HVAC & R heat exchangers, microchannel, header, brazing
The increased demand for aluminum microchannel condensers in the HVAC&R industry imposes new challenges on the design and manufacturing processes. An aluminum microchannel heat exchanger typically contains three major components: headers, microchannel tubes and multi-louvered fins. The parts are assembled and joined together by a brazing process. To ensure high thermal performance of the brazed product, microchannel heat exchanger designers must carefully consider manufacturing issues related to the brazing process during the design stage. For example, in addition to proper sizing of the header manifolds, it is essential to determine the optimal insertion length of the microchannel tube into the header during the header/tube assembly process. An oversized header diameter not only reduces heat exchanger compactness, but also leads to increased refrigerant charge. It further increases header wall thickness requirements due to increased hoop stress. Too short of an insertion length of the microchannel tube increases the risk of overflow of liquid filler metal into the microchannels during brazing and blockage of the refrigerant passages. On the other hand, small diameter header tubes and long insertion lengths of microchannel tube may contribute to a significant increase of refrigerant side pressure drop. This paper provides both experimental and analytical studies on the influences of header geometry and microchannel tube insertion length on the heat exchanger performance, such as refrigerant side pressure drop. Samples with various header/tube assembly design parameters are prepared and brazed using controlled atmosphere brazing (CAB) technology. Brazed samples are tested in a pressure drop measurement facility. Theoretical modeling of the brazed microchannel heat exchanger is performed and compared with the experimental results. This study provides important guidance and insights with respect to the design of microchannel heat exchangers under consideration of manufacturing issues.