Application of bimetallic finned tubes in boilers 

Why bimetallic finned tubes have become the standard for modern boilers

In our heat exchange design practice, we are seeing a clear shift: the use of bimetallic finned tubes in boilers has ceased to be a niche solution for aggressive environments and has become the basic standard for energy efficient systems. If ten years ago engineers were choosing between cheap carbon steel fins and expensive solid stainless steel profiles, today bimetal occupies a dominant position. This has nothing to do with marketing, but with the hard mathematics of operating costs. The combination of an inner tube made of stainless steel (or a copper-based alloy) and outer fins made of aluminum allows the weight of the heat exchanger to be reduced by 40-50% while maintaining the thermal resistance characteristic of monolithic structures.

The key problem that this technology solves is corrosion destruction under the insulation and the high cost of the material. In real operating conditions of boiler plants, especially those operating on solid fuel or biomass, the temperature of the flue gases often fluctuates in a range that causes condensation of acidic compounds. An ordinary steel pipe with welded fins under such conditions begins to rot on the outside, and the process is accelerated due to the capillary effect between the fins. A bimetallic pipe made by extrusion or high-frequency welding followed by processing eliminates this gap. We have recorded cases where switching to bimetal increased the service life of the economizer from 3 to 12 years without major repairs.

However, blind implementation of bimetal without taking into account hydraulic resistance and temperature expansion can lead to catastrophic consequences. One of our clients in Siberia experienced depressurization of the collector six months after the launch of a new boiler house. The reason lay not in the quality of the pipes, but in ignoring the linear expansion coefficient of aluminum, which is 1.5 times higher than that of steel. During cyclic heating and cooling, critical stresses arose in the pipe-collector transition zone. This article examines in detail the technical nuances that separate a successful project from an emergency situation, based on GOST standards and real cases of modernization of industrial boilers.

Technological features and physics of heat transfer

The use of bimetallic finned tubes in boilers is based on the fundamental principle of separation of material functions. The internal support pipe is responsible for strength, pressure and corrosion resistance from the working medium (water, steam, oil), while the external fins maximize the heat exchange area with the gas medium. The effectiveness of this tandem directly depends on the quality of contact between the two metals. Ideally, the thermal resistance of the contact should tend to zero. The extrusion method, in which an aluminum sleeve is stretched onto a steel base and then extruded into the shape of the fins, provides the best results. The pressure of the metal during the production process creates a monolithic structure, where heat transfer occurs almost as efficiently as in a solid product.

Let's consider the physics of the process in more detail. The thermal conductivity coefficient of aluminum is about 200-230 W/(m K), while for stainless steel it rarely exceeds 15-17 W/(m K). If we made the entire pipe from stainless steel, we would need to increase the surface area several times to compensate for the low conductivity of the material, which would lead to a colossal increase in the size and weight of the boiler. Using bimetal, we transfer the main volume of heat exchange to the zone of highly conductive aluminum. Gases wash the fins, quickly releasing heat, which is instantly transmitted through tight contact to the steel core and further to the coolant.

There is a misconception that any type of fin is equally effective. In practice, we see three main types of connection, and the choice between them determines the fate of the equipment. The first type is tension fins (L-shaped profile). This is the cheapest option, but it is absolutely not suitable for boilers with frequent thermal cycles. When heated, aluminum expands more than steel, the contact weakens, and an air gap is formed - an ideal heat insulator. The efficiency of such a unit drops by 30-40% after the first year of operation. The second type is high-frequency welding (HF-welding). Here the rib is welded to the pipe along the entire length of the contact. It is reliable, but the weld area is susceptible to corrosion unless it is made of compatible alloys. The third type is extruded fins. This is what we recommend for critical components of high-pressure boilers. The absence of welds at the interface between metals eliminates the point at which corrosion begins.

