18.2 Oil Drain Pots, Rectifiers, and Thermosiphon Oil Management

Ammonia (R-717) and the mineral or PAO oils specified for most industrial ammonia compressors are poorly miscible. That is the opposite of a typical HFC/POE supermarket rack, where oil is expected to travel with the refrigerant and return through suction. In an ammonia plant, oil leaves the compressor in discharge gas, is mostly knocked out in the oil separator, and the fraction that escapes still falls out in cold vessels, evaporators, and wet returns. If you do not collect it and send it home, it occupies evaporator surface, false-fills level columns, and starves the compressor of lubricant.

Density is the operator's friend. Liquid ammonia in the temperature range of industrial recirculators is roughly 40 lb/ft³ (it is lighter when warmer). Mineral refrigeration oil is typically around 55 lb/ft³. Oil sinks. It accumulates in the bottom of recirculators, intercoolers, flooded chillers, and dedicated oil pots piped off those bottoms. That is why frost on an oil pot is a diagnostic: liquid ammonia in contact with the shell drives a frost line; the unfrosted bottom is usually the oil layer. ANSI/IIAR 2-2021 and ANSI/IIAR 9 require that the plant have a means of removing oil. CIRO expects you to know both the physics and the valve arrangement.

Where oil goes and why the low side is the collection point

Discharge temperature keeps oil thin on the high side. Separators capture most of it for screws; reciprocating machines throw less oil but still lose some. What gets past the separator rides as mist into condensers and then into the high-pressure receiver. Oil does not usually build a deep puddle in a warm HPR the way it does in a −20°F recirculator—cold viscosity makes oil sticky and easy to trap on the low side. When oil is suspected in an HPR, treat draining as a high-side liquid job with full PPE and a written procedure; it is not the routine pot-drain you do on a recirculator drop leg.

Low-side collection points include:

• Recirculator / LPR drop legs and oil pots
• Suction accumulators and knockout drums
• Flooded chiller shells and surge drums
• Intercoolers on two-stage plants
• Evaporator headers that never get hot enough to push oil back

Overfeed ratios of 3:1 to 4:1 help wet coils, but they also sweep oil toward the recirculator. That is good if the pot is drained; it is bad if the pot is ignored until the pump suction is a milkshake of oil and ammonia.

Oil drain pots and the legal drain train

A typical oil pot is a small ASME vessel or a drop leg under the main vessel, with a liquid inlet from the vessel bottom, a vent back to the vessel vapor space (so the pot can equalize and so ammonia can boil off), a level indication (glass, frost band, or both), and a drain.

ANSI/IIAR 2-2021 §5.9.3 and ANSI/IIAR 9-2020 §7.2.5.3 allow oil removal by:

1. A rigid-piped oil return or transfer system (automatic return to the compressor oil circuit—less common as the only method on ammonia than on halocarbons).
2. A vessel (oil pot) with a shut-off valve in series with a self-closing shut-off valve.
3. A drain assembly at the collection point with, at minimum, the same shut-off plus self-closing pair.

The self-closing valve is the deadman (spring-return). You hold it open. If you slip, get chased by an alarm, or are overcome, it shuts. The hand valve upstream is for isolation and to throttle. IIAR 2 and 9 also require that any temporary rigid drain piping be supported and tight. Draining into an open bucket with a hose hung on a valve handle is not a procedure; it is an incident investigation waiting for a date.

A competent drain sequence (plant SOP governs; this is the physics):

• Confirm PPE, detectors, and that you are on the written oil-drain procedure.
• Close the liquid feed from the vessel into the pot if the design allows isolating the pot; leave the vent/equalizer to the vapor space open so liquid ammonia in the pot can boil back to the vessel.
• Wait. Cold oil holds a lot of dissolved and mixed ammonia. Time (sometimes hours) lets the pot “dry.” A rushed drain is an ammonia release through the oil valve.
• Drain slowly through shut-off then deadman into a closed, compatible container or to the still. Stop if you get white vapor instead of oil.
• Close up, reopen the pot to the vessel, and log quantity. Rising drain frequency means a separator problem, wrong oil, or a compressor throwing excess oil.

