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Formable duplex stainless steels for plate heat exchangers
This article elaborates on the opportunity of using less ductile duplex stainless steel alloys as a cost-effective alternative to high-alloy austenitic grades in welded plate heat exchangers . This is because the highest possible corrosion resistance is often required to meet the severe conditions encountered in the refining , chemical , and petrochemical industries .
By Claes Tigerstrand and Jan Li , Outokumpu
Shell-and-tube heat exchangers are typically selected for air coolers in the refining and petrochemical industries . However , plate heat exchangers ( PHE ) can provide more efficient heat transfer , result in a more compact design , and are less prone to fouling . The challenge of designing a PHE exists because of the complex shape of the plates , which require that the material from which they are made has a high formability both in its use as well as during its fabrication processes . The sealing properties of the stack of plates are critical to ensure optimal performance . The operating pressure cannot reach the same level as for a shell-and-tube tube heat exchanger . A consequence of the stacked-plate design is that it is also more vulnerable to crevice corrosion when the environment becomes more severe . Moreover , a PHE is per definition , in most cases , characterized as a piece of pressure equipment ( due to having internal pressures over 0.5 bar ), which implies that a high mechanical strength is an important material property to consider , ensuring the best possible structural performance . Thus , the critical design criteria and associated material properties typically requested for a PHE are listed below :
• High pressure – High mechanical strength
• Complex shape – High formability
• Corrosive media – High corrosion resistance
• Efficient heat transfer – High thermal conductivity
What about using duplex stainless steels for PHE ? The most commonly used materials for PHE are – not surprisingly – the highly ductile and corrosion-resistant austenitic stainless steel alloys , e . g ., 304L , 316L , and higher alloyed versions such as super-austenitic 6 % Mo grades for harsher environments . Other materials such as nickel and titanium alloys are also alternatives able to address extremely corrosive media , as well as copper alloys which provide very good heat transfer properties . Thus , if a corrosion resistant alloy , such as a stainless steel , is specified to resist corrosion in a severe environment , one start point could be : is duplex stainless steel a potential alternative for PHE because of its higher strength ? The acceptable design stresses given for different duplex alloys , in accordance with the applicable design standards for pressure equipment ( e . g ., ASME VIII , EN 13445 ), are up to two times higher than those provided by austenitic grades with no nitrogen addition . If this strength advantage can be exploited , then thinner gauges can be realized , or will allow higher working pressures depending on the prevailing design condition for the heat exchanger in question . The use of thinner plates in the heat exchanger also improves the heat transfer capacity . Additionally , duplex alloys provide somewhat higher heat conductivity than austenitic alloys with the same plate thickness . To further evaluate the potential of using a duplex alloy in PHE , the key parameter to focus on is its forming properties . Duplex stainless steels provide a
Fig . 1 . Comparison of mechanical strength vs . corrosion resistance .
Fig . 2 . Comparison of mechanical strength vs . ductility .
Heat Exchanger World November 2022 31