Forging the future

Florian Picard, Fives Cryo, France, introduces an innovative new heat exchanger solution.Improving heat transfer solutions still remains a key objective in any industrial sector. Many types of heat exchangers are available, but the brazed aluminium plate-fin heat exchanger (PFHE) stands out because of its structure, its manufacturing process, and also because of its numerous capabilities at cryogenic temperatures. Fives has developed the stainless steel plate-fin heat exchanger as a solution to the challenges of high temperatures, high pressures and corrosive environments.

PFHE APPLICATIONS AND DESCRIPTIONBrazed aluminium PFHEs are widely used in cryogenic processes, including both onshore and offshore applications, such as indus-trial gas, natural gas liquefaction or reliquefaction (LNG), recovery of natural gas liquids (NGL), helium refrigerators and liquefiers, hydrogen purification, ammonia and olefins processes, nuclear engineering and syngas production. Maximising thermal efficiency with high availability is the main purpose for such cryogenic processes. PFHEs promote exchange between many streams simulta-neously (cases with more than 12 streams are common) leading to efficient energy savings.

Fives’ brazed heat exchanger consists of a block (core) of alternating layers containing corrugated fins, separated by parting sheets (see Figure 1). Each layer is sealed along its edges with bars; the inlet and outlet ports of each stream are left open. After brazing the core in a vacuum furnace, headers and nozzles are welded to allow for connection to the inlet and outlet piping. The large range of complex configurations for layer geometries provides attractive applications for flow arrangements and saves cost and energy through a compact design, high thermal efficiency and multi-streams heat exchange. The layer arrangement imposes the overall heat transfer performances and temperature profiles, as well as distribution, longitudinal conduction and heat leaks.

Figure 1 - Structural description of PFHE

Cryogenics | Energy

MADE-TO-MEASURE DESIGNThese custom-made multi-stream heat exchangers are designed according to customer specifications. Several sets of operating condi-tions and additional constraints are usually specified for the same heat exchanger.

The design of such complex PFHEs is performed with advanced in-house software. Thermodynamic studies, design simulations, and process simulations are performed at every step of the designing process.

Every component of the structure is directly selected and manufactured on demand. Each piece of the heat exchanger is thus defined to achieve the best optimized heat exchanger configuration.

FINSA main feature and advantage of PFHEs is the large heat transfer surface area per unit volume. The configuration of the fins is versatile and can be fitted to find the best compromise between thermal, hydraulic and mechanical performances, thus maximizing the surface area of the wall between the two fluids while minimizing resistance to fluid flow through the heat exchanger. A large range of such fin types are available (Figure 2) and can be used in accordance with heat transfer purposes and pressure drop characteristics for each individual stream. Their dimensional parameters (thickness, height, serration, pitch, perforation) can be directly adapted to find the best dedicated overall heat exchanger solution. These fins typically have a height of 2 – 12 mm, thickness of 0.15 – 0.7 mm and a pitch of 1 – 4.5 mm.

Figure 2 - Typical shapes of fins (source: Alpema)

BRAZING PROCESSFrom a theoretical standpoint, brazing is a method of joining two pieces of metal together with a third molten filler metal (this brazing alloy having a lower melting point than the adjoining metal). The filler metal is heated slightly above its melting temperature. It then flows over the base metal by capillary action, before finally cooling down to join the work pieces together.

It is therefore possible to braze a large PFHE (approximately 20 m3), and to produce several cores a day. The heat exchanger is com-pletely brazed in one piece in a dedicated vacuum furnace, at about 600°C, over more than 10 hours. The set of components (bars, fins, parting sheets) is brazed all together in order to become a one-piece core.

Of all the methods available for metal joining, brazing is amongst the most versatile. Brazed joints have excellent tensile strength – they are often stronger than the two metals being bonded together.

NEW SOLUTIONFives has been developing brazed stainless steel PFHE technology (Figure 3). This new technology is proposed in order to extend the current design conditions range (limited to cryogenic temperatures and a maximum of 150 bars) to high temperatures, high pressures and corrosive environments (see Figure 4), while retaining key PFHE features such as efficiency, compactness, multi-streams capabilities, and good mechanical characteristics.

Figure 3 - Stainless steel PFHE Figure 4 - Operating ranges of aluminium and stainless steel PFHEs.

COMPACTNESSFor a given performance, the heat exchanger design is optimised to be as small as possible. Its weight can be advantageously reduced for the same duty, bringing some benefits to offshore or on-board applications for which a lighter weight results in a reduc-tion in CAPEX costs.

Moreover, such miniaturization with enhanced heat transfer capabilities encourages the development of complex and efficient small scale processes, while limiting the cost of production and the environmental footprint. Small heat exchangers can also be directly combined with the main dedicated operation unit, such as the distillation column or the absorber.

Another added benefit is that fluid inventory within the heat exchanger in operation is also reduced compared to a standard heat exchanger, increasing the level of safety of the whole process.

EFFICIENCYHeat exchangers are designed to maximize the heat transfer surface area between the streams, while minimizing resistance to fluid flow through the exchanger. For example, compared to a traditional shell and tube technology, stainless steel PFHEs have a larger surface area. A small temperature difference between cold and hot streams is then admissible: a pinch temperature of less than 3°C is still enough to maintain good heat transfer capabilities, while maintaining adequate pressure drops. This maximization of efficien-cy could bring a new solution to reduce OPEX costs, by improving process efficiency and limiting energy consumption.

MULTI-STREAMSSeveral streams can be combined into the same heat exchanger, thus extending the scope of possibility compared to two-stream heat exchangers. In the same process, several heat transfer units could then be integrated in the same heat exchanger (Figure 5). This process simplification should reduce the number of heat exchangers, limit the piping and support components, increase the efficiency of the system, and finally favor a higher potential return on investment.

Figure 5 - Assembly of heat exchangers.

MECHANICAL CHARACTERISTICSHigh temperature gradients and high pressure loads can be managed within this brazed PFHE structure. Mechanical stresses can be assessed using advanced simulation tools (see Figure 6). Moreover, any components of the heat exchanger can also be adapted in order to achieve the required performances.

The multi-chamber pressure vessel technology allows various pressures to be applied individually to each stream in a single heat exchanger. A high pressure difference between streams is thus admissible.

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CONTACT USFives Cryo25 bis rue du Fort - BP 87 - 88194 Golbey cedex - FranceTel.: +33 (0)3 29 68 00 00 - Fax.: +33 (0)3 29 31 22 18Email: bahx@fivesgroup.com – Website: www.fivesgroup.com

Figure 6 - Cartography of thermo-mechanical stresses.

COMMERCIAL PERSPECTIVESRecent advances in the design range and operational reliability could make PFHEs attractive in various industries, particularly off-shore applications such as intercoolers for gas compression systems, process heat exchangers for gas treatment or LNG industries, regasification, and LNG transportation, etc.

This new heat exchanger technology can also be cost effective in a wide range of applications, particularly in the field of energy.

CONCLUSIONThere is a current trend for ‘doing more with less’: improved production, quality, and safety with less CAPEX and OPEX. Numerous new solutions are emerging in every chemical engineering field to improve production management, opening up a broad range of options.

Heat transfer solutions proposed by Fives have been closely developed according to users’ needs. PFHE technology proposes enhanced features such as efficiency, compactness, multi-streams capabilities, and good mechanical characteristics. Today, alumi-nium PFHEs are widely used in cryogenic applications.

Stainless steel PFHE technology can be applied to high temperature, high pressure, and corrosive environments. This efficient, com-pact and multi-streams heat transfer solution brings many benefits in both capital and operational cost savings for many present and future applications.