| Recent years, fixed tubesheet heat exchanger with a central pipe (whosediameter is greater than other heat exchanger tubes) is often used for therequest of technology. But the relevant design method is not provided by theactual standards. In this paper, a mechanical model of this kind heatexchanger is developed. The analytical method and the correspondingformulas are presented. Three of this type of heat exchangers with differentparameters are analyzed. The results indicate that the central pipe onlyinfluence the stresses fields within a small area near itself.Based on the theory of calculating the axial tube stress of heat exchanger,an amendatory method to calculate the axial tube stress is presented. Theresults of some examples obtained by the present method are compared withthat obtained by the standard GB151-99 method. The comparison shows thatthe new method is more accurate.Finally, the tube buckling is analyzed as a lone tube by the traditionalmethod and as an elastic system by FEM. Two different mechanical models, inwhich the tubesheet is attached with or not with an unperforated rim, areanalyzed by traditional method. Four types of the fixed heat exchangers arecalculated. The results obtained by different methods are compared as follows:1) the critical buckling pressures obtained from two different mechanicalmodels both are calculated by traditional method;2) the critical bucklingpressures obtained from elastic system by FEM and from a lone buckling tubeby the traditional method. The results show that the compression stress will beoverrated if the unperforated rim is ignored by the traditional method and thecritical buckling load of the elastic system is higher than the one of the singletube. The design rule in the standard GB151-99, which defines the safetycoefficient of 2, is too conservative because it doesn't consider thecontribution of the unperforated rim. Therefore, a more economical safetycoefficient of 1.5 is suggested by this thesis.
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