GUIDES / GLOSSARY

AC and refrigeration glossary.

Plain definitions of the terms used in copper pipe, AC piping, brazing, refrigerants and insulation, with links to the guides and range pages that go into more detail.

By Saurabh Jain, Refrigeration Centre · Published 5 October 2026 · Updated 18 October 2026

Jump to: Copper pipe and sizes · AC and refrigeration systems · Joints and installation · Refrigerants · Insulation

Copper pipe and sizes.

ACR tube: Copper tube made for air-conditioning and refrigeration circuits. It is seamless, usually phosphorus-deoxidised copper (Cu-DHP, C12200), named by its actual outside diameter, and cleaned and capped at the mill so moisture and dirt stay out of the system. The usual specifications are ASTM B280, EN 12735-1 and IS 10773. More on ACR copper.
OD (outside diameter): The outside diameter of a tube. ACR tube is named by it: 3/8″ ACR tube measures 9.52 mm across the outside. Inch to mm sizes.
ID (inner diameter): The bore of a tube: the OD minus twice the wall thickness. A 3/8″ (9.52 mm) tube with a 0.81 mm wall has an ID of about 7.90 mm. Working out the ID.
Wall thickness: The thickness of the tube wall, given in millimetres. It sets the pressure a tube can take and its weight per metre, and R32 and R410A installation manuals set a minimum wall for each size. Wall thickness for R32 and R410A.
SWG (Standard Wire Gauge): The gauge the Indian trade often uses for copper wall thickness. A higher number means a thinner wall: 18 SWG is 1.22 mm, 21 SWG is 0.81 mm and 22 SWG is 0.71 mm. SWG to mm chart.
Temper: How hard the copper is, set by drawing and annealing. Annealed (soft, O temper) tube bends and flares; hard-drawn tube is rigid. Soft vs hard drawn.
Soft-drawn copper: Annealed copper tube supplied in coils. It bends by hand or with a bender and can be flared, so it suits split-AC line sets and small refrigeration lines.
Hard-drawn copper: Rigid copper tube supplied in straight lengths and joined by brazing with fittings. It must be annealed before it can be bent tightly or flared.
Coil and straight length: The forms copper pipe is sold in. At Refrigeration Centre, soft-drawn pipe comes in standard 50 ft (15.24 m) coils and hard-drawn pipe in 10 ft (3.05 m) straight lengths, and either can be cut to size. Coil and straight lengths.
Cu-DHP (C12200): Phosphorus-deoxidised copper with a high residual phosphorus content, the usual material for ACR and plumbing tube because it brazes well.
Type K, L and M: Plumbing water tube to ASTM B88, named by nominal size rather than actual OD. Type K has the thickest wall and Type M the thinnest. The nominal names do not match ACR sizes: 1/2″ Type L is 5/8″ OD. Type K, L, M vs ACR.
Weight per metre (kg/m): How copper tube is weighed and usually priced. With OD and wall in mm, kg/m = π × wall × (OD − wall) × 0.00894. Copper pipe weight calculator.

AC and refrigeration systems.

TR (ton of refrigeration): A unit of cooling capacity: 1 TR is 12,000 BTU per hour, about 3.52 kW. A 1.5 ton AC removes about 5.3 kW of heat. Pipe size by tonnage.
Liquid line: The smaller of the two AC pipes. It carries liquid refrigerant from the condenser side to the expansion device. Liquid and suction lines.
Suction line (gas line): The larger AC pipe. It carries low-pressure refrigerant vapour back to the compressor, runs cold in cooling mode and must be insulated.
Discharge line: The pipe that carries hot, high-pressure refrigerant gas from the compressor to the condenser.
Line set: The liquid and suction pipes, each insulated separately, that join an indoor unit to its outdoor unit.
Expansion valve (TXV, EEV): A metering device that drops the refrigerant’s pressure before the evaporator and controls how much enters it. A TXV is thermostatic; an EEV is electronically controlled. Expansion valves.
Capillary tube: A long, small-bore tube used as a fixed metering device in refrigerators and small AC units.
Service valve: The valve on the outdoor unit where the line set connects. It holds the factory charge in the outdoor unit until the piping has been tested and evacuated, and gives access for gauges.
VRF / VRV: Variable refrigerant flow: one outdoor system serving many indoor units through branched piping. VRV is Daikin’s name for the same idea. VRF piping.
Refnet joint (branch kit): A factory-made Y-branch or header that splits VRF piping to the indoor units. “Refnet” is Daikin’s name; other makers call them branch kits.
Superheat: How far the suction vapour’s temperature is above its saturation (boiling) temperature at the same pressure. Technicians use it to check the charge and the expansion valve.
Subcooling: How far the liquid refrigerant’s temperature is below its saturation (condensing) temperature at the same pressure.
Evacuation (microns): Pulling a deep vacuum on new piping to remove air and moisture before charging. Vacuum is measured in microns of mercury, and manuals commonly ask for 500 microns (about 67 Pa) or lower. Pressure testing and evacuation.
Recovery: Removing refrigerant from a system into a recovery cylinder, rather than releasing it, before the system is opened or brazed.

