The air conditioning and refrigeration industry continues to take steps internationally to minimize the potential effects of refrigerant leakage, release and misuse on global warming and ozone depletion, which are increasingly linked to human activity.
To curb the impact on our environment, many refrigerants previously used have been restricted and in some cases banned completely. Refrigerants are being identified with an associated Global Warming Potential (GWP) number. This GWP number compares the global warming potential of the subject refrigerant to the baseline or reference of carbon dioxide, refrigerant R-744, that has a GWP of 1. The higher the GWP number the greater risk that refrigerant poses to global warming. For example, R-22, a previously very common refrigerant has a GWP of 2400. R-134a, a refrigerant developed as a substitute for R-22, has a GWP of 1300 and R-410a, another low-GWP refrigerant, has a GWP of 1725. While much better in terms of GWP than their predecessors, both of these replacement refrigerants still have much greater potential impact than carbon dioxide.
Early refrigeration systems employed two common refrigerants, ammonia and carbon dioxide. Both of these refrigerants proved to be very troublesome in many ways. Ammonia was extremely toxic and caused great concern with respect to human health issues should a leak occur. Carbon dioxide operates at very high pressures (400+ psig in cascade systems to 2,000+ psig at transcritical high-side pressures) with discharge temperatures in the 300°+ range.
Standard copper tube was not suitable for use with ammonia, due to a propensity for corrosion in the presence of ammonia and moisture, and did not provide the strength necessary in economical wall thicknesses to handle the temperatures and pressures at which carbon dioxide systems operate. However, recent advances in copper tube manufacture utilizing a copper alloy (UNS C19400) that contains a small percentage of iron (97% copper minimum, 2.1% – 2.6% iron) has shown great promise in high pressure refrigeration systems, including those utilizing carbon dioxide (CO2) as well as other natural refrigerants, see table below for the chemical composition of alloy C19400.
Copper-iron tube is rated for pressures in the range of 90 Bar (1,305 psi) to 130 Bar (1,885 psi) or more at temperatures up to 300°F. Both copper-iron tube and fittings have been tested and certified as meeting the requirements of Underwriters Laboratories UL 207 Standard for Refrigerant-Containing Components and Accessories, Nonelectrical. For additional information related to copper-iron’s physical and mechanical characteristics, please review properties of Alloy C19400.
In designing a system, tube, fitting and joint ratings must be considered collectively, because the lower of the ratings (tube, fitting or joint) will govern the maximum installation design pressure.
Copper-iron tube and fittings are available in sizes from ⅜” O.D. to 2⅛” O.D. And the tube is internally cleaned and capped to the requirements of ASTM B280. Copper-iron tube is certified for pressure-temperature ratings via a performance standard, not through calculation from dimensional standards. Therefore, the physical properties and dimensions of the tube can vary from manufacturer to manufacturer, except for the outside diameter.
Additional information related to copper-iron tube and fittings can be obtained from the following:
Copper-iron alloy tube and fittings can be joined using the same brazing techniques and processes utilized for standard plumbing or ACR brazing applications. For brazed joints between tube and fittings manufactured from alloy C19400, which contain phosphorous (P), the use of brazing flux would not be required. However, when joining copper iron tube to other materials, that do not contain phosphorous (P), brazing flux would be required and brazing filler metals meeting the requirements of BAg series brazing alloys are highly recommended. See Brazed Joints.
Mechanically Formed Extruded Outlets
Though harder and less malleable than standard copper tube (UNS C12200) copper-iron tube has shown acceptable ability to be drilled and collared per the recommendations shown in Mechanically Formed Extruded Outlets. However, it is highly recommended prior to drilling the pilot hole, the tube being drilled to form the tee should be annealed prior to drilling the pilot hole. Pre-annealing of the main tube greatly increases the expected life of the drill head and collaring pins.