A spinning disc study of fouling of cold heat transfer surfaces by gel formation from model food fat solutions

The formation of immobile gels on heat transfer surfaces ('coring') caused by cooling fat solutions below their cloud point was studied using a novel spinning disc apparatus (SDA). The SDA features a cooled, removable heat transfer surface with well defined heat and mass transfer characteristics. Measurements of heat flux were combined with computational fluid dynamics simulations to yield reliable estimates of the surface temperature and shear stress. Fouling studies were performed with model solutions of 5 wt.% tripalmitin in a paraffin oil operating in the 'cold start' mode, wherein the experiment starts with the surface colder than the steady state, simulating one mode of operating a standard 'cold finger' experiment. Local heat flux measurements allowed the thermal fouling resistance to be monitored: deposit mass coverage and composition were also measured. The cold surface promotes the rapid formation of an initial gel layer, followed by a period of linear fouling, and finally falling rate fouling behaviour. The linear fouling rate was relatively insensitive to temperature and shear rate, while the fouling rate in the falling rate regime was found to depend on the temperature driving force for crystallisation kinetics. The solids fraction within the deposit layer increased over the duration of a 12 h fouling test, indicating rapid ageing. The rheological properties of the deposits were highly sensitive to solids fraction.