Piping, Tubing, Fittings, and Brazing
Tubing and materials
Know copper types K, L, and M (wall thickness decreases in that order) and ACR tubing, which is cleaned and capped and is standard for refrigeration. Line sizing balances pressure drop against oil return: oversized suction lines lose oil return, undersized lines lose capacity. Insulate the suction line, use inverted traps on long vertical risers, and add oil traps at the base of tall lifts.
Brazing practice
Brazing is done with filler alloys - typically 5% silver for copper-to-copper, 15% for copper-to-brass or steel - at temperatures well below the base metal melting point, with flux only where needed. The critical exam point: flow dry nitrogen through the system while brazing to prevent carbon/oxide scale from forming inside the tubing. Nitrogen is a purge, not a pressurizing gas during the braze.
Flares, evacuation, and charging
Flare fittings must be properly sized, lubricated minimally per manufacturer guidance, and torqued to specification - over-torquing cracks flares. After assembly, evacuate with a two-stage pump to 500 microns or below and perform a standing vacuum decay test to prove no leak and no moisture.
Traps
Do not use air or oxygen to leak-test; use dry nitrogen with a regulator. Do not skip the nitrogen purge because "it's a small joint." Do not assume a vacuum gauge on the manifold reads true deep vacuum - use a micron gauge at the system.
Worked example
A system pulled to 450 microns rises to 1,600 microns in ten minutes after isolation. That decay indicates moisture or a leak; a repeat triple evacuation with fresh oil and a dry-nitrogen sweep is warranted before charging.