How Do Finned Tube Heat Exchangers Work?

13 May.,2024

How Do Finned Tube Heat Exchangers Work?

The surface area which is available for the transfer of heat is extremely important in determining the general heat transfer. The finned tube heat exchangers are essential as they will maximize the transfer of heat across surface areas, and below is an overview of how they work.

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Device Functionality

These heat exchangers utilize tubes that feature fins or an external surface area that is extended. Their purpose is to boost the transfer of heat by boosting the transfer area that exists among tubes and the fluid that surrounds them. Finned tubes come in two types, which are transverse and longitudinal.

Transverse Fins

Transverse fins will be typically applied to turbulent and gas flows as well as cross-flow style exchanges. Tubes that use transverse fins are well suited to air coolers. This is because these fins appear in the form of hollow metallic discs which are spaced and then fitted along a finned tube’s length. The fin discs themselves may be tapered or flat, and the heated transfer coefficients along the fin’s surface will depend heavily on finned disc geometry.

Longitudinal Fins

Longitudinal style fins are well suited to applications where external tube flow is streamlined along the length of the cylinder. An example of this would be dual-pipe heated exchangers that have fluid that is highly viscous outside their finned tube. The majority of longitudinal fins within a tube will run along its length. They have a cross-sectional form which may either be tapered or flat.

Design

To design a finned tube heated exchanger in a manner that will boost the heated transfer area, the designer will need specific heat calculations as well as optimal spacing for the tubes to create the right environment. Generally speaking, a greater heat transfer region usually culminates in higher heated exchange efficiency.

This is because heated transfer coefficients near surfaces outside and inside these tubes will be calculated through the usage of correlations that have been experimentally determined. The heated transfer efficiency for the fins will also be calculated by utilizing corrections. Distinct correlation sets have been made available to calculate the transfer efficiency of heat for both transverse and longitudinal fins. When the fin region is multiplied by the finned heated transfer efficiency and then subsequently added to the bare tube region, the result will be an external heated transfer area that is effective.

The general heated transfer coefficient may be determined through the addition of heated transfer resistances which have been evaluated in the interior and exterior tube surface areas. For the external area, the value for an effective area will be utilized. Lastly, if the velocity of the external fluid is altered significantly, this means the heated transfer coefficients, as well as the needed tube area, should be reevaluated.

Applications

These heat exchangers are ideal in scenarios where you have a lower heated transfer coefficient outside the tubes. In this case, the additional heated transfer area which is generated by the fins will assist in ensuring the needed heat transfer rate has been made possible. This means they can be used for applications that entail external air and liquid within the tubes.

Choosing Fin Material for Industrial Heat Exchangers

Stainless, copper, E-coat, Heresite, or CuNi?

When Super Radiator Coils engineer coils for customers, materials are among our most important considerations. Things like applications, environments, and coil lifespan must all be factored in to the coil’s design. So too must price and industry standards, making material decisions all the more crucial.

Unlike light commercial applications, where planning and decision-making occur over weeks or months, industrial projects tend to move at a much slower pace, with project timelines often spanning months or years.

With industrial projects, quality and lifespan are often chief concerns. Therefore, these projects may involve considering different materials than those used in a typical commercial HVAC application.

For this post, we’ll take a look at five examples of fins made from materials one might see on an industrial heat exchanger. We’ll examine the relative cost, heat transfer performance, and corrosion resistance of each. For cost comparison purposes, all fins will be ranked from most expensive to least expensive.

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Option 1: Copper nickel fins

Cost: 1

Thermal conductivity: Copper nickel’s heat transfer capability varies depending on the alloy. The two main copper nickel alloy grades are 90/10 and 70/30 copper and nickel, respectively. Their thermal conductivity typically ranges from 17 Btu/hr × ft × F° to 23 Btu/hr × ft × F°[i].

Corrosion resistance: Copper nickel is very resistant to seawater corrosion and is often used in industrial marine environments because of that reason. This resistance is due to the formation of a thin, adherent protective surface film that develops quickly after exposure to clean seawater. This coating takes roughly two to three months to fully form, after which corrosion rates will continue to decrease.

Takeaways: It’s the most expensive material covered in the piece and its heat transfer is at the lower end, but its durability and corrosion resistance make it well suited for a bevy of industrial applications, especially in marine environments.

