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Fouling inside Finned Tubes

Fouling inside finned tube heat transfer surfaces is a more serious problem than equivalent fouling inside bare tubes.Extended surfaces are widely used in heat recovery applications such as gas turbine HRSGs,incineration heat recovery systems etc. As the fin surface increases,inside fouling arising out of say poor water treatment or scale,sludge formation becomes more of a concern. Tube wall temeprature increases significantly and the exit gas temperature from the heat transfer surface increases resulting in much lower steam generation compared to a surface with lesser fin density or fin area.My book Steam Plant Calculations Manual explains the calculations and reasons in detail.
Simply put the heat flux inside the tubes increases as the ratio of external to internal surface increases. As a result,the temperature drop across the fouling layer also is higher.Hence one has to be very careful while using large fin densities in  boiler evaporators or superheaters when the tube side fouling can be high due to scale or deposit formation.The following example illustrates this fact. This may also explain local tube failures in evaporators
Example:A water tube evaporator in a fume incineration plant is required to cool 150,000 lb/h of flue gases from 1000 F to 520 F.Gas analysis is: % vol CO2=7,H2O=12,N2=75,O2=6.Steam pressure is 285 psig and feed water is at 230 F.2x1.77 in carbon steel tubes are used.Assume that the gas side fouling is 0.001 ft2hF/Btu,metal conductivity =25 Btu/fthF,steam side coefficient=2000 Btu/ft2hF,heat loss=2 % and tube spacings are 4 in square,inline arrangement.Design the evaporator using 3 options:
bare tubes(cases 1 and 2)
finned tubes with 2x.75x.05x.157 serrated fins(cases 3 and 4)
finned tubes with 5x.75x.05x.157 serrated fins(cases 5 and 6)
and study the effect of 0.001 (clean) and 0.01 fouling factors inside tubes.
Results and Conclusions
The design and off-design calculation procedures are outlined in my books. Only the final results are presented here.Note that the evaporators were all designed for the same duty and initial fouling conditions and we are studying the off-design performance at higher fouling factor inside the tubes.
                       Table: Performance of Finned Surfaces in Fouled conditions
item case 1  case 2 case 3 case 4 case 5 case 6
gas temp in-F 1000 1000 1000 1000 1000 1000
exit temp-F 520 545 520 604 520 646
duty,MM btu/h 19.65 18.65 19.65 16.3 19.65 14.6
steam flow,lb/h 19,390 18,400 19,390 16,110 19,390 14,400
foul inside 0.001 0.01 0.001 0.01 0.001 0.01
heat flux 9314 8162 35360 23080 55790 30260
wall temp,F 437 516 490 680 530 760
fin temp,F - - 730 840 725 861
finning bare bare 2x.75x.05 2x.75x.05 5x.75x.05 5x.75x.05
tubes/row 20 20 20 20 20 20
no deep 60 60 16 16 10 10
surf area,ft2 5024 5024 6642 6642 9122 9122
gas pr drop 3.0 3.1 1.8 1.9 2 2.1
[heat flux-Btu/ft2h,gas pr drop-in wc,duty-MM Btu/h,fouling factor-ft2hF/Btu]
Conclusions
Tube wall and fin tip temperatures increase significantly as the inside fouling increases.For the same fouling factors,the bare tube wall temperature is 516 F,while that of the 2 fins/in surface is 680 F and that of 5 fins/in surface is 760 F.Hence one has to be very careful in selecting fin geometry and not indiscriminately use a large fin surface. Tube failures are even more likely when the gas temperatures are higher. In superheaters,this could be a more serious problem due to the high steam and tube wall temperatures and low tube side heat transfer coefficient. For various reasons explained in other web pages of mine and in my books,it does not make sense to use more than 2 or say 3 fins/in in superheaters. Be wary of designs using 5 fins/in or more in superheaters!! In addition to lower possible duty,you are decreasing the life of the surface and the larger surface area can be only misleading!! You are buying trouble with more finned surface area when it comes in the form of higher fin density,height!
The steam generation also decreases due to fouling though all the 3 options were designed for the same duty to begin with. We make only  14,400 lb/h with 5 fins/in and 16,110 lb/h with  2 fins/in,while the bare tube option makes 18,400 under the same fouling conditions.
The surface areas,as explained in my books and  several papers of mine can be very misleading.In fact the larger surface area design,as seen in this example, results in higher tube wall temperatures in both clean and foulded conditions(due to the higher heat flux)! So those of you using spread sheets  tabulating the boiler vendors with largest surface area and then buying those designs having the largest surface areas,be better informed! Unless one knows how to evaluate heat flux,tube wall temperatures,fin tip temperatures and off-design performance,selecting a design with largest fin  density/surface area can lead to problems!

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