Brazed Plate Heat Exchanger

In our brazing project, we utilized copper as filler metal for stainless-steel plates, chosen based on its optimal melting temperature. Through meticulous testing, we identified the suitable 30-micrometer thickness for consistent results. Our innovative brazing process, tailored for heating, holding, and cooling stages in a vacuum, ensures precise bonding without melting the base plates. Addressing challenges in material selection, filler metal thickness, and heat cycle optimization, our research advances efficient and reliable brazing practices for manufacturing of brazed plate heat exchangers.

Figure 1. A brazed plate heat exchanger.

Research Highlights

  • Copper, chosen as the filler metal, demonstrated optimal compatibility with stainless steel, ensuring a successful brazing process without compromising plate properties.
  • Careful experimentation with the thermal cycle proved crucial, highlighting the importance of maintaining the brazing temperature for an adequate duration. This approach facilitated effective filler metal penetration into the plates, ensuring a robust metallurgical bond without structural compromise.

Challenges

Addressing the challenges in the brazing process involves meticulous considerations. Selecting an appropriate filler metal is a primary challenge, as it requires a delicate balance—ensuring the filler’s melting point is below that of the plates to prevent plate melting, while enabling the filler to melt during brazing. The thickness of the filler metal plays a crucial role in establishing a robust metallurgical bond post-brazing. Furthermore, determining the optimal thermal cycle for the brazing process is equally critical. Achieving this delicate equilibrium is essential for successful and high-quality brazing outcomes.

Our Solution

In our approach, copper was employed because of its compatibility to stainless steel and low melting temperature. To determine the filler metal thickness, tests are conducted with filler metals of different thicknesses under consistent conditions. The results suggest that a thickness of 30 micrometers is suitable based on the thickness of the filler metal sheets. To optimize the brazing conditions, multiple experiments are performed by altering the heat cycle. The brazing heat cycle involves three stages: heating, holding, and cooling (Figure 2). It is crucial to emphasize that this process must occur in vacuum furnace. It is important to note that brazing must take place in a vacuum furnace to ensure the absence of air.

Figure 2. Brazing of plate heat exchangers. (A) Process heat cycle, and (B) Micrograph showing a proper connection of stainless steel and copper.

To optimize the brazing conditions, experiments were conducted multiple times by altering the heat cycle. The brazing heat cycle consists of three parts: heating, holding, and cooling. During the heating phase, the temperature increases at a constant rate. Once the brazing temperature is reached, it is maintained for a sufficient duration. Subsequently, the piece is cooled at a constant rate. It is essential to note that this process must be carried out in a vacuum furnace. As apparent in Figure 3, the copper filler metal penetrated the stainless-steel plates after the current brazing process.

Figure 3. Copper diffusion into stainless steel during the brazing process. (A) Metallographic sample, (B) EDS map of the material penetrated the plates.

Outcomes

  • Brazing at a temperature close to the melting point of the copper filler metal was achieved, and it provides the best results to not affect the mechanical and metallurgical properties of the plates.
  • Maintaining the brazing temperature for a sufficient duration allowed the filler metal to penetrate into the plates, successfully creating a metallurgical bond.
  • A high-quality connection was conducted using the minimum heating and cooling rates for brazing process during this project.

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