How to Select an Ammonia Compressor Lubricant: HTMO vs PAO
Ammonia compressor lubricant selection depends on three factors: its viscosity at the lubrication point once ammonia has dissolved into it, its behavior at the system’s coldest point, and its ability to withstand discharge temperature. Ammonia and hydrocarbon lubricants have limited miscibility in the cold sections of a plant. Therefore, most industrial ammonia refrigeration systems are designed for lubricant separation and drainage, not continuous circulation.
Hydrotreated mineral oil (HTMO) grades are suitable for conventional flooded systems. Polyalphaolefin (PAO) grades are preferred for low evaporating temperatures, high discharge temperatures, and in plants where evaporator fouling reduces capacity. Direct-expansion systems are an exception, as they require an ammonia-soluble lubricant.
Industrial ammonia systems are designed for lubricant separation and drainage, not circulation. The lubricant is a managed consumable, and poor selection leads to evaporator fouling and drainage issues rather than immediate failure.
The compressor's minimum viscosity requirement applies to the ammonia-diluted lubricant at operating temperature and pressure, not to fresh lubricant at its ISO grade.
Pour point is a qualitative flow/no-flow test. Products with identical pour points can differ significantly in measured low-temperature viscosity: two ISO VG 68 lubricants with a −42 °C (−44 °F) pour point measured 64,650 cP and 87,600 cP at −35 °C (−31 °F).
Published pressure-volume-temperature (PVT) measurements show 5.2 mass % ammonia dissolved in an ISO VG 68 Group II mineral oil at 70 °C (158 °F) and 30 bar absolute (435 psia), reducing the mixture viscosity to 8.09 mm²/s (cSt).
Polyol esters (POEs) react chemically with ammonia to form amides, viscous material, and sludge. POE contamination must be excluded from R717 circuits.
NEXT provides verified substitution statements for ten widely used ammonia lubricants across HTMO, naphthenic, and PAO chemistries.
How Ammonia Affects Compressor Lubrication
In most refrigeration systems, the lubricant travels with the refrigerant. It leaves the compressor, passes through the condenser and evaporator dissolved in the refrigerant, and returns as a single-phase mixture.
Ammonia breaks this pattern. Both HTMO and PAO have very low solubility in liquid ammonia and are denser than it. Any lubricant that escapes the compressor settles out as a separate layer at the bottom of vessels and evaporators instead of returning with the refrigerant.
This single property shapes the design of every industrial ammonia plant: a discharge-line separator or coalescer sits immediately downstream of the compressor to catch most of the lubricant before it enters the system. What gets past it collects at low points fitted with oil pots and drain valves. Operators drain it off and top up the sump on a routine schedule. In an ammonia plant, the lubricant is a consumable that leaves the machine, not a circulating charge that returns to it.
Selection, therefore, balances two competing demands. The lubricant must maintain enough viscosity in the compressor to carry the bearing and rotor loads, yet remain fluid enough at evaporator temperature to drain from the cold side of the system.
Why Diluted Viscosity Matters
The compressor manufacturer specifies a minimum kinematic viscosity at each lubrication point. That requirement applies to the refrigerant–lubricant mixture at operating pressure and temperature—not to fresh lubricant at 40 °C (104 °F).
Although the lubricant barely dissolves in liquid ammonia, the reverse is not true: under compressor pressures and temperatures, ammonia dissolves measurably into the lubricant, thinning it. How much depends on the base stock, which is why solubility and viscosity require PVT analysis at the actual discharge, separator, oil-cooler, and lubricant-supply conditions.
Glenn Short’s PVT research for the International Institute of Ammonia Refrigeration measured this directly for ISO VG 68 lubricants at a representative compressor condition:
| ISO VG 68 Lubricant | Solubility at 70 °C (158 °F) and 30 bar absolute (435 psia) | Viscosity at 70 °C (158 °F) and 30 bar absolute (435 psia) | Solubility at 67 °C (153 °F) and 15 bar absolute (218 psia) | Viscosity at 67 °C (153 °F) and 15 bar absolute (218 psia) | Viscosity after cooling to 42 °C (108 °F) |
|---|---|---|---|---|---|
| Group II mineral oil | 5.2 mass % | 8.09 cSt | 1.35 mass % | 14.4 cSt | 33 cSt |
| PAO | 4.1 mass % | 9.66 cSt | 1.48 mass % | 15.6 cSt | 35 cSt |
| Alkylbenzene | 6.0 mass % | 3.7 cSt | 2.0 mass % | ~8 cSt | 20 cSt |
| Ammonia-miscible PAG | 25.6 mass % | 2.3 cSt | 10.9 mass % | 6.95 cSt | 12.95 cSt |
Source: Short, G.D., “Assessment of Lubricants for Ammonia and Carbon Dioxide Refrigeration Systems,” IIAR, December 2017. Alkylbenzene values after Seeton (2014). Pressures are absolute.