It is important to understand the effect of fin pitch on boiler aerodynamics. Reducing the pitch increases the heat transfer area, but also sharply increases the resistance to gas flow. In our calculations, the optimal pitch for most industrial hot water boilers is the range of 2.5–3.5 mm. An attempt to save space and install fins with a pitch of 1.5 mm leads to the fact that the smoke exhausters operate at extreme conditions, consuming excess electricity and creating the risk of the draft overturning. We carried out measurements at a site in Kazakhstan, where replacing pipes with more densely finned ones without recalculating the ventilation system led to a 15% drop in boiler power due to insufficient combustion of the furnace.

The influence of temperature and pressure on material selection

The choice of a pair of metals for a bimetallic pipe is dictated by the parameters of the working environment inside the boiler. For low-temperature zones, such as air heaters or the last strokes of economizers, where the metal temperature does not exceed 250°C and the pressure is below 2.5 MPa, the classic “carbon steel + aluminum” pair works flawlessly. However, in areas of steam overheating or in the first passages of a convective shaft, where temperatures reach 450-500°C, the properties of aluminum change. The tensile strength of aluminum alloys at such temperatures decreases, and they can begin to “crawl” under their own weight or gas pressure.

In such cases, we switch to the “stainless steel + stainless steel” scheme or use special heat-resistant alloys for fins. Yes, this increases the cost of the structure by 2-3 times, but the alternative is deformation of the ribs and disruption of the geometry of the gas duct. It is also worth considering the phenomenon of intergranular corrosion. If the inner tube is made of AISI 304 steel and the outer fins are made of 1000 series aluminum, the galvanic couple in the presence of electrolyte (condensate) will work against us. The potentials of metals are too different. Therefore, it is critical to use protective coatings or select pairs with similar electrochemical potentials, or ensure complete dryness of the outer surface, which is almost impossible in the realities of burning wet biomass.

Application scenarios: from thermal power plants to private boiler houses

The use of bimetallic finned tubes in boilers most clearly demonstrates its effectiveness in two polar scenarios: the modernization of old coal-fired thermal power plants and the construction of new compact modular boiler houses. In the first case, the customer’s main pain is loss of productivity due to contamination of surfaces with scale and soot, as well as the inability to increase power without replacing the entire boiler body. The second is the requirement to fit high power into a minimum transport size for delivery by rail.

Case 1: Modernization of the economizer at a coal-fired thermal power plant (Power 150 MW)

The task was to increase the temperature of the feed water at the entrance to the drum by 15°C without increasing the hydraulic resistance of the path. The old smooth-tube packages occupied a huge volume and had a low heat transfer coefficient from the gases. The engineering solution was to replace the bundles with bimetallic pipes with extruded fins (steel 20 + aluminum AD1). The heat exchange area has increased 8 times with the same volume of occupied space. The result exceeded expectations: the water temperature increased by 18°C, which reduced fuel consumption by 2.3%. The payback period for the project was 14 months. An important nuance was the use of spiral scrubbers for cleaning pipes, since dense fins are prone to clogging with ash if the blowing is not organized correctly.

Case 2: Compact wood chip boiler for a woodworking plant

The key factor here was the hostile environment. Burning bark and wet woodchips generates flue gases with a high content of acidic condensates at temperatures below 140°C. Ordinary steel pipes rotted within 2 seasons. Solution: bimetallic pipes with an internal pipe made of acid-resistant stainless steel AISI 316Ti and external fins made of anodic coated aluminum. Stainless steel maintained pressure and protected against acid from the inside (condensation flows inside during stops), and aluminum provided heat removal. A special fin coating prevented pitting from chlorides contained in the bark. The service life of the node is predicted to be 10 years, which is 5 times higher than the previous figure. The cost of the solution was 40% higher, but the costs of equipment downtime during repairs covered this difference many times over.

These examples show that there is no universal recipe. One type of bimetal is suitable for clean gas, but a completely different one for waste incineration. A mistake in choosing the inner tube material can cost an accident, and a mistake in choosing the fin material can result in loss of efficiency. We recommend that you always conduct a chemical analysis of fuel and condensate before purchasing pipes.

