Development and Evaluation of Copper Tube and Fittings Used in R-410A Applications.

Jamison, Tommy L.; Stout, Charles A.

Why do some customers experience gas leaks in air conditioning and refrigeration units before ten years of usage? Context is required to answer such a question. Consensus standards (ASME B31.5 [2007], ASTM B280 [2008], ASME B 16.22 [2001]) governing the production and use of copper tube and fittings deem most of these products insufficient to meet the higher operating temperature and pressure requirements of R-410A and other refrigerants. Though these standards appear to err on the conservative side, data taken from refrigeration and air-conditioning products seem to suggest that equipment manufacturers (OEMs) overestimate the performance of materials. For example, according to ASME B16.22, a 3/8 in. (9.5 mm) copper fitting with a mininum wall thickness of 0.026 in. (0.660 mm) is rated at 660 psi (4.55 MPa) at 150°F (66°C). However, data taken from OEM (R-410A) products show internal wall thicknesses as low as 0.009 in. (0.229 mm) in some failed units. The standard calculation for a wall of 0.009 in. (0.229 mm) would result in an allowable operating pressure of less than 300 psi (2.07 MPa). Where then is the discrepancy? The purpose of this paper has four parts: 1. Baseline the critical material properties, in addition to wall thickness and burst pressure, necessary to meet the higher pressure and temperature requirements. 2. Confirm the manufacturing process parameters that ensure the best material properties. 3. Provide an accurate, performance-based (empirical) method to calculate maximum operating pressures for annealed copper tube and fittings. 4. Validate the improved material properties through accelerated life test equivalent to 30 years in an R-410A system. Four different types of tests were conducted to determine the long-term effects of increased pressure and temperature on air conditioning and refrigeration copper tube and fittings. The tube and fittings were produced with a proprietary manufacturing process designed to eliminate fatigue failure while minimizing deformation during strain hardening. Parameter changes in the casting, drawing, annealing, and forming processes for 7/8 in. (22.2 mm) to 21/8 in. (54.0 mm) diameters enabled the tube and fitting assemblies to strain harden (without hydro forming) and withstand significantly higher pressures at increased temperatures. The four tests are as follows: 1. Hydrostatic pressure test with a different assembly at each pressure and temperature setting (hoop strain): Pressures were set at 635 psi (4.4 MPa), 800 psi (5.5 MPa), 1000 psi (6.9 MPa), and 1500 psi (10.3 MPa) with temperatures set at 68°F (20°C), 180°F (82°C), and 250°F (121°C). 2. Hydrostatic pressure test using a single assembly per temperature (hoop strain hardening): Temperatures were set at 68°F (20°C), 180°F (82°C), and 250°F (121°C). At each temperature, pressures were set at 635 psi (4.4 MPa), 800 psi (5.5 MPa), 1000 psi (6.9 MPa), and 1500 psi (10.3 MPa). Measurements of changes in length and diameter of tube and fittings were taken at each pressure level. 3. Fatigue Test at 180°F (82°C) and 650 psi (4.48 MPa) for 1000; 5000; 10,000; 20,000; and 50,000 cycles. Each cycle started at 100 psi (689 kPa), pulsed to 650 psi (4.48 MPa), and repeated every 4 seconds. Measurements of changes in length and diameter of tube and fittings were taken at each increment of cycles. 4. Accelerated life test using a heat pump fitted with a line set assembly with sizes from 7/8 in. (22.2 mm) to 2-1/8 in. (54.0 mm). The heat pump was modified for a 5-minute heating and cooling cycle (2.5 minutes cooling and 2.5 minutes heating). Measurements of changes in length and diameter of tube and fittings were taken at 20; 379; 900; 3200; 5700; 8740; 13,980; and 18,060 cycles, providing an accelerated life estimate of approximately 32 years. At completion of testing, no failures were observed in the proprietary material. One failure was observed internal to the heat pump system utilized in the testing. This failure is attributed to the very thin wall inner-groove copper tubing.

COPPER tubes; GAS leakage; AIR conditioning; REFRIGERATION & refrigerating machinery; TEMPERATURE effect; REFRIGERANTS; MANUFACTURING processes

ASHRAE Transactions, 2011, Vol 117, Issue 2, p725

0001-2505

Conference Proceeding