Crystallization fouling in domestic appliances and systems

The complex problem of inorganic scale formation on heat transfer surfaces is of substantial importance for industrial and domestic systems. Formation of mineral scale, particularly calcium carbonate, has long been a concern in domestic appliances such as boilers, steamers and washing machines. Here, the formation of solid inorganic deposits was studied on solid surfaces and in the bulk solution as a result of evaporation of natural potable water. Scale precipitation rates were evaluated for metallic samples for various bulk temperatures, surface roughness values and as a function of time. The morphology of crystals from both surface and solution was evaluated. The concentration of calcium ions was evaluated using Atomic Absorption Spectrophotometry (AAS), in order to investigate the kinetics of the precipitation reaction. The microstructure and morphology of deposits on the surface were analysed using Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). The present work has demonstrated that fouling deposition rates increase as the evaporation process proceeds as expected. The nature of the material and the surface roughness both affect surface deposition kinetics. The complex relationship between the saturation ratio (as the bulk solution shows precipitation) and the kinetics of bulk and surface precipitation/deposition is demonstrated. The prevailing crystalline form of calcium carbonate as detected by SEM and XRD is aragonite. However, the polymorphic composition of surface deposition and the bulk precipitate is influenced by temperature.