Calculation of economic efficiency and payback period

When making equipment purchasing decisions, the CFO often looks only at the price per kilogram of pipe. This is a dangerous oversimplification. A bimetallic pipe weighs less than its pure stainless steel counterpart, but costs more than black steel with welded plates. To understand the real benefit, you need to consider TCO (Total Cost of Ownership). The formula is simple: initial cost + (downtime cost × repair frequency) + (energy cost × service life).

Let's look at the numbers. Let's take an economizer area of 1000 m².

• Option A (Steel St20 + welded strip fins):Low starting price. But due to the low thermal conductivity of steel and the presence of welds, the actual efficiency is 15% lower than the calculated one. After 4 years, replacement is required due to corrosion of the seams. Over 12 years, 3 replacements will be required. Plus excessive fuel consumption due to lower efficiency.
• Option B (Bimetal St20+Aluminium, extrusion):The starting price is 25% higher. Heat transfer is 30% higher due to better contact and properties of aluminum. Service life - 12+ years without replacement. Fuel savings due to better heat removal are about 3-4% annually.

Over a 12-year horizon, Option B turns out to be 35-40% cheaper in terms of the thermal energy produced. In addition, the weight of the bimetallic structure is lower, which reduces the load on the metal frame of the boiler and the foundation, giving additional savings on construction and installation work of up to 10%.

Another hidden factor is hydraulics. More efficient heat transfer allows the dimensions of the heat exchanger to be reduced. Less metal volume means less system inertia. The boiler reaches operating mode faster and reacts faster to load changes. For industrial enterprises where the steam consumption schedule is uneven, this is an opportunity to manage the process more flexibly and avoid releasing excess steam into the atmosphere, which is a direct loss of money.

Typical installation and operation errors

Even the highest quality bimetallic pipe can fail prematurely due to errors at the assembly stage or improper operation. In our practice, we have identified three critical problems that installation organizations face.

1. Damage to the fins during transportation and assembly.
Aluminum fins, especially thin ones (0.3-0.5 mm), are quite fragile under shock loads. A common sight on a construction site: pipes are removed from the packages by a crane without soft slings, the fins are wrinkled, and the density of the fins is disrupted. In places of deformation, the aerodynamics of the flow changes, zones of gas stagnation, local overheating or underheating appear. Moreover, crushed fins are more difficult to remove soot from.Recommendation:Request your supplier to pack in wooden boxes with spacers and use only textile slings when rigging. Deformed areas must be carefully straightened with special combs, and not left as is.

2. Expansion problems in tube sheets.
As mentioned earlier, the coefficient of thermal expansion of aluminum and steel is different. If the pipe is rigidly fixed in the tube sheet without compensation, at the first serious heating, enormous shear forces will arise. This can lead to flaring of the pipe or a crack in the grill itself. Proper installation involves the use of floating supports or special expansion joints that allow the inner steel pipe to move independently of the outer aluminum profile, or the use of structures where the load is transferred only to the steel core.Error:an attempt to weld aluminum fins to a steel chamber. This is a gross violation of technology, leading to instant destruction of the seam.

3. Incorrect blowing mode.
Steam or sonic blowers are often used to clean bimetallic surfaces. The mistake lies in choosing excess steam pressure. A jet of steam under pressure above 1.5 MPa, directed perpendicular to thin aluminum ribs, works like a sandblaster, gradually cutting off the metal. We saw bundles where, after a year of work, only “stumps” remained from the fins.Rule:The blowing pressure must be strictly regulated (usually no more than 0.8-1.0 MPa for aluminum), and the angle of attack of the jet must be as close as possible to the tangent in order to clean off soot and not cut metal.

