FOULING CONSIDERATIONS

In this section, several parameters of importance in understanding fouling will be explained and their importance estimated. 

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Fouling considerations Sensitive to particulate solids Yes Yes Moderately11 No No... 

Pressure drop per theoretical stage Fouling considerations i ... 

Mineral precipitation and complexation with proteins also contribute to fouling considerably. Adsorbed minerals may serve as salt bridges between the protein and the membrane, which aggravates fouling. In physicochemical conditions that promote calcium phosphate precipitation or calcium-protein complexation, membrane fouling in the filtration of nulk or whey is severe,... 

R. L. Webb, Performance, Cost Effectiveness and Water Side Fouling Considerations of Enhanced Tube Heat Exchangers for Boiling Service With Tube-Side Water Flow, Heat Transfer Engineering (3/3-4) 84-98,1982. 

Downcomer sealing. While pressure drop and fouling considerations set the minimum values for downcomer clearance, sealing considerations set the maximum value for downcomer clearance. If the service... 

Other energy considerations for cooling towers include the use of two-speed or variable-speed drives on cooling-tower fans, and proper cooling-water chemistry to prevent fouling in users (see Water, industrial water treatment). Air coolers can be a cost-effective alternative to cooling towers at 50—90°C, just below the level where heat recovery is economical. 

High Water Velocities. The abiUty of high water velocities to minimize fouling depends on the nature of the foulant. Clay and silt deposits are more effectively removed by high water velocities than aluminum and iron deposits, which are more tacky and form interlocking networks with other precipitates. Operation at high water velocities is not always a viable solution to clay and silt deposition because of design limitations, economic considerations, and the potential for erosion corrosion. 

Aromatic polyamide (aramid) membranes are a copolymer of 1-3 diaminobenzene with 1-3 and 1-4 benzenedicarboxylic acid chlorides. They are usually made into fine hollow fibers, 93 [Lm outer diameter by 43 [Lm inner diameter. Some flat sheet is made for spirals. These membranes are widely used for seawater desalination and to some extent for other process applications. The hollow fibers are capable of veiy high-pressure operation and have considerably greater hydrolytic resistance than does CA. Their packing density in hoUow-fiber form makes them veiy susceptible to colloidal fouling (a permeator 8 inches in diameter contains 3 M fibers), and they have essentially no resistance to chlorine. 

For compressors in general and for some types in particular, the cleanliness of the gas stream is the key factor in a reliable operation. Moisture or liquids in various forms may be the cause of an early failure or in some-cases a catastrophic failure. Corrosive gases require material considerations and yet even this may not entirely solve the loss of material issue that can certainly cause early shutdowns or failures and high maintenance cost. Fouling due to contaminants or reactions taking place internal to the ( i-pressor can cause capacity loss and the need for frequent shutdowns. 

A critical consideration with UF technology is the problem of fouling. Foulants interfere with UF by reducing product rates- sometimes drastically-and altering membrane selectivity. The story of a successful UF application is in many respects the story of how fouling was successfully controlled. Fouling must be considered at every step of UF process development in order to achieve success. 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

Corrosion and fouling will occur, though normally considerably le.ss than that experienced in the open system. 

The percentage effect of the fouling factor on the effective overall heat transfer coefficient is considerably more on units with the normally high value of a clean unfouled coefficient than for one of low value. For example, a unit with a clean overall coefficient of 400 when corrected for 0.003 total fouling ends up with an effective coefficient of 180, but a unit with a clean coefficient of 60, when corrected for a 0.003 fouling allowance, shows an effective coefficient of 50.5 (see Figure 10-39). 

Gilbert, P. T., Considerations arising from the use of dissimilar metals in seawater piping systems , 5th International Congress on Marine Corrosion and Fouling, Barcelona (1980)... 

Most of the published evidence suggests that marine fouling cover— particularly where it is continuous and well established — reduces corrosion rates of steels . Indeed, 35%o seawater is by no means the most corrosive of saline environments towards steel. Brackish water, as found in estuarine or certain other coastal areas, is considerably more aggressive towards steel, and careful design measures should be taken to ensure that effective corrosion control is achieved in such circumstances. 

Under the sludge considerable orange-red tuberculation corrosion deposits may develop. In cause-and-effect fashion, the tubercles grow cause fouling, permit under-deposit corrosion to persist, and generally act as a binding agent for carbonates, silicates, and other precipitates. 

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