Two-phase forced-convective fouling under steam generator operating conditions

Two-phase forced-convective fouling can occur in adiabatic two-phase flow and in diabatic two-phase flow, where it can be a significant contributor to fouling under flow-boiling conditions. For recirculating steam generators (SGs), it is, therefore, of significance to steam separators, tube support plates, tubesheet and the tube bundle. Loop test data are presented on forced-convective fouling rate of iron corrosion products under a range of conditions relevant to the secondary-side of recirculating SGs. The measurements were performed using a number of corrosion products (magnetite, hematite and lepidocrocite) under a range of water chemistry conditions, with several different amines. The measurements were limited to the straight-tube geometry. Comparable fouling data are given for flow-boiling conditions. A SG artefact was examined to corroborate the loop data. The rate constants for the forced-convective fouling measurements are compared with those for flow-boiling fouling. Their relative magnitudes can vary greatly, depending on the chemistry and thermohydraulic conditions. Boiling fouling dominated over forced-convection fouling for hematite and lepidocrocite particles, likely because of particle-bubble interactions. Forced-convective fouling rate was only slightly lower than boiling fouling for magnetite. For the region of cross-flow (upper tube bundle), deposits show significant thickness variation. Four or five deposit thickness peaks are noted, approximately equally spaced circumferentially. It is hypothesized that the fouling pattern is developed due to the cross-flow pattern present in the tube bundle. The possible interactions between the force-convective and nucleate-boiling fouling streams are briefly discussed. A method is presented for the superposition of the forced-convective and nucleate boiling fouling components. This method is based on the Chen heat transfer correlation. (author)