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Fouling Focus
Heat exchanger fouling in practice – understand & mitigate
Part 5 – fouling mitigation using design and process-related changes
In this series of articles we will look at how heat exchangers foul , how to understand the root causes of fouling , and how to mitigate the impact of fouling . The material presented is based entirely on the author ’ s experience and analysis of operating situations in the Oil & Gas industry . However , many theories and varied experiences exist across the industry and amongst researchers .
About the author
Himanshu Joshi retired from Shell in 2021 after 34 combined years with ExxonMobil and Shell , during which he specialized in heat exchangers and fouling . He was part of a team that was granted a patent related to fouling deposit analysis at ExxonMobil , and led applied fouling R & D projects at both companies . He has made several presentations about the field aspects of fouling and fouling mitigation , and deployed many mitigation technologies in the field . He can be reached by email at alph . hmj @ gmail . com .
By Himanshu Joshi , Heat Exchanger Specialist
In Part 4 we saw techniques which use external hardware to mitigate fouling . This month we will continue to look at fouling mitigation , but with changes to the heat exchanger geometry or to the fluid conditions like flow and temperature . Recall from Part 1 these most likely causes of fouling - single phase fluids foul due to deposition of particles , boiling services foul due to high wall temperatures leading to a wet-dry wall condition , and condensing services foul due to precipitation of salts at the tube wall and flow patterns which enable deposition or prevent the redissolution of the salts .
Shear stress ( velocity ) in single phase Deposition of particles is controlled by two competing effects , fluid shear at the wall ( τ ) and the attraction between the solids and the tube surface . For a specific situation where the type of particles and the tube metallurgy ( surface characteristics ) are fixed , shear stress controls the rate of deposition . This behavior is seen both in liquids and gases . Increasing the shear stress is a very effective method to minimize fouling driven by deposition . Although shear stress correlates with velocity ( V ), it is fundamentally a better parameter to use because it accounts for fluid properties , mainly the viscosity . Field and pilot-plant data has shown that the rate of fouling ( increase in fouling resistance per day ) varies as ( τ ) -a on the tube side , and ( τ ) -b on the shell side , where a is in the range of 1.0-1.2 and b is about 0.6 . A substantial amount of data is available for the tube side , but relatively little for the shell side , so the latter relationship has more uncertainty . Another uncertainty on the shell side is that there are many flow paths and there is no exact calculation of velocities and shear stresses . Most commonly we use the crossflow shear stress . The exponential relationship between fouling rate and shear stress means that as shear increases the fouling rate will eventually flatten out , see Fig . 1 .
Data for the tube side shows this happens at about 10-12 Pa , beyond which little can be gained by increasing shear stress . The recommended value of tube side shear stress for minimum fouling is 10 Pa , which for most liquids will correspond to velocities between 2.0-2.5 m / s . Fig . 1 shows the fouling rates for three different fluids , varying by an order of magnitude , but they reach exponential lows at similar values of shear stress . The best method to increase tubeside velocity is by changing the number of tube passes . Changing two passes to four doubles the velocity and reduces the fouling rate by about 60 %, or four passes to six increases velocity by a factor of 1.5 and reduces fouling by about 40 %. The price paid for an increase in velocity is increased pressure drop which varies as V 2 * L , where L is the length of the flow path . Thus , when two passes are changed to four , the velocity and flow length both double , and the new clean pressure drop is approximately eight times . That much higher pressure drop may not be available but note that fouling also increases pressure drop and it may be that a factor of eight is already encountered in actual operation for severe fouling situations . For the shell side , from a practical standpoint it is not possible to increase velocity beyond about 0.75 m / s for liquids . This is because as the baffle spacing is narrowed to increase velocity , the flow gets diverted away from the crossflow component to the various “ leakage ” paths and the crossflow shear does not increase significantly . There is also
Rate of Fouling [ m2-C / W / day ]
0 2 4 6 8 10 12 14 16 Shear Stress [ Pa ]
Fig . 1 Dependence of fouling rate on tubeside wall shear stress . www . heat-exchanger-world . com