Chemical Processing - November 2007 - (Page 35)

Chemical treatment. Biocides or oxidizing agents can be used on the tubes to control biological activity. After a biocide treatment, application of biostats, a milder form of chemical control, can control future growth. In recent years, chemical treatment has fallen out of favor due to the toxic nature of the substances used. Coatings can significantly improve heat transfer and overall performance. Retubing. If severe pitting can’t be alleviated, it may be necessary to resort to complete retubing of the heat exchanger, particularly in older equipment that has run for years. Choosing a more appropriate tube material or accelerating maintenance can stave off recurring fouling and corrosive conditions; however, retubing is extremely costly and may have to be done several times to prolong the working life of the apparatus. The bare tube surfaces will inevitably sustain pitting and bacterial build-up as long as they remain uncoated. None of these options is as viable or cost-effective as coatings. What about heat transfer? Decades of service history and studies have proven that coatings can significantly improve heat transfer and overall performance. While the thermal conductivity of the coating alone is less than that of the metallic parent tube, this is offset by several factors. The first is normal design margin. Generally, heat exchangers are designed with a fouling factor of 0.001 or 0.002 ft2 °F/Btu. Adding a coating to the tube ID impacts the thermal duty by only a factor of 0.0006 – 0.0008 ft2 °F/Btu the same at fully dry film thickness. Morewww.chemicalprocessing.com over, the coating can either totally eliminate the subsequent fouling or greatly reduce the accumulation of typical micro- or macro-fouling, thus enabling greater heat transfer. The second major factor is boundary-layer drag reduction. About 70% of the total heat-transfer resistance of an exchanger is a result of boundary layer drag. Tube wall friction due to fouling reduces designed flow and creates an insulating barrier of low velocity fluid. Polymer coatings cut the friction at the tube wall by a factor of 40 compared to bare metal. Less friction decreases the boundary layer drag and substantially opens up the flow profile. Data from two refineries showed flow rate improvements of 80% and 100%, respectively, in coated tubes compared to new “bare” tubes in the same fluid train. This increase in flow and the low surface energy of the coating contributes to the improved overall thermal efficiency of the heat exchanger. Coated tubes have maintained 100% of their heat transfer efficiency over years of service without cleaning cycles. One five-year study that took monthly data on waterand process-side temperature differentials showed that coated heat exchangers remained at optimal thermal duty over that period while bare tubes saw performance drop by more than 50%. An economical proposition The cost of coatings is easily only one-quarter that of retubing. Once the tubes are recoated after their first 10-year period, they remain functional in perpetuity, requiring minimal maintenance for the rest of the life of the heat exchanger. Over 12 years, the savings from coating the tubes in a single large heat exchanger could exceed $8 million. http://www.ballvalve.com http://www.ballvalve.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - November 2007

Chemical Processing - November 2007 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor PAT isn't Standing Pat Pursue the Perfect Plant Eliminate Exchanger Tubing Troubles Portable Conveyor Speeds Paint Production Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - November 2007

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