At a milder condition of 67 °C (153 °F) and 15 bar absolute (218 psia), solubility in the mineral oil falls to 1.35 mass % and the mixture viscosity recovers to 14.4 cSt, rising to 33 cSt after cooling to 42 °C (108 °F). The practical point is that the same lubricant presents a different viscosity at every station in the circuit, and the compressor requirement must be met at the diluted condition, not on the fresh-oil data sheet.
The Direct-Expansion Exception
Direct-expansion (DX) ammonia systems invert the logic above. A DX evaporator carries a much smaller refrigerant charge—which is why this configuration is chosen when charge inventory is restricted—and it has no collection vessel from which to drain lubricant. The lubricant must return with the refrigerant.
An immiscible lubricant cannot do this. It plates out as a film on the evaporator tube wall, thickens as the mixture cools along the tube, raises pressure drop, and slowly starves the compressor sump. Failure is rarely immediate; capacity fades and the oil level falls.
DX service therefore requires a lubricant with high ammonia solubility, so that dissolved ammonia keeps it thin and mobile inside the evaporator. NEXT 717-68-DX is a polyalkylene glycol (PAG) lubricant formulated for exactly this duty. It is not interchangeable with the HTMO, PAO, or HTMO/PAO products used in flooded systems, in either direction. One further consequence of the chemistry: ammonia is strongly hygroscopic and concentrates water on the low-temperature side, and a PAG holds water rather than shedding it, so DX lubricants require consistent moisture monitoring.
HTMO vs PAO
Both chemistries suit flooded ammonia systems; they diverge under severe conditions. HTMO covers most conventional duties. PAO stays thinner at low evaporating temperatures and tolerates higher discharge temperatures. The differences appear in three places: low-temperature viscosity, evaporator fouling, and behavior at the discharge end.
Low-Temperature Viscosity — and Why Pour Point Misleads
The pour point (ASTM D97) records the temperature at which a lubricant stops flowing in a standardized test jar. It is a flow/no-flow result: it says nothing about how viscous the lubricant is at any temperature above that point, and the method has known repeatability limitations. Yet pour point is often the only low-temperature figure on a data sheet, which makes it the number most selections are wrongly based on.
Measured low-temperature viscosity tells a different story. The table below compares ISO VG 68 HTMO ammonia lubricants at −35 °C (−31 °F):
| Product | Pour Point | Brookfield Viscosity |
|---|---|---|
| NEXT 717-68 | −42 °C (−44 °F) | 64,650 cP |
| Competitor A | −42 °C (−44 °F) | 87,600 cP |
| Competitor B | −42 °C (−44 °F) | 78,000 cP |
| Competitor C | −39 °C (−38 °F) | 94,000 cP |
| Competitor D | −39 °C (−38 °F) | 110,200 cP |
The first three products share an identical pour point, yet NEXT 717-68 measures 26% lower than Competitor A at the same temperature. Across the full set, NEXT 717-68 runs 17% to 41% lower than the four listed competitors. Lower viscosity at evaporator temperature means faster drainage and less accumulation—a difference the pour point column cannot reveal.
Base stock chemistry impacts performance more than formulation does. At −35 °C (−31 °F), the PAO grade NEXT 717-68-SYN measures 14,040 cP compared to 64,650 cP for its HTMO counterpart—more than four times thinner. At −40 °C (−40 °F), the gap remains: 23,400 cP compared to 90,200 cP. The PAO grade continues to provide measurable viscosities down to −54 °C (−65 °F), where the HTMO grade no longer flows. This low-temperature PAO performance is not reflected in a pour point comparison.
| Temperature |
NEXT 717-68 (HTMO) Brookfield Viscosity (cP) |
NEXT 717-68-SYN (PAO) Brookfield Viscosity (cP) |
|---|---|---|
| −30 °C (−22 °F) | 24,625 | 6,200* |
| −35 °C (−31 °F) | 64,650 | 14,040 |
| −40 °C (−40 °F) | 90,200 | 23,400 |
| −45 °C (−49 °F) | — | 59,040* |
| −50 °C (−58 °F) | — | 145,000 |
| −54 °C (−65 °F) | — | 233,000 |
Lubricant Accumulation and Evaporator Efficiency
Lubricant carried into the evaporator does not remain in suspension. It forms an insulating film on the heat transfer surface, with thickness depending on refrigerant velocity, tube geometry, carryover rate, and, critically, the lubricant’s viscosity at evaporator temperature. A more viscous lubricant in the cold drains more slowly, accumulates faster, and increases insulation.
