Microchannel heat exchangers: fabrication process
Air-conditioning and refrigeration industries have been evolving rapidly over the past decades. The latest energy efficiency regulations and phase-out of ozone-depleting refrigerants spur further development of air-to-refrigerant heat exchangers, making microchannel heat exchangers the better alternative to conventional heat exchangers for the majority of HVAC manufacturers. Heat transfer technology based on microchannel coils offers the largest potential for improving energy efficiency, reducing refrigerant volumes required, while also providing a number of other benefits.
The traditional way of manufacturing finned tube heat exchangers by mechanical tube expansion have the disadvantage of lack of sufficient contact between tubes and fins. On the other hand, the brazing process that metallurgically bonds fins and tubes in microchannel coils eliminates the drawback of contact resistance.
Brazing of aluminium involves joining of components with an aluminum-silicon alloy (AlSi) whose melting point is appreciably lower than that of aluminium. Heat exchanger assembly can be heated to a temperature higher than the melting point of brazing alloy but below that of the aluminium, leading to forming a metallurgical bond between the joining surfaces upon cooling. Meanwhile, corrosion-resistant oxide film melts at a much higher temperature than aluminium and, therefore, must be removed before brazing. Flux, a potassium aluminum fluoride salt, is then used to dissolve the oxide film barrier and prevent further oxidation during the brazing process.
The whole fabrication process for the microchannel coils consists of the following steps:
- Assembling heat exchanger core. The components of a heat exchanger – manifolds, flat tubes, fins, and others – are assembled and fixed in place by a core builder. Fixturing strips maintain the geometry of the heat exchanger during the brazing process.
- Thermal degreasing. At this step, impurities, residual lubricants and oils are removed from the surface by applying a specific temperature.
- Fluxing. At this stage, flux is applied to the coil as an aqueous suspension. Then, air blow-off is used to remove excess flux slurry and evenly distribute it throughout the coil. The pre-fluxing technique can also be used for specific heat exchanger components like manifolds and flat tubes. Pre-fluxing includes spraying a mixture of flux and silicon powder using a binder.
- Drying. The next phase is the removal of residual moisture from the fluxing stage before brazing. Drying is carried out at coil surface temperatures between 200 and 250°C.
- Brazing. Brazing is an industrial process used to join heat exchanger components into a rigid assembly. The process is carried out in an inert (nitrogen) atmosphere of brazing furnace (Controlled Atmosphere Brazing, CAB). As the coil travels inside a tunnel-type furnace, temperature increases, and flux starts melting at the temperature of about 565°C, followed by the melting of the brazing alloy at about 580°C. Oxygen and moisture are at the lowest concentrations at this stage; filler metal flows into the joints by the capillary process.
- Cooling. Solidification of the filler metal takes place at the cooling stage whereby a metallurgical bond is formed between all parts of the heat exchanger assembly. The flux residue remains on the heat exchanger surface as a thin film (1 to 2µm).
- Testing. This last stage consists of a series of checks, including leakage tests, pressure tests, geometry checks, brazing quality control, and the coil becomes ready for packaging or further manufacturing stages such as bending or coating.
Research and advancements in the area of aluminium brazing are crucial to the continued development and predominance of microchannel heat transfer technology. Brazing techniques become more precise and drive technology towards lower cost, higher strength, and achieving greater corrosion resistance of end products.
