Are Internal Fins Safe to Use in Cooling Water Service?
Craig Thomas
Technical Sales Director - Finned Tubing at NEOTISS High Performance Tube, Inc.
The use of ID-enhanced tubing is widespread in the Chiller Market and yet remains underutilized in the hydrocarbon process market due to uncertainty regarding fouling and cleaning.?Two industries, similar cooling water conditions, and yet two very different approaches suggests a need for more industry wide education and exchange of best practices knowledge.?
Assumptions
Most heat exchangers on open loop water cooling circuits experience some degree of tube side fouling and will eventually require cleaning. The most significant variables in fouling rate are water quality, water velocity, tube wall temperature, and water treatment methods. However, the ID surface geometry can also be a significant factor by increasing the turbulence and shear stress at the tube wall. This will not only increase the heat transfer rate but can also reduce the fouling rate compared to a smooth surface.
Industry Experience
In cases where internally enhanced tubes do foul, they tend to maintain their performance advantage over smooth surfaces.? Spiral grooved tubes can be successfully cleaned using similar methods to smooth tubes. (From ASHRAE Journal Newsletter, August 24, 2021). It is safe to assume that if there were a serious problem with fouling or cleaning of spiral grooved tubes then the commercial chiller market would have discontinued this 30+ year practice of using ID-enhanced tubes. NEOTISS’s HPT Thermo-C titanium condenser tubing has an internal rib surface and has been used in over 200 direct seawater cooling installations in the past 20 years.? Service companies that maintain chillers in the field have commented that most types of fouling can be removed by hydro brush cleaning and that suppliers like Goodway offer dual diameter brushes that have been optimized to clean spiral grooved ID surfaces.? In the case of hard scale deposit, a chemical descaling may be required and then the hydro brush cleaning follows. ?Some installations use online or automatic brush cleaning systems that operate on a repeating reverse flow cycle.? ?
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Past Research
Tests with cooling tower water comparing different fin geometries showed that the lower rib density and wider rib spacing led to a lower overall fouling rate (R. Webb).? The ratio of rib pitch to rib height of 3.5 was found to be optimal (P. Thor). Tests in seawater have confirmed that the shear stress in spiral-fluted tubes can lift the oxidized film from the wall to reduce rate of deposition (C.B. Panchal). Tests with crude oil fouling also showed less than half the fouling rate of smooth bore (H. Joshi).
New Research
NEOTISS Inc. is undertaking new research in this area with several partner companies to better understand the nature of cooling water fouling and cleaning on enhanced surfaces.? This will include testing with natural seawater and with cycled cooling tower water to simulate scale fouling and comparing rate of deposition with a smooth bore surface.? In addition, CFD modeling to evaluate the local velocity profile along the ID surface will be considered in an effort to optimize the fin geometry for the lowest possible fouling rate.
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1 年You add a outer Tube over the finned Tube you can cool or heat different fluids without comprise of the metal in contact. Acid and seawater for example.
Retired - Director at Allegheny Technologies Incorporated (ATI)
1 年Wow! Something completely new to me, but apparently quite effective. The sciences of heat transfer, fluid dynamics and metallurgical corrosion are impressive when combined to improve efficiency.
Heat exchanger supply for all your Heat Transfer equipment requirements.
1 年ASL Group airside cleaning and internal tube cleaning
Principal / Sen. Consultant - American European Consulting Company (AECC)
1 年Neotiss Finned Tubing
Engineering Laureate - Energy,Heat Transfer and Sustainability Technology- Water and Protection Business at DuPont
1 年In Webb's book, "Principles of Enhanced Heat Transfer", he shows on page 267 a flow field in between two ribs which, at least from the text appears to be the result of some CFD studies. There is as one might expect a recirculation zone on the lee side of a rib, and then a build up of a boundary layer in front of the next flight before a small recirculation zone develops right in front of the next flight. This all takes place within the height of the rib. Above that are turbulent eddies in the main flow. Whether the recirculation zones act to scrub or accumulate fouling I guess depends on the overall velocity through the tube which in turn drives the upper boundary of the eddies. Once again, in any case, smooth tube or enhanced, higher velocity is your friend to suppress fouling.