NEXT’s film-thickness measurements show a PAO ISO VG 68 lubricant forming a film of approximately 17 µm (0.00067 in.), compared to 36–47 µm (0.0014–0.0019 in.) for a typical HTMO grade under the same conditions. Independent research indicates that tube-side heat transfer coefficients in R717 measurably decrease at oil concentrations between 0.1% and 1%.
A field case study demonstrates the operational costs associated with lubricant accumulation:
Cold Storage Facility, 2 × GEA W-3 Screw Compressors: This facility previously used a competitor’s product and required a −38 °C (−36 °F) set point to maintain cooling capacity. After switching to the NEXT HTMO grade, system fouling decreased. This allowed the plant to raise its operating temperature from −38 °C (−36 °F) to −34 °C (−29 °F) while maintaining the same cooling output.
Discharge Temperature, Carryover, and Fluid Life
Ammonia has a high ratio of specific heats, causing it to heat up more during compression than most refrigerants at the same pressure ratio. Industry guidelines limit reciprocating compressor discharge to 121 °C (250 °F); conventional refrigeration screw compressors typically discharge at 77–82 °C (170–180 °F). Certain high-temperature heat pump screw compressors have model-specific discharge limits up to 140 °C (284 °F), but these limits apply only to the machines for which they are published.
Oxidation, driven by air and moisture entering the system, is the primary lubricant aging mechanism in ammonia service, and it accelerates as it progresses: oxidation products increase viscosity and acidity, higher viscosity slows oil return, and slower return means more lubricant remains hot in the system. Antioxidant additives interrupt this cycle.
PAO base stocks are inherently more oxidation-stable than HTMO base stocks. This is a base-stock property, not a guarantee of longer service life in a finished lubricant. Antioxidant formulation, discharge temperature, air, moisture, and contamination routinely outweigh the base-stock difference. Service life is formulation-specific and determined through fluid analysis, not chemical composition.
Carryover also rises with discharge temperature. Mechanical separators catch liquid droplets, not evaporated lubricant, so the lubricant’s volatility at the actual discharge temperature sets the floor on consumption.
HTMO or PAO: The Decision
| Condition | HTMO or HTMO/PAO Blend | Full PAO |
|---|---|---|
| Evaporating temperature above −25 °C (−13 °F) | Adequate | Not required |
| Evaporating temperature from −25 °C (−13 °F) to −35 °C (−31 °F) | Advised | Not required |
| Evaporating temperature from −35 °C (−31 °F) to −45 °C (−49 °F) | Suitable | Preferred |
| Evaporating temperature below −45 °C (−49 °F) | Not recommended | Preferred |
| Discharge temperature below 120 °C (248 °F) | Adequate | Not required |
| Discharge temperature above 140 °C (284 °F) | Deposit risk and shortened fluid life | Preferred |
| Evaporator fouling or capacity loss reported | Contributing factor | Corrective |
| Retrofit from naphthenic oil or alkylbenzene | Use the seal-conditioned grade | Not suitable without seal review |
| Direct-expansion evaporator | Not suitable | Not suitable |
Compatibility, Mixing, and What Actually Goes Wrong
Chemical compatibility and operational suitability are separate questions, and a changeover must pass both. Two lubricants can blend without separation and still cause a leak, a blocked filter, or a foaming sump.
Seals
Highly refined HTMO and PAO base stocks tend to shrink certain elastomers; naphthenic oils swell them. A system that has run for years on naphthenic oil has seals conditioned to that swelling. Moving it directly onto a highly refined base stock will cause those seals to shrink and leak. The transition requires a seal-conditioned grade. NEXT 717-68-SC carries seal conditioners for exactly this scenario and is the only grade NEXT recommends when replacing naphthenic lubricants.
Deposits
HTMO grades dissolve and dislodge the deposits that naphthenic products leave behind. Long-term, this cleans the system; short-term, it loads the filters. Monitor filters closely in the weeks after a naphthenic changeover.
The Ester Rule
Esters do not belong anywhere near ammonia. POEs react chemically with ammonia to form amides—solids, viscous liquids, or sludge that block orifices and foul heat-transfer surfaces. The most common contamination path is the ammonia/CO₂ cascade system: the CO₂ side typically runs on POE at higher pressure, so a leak in the cascade heat exchanger pushes POE into the ammonia circuit.
Mixing and Foaming
HTMO and PAO are chemically compatible and blend without separation, but a changeover between them still has consequences in both directions. Moving from HTMO to PAO can shrink seals but usually improves oil return substantially. Moving from PAO back to HTMO can impair oil return through higher low-temperature viscosity. Neither direction is prohibited; both require planning.
