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Unexpected fouling in condensers on Central Electricity Generating Board power stations operating on the river Trent in the UK in the 1970s prompted an extensive investigation of the phenomenon. Fouling was caused by deposition of calcium phosphate rich scale on the cooling water side. A fouling test rig was specially constructed to study the performance of different chemical treatments. The rig ran two tubes in parallel, one with undosed water, and the latter's data sets from over 100 "control" tests were analyzed to determine the mechanism and rate of fouling. The trends could not be explained in terms of conventional scaling indices. Insight into the mechanism was provided by separating precipitation (loss from solution) and deposition (adhesion of some of the precipitated mass to the heated surface): scaling was found to be caused by the deposition of particulates in the recirculating cooling water, driven by the change in pH as the water went through the evaporative cooling stage. Quantitative expressions for fouling were generated from tests on 1.83 m long tubes and were found to give a reasonable prediction of the fouling behavior observed on a full sized (18.3 m long) single condenser tube on an operating condenser. The results indicated that recirculating water systems need to be approached very differently to once-through systems, and the use of scaling indices for such systems is not recommended. [ABSTRACT FROM AUTHOR]

The effect of fouling in heat-transfer devices (HTDs) is complicated by aging of the fouling deposits. Aging is, like deposition, often sensitive to temperature, so that heat transfer, deposition, and aging are coupled phenomena. Ishiyama et al. ( AIChE J. 2010;56:531-545) presented a distributed model of the aging of deposits formed by chemical reaction fouling and illustrated its effect on thermal and hydraulic performance of a HTD operating in the turbulent flow regime. Two-layer models, simpler than the distributed model, are explored. The deposit is considered to consist of two layers, fresh and aged; this simple picture is shown to be sufficient to interpret thermal and hydraulic aspects of deposit aging when HTDs are operated at constant heat flux (as reflecting laboratory experiments) but not in cases where the constant wall temperature approximation is more realistic. © 2011 American Institute of Chemical Engineers AIChE J, 2011 [ABSTRACT FROM AUTHOR]

Abstract: A mathematical analysis is presented for the unsteady conduction of heat through a growing fouling layer on a planar surface. The aim is to identify when the assumption of a steady state linear temperature profile in the layer, commonly employed both industrially and academically to obtain an estimate of the fouling layer thickness, is not valid. Deviations from the linear profile arise from the volumetric heat capacity of the fouling layer being finite and non-zero, causing thermal inertia to play an important role in this system. The magnitude of the deviations depends upon the numerical values of the thermal diffusivity, the growth rate of the fouling layer, the heat transfer coefficient and the thickness of the fouling layer. A series of criteria for when large errors in the estimated fouling layer thickness (greater than 10%) can arise are presented, for combinations of boundary conditions where the analysis can be applied. A case study of a typical gas-side particulate fouling system shows that estimating the thickness using the linear profile assumption will lead to a 10% error in the estimated fouling layer thickness. [Copyright &y& Elsevier]

Abstract: 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 5wt.% 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 12h fouling test, indicating rapid ageing. The rheological properties of the deposits were highly sensitive to solids fraction. [Copyright &y& Elsevier]

Abstract: Complete elimination of fouling in heat transfer equipment is rarely achieved in practice, so cleaning of fouled units is a regular task in the process industries. Algorithms for scheduling cleaning have, to date, minimized the net losses due to fouling by focusing on when and which units to clean. In contrast, this paper focuses on when and how to clean a unit, when more than one cleaning method is available. The model formulation is approached by idealizing a foulant deposit as consisting of two layers, soft (fresh) and hard (aged). The hard material is formed through the ageing of the soft material. Hard deposits are more difficult to remove and require time- and cost-intensive cleaning methods (such as mechanical cleaning). Soft deposits are removed through less time- and cost-intensive methods such as chemical cleaning by recirculation of solvents. The hard deposit usually consists of more thermally conductive material and hence, for a given thickness, has a lower thermal resistance compared to the soft deposit. This work introduces a new methodology to identify optimum cleaning cycles (OCCs) under the presence of both soft and hard deposit, when two cleaning methods (solvent and mechanical cleaning) are available. The analysis of OCCs is extended and a new concept called the ‘cleaning supercycle’ is elaborated, which can be related to the optimal time between plant shutdowns. [ABSTRACT FROM AUTHOR]

The effect of surface coating on the detachment of a complex microstructured food material, was investigated using an improved version of the millimanipulation device described by Ali et al. (2015 Food & Bioproducts Processing , Vol. 93, 256–268). The test material was baked sponge cake batter, which contains approximately 27 vol% bubbles in a ‘continuous’ phase of emulsified oil in a flour/syrup suspension. Detachment in the dry state was studied for aluminium, 304 stainless steel and seven different fluoropolymer coatings. The surfaces differed in surface energy and roughness. The shear force required to detach baked cake, the work done, and the mass of residue remaining on the surface were measured. Virtually all samples detached by cohesive or mixed failure, where adhesion to the surface was stronger than or comparable with cohesive interactions within the cake. The shear force was almost independent of surface composition, energy and roughness, but strongly related to the oil content of the cake. The mass of residue was found to be linearly dependent on the calculated work of adhesion of oil to the surface in an aqueous environment. The quantitative findings are consistent with confocal microscopy images of uncooked batter contacting polar and non-polar surfaces which show very different oil spreading behaviour at the batter-substrate interface. The ability of oil to replace water from a surface is shown to be a key factor determining adhesion of these materials. [ABSTRACT FROM AUTHOR]