Bimetallic finned tubes: thermal conductivity 

Bimetallic finned tubes: thermal conductivity as a key efficiency factor

Heat transfer in bimetallic finned tubes directly depends on the quality of the contact between the core and the fins, and not just on the material of the components themselves. In our engineering practice, we have repeatedly encountered situations where the declared characteristics of a heat exchanger did not coincide with the actual performance at the site due to microscopic gaps in the welding zone. Bimetallic finned tubes provide an optimal balance between the corrosion resistance of the outer layer and the high thermal conductivity of the inner core, but this balance can be easily upset if the wrong production technology is chosen. For engineers designing heating systems or industrial heat exchangers, understanding the physics of the process is more important than just looking at a catalog with numbers.

The market is saturated with offers where manufacturers sacrifice contact quality to reduce costs, which leads to a drop in system efficiency by 15–20% already in the first year of operation. We analyzed hundreds of samples and found that true thermal conductivity is determined not so much by wall thickness as by the method of fin formation. In this article, we will analyze the technical nuances that distinguish reliable equipment from defective ones, based on data from laboratory tests and real cases from the oil and gas industry and housing and communal services.

Physics of the process: why core material is critical for heat transfer

The main purpose of a bimetallic structure is to combine the advantages of two different metals. The inner tube (core) is usually made of steel or stainless steel for strength and high pressure resistance, while the outer fin is made of aluminum or copper due to their outstanding thermal conductivity. The thermal conductivity coefficient of aluminum is about 200–230 W/(mK), which is almost four times higher than that of carbon steel. However, this theoretical advantage only works if there is perfect thermal contact between the layers.

In our laboratory, we conducted comparative tests of pipes manufactured by mechanical bonding and extrusion or high-frequency welding. The results showed a shocking difference: when mechanically stretching an aluminum tape onto a steel pipe, the thermal resistance of the contact zone can reach values ​​that negate the benefits of using aluminum. When heating and cooling in a cyclic mode, different coefficients of linear expansion of metals lead to the formation of microgaps. The air in these gaps acts as an insulator, dramatically reducing the overall efficiency of heat transfer.

High frequency welding (HF welding) solves this problem by creating a monolithic connection at the molecular level. In such pipes, heat loss in the transition layer is minimal, and the system operates as if it were a solid piece of metal with average characteristics. For projects where every unit of energy is important, such as heat recovery systems or cryogenic plants, the use of pipes without a welded contact is unacceptable. Engineers should require thermal contact resistance test reports from suppliers, not just material certificates.

The choice of core material is also dictated by the coolant environment. If aggressive chemicals or water with a high oxygen content circulate inside the pipe, ordinary carbon steel will quickly corrode, even if the pipe looks perfect on the outside. This is where AISI 304 or 316L stainless steel comes to the rescue as a core. Although the thermal conductivity of stainless steel is lower (about 15–17 W/(m K)), its durability in aggressive environments compensates for this disadvantage by eliminating the need for frequent replacement of equipment. In the long run, such a pipe saves more money than a cheap black steel equivalent.

One of our clients in the petrochemical sector was faced with the problem of a drop in heat exchanger efficiency after 8 months of operation. Analysis showed that the pipes used had a mechanical connection that failed due to vibration and thermal cycling. Replacing the batch with welded bimetallic pipes returned the system performance to the design values. This case emphasizes: do not chase the low price per meter, consider the cost of ownership over 10 years.

Comparison of core and fin materials

Production technologies and their impact on heat transfer coefficient

The fin manufacturing method determines not only the initial efficiency of the pipe, but also its ability to maintain these properties for years. There are three main ways to create bimetallic pipes: roll forming, winding followed by soldering/welding and extrusion. Each method has its own physical limitations, which directly affect the final thermal conductivity of the system.

Knurling is the most common method for mass production of heating radiators. Aluminum tape is wound onto the pipe and crimped by rollers under high pressure. The problem with this method is that contact is achieved only through friction and plastic deformation. Over time, especially at temperatures above 90°C, the metal “gets tired” and the tightness of the seal decreases. In our tests, knurled pipes lost up to 12% of thermal output after 500 heating and cooling cycles. This is critical for systems where the coolant temperature constantly fluctuates.

