Scaling of Inorganic Minerals from Potable Water in Appliances and Systems

Scale formation has long been a concern in domestic appliances such as electric boilers, steam irons, washing machines, dishwashers, coffee makers and potable water distribution systems in general. Potable water contains a variety of ions prone to precipitate over a range of concentrations depending on water quality and its geographic location. The kinetics and mechanisms of mineral scaling from potable water in the open and closed household systems have received no attention. The present study aims to gain an understanding of the mechanisms and structure of inorganic fouling from potable water in household appliances conditions. Four experimental setups and methodologies were developed to achieve the work goals, namely boiling static setup, flow visualization rig, flow-evaporation scaling cell, and pressurised flow-evaporation scaling cell. The bulk precipitation in closed systems during pool boiling conditions was evaluated. Then, the surface deposition of inorganic minerals from potable water under the convective heat transfer was investigated. Finally, the effects of water evaporation were examined at various surface conditions, key operating conditions, and water ionic species. The effects of solution temperature, surface material, surface roughness, evaporation area, water composition, heating and cooling rates on the bulk precipitation and surface deposition in closed systems were investigated using the boiling static setup. The solution temperature promotes the bulk precipitation and the polymorphic transformation from both calcite and vaterite to aragonite. The results also showed that the faster the cooling and heating rates the lower the scaling rate. However, CaCO3 formed in the cooling period was greater than in the heating period. The inorganic scaling kinetics during the convective heat transfer were studied using a once-through flow visualisation setup through investigating the effects of surface temperature, material, roughness, flow rate and vapour bubbles. The study showed that all parameters, especially vapour bubbles, play a vital role in the scaling kinetics and scale morphology. The influence of surface temperature, pressure, surface roughness and the ionic species on the fouling mechanism and structure was evaluated during the boiling convective heat transfer using the flow-evaporation scaling cell. The water species examined include sodium, chloride, magnesium, sulphate, total organic carbon (TOC) and zinc. The pressure significantly minimises the fouling resistance as well as the amount of the deposits. However, the impact of different ionic species varies from negligible to strong. Understanding the kinetics of surface deposition and bulk precipitation of inorganic minerals from potable water allows for better design and development of household appliances and systems.