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Option 2: 316 stainless steel fins

Cost: 2

Thermal conductivity: The heat transfer capability of 316 stainless is relatively poor. While better than type-304, 316’s is still only between 17 Btu/hr × ft × F° and 13 Btu/hr × ft × F°, which is at the lower end of the spectrum covered in this piece.

Corrosion resistance: Unlike type-304, 316 stainless contains molybdenum, which helps provide greater corrosion resistance against things like localized attack by chlorides as well as general corrosion from reducing acids like sulfuric acid. Thus, type-316 is typically used in harsher corrosive environments than 304.

Takeaways: Type-316 stainless is tough and durable, capable of withstanding harsh industrial environments. However, its relatively poor heat transfer capability makes it best suited for environments where resilience and lifespan are more valued than heat transfer.

Option 3: 304 stainless steel fins

Cost: 3

Thermal conductivity: Like type-316, poor heat transfer is among the largest drawbacks of using stainless for heat exchangers. Its heat transfer capability (9.24 Btu/(hr × ft × F°[ii]) is worse than aluminum (136 Btu/hr × ft × F°[iii]) and far worse than copper (231 Btu/hr × ft × F°[iv]).

Corrosion resistance: Stainless provides excellent corrosion resistance and is a great choice if corrosion mitigation is an application’s primary concern. Due to its chromium content, stainless undergoes passivation in wet environments, which forms a thin inert surface film of chromium oxide when exposed to even small amounts of oxygen. The film inhibits additional corrosion by blocking oxygen diffusion to the surface of the steel, which prevents corrosion from spreading.

As far as corrosion from acids and bases, room temperature 304 stainless is only resistant to 3% acid and may not be a practical choice for acidic environments. 304 can also be susceptible to crevice corrosion by chlorides.

Takeaways: Performance is similar to 316, but 304 is less corrosion resistant.

Option 4: Copper fins

Cost: 4

Thermal conductivity: Copper is among the most thermally conductive substances on Earth, making it extremely effective in heat exchangers. At 231 Btu/hr × ft × F°, copper’s thermal conductivity rating is 60% greater than that of aluminum and 3,000% that of stainless.

Corrosion resistance: In clean air, water, and deaerated non-oxidizing acids, copper corrosion occurs at very low rates. However, in harsher environments, copper oxidizes over time, resulting in a green patina. This patina protects the copper from corrosion to a certain degree, but not from aggressive corrosives like acid rain.

Takeaways: Uncoated copper fins likely aren’t feasible for most harsh industrial environments, especially acidic ones. However, if heat transfer is the top priority and the coil’s application doesn’t involve acids, copper fins are a very good option.

Option 5: Heresite P413-coated aluminum fins

Cost: 5

Thermal conductivity: Coating aluminum fins with Heresite has a less than 1% negative effect on a coil’s heat transfer capabilities. It’s effectively the same as non-coated aluminum and copper. Heresite is a good option for applications that want the heat transfer benefits of copper or aluminum, but with far greater resistance to corrosives.

Corrosion resistance: Heresite provides terrific corrosion resistance and can tolerate a multitude of corrosive atmospheres. It’s particularly effective in coastal and marine salt air applications, having passed 1,000 hours on the ASTM G85-A3 Acidified Synthetic Sea Water Testing (SWAAT) test. Heresite also meets the ISO 12944-9 (formerly ISO 20340) standard, having withstood 4,200 hours (25 cycles) alternating between salt spray, UV radiation, and temperature shocks. Heresite is the first HVAC-R and radiator coating to meet this standard.

Takeaways: Heresite-coated aluminum fins are suitable for a variety of options. For applications where price, heat transfer, and corrosion resistant are all equally valued, Heresite should be considered, especially for marine and salt air environments.

Option 6: E-coated aluminum fins

Cost: 6

Thermal conductivity: Like Heresite, e-coat reduces heat transfer by less than 1%, with little difference between a coated coil and a bare one as far as heat transfer goes.

Corrosion resistance: E-coat also provides terrific corrosion resistance, having passed 3,000 hours on the ASTM G85-A3 (SWAAT) test. E-coat has not been tested against the ISO 12944-9 standard.

Takeaways: Like Heresite, E-coated aluminum fins are a good option for a variety of applications, such as coastal areas, or industrial environments with high humidity and aggressive atmospheres like power plants, refineries, or steam turbines.

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Contact us to discuss your requirements of Industrial Finned Tube Heat Exchanger. Our experienced sales team can help you identify the options that best suit your needs.

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