Evaluation and modification of air-cooled heat exchanger header box design procedure

[摘要] ENGLISH ABSTRACT: The purpose of the heat exchanger header box is to act as a high pressure manifoldthat redistributes process fluid from a central source to a bundle of finnedtubeswhere it can be cooled. South African law requires that pressure vessels likethis one must to be designed according to a pressure vessel design code, such asthe American Society of Mechanical Engineers' (ASME) pressure vessel designcode.The two most commonly used header box designs are a plug type and a covertype header box. For the design of a plug type header box, ASME Section VIIIDivision 1 provides a full complement of rules and formulae necessary for producinga satisfactory pressure vessel design. Such a set of rules and formulae arehowever not available for cover type header boxes.To overcome this problem, industrial manufactures have developed in-housedesign codes focussing specifically on the design of cover type header boxes.These in-house codes draw in part on existing formulae in the ASME code, butrely primarily on calculating the strength of a header box using a simplified modelconsisting of simply supported beams, each simulating a different part of theheader box.Recently there has been some concern regarding the accuracy and validity ofthese in-house design codes. This project has sought to address some of theseconcerns by evaluating one such in-house design code and comparing its resultswith those from finite element analyses.For the purpose of this comparison it is shown that the header box's structurecan be simplified to a representative 2D finite element model while still yieldingtrustworthy results. The results from the comparisons with these 2D modelsshowed that the in-house methods produced non-conservative estimationsfor certain stresses in a header box. However, after analysing multiple sampledesigns, it was also found that header boxes that had been designed using thismethod were all substantially over-designed.Two major contributing factors that lead to this over design were identified.The first was thatmaterial and manufacturing constraints frequently necessitatethat material thicknesses be increased. The second factor was that because ofthe concern linked to the validity of these in-house design methods, designersare inclined to further increase the material thicknesses to account for any uncertainty.To address this problem, a new 2D finite element software package was developed,on an open source platform, to accurately analyse the structure of covertype header boxes. The software is designed to integrate directly with the platformcurrentlybeing used to performthe calculations for the in-house code andincludes an optimiser that takes the material and manufacturing constraints intoaccount. The software can be used to validate any existing designs, as well asproviding optimal designs and accurate stress predictions for new header boxes,thus reducing uncertainty in the design process.

[发布机构] Stellenbosch University