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Are fin tubes more likely to foul than smooth tubes ?
Be careful how you answer … Below is a photo of a reboiler bundle pulled for cleaning that shows the smooth tubes completely covered in scale and other deposits while the single fin tube remains relatively clean . This is the opposite of what most engineers would predict . How do we explain this ?
About the author
Craig Thomas is the Director of Technical Sales for NEOTISS , Inc ., a manufacturer of highperformance heat transfer tubing with operations in USA , France , China , and India . Craig has 30 + years of experience in applications engineering related to shell & tube heat exchangers . He is a member of the National Association of Corrosion Engineers , The Materials Technology Institute , Heat Transfer Research Inc ., and The American Society of Heating , Refrigerating and Air- Conditioning Engineers .
By Craig Thomas – Director of Technical Sales , NEOTISS , Inc .
A reboiler test case In reboilers , where crystallization fouling is common and tends to form a hard scale on the tube surface , it has been shown that for the same heat duty a low-fin surface will foul less than a smooth surface . The reasons for this are twofold . The primary factor is the metal surface temperature . In the case of the fin tube , the same heat energy from the tube side is transferring to the shell side through 250 % more surface area as compared to the smooth tube . Therefore , the fin tube is operating at a lower heat flux compared to the smooth tube . The resulting skin temperature of the fin tube will be lower than the smooth tube and therefore less prone to scale formation . The higher the tube wall temperature the more likely that minerals in the boiling fluid will come out of solution to form a scale deposit on the tube surface . A secondary factor , besides temperature , could be that the mechanics of the scale formation . Some have theorized that there may be a breaking away or sluffing off effect in normal operation as scale attempts to form a continues structure but then breaks off and falls away . This may be more likely to happen on a finned surface as compared to smooth .
What about other services ? In overhead condensers and light-ends coolers , low fin tubes have been used throughout the refining industry since the 1950s with a high degree of success . One major end client using HPT fin tubes since 1975 has reported excellent results in services such as : catalytic cracking unit fractionator OHC , catalytic light ends splitter OHC , pipestill vacuum OHC , power former reactor effluent cooler , naptha debutanizer OHC , alkylization unit deisobutanizer condenser , fuel gas MEA regenerator OH , lube oil cooler . In some heavier saltine fouling services , fin tubes have been used with an online water wash system as a way to keep them clean and extend run time and ensure the full heat transfer benefit between planned maintenance cycles . Low-fin tube has also been used in very heavy / dirty services . One operator replaced bare tubes with low fin tubes in an asphalt cooler ( 7 % API asphaltic residual shell side ) as part of a debottlenecking project . They found that the fin surface was effective and maintained a higher average heat duty than the bare tube within their 18-month cleaning cycle .
What about cleaning ? This would depend on the type of fouling but in the above referenced asphalt cooler case , the fin tube bundles cleaned by hydro blasting in less time than the original smooth bundles . The same was observed by another operator with cracked gas coolers . One explanation for this may be the larger cleaning lane in the bundle given the smaller root diameter of the fin tubes .
So how do I know when a service is safe for low-fin tubes ? Each case must be evaluated on its own merits because there are many types of fouling and not all services are appropriate for fin tubes . Ask for and check references on experience . If there is a removable bundle that is already on a planned cleaning cycle , even if the fin tube fouls over time , it may still deliver a higher average heat duty over the same mean time between cleaning as compared to a smooth tube . A pragmatic problem-solving approach is always better than a blind policy approach . The main point here is not to rule out the low fin tube just based on a general policy that it can only be used in very clean , non-fouling environments . You might even be able to solve a fouling problem using fin tubes as shown in the reboiler case photo below .
Fouling issue in a reboiler case study , solved using fin tubes .
36 Heat Exchanger World April 2023 www . heat-exchanger-world . com