Quality standards and certification (GOST, ISO, EAC)

The market is saturated with offers, but not all bimetallic pipes are the same. When purchasing boilers that are subject to registration with Rostekhnadzor or similar bodies of the EAEU countries, certificates are required. Key documents to rely on:

• GOST R 53922-2010(and analogues in the CIS countries): Regulates the technical specifications for steel electric-welded straight-seam pipes. Although it describes the base, for bimetal, additional specifications (Technical Conditions) of the manufacturer are important, where the parameters for connecting the layers are specified.
• ISO 9001: Certification of the manufacturer's quality management system. Having this certificate ensures that the plant controls the extrusion or welding process and does not rely on luck.
• EAEU Certificate of Conformity (EAC): Confirms the safety of products for use under pressure. Pay attention to the HS code and the hazard group of the equipment.

Pay special attention to bond strength test protocols. A high-quality bimetallic pipe must withstand a shear force of at least 20-25 MPa. If a supplier can't provide a lab report on metal bond strength, that's a red flag. Poor contact will mean that after a year of operation, your “bimetallic” pipe will turn into two separate parts with an air gap between them, and you will pay for aluminum that does not work.

Also check the chemical composition. Unscrupulous manufacturers often use recycled aluminum for fins. This material has slag inclusions, which reduce thermal conductivity and ductility. Visually, this can be seen by the matte, heterogeneous color of the ribs and the presence of micropores. True virgin aluminum has a uniform silvery sheen.

How to choose a reliable supplier: checklist

Choosing a supplier of bimetallic pipes is not just about finding the lowest price in the price list. This is the choice of a technology partner on which the continuity of your energy supply depends. Here is the algorithm of actions that we recommend to our clients when conducting a tender:

1. Request a reference list.Don't believe the words “we supply everyone”. Ask for contacts of 2-3 facilities where their pipes have been operating for more than 3 years under similar conditions (temperature, fuel type). Call the main power engineer for this facility. Ask directly: “Were there any fistulas? How does the fin hold up after cleaning?”
2. Check production capacity.If possible, visit the factory. You should not be interested in the sales office, but in the extrusion or welding workshop. Do they have their own quality control? Есть ли установка для неразрушающего контроля (вихретоковый или ультразвуковой) каждой погонной метры трубы? Отсутствием такого контроля грешат многие мелкие перекупщики.
3. Уточните сроки и логистику.Биметаллические трубы — продукт специфический, он не лежит на складе в любых объемах. Реальный срок производства партии под заказ составляет 3-5 недель. Если вам обещают отгрузку “завтра” большой объем — скорее всего, вам предлагают неликвид или трубы с нарушением геометрии, от которых отказались другие заказчики.
4. Требуйте образец для тестов.Перед подписанием крупного контракта закажите пробную партию (50-100 метров). Проведите свои испытания: разрежьте трубу, проверьте плотность прилегания, попробуйте согнуть ребро (оно не должно отломиться у основания). Положительный опыт с пробной партией страхует от многомиллионных убытков.

В контексте выбора надежного партнера важно обращать внимание на компании с широким спектром компетенций в области теплообмена и работы со сложными сплавами. For example,Wuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.специализируется не только на стандартных решениях, но и на разработке высокотехнологичного оборудования для экстремальных условий. Их опыт в производстве титановых кожухотрубных теплообменников, аппаратов ASME высокого давления и пучков из специальных сплавов (никелевых N06625, медно-никелевых, морской латуни C46400) демонстрирует глубо понимание металлургии и физики процессов. Такая экспертиза критически важна, когда речь заходит о подборе материалов для биметаллических труб: способность работать с коррозионностойкими сталями (316, 321) и гарантировать качество соединений согласно международным стандартам PED и ASME говорит о том, что поставщик способен обеспечить надежность даже в самых агрессивных средах нефтепереработки, химической промышленности или опреснения воды. Сотрудничество с такими производителями, предлагающими индивидуальные решения и полный цикл контроля качества, минимизирует риски при модернизации энергообъектов.

