Performance restoration technique developed for fouled heat exchanger

Heat exchanger (HE) is one of the important equipments for satisfactory operation of installations like power plants, chemical plants, particle accelerators etc. The performance of HE depends on the material of construction (MOC) as well as good engineering practice adopted, and performance deterioration takes place due to surface deposition, making it a thermal insulator. In Indus Electron Synchrotron Accelerator, RRCAT, Plate Heat Exchangers (PHEs, heat exchangers having corrugated plates) are installed to dissipate heat from primary process coolant (deionised water) to secondary coolant (soft water) through parallel SS 316 corrugated plates. For achieving precise electron beam stability, the process cooling water temperature is maintained within ±1°C. Deposition of scale takes place in secondary coolant side as Saturation Index (SI) of cooling tower water is maintained at + 0.5 to have mild scale of calcium carbonate on pipeline and other wetted parts of the loop to prevent corrosion. This forms scale in HE and affects the heat transfer coefficient, requiring routine cleaning to remove scale of PHE to have designed performance. A hard and sticky scale was formed in PHE and the problem could not be addressed by standard reconditioning techniques available. Samples were systematically analysed in our laboratory to know the content of the deposit so that suitable method could be applied to remove the foulants to clean the HE. About 48.52 % of deposit was found to be acid soluble, whereas approximately 44.14% of deposit dissolves in alkali. The remaining residue (7.43%), neither dissolved in acid nor in alkali, may be mostly dust. The cleaning solution was formulated in-house to remove the scale from heat exchanger plates. Sulfamic acid solution at 80 °C was used to decompose calcium scale to liberate carbon dioxide, whereas sodium hydroxide solution was used to remove remaining scale. The performance of the heat exchangers was restored. The developed formulation is believed to be most effective for all heat exchangers used in water application. (author)