A. Background and objectives

The double pipe heat exchanger in its classical form is the simplest apparatus for exchanging heat between two fluids, consisting of a "tube inside a tube", with suitable connections for both fluids, as shown in Figure 1. such designs are still used occasionally for very small flow rates, very high pressures, special metals etc., but are virtually limited to laboratory-type apparatus. In the 1930's the sectional "hair-pin" design was introduced and the advantages of longitudinal finned tubes were realized, increasing the inherently low heat transfer coefficient of viscous fluids and gases by a factor between about 5 and 20.

While the performance of plain double pipe heat exchangers can be predicted by correlations for flow in the tube and in the annul us, this is not the case for longitudinal fins, which act like parallel plates and exhibit a wide transition region between laminar and turbulent flow. In the 1940's, data on longitudinal finned tubes were in proprietary domain of US manufacturers De Lorenzo (1945), Gunter and Shaw (1942). A predictive method developed from the Braun Fin Co. data (as of 1940) was published in graphical form by De Lorenzo and Anderson (1945), and still represents the only data-based source in the open literature.

The De Lorenzo and Anderson method was made popular by being included in the D. Q. Kern book (1951) and in many other later articles and Handbooks. A review article by Purohit (1983), suggests the use of the Hausen tube-side laminar flow method, but does not deal with the all important transition region, especially for the "cut-and-twist" longitudinal fins. In these revised HEDH Sections on double pipe and multitube heat exchangers, these shortcomings of the published literature are addressed. The author wishes to acknowledge the value in his work of some unpublished notes by Gardner (1980).