9.5 Refrigerant Line Sizing, Piping Materials, Brazing, Oil Return, and Pipe Support

ACR Tube Is Not Plumbing Tube

Refrigerant lines are run in ACR tube — air conditioning and refrigeration tube manufactured to ASTM B280. Two properties define it. First, it is cleaned, dehydrated, capped and shipped charged with nitrogen, so the bore arrives free of the oil films, moisture and oxides that would poison a sealed system. Second, and the point exam writers love, ACR tube is specified by outside diameter (OD). A "7/8-inch" refrigerant line is 7/8 inch across the outside.

Plumbing tube to ASTM B88 — Types K, L and M — is specified by nominal size, and its actual outside diameter is 1/8 inch larger than the nominal call-out. So 3/4-inch nominal Type L plumbing tube measures 7/8 inch OD and is dimensionally the same tube as 7/8-inch ACR. Order or cut from the wrong system and nothing fits.

Type L is the normal wall thickness for field-run refrigerant lines; Type M is thin-wall and is not used for refrigerant service. Joints are made with wrought copper fittings to ASME B16.22 rather than cast fittings, because wrought fittings have a uniform, controlled socket and better fatigue behavior. Use long-radius elbows wherever layout allows — they cost roughly one-third less pressure drop than a standard 90-degree ell of the same size. IMC Section 1107 requires piping material rated for the operating temperature and pressure and compatible with the refrigerant and lubricant, and it prohibits magnesium alloys in contact with halogenated refrigerants.

Three Lines, Three Different Sizing Rules

A split-system line set is not one pipe sized one way. Each leg is sized on its own basis, and mixing the rules is the classic wrong answer.

Liquid line. Sized for pressure drop, with the goal of delivering a solid column of liquid to the metering device. Every psi of drop — friction plus vertical lift — eats subcooling; lose it all and the line flashes, the sight glass bubbles and the expansion valve starves. Vertical lift alone costs roughly half a psi per foot of rise for common HFC liquids, so a condenser 40 feet above the evaporator can consume most of the available subcooling before friction is counted. Liquid velocity is deliberately kept low; published guidance keeps it under about 300 feet per minute (fpm) on lines with solenoid valves so that closing the solenoid does not produce liquid hammer. The liquid line is always the smallest line in the set.

Suction line. Sized for pressure drop and minimum velocity at the same time. Suction pressure drop is expensive — as a rule of thumb, 1 psi of suction drop costs about 1 percent of capacity and raises power draw — but a suction line sized purely for low drop will be too large to carry oil back. Because some compressor oil circulates with the refrigerant continuously, the vapor has to move fast enough to entrain it. Commonly published minimums are about 500 to 750 fpm in horizontal runs and 1,000 to 1,500 fpm in vertical risers, and sources differ within those ranges; the upper bound of roughly 4,000 fpm is set by noise and erosion. The manufacturer's line-sizing table for the specific equipment and refrigerant governs — these figures are design rules of thumb, not code values.

Hot-gas discharge line. Sized on the same velocity-plus-drop logic as the suction line. Discharge gas is dense, so the line is smaller than the suction line for the same tonnage, but it still has to carry oil up any riser.

Risers, Traps and the Double Riser

Vertical suction risers are where oil return is won or lost. Oil climbs as a film on the tube wall, so the larger the tube, the higher the center-line velocity needed to keep that film moving. Systems with capacity modulation are the hard case: a riser sized for full load can fall well below the entrainment velocity at part load.

• P-trap at the base of every suction riser. The trap collects oil into a slug that the vapor can push up the riser in one lift instead of trying to drag a film the whole way.
• Additional traps on tall lifts, per manufacturer instruction. One manufacturer's published rule, typical of the genre, is a single trap at the bottom for a 5 to 50 foot lift, a second trap halfway up between 50 and 100 feet, and traps at one-third intervals above 100 feet. Numbers vary by brand and refrigerant — follow the installation instructions for the equipment in front of you.
• Inverted trap at the top of the riser. Running the line up to the top of the evaporator or over an inverted loop keeps liquid refrigerant and oil from draining back into the compressor or evaporator on the off cycle.
• Slope and low points. Horizontal suction lines should be level or sloped slightly toward the condensing unit, with no dips or sags that can pool oil. Use hard-drawn tube on long horizontal runs so the line does not develop bellies between hangers.
• Double riser. When a single riser cannot hold velocity at minimum load, run two risers with a trap between them. At part load the trap fills with oil and seals the large riser, forcing all vapor up the small riser at full velocity; at full load both carry. The connecting trap must be close-coupled to hold as little oil as possible, and the second riser must enter the main suction line from the top so it cannot drain back.

Equivalent Length Drives the Line Size

Line-sizing tables are published per 100 feet of pipe, so you cannot enter one with a tape-measure length. Total equivalent length (TEL) is the measured run plus the equivalent straight-pipe length of every fitting, valve and accessory:

$TEL = L_{measured} + \sum L_{fittings}$

Representative equivalent lengths in feet of straight copper tube, from the ASHRAE Handbook fitting-loss tables reproduced in most manufacturer piping guides:

Notice the scale: a single globe or solenoid valve in a 1-1/8-inch line adds about 29 feet of equivalent pipe — more than most elbows in the whole job combined. That is why service valves and accessories, not elbows, usually decide whether a line set needs to move up a size.

