Resistance scale

Industrial materials without sufficient scaling resistance frequendy fail after a short period of time as a result of rapid oxidation or hot corrosion, in conjunction with severe spalling owing to poor adherence of the scale to the metallic component. As a result, the permissible limits of metal loss often are exceeded and expensive, and premature replacement of parts is requited. Extensive efforts are made to develop alloys which are not simply strong at elevated temperatures but which also possess the adequate surface stabiUty. 

R Thermal resistance, equals x/kA, 1/UA, l/hA Ri, Ro, R l, R for thermal resistance of sections 1, 2, 3, and n of a composite body Rj for sum of individual resistances of several resistances in series or parallel R -, and for dirt or scale resistance on inner and outer surface respectively Ratio of total outside surface of finned tube to area of tube having same root diameter (s-K)/J (h- F)/Btu... 

Normally, dirt and scale resistance must he considered on both sides of the tube wall. The area dA is any convenient reference area. 

Sometimes one of the fluid-side scale resistances can be neglected or assumed to be so small as to be of little value, in which case only the significant resistances and/or film coefficients need to be used in arriving at the overall coefficient, U. Note that Aq, Aj, and can be substituted by d , dit, and d g respectively. Theoretically, d g and should be the logarithmic average, but for most practical cases, the use of the arithmetic average is completely satisfactory. 

Fig. 7.19 Scaling resistance of high-alloy irons (after Hallett ). Curve a Niresist (Fe-14-6Ni-2-lCr-7-2Cu). Curve b Silal (Fe-5-7Si). Curve c Niresist (Fe-14-4Ni-4-8Cr-7-4Cu). Curved Nicrosilal (Fe-4-6Si-22-5Ni-2-5Cr). Curve e Nicrosilal (Fe-4-9Si-22-8Ni-4-6Cr).

Assuming true cross-flow prevails in the shell, what value of scale resistance could be allowed if these units were used ... 

For conditions of turbulent flow the transfer coefficient for the water side, hi = u<>. Ri the scale resistance is constant, and h the coefficient for the condensate film is almost independent of the water velocity. Thus, equation 9.201 reduces to ... 

The thermal conductivity of cupro-nickel alloys = 50 W/m K and, from Table 9.16, scale resistances will be taken as 0.00020 m2K/W for the water and 0.00018 m2K/W for the organic. 

In a countercurrent-flow heat exchanger, 1.25 kg/s of benzene (specific heat 1.9 kJ/kg K and specific gravity 0.88) is to be cooled front 350 to 300 K with water at 290 K. In the heat exchanger, tubes of 25 mm external and 22 mm internal diameter are employed and the water passes through the tubes. If the film coefficients for the water and benzene are 0.85 and 1.70 kW/nr K respectively and the scale resistance can be neglected, what total length of tube will be required if the minimum quantity of water is to be used and its temperature is not to be allowed to rise above 320 K ... 

Air content of freshly mixed concrete by the pressure method Air content of freshly mixed concrete by the volumetric method Unit weight, yield, and air content of concrete Specific gravity, absorption, and voids in hardened concrete Resistance of concrete to rapid freezing and thawing Scaling resistance of concrete surfaces exposed to deicing chemicals... 

ASTM, Standard Specification C672 Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals, in Annual Book of ASTM Standards, Vol. 04.02, American Society for Testing and Materials, West Conshohocken, PA, 1993. 

Fig. 6.21 Arrhenius plot of the relaxation time (r(Q=14.5 nm )) for dPPO and dPPO-LiCl04 (Li 0=l 16) upper curves). Solid symbols were derived from NSE (INI 1) results, open symbols from inverted TOP (IRIS). The solid and dashed lines represent the temperature dependence of the scaled viscosity for the two systems. The dash-dotted line is the scaled resistivity. (Reprinted with permission from [314]. Copyright 2002 American Institute of Physics)...

The individual coefficients for the inside and outside films, and h, can be combined with the coefficient for conduction through the wall [eqn. (84)] and allowance made for any scale resistances, R, to give an expression for the overall heat transfer coefficient, h, based on an area, A... 

For added corrosion and temperature resistance, the nickel content of tubes, and sometimes the Moly (molybdenum) content as well, are increased. Tubes with a high nickel content are classified as 300 series stainless steels. A 0.5 percent silicon content is used to enhance the tube s oxidation or exterior scaling resistance. 

B Silicon (2-391) added to increase scaling resistance... 

Silicon added for increased scaling resistance at high... 

Oxidation. Immense progress in technology has imposed everinereasing demands oil the mechanical and chemical properties, in particular the oxidation and scaling resistance, of metallic ntalerials. 

Fig. 9. (a) Scaled resistivity p(T)/p (300 K) vs. T/Tq for YbFe4Sbi2> where Tq is the scaling temperature. Inset shows the pressure dependence of Tq. (b) Scaled resistivity of CeFe4Sbi2 vs. T/Tq. Inset shows pressure dependence of Tq. The room temperature resistivity of both compounds was about 0.8 m cm at ambient pressure (E.D. Bauer et al., 2000). 

Neglecting the wall and scale resistance, the clean overall coefficient is ... 

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