Fin fouling

In a forced-draft air cooler, cool air is blown through the underside of the fin tube bundle. In an induced-draft air cooler, cool air is drawn through the underside of the fin tubes. Either way, road dust, dead moths, catalyst fines, and greasy dirt accumulate along the lower row of tubes. As the tubes foul, they offer more resistance to the airflow. However, note that 

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Evaporators—These usually utilize a fin-tube design. Spirally finned tubes of 1.25 in to 2 in outer diameter (OD) with three to six fins per ineh are eommon. In the ease of unfired designs, earbon steel eonstruetion ean be used and boilers ean run dry. As heavier fuels are used, a smaller number of fins per ineh should be utilized to avoid fouling problems. 

Fluid has low film rate and is non-fouling (finned tubes can be used). 

The usual applications for finned tubes are in heat transfer involving gases on the outside of the tube. Other applications also exist, such as condensers, and in fouling service where the finned tube has been shown to be beneficial. The total gross external surface in a finned exchanger is many times that of the same number of plain or bare tubes. 

This tube has a ratio of outside to inside surface of about 3.5 and is useful in exchangers when the outside coefficient is poorer than the inside tube coefficient. The fm efficiency factor, which is determined by fm shape and size, is important to final exchanger sizing. Likewise, the effect of the inside tube fouling factor is important to evaluate carefully. Economically, the outside coefficient should be about V5 or less than the inside coefficient to make the finned unit look attractive however, this break-even point varies with the market and designed-in features of the exchanger. 

Figure 10-144. Approximate relationship of the overall coefficient fouled, and the fouling factor of inside tubes for predicting the economical use of finned tubes in shell and tube units. (Used by permission Williams, R. B., and Katz, D. L. Performance of Finned Tubes and Shell and Tube Heat Exchangers, 1951. University of Michigan. Note For reference only, 1950 costs.)...

Figure 10-148 presents a recommended pressure drop correlation for low-fin tubes in shells and is based on clean tube pressure drop with no dirt sealing the leakage clearances between tubes and baffle holes or baffle-to-shell clearances. A fouled condition pressure drop may be an indeterminate amount greater. The authors state that this University of Delaware correlation has some lactors built in that limit the deviations to a relatively small range. Figure 10-148 has... 

The double-pipe longitudinal finned exchanger is designed by adding the fouling factor to each respective film coefficient before calculating the overall... 

Fouling on the outside of finned surfaces is usually rather small, but must be recognized. Values of 0.0001-0.0015 usually satisfy most fin-side conditions. Finned surfaces should be cleaned periodically to avoid excessive buildup of dust, oil films, bugs, etc. 

Since no serious problems exist with corrosion, the materials of constmction can be cheaper and the thicknesses reduced. Fouling coefficients do not need to be incorporated into the design calculations, as the surfaces remain clean indefinitely. This in itself provides a secondary benefit in that the pitching of fins may be reduced without the risk of reducing flow. 

Typical values for the fouling coefficients and factors for common process and service fluids are given in Table 12.2. These values are for shell and tube exchangers with plain (not finned) tubes. More extensive data on fouling factors are given in the TEMA standards (1999), and by Ludwig (2001). 

Webber, W. O. (1960) Chem. Eng., NY 53 (Mar. 21st) 149. Under fouling conditions finned tubes can save money. 

The total airflow discharged by the fan remains constant, regardless of the fin tube fouling. 

Actually, retrofitting a tube bundle with low fin tubes often reduces heat-transfer capacity. This happens when the controlling resistance to heat transfer is shell-side fouling. The fouling deposits get trapped between the tiny fins. This acts as an insulator between the shell-side fluid and the surface of the tubes. In severe shell-side fouling services, I have replaced fin tubes with bare tubes, and doubled the heat-transfer duty on the exchanger. 

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