Brazing: Nitrogen, Filler Metal and Temperature

Soldering uses filler metals that melt below 840 degrees F; brazing uses filler metals that melt at or above 840 degrees F. Refrigerant joints are brazed. The IMC does allow soldered joints in a narrow case — Group A1 refrigerants at pressures not exceeding 200 psi — but field practice on modern high-pressure refrigerants is brazing.

Purge with nitrogen while you braze. This is a code requirement, not a preference — but tab the right section. IMC Section 1108.3.1, "Brazed joints," is the purge rule; Section 1108.2 next to it is "Preparation of pipe ends" (cut square, ream, chamfer), and a candidate tabbed to 1108.2 lands on the wrong paragraph. Section 1108.3.1 requires the piping being brazed to be purged of air and filled with an inert gas — oxygen-free nitrogen, helium or argon — pre-purged with five volume changes at a minimum velocity of 100 feet per minute, with the gas maintained between 1.0 psi and 3.0 psi during brazing. Heated copper in the presence of oxygen forms black cupric oxide scale on the inside of the tube; it flakes off in service and plugs thermostatic expansion valve screens, capillary tubes and filter-driers. In the field, regulating dry nitrogen to roughly 2 psig puts you inside that 1-to-3 psi code band, and a trickle of about 2 to 5 cubic feet per hour is the usual trade practice for holding it there — the psi range is the code number, the cfh figure is not. Too much flow chills the joint, blows the molten alloy out of the socket and leaves a porous, leaking braze.

Filler metals.

• BCuP (phos-copper) alloys — BCuP-2 at essentially no silver, BCuP-3 and BCuP-5 at 5 and 15 percent silver. The phosphorus is self-fluxing on copper-to-copper, so no flux is used and no flux residue can wash into the system. Never use a BCuP alloy on steel or high-nickel alloys; phosphorus forms brittle phosphides at the joint.
• BAg silver brazing alloys with flux — used for copper to brass, bronze or steel, such as a copper line to a steel accumulator or a brass service valve. Apply flux to the joint, not to the rod alone, and clean the residue off afterward.

Never braze on a system that still holds refrigerant pressure, and never braze on a line under vacuum.

Pressure Test, Then Evacuate

Test with dry nitrogen. IMC Section 1110.3 limits field test gases to oxygen-free nitrogen, helium or argon and prohibits oxygen, air and combustible gases or mixtures containing them as a test medium; carbon dioxide is allowed for R-744 systems and water for R-718. Systems erected on the premises with tubing not exceeding 5/8 inch OD may use the refrigerant named on the nameplate as the test medium. Section 1110.5 then requires the assembled piping to pass a pressure test and a leak test.

Why oxygen is an absolute prohibition: oxygen in contact with compressor oil under pressure can detonate. Compressed shop air is equally unacceptable — it carries oxygen plus water vapor, and the moisture it deposits will not come out with a normal evacuation. Always use a regulator on the nitrogen cylinder; cylinder pressure is thousands of psi and will burst tube and components.

Then pull a deep vacuum. Evacuation removes non-condensables and boils off moisture; it is not a leak test and never a substitute for one. Connect the vacuum rig with large-bore hoses through both service ports and read a micron gauge placed at the system, not at the pump. The widely used field target is 500 microns. Then valve off the pump and run a decay (rise) test: a reading that rises slightly and levels off below roughly 500 microns indicates a tight, dry system; a rise that levels off in the low thousands of microns points to remaining moisture boiling off; a reading that climbs steadily without leveling means a leak or an open connection. Manufacturer instructions set the binding number for the equipment you are commissioning.

Support, Protection, Insulation and Penetrations

Support spacing is code, not preference. The IMC gives the intervals in Table 305.4:

Refrigerant line sets are copper tubing, so the governing figures are 8 feet horizontal and 10 feet vertical. Be alert on the exam: plumbing codes grade copper tube supports by diameter — six feet for the smaller sizes — so a support question must be answered from the table in the book the item cites.

Other requirements that show up as items:

• Dissimilar metals. IMC Section 305.2 requires hangers and supports in contact with the pipe to be compatible with the piping material so galvanic corrosion is not set up; hangers must be attached to the building structure in an approved manner (Section 305.3).
• Protection through framing. IMC Section 305.5 requires steel shield plates at least 0.0575 inch thick (No. 16 gage) where nonferrous piping passes through a stud or plate less than 1-1/2 inches from the nearest edge, extending at least 2 inches beyond the plate.
• Installation clearances. IMC Section 1109 requires exposed refrigerant piping in open spaces to be at least 7 feet 3 inches above the finished floor, prohibits it in exit corridors, interior stairs and exit passageways, and requires a fire-resistance-rated shaft where it penetrates more than one floor-ceiling assembly.
• Insulation. The suction line is insulated for its full length, including through the wall, with closed-cell elastomeric insulation - the vapor barrier is the insulation itself, so seams and butt joints must be glued, not taped. The liquid line is normally left bare so it can shed heat and hold subcooling; insulate it where it runs through a hot attic or is strapped to the suction line, since heat gain there produces flash gas at the metering device.
• Vibration and rooftops. Isolate the line set from the compressor with a loop or vibration absorber, and never hard-clamp tube directly to framing - use isolated clamps or sleeves so vibration is not transmitted into the structure. On roofs, carry lines on cushioned pipe supports or blocks that do not penetrate the membrane, spaced no wider than the table above, and jacket the insulation against ultraviolet light and hail.
• Penetration sealing. Sleeve the exterior wall penetration, slope it to drain outward, and seal the annular space with an approved sealant. It is a weather, pest and energy-code air-sealing detail all at once, and a leaking line-set penetration is one of the most common callbacks on new installations.