A component-level analysis of brazed-type capillary tube-suction line heat exchangers

Despite the important role played by capillary tube-suction line heat exchangers in the performance of small refrigerators, they are still sized through trial-and-error procedures. In this context, the present work aims, for the first time, to comprehensively analyze the impact of geometric parameters often overlooked in the design of brazed-type heat exchangers, such as the adiabatic entrance and heat exchange lengths, as well as the width and height of the welded region. To this end, a mathematical model was developed and the walls of both tubes were discretized in a 3D mesh so that the heat transfer could be calculated over a wide range of conditions without the need of an effectiveness correlation. The model was validated within ±10 % error bands for the mass flow rate against 54 points found in the literature for R600a. Moreover, to extend the applicability of the analysis, the simulations were performed considering not only sub-cooled but also two-phase conditions at the capillary tube inlet. For instance, for sub-cooled inlets, the mass flow rate always increases for larger heat exchanger lengths if flashing occurs outside the entrance region. For two-phase inlets, the trends are opposite and the mass flow rate tends to decrease as the heat exchanger length increases. It was also seen that the width of the brazed region and the suction line inner diameter have a minor impact on the mass flow rate. Finally, the heat exchanger effectiveness was found to be nearly insensitive to the adiabatic entrance length.