High frequency welding (HF welding) is a more technologically advanced process. The tape is fed continuously, and before contacting the pipe, its edges are heated by high-frequency current to the point of melting. As a result, a continuous weld is formed along the entire length of the contact. Such pipes can withstand pressures up to 40 bar and temperatures up to 200°C without loss of tightness and thermal contact. For industrial boiler houses and process lines this is the only acceptable option. The production cost is higher, but the reliability is worth the investment.

Extrusion pipes, where aluminum fins are extruded from a solid sleeve placed on a steel pipe, have a maximum heat transfer area and the absence of joints. However, they are limited in the diameter and complexity of the fin shape. In the context of thermal conductivity, extrusion gives an excellent result, as it eliminates any air gaps. But it is worth remembering that such pipes are more difficult to repair and are less flexible when installed in cramped conditions.

An important parameter that is often overlooked is the pitch and height of the fin. An increase in surface area does not always lead to a proportional increase in heat transfer. If the fins are located too tightly, the air between them stagnates, forming a thermal plug. The optimal pitch depends on the speed of the air flow. For natural convection, high and sparse fins are needed, for forced airflow by a fan - frequent and low ones. Our engineers use CFD (Computational Fluid Dynamics) calculation models to select the geometry for a specific fan, which helps avoid design errors.

Pay attention to the quality of finishing. The sharp edges of the fins can injure installers and contribute to the accumulation of dust, which over time acts as a heat insulator. Quality bimetallic pipes are hydroformed or have rounded edges, which also improves the aerodynamics of air flow around the pipe.

Calculation of heat transfer and selection of parameters for specific tasks

When designing a heating or cooling system, it is not enough to simply select a pipe with the maximum number of fins. It is necessary to carry out an accurate thermal calculation, taking into account the temperature difference between the coolant and the environment, air flow speed and required power. The formula for calculating heat flow Q = k · A · ΔT, where k is the heat transfer coefficient, A is the surface area, ΔT is the temperature difference, seems simple, but in practice each factor is fraught with pitfalls.

The k coefficient for bimetallic pipes is not a constant. It depends on the mode of fluid flow inside the pipe (laminar or turbulent) and the nature of the air flow around the outside. In real conditions, the value of k can vary from 15 to 40 W/(m²·K) depending on the design of the radiator. We recommend incorporating a safety factor of at least 1.15 into the design to compensate for possible surface contamination and loss of efficiency over time.

Surface area A is increased by 5–10 times due to fins compared to a smooth pipe. However, the effective area is always less than the geometric one due to the finning ratio. Some of the heat is wasted to warm the fin itself from the base to the edge, and if the fin is too long, its end remains cold and useless. Fin efficiency for aluminum is typically 0.85–0.95, but for steel it drops to 0.6. This is another argument for using aluminum for the outer part, even if the core is steel.

Temperature difference ΔT is the most controllable parameter. Increasing the temperature of the coolant increases efficiency, but accelerates wear of seals and promotes the formation of scale. In modern energy-efficient buildings, they try to operate at low temperatures (55/45°C instead of the old 90/70°C). In such conditions, the role of bimetallic pipes with a large dispersion area becomes critical. Conventional cast iron or steel registers simply cannot provide comfort at low water temperatures without increasing their dimensions significantly.

Practical example: to heat a warehouse with an area of ​​2000 m² with a ceiling height of 6 meters, we needed to calculate a system based on bimetallic registers. Using smooth pipes would require installing 400 meters of pipeline, which would take up all the usable space. The use of finned tubes with a pitch of 3 mm made it possible to reduce the length to 120 meters while maintaining the same thermal power. In addition, due to the smaller mass of the coolant, the system began to respond faster to automation commands, saving gas during transition seasons.

When ordering equipment, always ask the manufacturer for a passport with real heat transfer measured in an accredited laboratory in accordance with GOST 31311 or European standards EN 442. Figures “out of the blue” or taken from reference books of the 80s can lead to the fact that in winter the room will be cold, and reworking the system will cost more than the initial savings.

Application in industry and housing and communal services: real operating scenarios

The scope of application of bimetallic finned pipes is extremely wide, from private homes to giant refineries. However, the equipment requirements in these sectors differ dramatically. Understanding the specifics of each niche helps to avoid fatal mistakes when purchasing.

Oil and gas industry and chemical production.The main enemies here are corrosion, high pressure and extreme temperatures. Pipelines transport hot oil products, steam or aggressive reagents. In such conditions, pipes with a core made of alloy steel (12Х18Н10Т) and fins made of AD1 aluminum or copper are used. The key requirement is the tightness of the connection. Any fistula means a stop in production and the risk of an environmental disaster. We supplied batches of pipes for pyrolysis plants, where temperatures reach 400°C. Only welded bimetallic structures withstood such loads, while analogues with adhesive joints failed in the first week.

