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David C. Silverman |
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Heat Exchangers- Effect of Fouling on Heat Transfer Rates After some operational period, surfaces of heat exchangers are no longer in the pristine condition found upon installation. The surfaces can become coated with deposits from the process or cooling streams (e.g. particulates) or become coated with corrosion products or microbiological films. No matter the cause, this additional structure on the surface creates an additional resistance to heat transfer and a decrease in performance. This resistance is represented by the term "fouling factor" or "fouling resistance" (Rf) which must be considered when designing or otherwise analyzing performance of a heat exchanger. The important point is that this fouling tends to increase the temperature at the wall. Such an increase in temperature can increase the fouling or corrosion rate in addition to decreasing operational effectiveness of the exchanger.As discussed in the section Heat Exchangers-Analysis in the Absence of Fouling the overall heat transfer rate coefficient is related to the heat transfer rate and temperature difference by 
or
(20)where U is the overall heat transfer coefficient, QTotal is the heat transfer rate, A is the area, and ΔT is the temperature difference. "Mean" is the mean temperature difference across the tube and "LMTD" is the log mean temperature difference using only the temperatures at the hot and cold ends. Both subscripts are defined in Heat Exchangers-Analysis in the Absence of Fouling . The fouling factor is defined as 
(21)Note that the heat transfer coefficient of the fouled exchanger is smaller than that of the clean exchanger making Rf positive. The value of 1/Rf is subtracted from Uclean to obtain Ufouled. Fouling factors are obtained experimentally by determining the value of the heat transfer coefficient for both the clean and fouled exchanger. Some reported values are in the table below. They are provided as examples of order of magnitude only and should not be used for any design calculations. Also note that fouling factors vary with the inverse of the fluid velocity raised to a power "n", where "n" lies in the range of about 1 to 2 (1/(velocity)n).
As shown in equations (20), the overall heat transfer coefficient is proportional to the overall heat transfer rate and inversely proportional to the temperature difference between the streams and the required area. The example from Heat Exchangers-Analysis in the Absence of Fouling can be used to provide an idea of the effect of fouling on heat transfer and wall temperature. Water at the rate rate of 1.5 kg/min is heated from 25C to 50C by an oil. The fluids are in a counter-current flow arrangement and the oil enters at 150C and leaves at 100C. The overall heat transfer coefficient as calculated from equation (19) is 2.065 cal/hr-m2-C. The calculated area is 14.8m2. Now suppose that the cooling water is seawater with a fouling factor of 0.001 hr-ft2-oF/BTU or 7.4 s-cm2-oC/cal . This fouling factor is Rf in equation (21). Equation (21) is used to estimate the fouled overall heat transfer coefficient. Converting the fouling factor to square meters and hours and inverting the equation results in 
Solving for Ufouled results in a value of 1.98x105 cal/hr-m2-C, about a 5% decrease in the heat transfer coefficient. If the surface area remains at about 14.8 m2, the log mean temperature difference becomes 85.3C. Assuming that the oil still must be heated between 100C and 150C and the inlet water temperature is 25C, the outlet water temperature would increase by several degrees. While this increase in temperature seems small, it can have a significant impact on the fouling rate. The reason is that fouling is an activated process. It can be assumed to be proportional to exp(-Eactiviation/T). If one assumes an activation energy of about 10000 cal/mol, a 5C change in surface temperature results in about a 20% increase in fouling rate (deposition and crystal growth) at 85C. The result is more rapid build-up resulting in higher temperatures and an accelerating fouling process. A corrosion process could increase in a similar fashion because it, too, is an activated process. Minimizing fouling is important in heat exchangers. Detailed information on materials of construction of water cooled heat exchangers, water chemistry, and chemical water treatments can be found in "Guidelines for Troubleshooting Water Cooled Heat Exchangers", A. J. Freedman, A. S. Krisher, and D. Steinmeyer, Materials Technology Institute, 2004. Fouling can also occur in air cooled exchangers depending on cleanliness of the air and moisture content especially if condensation is possible. In addition, fouling can roughen the surface. The effect on heat transfer is complex. On the one hand, surface roughening in and of itself can increase the heat transfer coefficient in the fluid. But, since the roughening is usually accompanied by an increase in thickness of the wall, the resistance to heat transfer through the wall can be increased, sometimes significantly. In addition, the structure of the fouling can be strongly influenced by the local velocity field. Laboratory simulation as outlined in Laboratory Corrosion Testing Under Heat Transfer Conditions - a Critique can be difficult for fouled heat exchangers. Resorting to a test heat exchanger has sometimes been the only way to simulate some of these complex situations. Previous Page: Heat Exchangers-Analysis in the Absence of Fouling |
David C. Silverman, Ph.D. - Primary Consultant |