PTQ Q1 2023 Issue

Figure 3 Tubeside fouling photos with plugging of tube holes

Terms: U clean – Overall clean heat transfer rate, Btu/hr ft² °F U fouled – Overall fouled heat transfer rate, Btu/hr ft² °F U working or U design – result of Q/Area * LMTD, Btu/hr ft² °F OAFF – overall fouling factor incorporating shellside and tubeside fouling factors, hr ft 2 °F/Btu

refinery vacuum distillation ejector system is the size of the first intercondensers. When considering the excess area 0.004 hr ft² °F/Btu overall provides, seemingly twice the area necessary versus a clean design, it can be compelling to consider a lower overall fouling factor. Referring to the previous table, for the first intercondenser, if 0.002 OAFF was applied rather than 0.004, U design might range between 158 to 214 Btu/hr ft² °F and as such the condenser would be about 25% smaller in surface area and the overall sys- tem about 15% less expensive. A careful assessment of OAFF is important. This is ever- more true as refiners strive for longer periods between turnarounds. A changing crude slate can introduce risk as lower-cost, poorer-quality crude blends are processed that may have a higher fouling tendency. Rather than a sched- uled turnaround every four years, refiners wish to push that to six years or longer. More importantly, an unplanned shutdown to address a performance shortcoming due to fouling is costly, stressing the importance of establishing an appropriate OAFF. Shellside fouling The extent to which fouling impedes heat transfer on the shellside varies from refiner to refiner. There are services where shellside fouling is of little worry. In other cases, foul- ing can become surprisingly high. For some refiners, the uncertainty juxtaposed with the importance of not having an unplanned shutdown led to restrictions for the maxi- mum U design permitted. As an example, a refiner may limit U design for a first intercondenser to less than 100 Btu/hr ft² °F.

Q – thermal duty or heat released, Btu/hr Area – condenser heat transfer area, ft² LMTD – log mean temperature difference, °F

Overall fouling factor (OAFF) is an important variable to assess. Crude oils can have different fouling tendencies or foul very little, and the cooling water side can also present fouling issues. A typical vacuum distillation ejector system will have the following condenser overall design heat trans- fer rates (U design ) and consider the cleanliness factor and excess area an OAFF of 0.004 hr ft 2 °F/Btu provides:


Typical U design OAFF Cleanliness




1st Intercondenser


0.004 0.004 0.004

40-52% 48-60% 56-68%

90-150% 67-108% 47-78%

2nd Intercondenser 100-130



The rates can differ from these bespoke typical ranges based upon the amount of non-condensible gases and hydrocarbon loading, be it vapour and/or liquid. Overall fouling factors for refinery service have generally ranged between 0.0015 and 0.005. The greatest Capex factor for a


PTQ Q1 2023

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