It is in such extreme conditions that the specialized equipment produced by the company manifests itself.Wuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.. Specializing in the development and production of heat exchangers for oil refining, the chemical industry and energy, the company creates solutions where reliability comes first. Wuxi Kaisheng products, including ASME high-pressure heat exchangers, air coolers and waste heat boilers, are made from a wide range of materials, from carbon and stainless steel to titanium, C46400 marine brass, copper-nickel alloys and N06625 nickel alloys. The company's PED and ASME certified equipment demonstrates exceptional corrosion resistance and the ability to operate under high pressures and extreme temperatures, making it an ideal choice for demanding industrial processes such as seawater desalination or shipbuilding.

Central heating systems (HCS).Russian heating systems are known for their unstable pressure and low quality coolant (high hardness, presence of sludge, oxygen corrosion). Cast iron radiators are becoming a thing of the past due to low efficiency and heavy weight. Bimetallic sectional radiators and registers have become the de facto standard. They can withstand water hammer up to 25–30 atm, which is typical for high-rise buildings. Aluminum fins ensure rapid heating of the room, and the steel interior protects against corrosion. It is important to note that for autonomous systems of private houses, where the pressure is stable and the water is prepared, cheaper aluminum radiators can be used, but for centralized networks, bimetal is the only correct solution.

Drying chambers and agro-industrial complex.Dryers for grain, wood or vegetables require uniform heating of large volumes of air. Bimetallic heaters are installed in air ducts. Not only heat transfer is important here, but also hygiene. The smooth surface of aluminum fins is easy to clean and does not accumulate dust as intensively as complex cast structures. In one of the projects to modernize a grain drying complex, replacing old smooth pipes with finned ones made it possible to reduce the drying time of a batch of grain by 18%, which directly affected the enterprise’s profit during the harvest season.

Energy.Turbine condensers and boiler air preheaters use special types of finned tubes with spirally wound fins. They operate under conditions of constant blowing of flue gases or steam. Materials are selected taking into account abrasive wear. Double fins or perforated tapes are often used to intensify heat transfer. The service life of such equipment is expected to last for decades, so quality control at the production stage is stricter here than anywhere else.

Regardless of the industry, the general trend is the transition to individual heating units (IHP) with automatic regulation. Bimetallic pipes, due to their low inertia, are ideal for such dynamic systems. They quickly respond to changes in valve position, allowing you to accurately maintain the room temperature and save resources.

Quality standards and certification: what to look for when purchasing

The heat exchange equipment market is full of counterfeits and low-quality products that only superficially resemble certified products. To protect your project from risks, you need to clearly understand the system of standards and requirements. In Russia and the CIS countries, the main document is GOST 31311-2005 “Heating devices. General technical conditions”, as well as GOST R 58093-2018 for pipes with fins.

An EAC (Eurasian Conformity) certificate of conformity is required for legal sale in the territory of the Customs Union. However, the presence of a piece of paper does not guarantee the quality of a particular batch. We advise you to pay attention to the following parameters in the accompanying documentation:

• Core steel grade.Must be specified specifically (for example, St20, 09G2S, AISI 304), and not just “steel”. The difference in price and properties between these brands is colossal.
• Wall thickness of the inner pipe.Для систем отопления минимум должен составлять 3–3.5 мм для стали. Тонкостенные трубы (2 мм) могут не выдержать гидроудара в старой системе.
• Метод соединения.В паспорте должно быть прямо указано: “высокочастотная сварка” или “экструзия”. Если написано “контактное соединение” без уточнения, скорее всего, это обычная натяжка.
• Испытательное давление.Оно должно превышать рабочее минимум в 1.5 раза. Если рабочее 16 бар, испытательное должно быть не менее 24 бар.

Международные стандарты ISO 9001 подтверждают, что у завода налажена система менеджмента качества, но это не заменяет приемочных испытаний продукции. Для экспортных контрактов часто требуются сертификаты PED (Pressure Equipment Directive) для Европы или ASME для США. Наличие этих маркировок говорит о том, что производитель прошел аудит независимых инспекционных органов. Например, продукция компании «Уси Кайшэн» полностью соответствует этим строгим международным нормам, что позволяет ей успешно поставляться заказчикам по всему миру для реализации самых амбициозных проектов в области энергосбережения и тяжелой промышленности.

