Compact Heat Exchangers

The process of heat exchange between two fluids that are at different temperatures and separated by a solid wall occurs in many engineering applications. The device used to implement this exchange is termed a heat exchanger. Decreasing size and increasing heat load is the typical feature of the modern-day heat exchanger industry. In forced-convection heat transfer between a gas and a liquid, the heat transfer coefficient of the gas may be 10 to 50 times smaller than that of the liquid. The use of specially configured surfaces can reduce the gas-side thermal resistance. For heat transfer between two gases, the difficulty in inducing the desired heat exchange is even more pronounced. In this case especially, the use of enhanced surfaces can substantially reduce heat exchanger size. This is the motivation behind the design of a category of heat exchangers with reduced size and greatly enhanced gas-side heat transfer, which are referred to as “compact.” A compact heat exchanger is generally defined as one which incorporates a heat transfer surface having a high “area density.” In other words, it possesses a high ratio of heat transfer surface area to volume. Quantitatively, Shah arbitrarily defines a compact heat exchange surface as one that has an area density greater than 700 m2/m3. Figure 2.1 shows a spectrum of surface area density for heat exchangers. The range of surface area density (and hydraulic diameter) is given for various types of heat exchange surfaces, with the dividing line for compactness clearly marked.