26.2 Liquid, Suction & Heat-Pump Dryers, Filters & Oil Traps

26.2 Liquid, Suction & Heat-Pump Dryers, Filters & Oil Traps

The next Trade D cluster is the line accessories that keep the circuit clean and the oil moving: liquid-line dryers/filters, suction-line dryers/filters, heat-pump line dryers/filters, and oil traps. Chapter 25 hung the copper. Section 26.1 hung the TXV. This section is what you put in that piping so moisture, acid, scale, and oil do not kill the compressor. Refrigeration & Air Conditioning Technology, 9th Edition (2021) is the component book. FBC Mechanical 2023 Chapter 11 still wants listed refrigerant piping and accessories. EPA Section 608 still forbids venting while you change a drier. Class B still stops at 25 tons / 500,000 Btu in any one system.

Quick Answer: A liquid-line filter-drier sits in 100 percent liquid before the metering device. After a burnout, add a suction-line filter-drier and watch its pressure drop. Heat pumps need biflow driers (or two driers with checks) because liquid reverses. Oil traps go at the base of a vertical suction riser. A suction drier is not a permanent extra liquid drier, and an oil trap is not an oil separator.

What a filter-drier actually does

A filter-drier combines a desiccant (molecular sieve and/or activated alumina) with a filter screen or felt. The desiccant holds water. Alumina and some burnout cores also hold acid. The filter catches particles — copper oxide from a nitrogen-less braze, carbon from a motor burn, solder balls. Moisture plus POE oil plus leftover mineral oil is how you grow sludge that plates a TXV needle. POE is hygroscopic; cap the drier and the oil until the second they go into the circuit (Chapter 23.2).

Liquid-line placement is standard on every DX circuit: after the condenser (and after the receiver if there is one), before the metering device, in a run that is 100 percent liquid. Vapor in a liquid drier means you undersized the condenser, undercharged, or put the drier too high on a trap that flashes. The arrow on a one-way drier points toward the TXV in cooling. A sight glass / moisture indicator usually sits after the drier so you can see liquid and a color change (dry versus wet) once the circuit is running. Bubbles in the glass can be undercharge, a restriction (including a plugged drier), or a liquid line that is flashing because it runs through a 130°F attic without enough subcooling — do not automatically add refrigerant.

Sealed driers are typical on residential line sets. Replaceable-core shells on racks and large packages need isolation valves so you can pump down (Section 26.3), change cores, and not vent the charge. Size the drier for the liquid mass flow and the moisture load of the refrigerant, not for whatever 16-cubic-inch can is on the truck. An oversized drier that is still in 100 percent liquid is usually harmless; an undersized drier that goes into high pressure drop starves the TXV just like a kinked liquid line.

Suction-line driers — after a burnout

A suction-line filter-drier is cleanup hardware. After a hermetic or semi-hermetic burnout, acid and carbon are in the oil and on every surface. Recover, flush as the listing allows, change the oil, replace the liquid-line drier, and install a suction drier (often an HH / acid-capable core) in the suction line as close to the compressor inlet as practical, with access pressure taps on both sides so you can read pressure drop.

When suction-drier ΔP climbs past the manufacturer’s limit — commonly in the 2–3 psi neighborhood on small systems, follow the tag — the core is loaded. Replace it, or remove a temporary burnout drier once acid tests clean, and leave a proper liquid-line drier in place. Leaving a clogged suction drier in forever is how the replacement compressor is starved of mass flow and oil return. Putting a one-way liquid drier in the suction line with the cooling arrow toward the compressor is the wrong vessel: the desiccant and the check are not built for low-side vapor and reversing oil slugs.

A plain suction filter (no desiccant) is a particle strainer some racks use; it is still not a substitute for a burnout drier when the oil is acid. Test oil acidity after a burn; one core change is often not the last.