В нашей практике был случай, когда партия труб, имевшая все сертификаты, пришла с браком геометрии: ребра были деформированы при транспортировке из-за отсутствия жесткой упаковки. Сертификат был настоящим, но логистика подвела. Поэтому при заключении договора важно прописывать требования к упаковке (деревянные паллеты, стрейч-пленка, угловые защитники) и условиям поставки (Incoterms). Качество продукта заканчивается там, где начинается халатность при погрузке.

Также стоит проверить репутацию завода-изготовителя. Крупные производители имеют собственные испытательные стенды, где каждая партия проходит проверку на герметичность и теплоотдачу. Маленькие гаражные мастерские часто покупают готовые трубы и просто красят их, выдавая за свой продукт. Запросите видео с производства или отчеты о последних испытаниях — честный поставщик не откажет в такой информации.

Frequently Asked Questions

Какая реальная разница в теплоотдаче между биметаллическими и алюминиевыми радиаторами?

В идеальных лабораторных условиях алюминиевые радиаторы показывают теплоотдачу на 10–15% выше за счет отсутствия стальной прослойки. Однако в реальных системах центрального отопления с агрессивным теплоносителем алюминиевые радиаторы быстро деградируют из-за коррозии, теряя до 30% мощности за 3–5 лет. Биметаллические трубы сохраняют стабильные характеристики на протяжении 20–25 лет. Таким образом, начальный выигрыш алюминия нивелируется быстрым старением, делая биметалл более выгодным в долгосрочной перспективе.

Можно ли использовать биметаллические трубы в системах с антифризом?

Да, можно, но с осторожностью. Теплопроводность смесей на основе этиленгликоля или пропиленгликоля ниже, чем у воды (примерно на 15–20%). Это значит, что для получения той же мощности потребуется увеличить площадь поверхности батарей или повысить температуру подачи. Кроме того, некоторые виды антифризов могут агрессивно воздействовать на алюминиевое оребрение при наличии протечек или конденсата. Рекомендуется использовать составы с ингибиторами коррозии, специально предназначенные для алюминиевых сплавов, и регулярно проверять систему на герметичность.

Как определить качество сварки оребрения без разрушающего контроля?

Визуально оценить качество сварного шва сложно, так как он находится под лентой. Однако есть косвенные признаки. Во-первых, попробуйте аккуратно покрутить ребро рукой (на холодном образце). Если оно люфтит или проворачивается — это брак (механическая натяжка или плохая сварка). Во-вторых, посмотрите на торец трубы: у качественной сварной трубы виден четкий контур приварки ленты к основанию без зазоров. В-третьих, запросите у продавца результаты ультразвукового контроля (УЗК) выборочных образцов из партии. Надежные заводы проводят такой контроль регулярно.

Conclusion and recommendations for choosing a supplier

Биметаллические оребренные трубы представляют собой вершину эволюции в технологии теплообмена, сочетая прочность стали и эффективность алюминия. Однако магия работает только при условии соблюдения технологий производства и правильного подбора параметров под конкретную задачу. Попытка сэкономить на качестве соединения или толщине металла сегодня обернется дорогостоящим ремонтом и потерями энергии завтра.

Мы рекомендуем подходить к выбору оборудования системно: начать с аудита вашей системы (давление, качество воды, температурный режим), затем выбрать тип соединения (строго сварка для промышленности) и только потом сравнивать коммерческие предложения. Не стесняйтесь запрашивать образцы для независимой экспертизы — хорошая труба не боится проверки.

Если вы ищете надежного партнера для поставки сложного теплообменного оборудования, способного обеспечить полный цикл от расчета до монтажа и гарантировать соответствие ГОСТ и международным стандартам, обратите внимание на компаниюWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.. Мы предоставляем высококачественные индивидуальные решения, используя передовые материалы, такие как титан, различные сплавы никеля и меди, а также нержавеющие стали. Наша продукция, включая гофрированные трубные пучки и трубные решетки, уже доказала свою эффективность в суровых условиях эксплуатации по всему миру. Мы не просто продаем трубы, мы предоставляем инженерную экспертизу, которая спасает ваши проекты от ошибок.

Узнать актуальные цены на биметаллические трубыorсвязаться с нашим техническим отделомдля консультации по вашему проекту. Помните: правильная труба — это половина успеха вашей системы отопления.