Manufacturing & Testing of Air-to-Refrigerant Heat Exchangers Based on 0.8mm Diameter Tubes

Recent research has shown that moving towards small tubes with diameters less than 5 mm bring significant advantages in air-to-refrigerant heat exchangers. These advantages include compactness, high airside heat transfer coefficient, reduced refrigerant charge and reduced weight. Tubes with Outer Diameter of 0.8 mm were shown to provide very high heat transfer coefficient resulting in heat exchanger with a high degree of compactness. However, using such small tube diameter comes with manufacturing and field performance challenges. Manufacturing challenges include the management of many hundreds of small tubes, tubes geometrical requirements, developing a method to ensure a specific gap between the tubes at any given point is kept, maintaining the desired geometrical shape of the complete heat exchanger and, the biggest challenge, sealing the tube to header joints. Field challenges include areas such as fouling and water drainage. This paper presents the lessons learned from manufacturing and testing of air-to-refrigerant heat exchangers that use such 0.8 mm OD tubes. Three prototypes were manufactured, the first one is made of stainless steel (SSHX), the second one is made of copper (CuHX), and these two were designed to have the same pressure drop and heat transfer performance. The third one is a larger version of the second prototype (CuHX-10kW). These prototypes are fabricated using different manufacturing techniques and lessons learned are discussed. One of the observations was that the uncertainty in tube spacing due to manufacturing has a significant impact on the performance. This points to the need for robust optimization methods in design of such heat exchangers for which the manufacturing technologies are still being developed. Hydrostatic tests as well as performance tests in a closed loop wind tunnel were conducted. Test results of three heat exchangers using air and water are discussed. The energy balances were within ±5% and uncertainties in averaged capacity were within ±3% for the three prototypes.