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Materials
A heat-exchanger plate with its complex shape really challenges the material performance . The formability would be typically highest when biaxially strained , and lowest in the plane strain condition and therefore complex parts such as a heat-exchanger plate often fail under this condition . The sheet thickness also influences the formability – a thinner gauge depresses the FLC , which brings further challenges when a duplex alloy is opted for , when considering that its higher strength enables plate thicknesses to be reduced . To evaluate if a duplex alloy is feasible or not for use , one approach could be to perform a forming trail using the existing tooling designed for the austenitic stainless steel . A grid pattern is then typically applied to the blank , This makes it possible to measure the strains after forming with a camera and compare them with the FLC for the same alloy and thickness . Let us illustrate the situation using an experiment with 0.6 mm thick DX 2205 . This was carried out as the initial development work for a case study presented below in which the goal was to replace 316L for a welded PHE . A cut piece of the heat-exchanger plate is displayed in Fig . 5 . The result of the strain measurements are depicted in Fig . 6 , where the measured data points indicate the most prone area to cracking is in the bottom radius of the protrusion in the rolling direction of the sheet , where plane strain conditions prevail . The margin of failure is about 5 % strain . However , the most strained area is located at the end point of the ridge where thinning has occurred due to stretch forming conditions . Yet , the margin of failure is about a 15 % -strain . A common rule of thumb is to include a safety margin of about 10 – 15 % strain below the FLC to avoid failure . The insight gained from this test was that it is possible to use the existing tooling for DX 2205 without causing any failure . Nevertheless , to ensure a robust and predictable solution minor changes in the pattern were recommended , such as smoothing out the critical radii to reduce both the critical plain strain conditions and to reduce the degree of thinning . Important to mention here is that a finite element simulation is a very effective tool to use prior to conducting further prototype testing in order to optimize the shape of the corrugation pattern .
Fig . 5 . A cut piece of the heat exchanger plate displays the pressed pattern .
Case study : Plate heat-exchanger solutions using duplex for refining and petrochemical industries Heat-exchanger plates made of duplex stainless steel were developed by the Chinese heat exchanger manufacturer Gansu Lanke Petrochemical Equipment Co ., Ltd . ( hereinafter referred to as LANPEC ) under the lead of Prof . Chunmei Chang . The plate design , which was considered previously impossible to produce , was made using duplex grades . This result came about due to intensive technical cooperation between Outokumpu and LANPEC . Only minor modifications to the pressing die were implemented for the final solution ( as explained in the previous section ). The first solution in 2006 was an all-welded PHE using 0.6 mm thick DX 2205 plates , and replaced underspecified 316L for an overhead condenser of an atmospheric distillation column in an oil refinery . The solution provided a considerably longer service life when compared to that of 316L , which lowered the OPEX ( e . g . shutdown , repair and replacement costs ) for the plant . This experiment was a great success and has been followed by numerous others since then ( another case depicted in Fig . 7 ). In recent years , refining and petrochemical plants have encountered even more corrosive sour crudes , which also reach the limit of DX 2205 for some PHE applications . Therefore , our joint development team has continued to work on the development of more resistant – but
Areas most prone to cracking according to FLC true major strain
0.22 0.19 0.17 0.15 0.12 0.10 0.08 0.06 0.03 0.01 -0.01
Source : Outokumpu internal R & D report . Fig . 6 . FLC compared with strain measurements of the heat exchanger plate .
Most strained areas = thinning !
Phi 1
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Measured strains in heat exchanger plate plotted in the FLD
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Heat Exchanger World November 2022 33