Мы в своей работе придерживаемся принципа прозрачности. Если ваши условия эксплуатации выходят за рамки стандартных (например, температура выше 500°C или наличие фтороводорода в газах), мы честно скажем, что стандартный биметалл не подойдет, и предложим альтернативу из специальных сплавов, даже если маржа с такой сделки будет ниже. Наша репутация строится на том, что оборудование работает, а не на том, что оно было продано.

Frequently Asked Questions

Какова максимальная рабочая температура для алюминиевого оребрения?

Стандартные алюминиевые сплавы (серии 1000, 3000, 6000) начинают терять прочностные характеристики при температуре металла выше 250-280°C. Для длительной эксплуатации в зонах с температурой дымовых газов выше 400°C (где температура стенки трубы может превышать 300°C) использование алюминия не рекомендуется. В таких случаях необходимо применять биметаллические трубы с оребрением из нержавеющей стали или легированных жаропрочных сплавов. Превышение температурного порога приведет к необратимой деформации ребер (“оплыванию”) и потере эффективности теплообмена.

Можно ли ремонтировать поврежденные ребра на месте эксплуатации?

Локальный ремонт отдельных поврежденных ребер возможен, но экономически и технически нецелесообразен для больших площадей. Мелкие замины можно расправить специальной гребенкой. Однако, если оребрение разрушено коррозией или механически срезано на значительном участке, замена отдельной трубы в пучке часто требует остановки котла и частичной разборки каркаса. В нашей практике мы рекомендуем иметь на складе страховой запас труб (3-5% от общего количества) для оперативной замены вышедших из строя элементов во время плановых ремонтов, вместо попытки “латать” старые.

В чем разница между экструдированным и накатным оребрением?

Главное отличие — в надежности теплового контакта. При накатном методе (rolling-on) алюминиевая лента навивается на трубу и поджимается роликами. Со временем, из-за разницы температурных расширений, контакт может ослабнуть. Экструдированное оребрение создается путем выдавливания металла из гильзы, натянутой на трубу, образуя с ней единое целое без швов и зазоров. Для котлов, работающих в режиме частых пусков-остановов (пиковые нагрузки), экструзия является безальтернативным вариантом, гарантирующим стабильную теплопередачу на протяжении всего срока службы.

Подвержены ли биметаллические трубы электрохимической коррозии?

Да, пара “сталь-алюминий” является гальванической. Однако в конструкции трубы этот риск минимизирован тем, что контакт металлов происходит внутри, в сухой среде, без доступа электролита. Основная опасность возникает снаружи, если конденсат с кислотными примесями попадает в зазоры между ребрами и трубой (для неэкструдированных типов). Для защиты внешних поверхностей в агрессивных средах мы рекомендуем использовать трубы с оребрением из коррозионностойких алюминиевых сплавов или наносить специальные полимерные покрытия, устойчивые к высоким температурам.

Conclusion and next steps

Применение биметаллических оребренных труб в котлах — это инвестиция в надежность и энергоэффективность вашего предприятия. Переход от традиционных решений к биметаллу позволяет сократить расходы на топливо, уменьшить габариты оборудования и забыть о частых ремонтах из-за коррозии. Однако успех зависит от правильного подбора марки сталей и сплавов под конкретные условия сжигания, а также от квалификации монтажной бригады.

Не рискуйте безопасностью своего производства, покупая трубы у непроверенных поставщиков с сомнительной геометрией. Наши инженеры готовы провести бесплатный аудит вашего текущего теплообменного оборудования, рассчитать потенциал экономии и подобрать оптимальную конфигурацию биметаллических труб именно для вашего котла. Мы предоставляем полную техническую документацию, сертификаты ЕАЭС и гарантию на герметичность соединения слоев.

Если вы планируете модернизацию котельной или строительство нового объекта, свяжитесь с нами сегодня для получения детального технико-коммерческого предложения. Мы поможем избежать ошибок, которые стоят миллионов, и обеспечим ваш объект теплом на десятилетия вперед.

Contact us todayдля консультации с ведущим инженером проекта.