Heat-pump line driers — biflow

On a split heat pump, the small line is the liquid line in cooling and still carries liquid in heating, but flow reverses. A one-way drier with a check will block reverse flow or blow desiccant the wrong way. The listed answer is a biflow (bidirectional) liquid-line filter-drier, or two driers with check valves so each direction has a legal core. Mount it where the manufacturer shows — usually in the outdoor-unit liquid line or in the field line that is liquid in both modes.

The large vapor line also reverses role (suction in cooling, hot gas in heating). You do not treat that vapor line as a second liquid drier location. Suction-line burnout driers on heat pumps must be placed so they are actually in suction toward the compressor in both modes — typically between the reversing valve and the compressor, not between the indoor coil and the reversing valve. Put a suction drier on the indoor vapor line of a heat pump and in heating you have a drier in the hot-gas path.

Worked burnout / HP check. A 4-ton heat pump in Sarasota takes a shorted scroll. The replacement compressor gets: new POE, a new biflow liquid drier, a suction burnout drier between the reversing valve and the compressor suction, nitrogen-purge brazes, evacuation, weigh-in, then ΔP logged on the suction drier at the first two visits. Skipping the biflow and reusing the old one-way liquid drier is how heating season clogs the TXV.

Oil traps on vertical suction risers

Oil travels with suction vapor when velocity is high enough. On a vertical suction riser — indoor coil in a first-floor closet, condensing unit on a two-story roof, or a penthouse compressor above a ground-floor cooler — velocity can fall at light load and oil drains back down the riser, logging at the bottom and starving the crankcase.

An oil trap (a P-trap in the suction line) at the base of the riser holds that oil so that when the compressor starts or load rises, a slug of oil is lifted up the riser. Tall risers may need intermediate traps at the interval the manufacturer or piping guide specifies. A double suction riser (small pipe plus large pipe, trapped so only the small riser carries oil at low capacity) is the commercial answer when a compressor unloads and a single large riser would be too slow. That is a Class A rack conversation more often than a 3-ton split, but Class B still has to name it.

Rules that keep the trap from becoming a restriction:

• Trap the suction riser, not the liquid line and not the discharge line as a default. Discharge oil separators (with a return to the crankcase) are a different accessory.
• Put the trap at the bottom, not at the top. An inverted loop at the top of a discharge riser is a different oil-management trick; do not invert a suction P-trap at the roof and call it done.
• Size the riser for oil-return velocity on the manufacturer’s equivalent-length table (Chapter 25). A trap cannot fix a 1-3/8-inch suction riser on a 2-ton compressor.
• A suction accumulator (Chapter 23.3 / 26.3) protects against liquid slugging; it is not an oil trap. An oil trap is not a filter-drier.

Florida attics and two-story line sets make risers common. A Jacksonville townhouse with the air handler in a first-floor closet and the condensing unit on a second-story pad needs the trap the equivalent-length table called for, not a straight 7/8-inch riser because it looked clean.

Florida scenario

A certified Class B shop in Duval County replaces a burned 3-ton scroll on a split heat pump. The helper (1) changes the compressor but reuses the old one-way liquid drier with the arrow toward the indoor coil, (2) skips a suction drier because the new compressor is hermetic, (3) puts a temporary suction drier on the indoor vapor line between the A-coil and the outdoor unit, and (4) runs a 25-foot vertical suction riser with no P-trap at the base. Four failures: heat pumps need biflow liquid driers; burnout cleanup wants a suction drier between the reversing valve and the compressor; a suction drier on the indoor vapor line sits in hot gas in heating; the riser needs an oil trap at the bottom. Class B is legal on 3 tons. If the same owner had a 40-ton rack with replaceable cores, Class B stops at the bid table — the core-change theory still shows up on the exam.

Traps: (1) One-way liquid drier on a heat pump. (2) No suction drier after a burnout. (3) Suction drier left in forever at high ΔP. (4) Suction drier on the indoor vapor line of a heat pump. (5) Oil trap at the top of the riser. (6) Calling an accumulator an oil trap. (7) PVC on the refrigerant drier connections.