchangers, and plate heat exchangers (PHE) may have either parallel flow or counterflow arrangement. Finned‐tube heat exchangers and plate‐fin heat exchangers typically have a crossflow arrangement. These heat exchangers are depicted in Figure 4.1a–e.

A special and important class of heat exchangers is used to achieve a very large heat transfer area per volume. Termed compact heat exchangers, these devices have dense arrays of finned tubes or plates and are typically used when at least one of the fluids is a gas, and is hence characterized by a small convection coefficient. PHE, finned‐tube heat exchangers, and plate‐fin heat exchangers are in the class of compact heat exchangers.

The surface area density β (m²/m³), which is defined as the ratio of the heat transfer area to the volume of the heat exchanger, is often used to describe the compactness of heat exchangers. The compactness ...