Resistance fouhng

These include glass, sihcon carbide, and similar variations. Even larger tubes are available in these materials, up to 19-mm diameter. They have high thermal conduc tivities and are usually very smooth surfaces to resist fouhng. Very high material/fluid compatibility is seen for these products, not many fluids are excluded. Brittleness is a consideration of these materials and a complete discussion of the service with an experienced suppher is warranted. The major selection criteria to explore is the use of O rings and other associated joints at tubesheet. The shell is steel in most cases. 

Work in connection with desahnation of seawater has shown that specially modified surfaces can have a profound effect on heat-transfer coefficients in evaporators. Figure 11-26 (Alexander and Hoffman, Oak Ridge National Laboratory TM-2203) compares overall coefficients for some of these surfaces when boiling fresh water in 0.051-m (2-in) tubes 2.44-m (8-ft) long at atmospheric pressure in both upflow and downflow. The area basis used was the nominal outside area. Tube 20 was a smooth 0.0016-m- (0.062-in-) wall aluminum brass tube that had accumulated about 6 years of fouhng in seawater service and exhibited a fouling resistance of about (2.6)(10 ) (m s K)/ J [0.00015 (fF -h-°F)/Btu]. Tube 23 was a clean aluminum tube with 20 spiral corrugations of 0.0032-m (lA-in) radius on a 0.254-m (10 -in)... 

Among the topics of the TEMA Standards are nomenclature, fabrication tolerances, inspeclion, guarantees, tubes, shells, baffles and support plates, floating heads, gaskets, tube sheets, channels, nozzles, end flanges and bolting, materi specifications, and fouhng resistances. 

Overall coefficients are determined hke shell and tube exchangers that is, sum all the resistances, then invert. The resistances include the hot-side coefficient, the cold-side coefficient, the fouhng factor (usually only a total value not individual values per fluid side) and the wall resistance. 

Problems encountered with filtration ate that membrane fouhng can occur, which causes a decline in flux with time under constant operating conditions. Furthermore, concentration polarization, the effect that the increased concentration of components on the membrane surface reduces the flux due to the additional hydrodynamic resistance, is observed. This effect can be minimized in cross-flow filtration, by applying high flux rates across the membrane surface (Wang et al, 1979 Lee, 1989). 

The overall coefficient includes the following heat transfer resistances (1) process side film (2) process side fouling (3) wall resistances (4) utility side fouhng, and (5) utility side film. The development of the relationship between U and these individual resistances is given in myriad heat transfer texts. 

Blatt WF, Dravid A, Michaels AS, and Nelson L. Solute polarization and cake formation in membrane ultrafiltration Causes, consequences and control techniques. In Membrane Science and Technology, ed., Fhnn JE, Plenum Press, New York, 1970, pp. 47-97. 40. Blanpain-Avet P, Doubrovine N, Lafforgue C, and Lalande M. The effect of oscillatory flow on crossflow microfiltration of beer in a tubular mineral membrane system—membrane fouhng resistance decrease and energetic considerations. J. Membr. Sci., 1999 152(2) 151-174. 

With large holes (16 to 25 mm), these trays are some of the most fouling-resistant and corrosion-resistant devices in the industry. This defines their main application highly fouhng services, slurries, and corrosive services. Dual-flow trays are also the least expensive and easiest to install and maintain. 

Membrane processes have been widely adopted over the last 30 years in spite of inherent hmitations such as fouHng, thermal and chemical resistance and maximum achievable purity. One reason for their wide success is the emergence of integrated/hybrid membrane processes for several appHcations, some of which are discussed in Chapter 3. The principal characteristics of membrane processes are Hsted in Table 1.3. AH processes except dialysis, ED and EDI are pressure driven, and aU except PV and MD do not involve a phase change. 

Chang Y, Shu S-H, Shih Y-J, Chu C-W, Ruaan R-C, Chen W-Y. Hemocompatihle mixed-charge copolymer brushes of pseudozwitterionic surfaces resistant to nonspecific plasma protein fouhng. Langmuir 2010 26 3522-30. 

Listiarini, K., D. D. Sun, and J. O. Leckie. 2009b. Oiganic fouhng of nanofiltration membranes Evaluating the effects of humic acid, calcium, alum coagulant and their combinations on the specific cake resistance. J. Memb. Sci. 332 56-62. 

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