Chemical Processing - March 2008 - (Page 43)

PLANT INSITES < > Original plan Hot out Cold endout Hot Cold Cold in end Cold in out Cold out Hot end in Hot Cold end Hot Hot in Hot out Hot out Cold in Cold in Cold end Cold end Hot in Hot end Hot in out Cold Cold out Hot end Future exchangers on top instead One low temperature exchanger Future exchangers on top instead One high temperature exchanger One low temperature exchanger One high temperature exchanger Reversed piping Future exchangers in middle instead Reversed piping Two high temperature exchangers Future exchangers in middle instead Two high temperature exchangers Better sequencing of exchangers offered significant savings. remaining in service. the immediate response was “Just design the new four pack to have two new shells on top.” however, that means the future exchangers would be inserted either into the middle of the sequence or at each end of the set of four (Figure 1). For heat recovery, putting the two shells in the middle is preferred. this optimizes the heat transfer by maximizing the overall exchanger corrected temperature difference. Additionally, the piping modifications are simple: the pipe segments connecting the exchanger banks are simply unbolted and re-used as-is. however, there’s a downside. Both new exchangers would handle relatively high temperatures. putting one of the future exchangers at the cold end of the sequence reduces the mechanical design and materials severity of its service — saving up to $100,000. however, by rethinking the layout the plant was able to double the savings. switching to a current installation of a 1 × 1 followed by a 1 × 3 assembly allows both future exchangers to use the less expensive specification (Figure 2). Getting this configuration requires moving one of the existing shells from the bottom of the first stack to the www.chemicalprocessing.com Figure 1. Four new exchangers were configured as a 2 × 2 stack, with two future exchangers to be added on top. >> Significant savings Hot end Hot in Hot 720°F in Cold out720°F Cold664°F out 664°F Hot end Coldout Hot end Hot out Cold in Cold in Cold end Hot in Cold out Cold out Hot end Hot end Hot in Hot 464°F out 464°F Cold in Cold in 378°F 378°F Cold end Coldout Hot end Switching to 1 x 3 stack Switching to 1 x 3 and rotate 180°) (Shift bottom exchanger stack (Shift bottom exchanger and rotate 180°) Future exchangers on side Future exchangers on side Two low temperature exchangers Two low temperature exchangers Figure 2. Reconfiguration puts both future exchangers at the cold end of the sequence, markedly lowering their cost. bottom of the second stack and rotating it 180°. to minimize the amount of high-alloy piping the hot end of the exchangers was set as high as possible. even an apparently obvious and easy configuration demands careful thought when expansion is likely. proper pre-planning often can make future work dramatically cheaper and easier. CP By Andrew Sloley, contributing editor ASloley@Putman.net March 2008 • 43 http://www.chemicalprocessing.com

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Chemical Processing - March 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Distillation is Bubbling Feel Secure About Vulnerability Assessments The Door Opens For Membranes Achieve Effective Heat Exchanger Control Epoxy Maker Finds the Right Glue for Its Business Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - March 2008

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