press fitting, flame free, accelerated testing, aluminum, copper
Failed brazed joints causing refrigerant leakage is a multi-billion dollar problem for the global HVAC&R industry. Leaks are frequently caused due to mechanical fatigue from extreme pressure cycling, temperature cycling including exposure to freeze/thaw cycles, or vibrational wear induced from rotating electrical machinery. In this study, a new, cost-effective type of press fitting suitable for a wide variety of refrigerants and applications is investigated experimentally. Three tests to accelerate mechanical fatigue were devised to simulate real world extreme conditions to determine possible failure modes of refrigerant components and joining technologies. The first test is a combined thermal/pressure shock test designed to simulate abrupt temperature and pressure changes due to start/stop cycles and frost/defrost mode changes. Field failures of brazed joints have been detected due to water being trapped in tight spaces and expanding during freezing, causing high stress on brazed joints. The second test is a vibration test, designed to simulate vibrational loads induced from rotating components in the system. The third and final series of testing is a freeze/thaw cycling profile which simulates ice buildup and defrost observed during heat pump operation. The investigated press fittings are made from aluminum and utilize o-ring seals. Test specimen of six different sizes designed to connect to aluminum tubes having outer diameters between 9.5 mm and 28.5 mm were subjected to the accelerated tests described above. Pressure, temperature, strains and acceleration were measured and analyzed. At the end of each test the exposed specimen were carefully investigated for possible damage and leak tightness confirmed. The results were also compared to earlier test results conducted with an all-copper version of same press fitting technology.