Joints and installation.

Brazing: Joining metals with a filler that melts above 450 °C but below the melting point of the parts. It is the standard joint for refrigerant copper. How to braze copper pipe.
Soldering: Joining metals with a filler that melts below 450 °C. It is not used for refrigerant joints. Brazing vs soldering.
BCuP (copper-phosphorus brazing alloy): The usual filler for copper-to-copper joints, with 0–15% silver. The phosphorus fluxes copper, so no added flux is needed on copper; it must not be used on steel or iron. Which brazing rod to use.
BAg (silver brazing alloy): A silver-based brazing filler, used with flux for copper-to-steel and copper-to-brass joints.
Flux: A paste or powder that dissolves oxides and protects the joint while brazing. It is needed on brass and steel joints, and its residue must be washed off afterwards.
Nitrogen purge: A gentle flow of dry nitrogen through the pipe while brazing, so the inside does not form black oxide scale. How to purge with nitrogen.
Capillary action: The pull that draws molten filler into the narrow gap between a tube and its fitting. It is why a brazed joint needs a close, clean fit.
Flare joint: A mechanical joint: the tube end is opened into a 45° cone and clamped with a flare nut against a matching fitting or valve, so it can be undone. Flare or brazed.
Flare nut: The brass nut that clamps a flare. Nuts for R410A and R32 differ from R22 nuts in some sizes, so match the nut to the refrigerant. Flare nuts.
Swage: An expanded tube end that another tube of the same size slides into, forming a socket for brazing without a coupling. It is made with a swaging tool. Tools.
Pressure test: Filling new piping with dry nitrogen to the test pressure in the installation manual and checking that it holds, before the piping is evacuated.
Pinch-off: Sealing a process tube by crimping it flat with pinch-off pliers.

Refrigerants.

HCFC: Hydrochlorofluorocarbon refrigerants, such as R22. They deplete the ozone layer and are being phased out under the Montreal Protocol.
HFC: Hydrofluorocarbon refrigerants, such as R32, R410A, R134a and R404A. They do not harm the ozone layer but do have a global-warming potential.
HFO: Hydrofluoro-olefin refrigerants, such as R1234ze, with a very low global-warming potential.
Hydrocarbon (HC) refrigerants: Refrigerants such as R290 (propane) and R600a (isobutane). Their global-warming potential is very low, but they are flammable (class A3).
GWP (global warming potential): How much a gas warms the climate compared with the same mass of carbon dioxide over 100 years. R410A is 2,088 and R32 is 675. R22 vs R410A vs R32.
ODP (ozone depletion potential): How much a gas damages the ozone layer compared with R11. R22 has an ODP above zero; HFCs and HFOs have zero.
A2L: A refrigerant safety class under ASHRAE 34 and ISO 817: “A” means lower toxicity and “2L” lower flammability. R32 and R1234ze are A2L, so they need tools rated for them. R22, R410A and R134a are A1 (non-flammable); R290 and R600a are A3 (higher flammability).
Refrigerant blend: A mixture of refrigerants, such as R410A, R407C or R404A. Blends are charged as liquid so that the mixture entering the system keeps the right proportions.
Temperature glide: In some blends, such as R407C, the boiling temperature changes as the refrigerant evaporates, because the components boil at different temperatures. R410A has almost no glide.

Insulation.

Closed-cell elastomeric insulation: Flexible foam insulation, such as nitrile rubber (NBR) and EPDM, whose sealed cells resist moisture. It is the usual insulation for refrigerant and chilled-water pipe. Nitrile, EPDM and XLPE.
Nitrile rubber (NBR): The usual insulation for AC pipe, sold as tube and sheet. The tube’s inner diameter (ID) is matched to the pipe’s OD. Nitrile tube sizes.
Class 0 and Class 1: Fire ratings under the British Standard BS 476. Class 1 is the best class for surface spread of flame; Class 0 also limits fire propagation. Class 0 vs Class 1.
Dew point: The temperature at which moisture in the air condenses. Pipe insulation must keep its outer surface above the dew point, or the pipe sweats. Why pipes sweat.
XLPE: Cross-linked polyethylene: a lighter, lower-cost closed-cell foam insulation.

Cannot find a term?

MORE GUIDES

AC copper pipe size by tonnage

Copper pipe weight calculator and thickness chart

How to braze copper pipe

All guides