Stills, rectifiers, and getting oil back to screws

A drain pot separates by gravity and boiling. An oil still (oil rectifier, oil stillpot with heat) goes further: it applies heat—electric, hot gas, or high-pressure liquid—so remaining ammonia is distilled out of the oil. Ammonia vapor is piped to a wet suction or vessel vapor space. The leftover oil is dry enough to return to the screw compressor oil system (separator, reservoir, or injection manifold) through a pump, a pressurized transfer, or a carefully controlled drain into the oil circuit.

That return path is why plants bother. Screw compressors need a continuous oil loop for sealing the rotors, lubricating bearings, and carrying heat to the oil cooler. Oil that stays in a blast-freezer recirculator is missing from that loop. Automatic rectifier packages still fail: heaters burn out, the liquid feed to the still sticks open and dumps the recirculator into the oil system, or the return check leaks high-pressure oil into the low side. Treat rectifier valves as you would any other automatic isolation—know the fail position and the hydrostatic implications.

Reciprocating ammonia machines use less oil circulation; recovered oil may be filtered and reused only if it meets the OEM spec. Do not pour mystery crankcase oil from a drain pan back into a screw separator.

Thermosiphon oil coolers (high-side, no pump)

Screw oil must be cooled. Thermosiphon oil cooling (TSOC) uses high-side liquid ammonia as the coolant. A thermosiphon receiver (sometimes a dedicated vessel, sometimes a volume in or beside the HPR) sits above the oil cooler. Liquid flows down to the cooler by gravity. Heat from the oil boils some of that liquid. The two-phase mixture is less dense than the liquid column in the supply drop, so it rises back to the receiver. Separated vapor leaves to the condenser; liquid stays in the thermosiphon loop.

There is no refrigerant pump in a true thermosiphon. The driving force is elevation and density difference. Manufacturers commonly require several feet of elevation; some published ammonia thermosiphon-receiver catalogs specify on the order of 6 ft (72 in) between receiver and cooler. If someone “simplified” the piping and returned two-phase fluid into the condenser inlet instead of the receiver, condenser performance suffers because liquid is being thrown at coils that should see vapor. If the supply valve is throttled, the cooler starves, oil temperature climbs, and the compressor oil-temperature cutout trips—the same symptom as a fouled cooler or a condenser that cannot hold head pressure for the loop.

Thermosiphon liquid is high-side inventory. It is part of the plant charge and part of the HPR/thermosiphon-receiver level problem. Low HPR level can starve both expansion devices and oil cooling. High oil temperature on a CIRO operating screen with otherwise normal suction is often a thermosiphon or condenser-approach problem, not a “need more oil” problem.

The cooler itself is typically an ASME exchanger (shell-and-tube or plate-and-shell). If you can isolate it with stop valves, it needs relief—vapor relief for fire case on the ammonia side, and a hydrostatic path if oil or liquid can be trapped. Do not isolate a liquid-full oil cooler “for a minute” without that path.

Wrong oil and water in oil

Use the compressor manufacturer’s ammonia lubricant. Traditional plants run naphthenic mineral oil; some screws specify PAO or other ammonia-rated synthetics. POE oils designed for HFCs are the wrong chemistry for a mineral-oil ammonia plant: they are hygroscopic, they can attack paints and elastomers not chosen for them, and they change separator performance. Mixing leftover “refrigeration oil” from another facility is a classic way to create a mayonnaise emulsion that will not separate in the pot.

Water in oil (or wet ammonia that hydrates the oil) drops lubricity, promotes corrosion and acid, and at low evaporator temperatures can form ice or sludge that blocks strainers and level columns. Wet oil also changes the frost picture on pots and can make a rectifier work overtime. Sources include poor-quality ammonia, atmospheric moisture during sloppy charging or oil addition, and leaking oil coolers on water-cooled packages. Check oil with the OEM’s method (crackle test is a field clue, not a lab); treat wet oil as a mechanical-integrity and charging-practice problem, not a reason to keep adding makeup oil forever.

Makeup oil belongs in the compressor oil system per procedure—not into a recirculator “to help the pump.” Extra oil in the low side is a heat-transfer penalty and a false-level hazard, which the next section takes up.