Thermal Stresses in U-Bends

Abstract

The tubular heat exchanger, being a device to transfer heat between two fluids at different temperatures, is a natural candidate for thermal expansion related problems. Indeed thermal stresses merit careful analysis in the design of all kinds of heat transfer apparatus. In shell-and-tube exchangers, the thermal stress problem usually stems from the differential expansion between the shell and the tubes. The tubes are in (turbulent) contact with both fluids undergoing the heat transfer process, whereas the shell is in contact with the shell-side fluid only. The resultant differential expansion between the shell and the tubes, sometimes abetted by the difference in the thermal expansion coefficients of the tube and the shell materials, is a major source of structural problems in fixed tubesheet heat exchangers. Chapter 9 deals with the methods to predict the stress field in fixed tubesheet exchangers. As stated in Chapter 1, this drawback of the fixed tubesheet design is eliminated by resorting to U-tube construction, wherein the longitudinal expansion of the shell is physically isolated from that of the tubes. This inherent feature of U-tube design has fostered the mistaken belief that U-tube designs are immune from axial thermal expansion problems. While the relative differential expansion between the shell and the tube bundle is certainly eliminated as a potential design problem in the U-tube construction, the thermal stress problem now manifests itself in the tube bundle. The straight tube segments in different tube passes are, in general, subject to different flow patterns and to different fluid temperatures. As a consequence, the two constituent legs of a U-tube expand or contract by unequal amounts. Since the U-tubes are clamped (“built-in”) at the tubesheet for all intents and purposes, the semicircular bends at the rear end of the bundle must absorb most of the differential expansion between the adjoining legs. Often, the attendant stresses are quite high.