Choice Of Materials For Refrigerant Piping

Copper is the material almost universally used for halocarbon piping, with no low-temperature limitations in industrial refrigeration practice. For ammonia systems, since copper is not acceptable, all pipe is steel in one form or another. While aluminum is often used for the tubes of air-cooling coils, it is hardly ever used for liquid and vapor piping in a plant. Cast iron pipe should not be used and wrought iron pipe should not be used for liquid lines.

The most commonly used steel for ammonia systems is carbon steel of type A53 or A106. Type A53 F is butt-welded and should not be used, on the other hand A106 is seamless, so is acceptable. The ANSI/ASME Refrigeration Piping Standard14 permits the use of A53 or A106 down to temperatures of -29°C (-20°F). One of the steels permitted by the ANSI/ASME standard below this temperature is type A333. Compared to A53 or A106, A333 pipe is approximately three times as expensive, the the cost factor is even higher for valves and fittings. A53 and A106 steels may be used below -29°C (-20°F) provided they pass impact tests at the temperature at which they will be operated. Obviously, impact testing adds to the cost. Another option offered by the standard is to design for the circumferential or longitudinal tensile stress to be 40% of the allowable stress given for the material. Refrigeration interests in recent years have been attempting to get A53 and A106 steels approved for below -29°C (-20°F) operation on the basis that with ammonia the pressure of the refrigerant will be in the range of atmospheric, so that pressure stress on the pipe will be negligible.

At temperatures lower than -46°C (-5°F) many designers uses stainless steel for ammonia and hydrocarbons.