Fouling of surfaces

Fouling of surfaces, resulting in decreased heat transfer efficiency... 

M. R. Detty, M. D. Drake, Y. Tang and F. V. Bright, Hybrid antifouling coating compositions and methods for preventing the fouling of surfaces subjected to a marine environment, US Pat., 1 244 295 B2, 2007. 

Scraped-surface exchangers are particularly suitable for heat transfer with crystallization, heat transfer with severe fouling of surfaces, heat transfer with solvent extraction, and heat transfer of high-viscosity fluids. They are extensively used in paraffin-wax plants and in petrochemical plants for crystallization. 

Gudmundsson, J.S., 1977, Fouling of surfaces. PhD Thesis, University of Birmingham. 

In the past, waste e.g. municipal waste, has been incinerated in the "as received" condition in mass bum equipment, but modem methods involve the pretreatment of the waste before firing as for fluidised bed operation. The higher costs that this entails are generally offset by improvements in operation not least in respect of fouling of surfaces, due to the opportunity to maintain consistent conditions in the equipment. 

Chapter 19 deals with the physicochemical aspects of the most ubiquitous interface in living systems, the biological membrane. We conclude with Chapter 20, which deals with bioadhesion, that is, the accumulation of biological cells at interfaces. Bioadhesion may lead to adverse effects—for example, fouling of surfaces—but in other applications it is desired—for example, immobilization of cells in bioreactors. 

The position of reactant addition points should be considered. The order and location of addition of key reactants may be critical, especially when proper dispersal is necessary to avoid coagulation or fouling of surfaces. However, if all ingredients are added together, this may be of little importance. 

Chemical compounds were applied for the inhibition of metal corrosion in aqueous environments as ee ly as the 1930s. The choice of inhibitor is greatly influenced by the pH of the water. Deposits on metal surfaces arising from scale formation, fouling, and microbial activity cause accelerated corrosion as a result of differential aeration. Fouling of surfaces may lead to overheating, unscheduled shutdown time, and loss in efficiency. 

Fouling of the pH sensor may occur in solutions containing surface-active constituents that coat the electrode surface and may result in sluggish response and drift of the pH reading. Prolonged measurements in blood, sludges, and various industrial process materials and wastes can cause such drift. Therefore, it is necessary to clean the membrane mechanically or chemically at intervals that are consistent with the magnitude of the effect and the precision of the results requited. 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

With normal excess ammonia the gas/liquid ratio is about 3,500 mVm. At this high ratio there is danger of fouling the surface with tariy reaction products. The ratio is brought down to a more satisfactory value of 1,000 to 1,500 by recycle of some of the effluent. 

RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

If an appropriate thermal feedback mechanism is not provided, the reaction occurs at the lower stationary state where the reaction rate may be negligible. The reaction could be extinguished, if the temperature of the feed entering the reactor drops below some critical value due to fouling of the heat exchange surface. 

The fouling of the probe when inserted into a duct or pipe acts as an isolating layer and increases the measurement error. To avoid this conduction error, the probe should be a poor heat conductor. In measuring surface temperatures, the probe should not have an insulating effect, as this will change the temperature in the measuring point. 

A, = area of inside of. surface for heat transfer, such as coil, flat surface, or other barrier, sq ft/ft h = inside heat transfer fluid side coefficient, in coil, flat plate, or otlier barrier, Btu/hr/sq fl/°F ro = fouling resistance (factor) associated wTth fluid on outside (tank process side) of heat transfer... 

The spray-filled tower, Figure 9-100, is also an atmospheric type, containing no fill other than the water sprays and no fans. The water-air contact comes about due to the spray distribution system [144], This design is often used where higher water temperatures are allowed, and the situations where excessive contaminants building up in the water would cause fouling of other direct contact heat transfer surfaces. 

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