Foaming is a failure mode introduced by mixing. Refrigeration lubricants contain defoaming agents at tuned concentrations. Blending two products can cause the combined defoamer concentration to be too low or too high—both lead to foaming, which increases carryover, degrades lubrication, and makes oil-level readings unreliable.
Verified Substitutions
| Existing Lubricant | Base Stock | NEXT Replacement | Compatibility | Foaming | Properties |
|---|---|---|---|---|---|
| CPI-1008 | HTMO | NEXT 717-68-SC | Pass | Pass | Pass |
| CPI-1009 | HTMO | NEXT 717-68 | Pass | Pass | Pass |
| Petro-Canada Reflo 68A | HTMO | NEXT 717-68 | Pass | Pass | Pass |
| Klüber RHT-68 | HTMO | NEXT 717-68 | Pass | Pass | Pass |
| Mobil Gargoyle Arctic 300 | Naphthenic | NEXT 717-68-SC | Pass | Pass | Pass |
| Suniso 3GS | Naphthenic | NEXT 717-68-SC | Pass | Pass | Pass |
| CPI 4600-68 | PAO | NEXT 717-68-SYN | Pass | Pass | Pass |
| Mobil Arctic SHC 226 | PAO | NEXT 717-68-SYN | Pass | Pass | Pass |
| Sabroe PAO | PAO | NEXT 717-68-SYN | Pass | Pass | Pass |
| Klüber R-200 | PAO | NEXT 717-68-SYN | Pass | Pass | Pass |
The NEXT Ammonia Product Range
| Product | ISO VG | Base Stock | Pour Point | NH3 Solubility | Duty |
|---|---|---|---|---|---|
| NEXT 717-68 | 68 | HTMO (hydrotreated mineral oil) | −42 °C (−44 °F) | Very low | Standard flooded ammonia refrigeration |
| NEXT 717-68-SC | 68 | HTMO with seal conditioners | −42 °C (−44 °F) | Very low | Retrofit from naphthenic oil or alkylbenzene lubricants |
| NEXT 717-68-SYN | 46, 68 | PAO | −57 °C (−71 °F) | Very low | Low-temperature duty, oil return, and energy efficiency; NSF H1 registered |
| NEXT 717-68-XLT | 68 | HTMO/PAO | −51 °C (−60 °F) | Very low | Operation below −40 °C (−40 °F) where PAO cannot be used |
| NEXT 717-DX | 46, 68, 100 | PAG | −42 °C (−44 °F) | High | Direct-expansion ammonia systems |
| NEXT 717-100 | 100 | HTMO | −33 °C (−27 °F) | Very low | Ammonia heat pump duty |
| NEXT 717-100-SYN | 100, 150 | PAO | −57 °C (−71 °F) | Very low | High-temperature heat pump duty; NSF H1 registered |
| NEXT 717-100-XLT | 100 | HTMO/PAO | −51 °C (−60 °F) | Very low | Extended low-temperature range |
What type of oil is used in ammonia refrigeration compressors?
Industrial ammonia systems run on hydrotreated mineral oil, polyalphaolefin (PAO), or blends of the two. Mineral grades cover conventional flooded refrigeration at evaporating temperatures above roughly −35 °C. PAO grades cover low evaporating temperatures, high discharge temperatures and extended drain intervals. Blends sit between the two where full PAO is not justified.
Direct-expansion systems are the exception and require an ammonia-miscible polyalkylene glycol (PAG) instead, because they depend on the lubricant returning with the refrigerant.
The deciding inputs are the compressor type and OEM viscosity requirement, the evaporating temperature, the measured discharge temperature, and the lowest temperature the lubricant reaches anywhere in the system — which is usually not inside the compressor.
Is ammonia really insoluble in refrigeration oil?
Incorrect, and this is one of the most persistent errors in the field.
Measured pressure-volume-temperature work puts ammonia solubility in an ISO VG 68 Group II mineral oil at approximately 5.2 % by mass at 70 °C and 30 bar, with viscosity falling to about 8.09 cSt. A PAO at the same conditions carries about 4.1 % and delivers 9.66 cSt.
Both sit at or below the 10 cSt working minimum. The practical consequence: an ISO VG 68 lubricant in an ammonia compressor is not delivering ISO VG 68 performance, and the higher the condensing pressure, the further it falls. This is why ammonia heat pumps commonly move to ISO VG 100.
Why is pour point a poor way to compare ammonia lubricants?
Because pour point records the temperature at which a sample stops flowing in a test jar. It says nothing about how viscous the fluid became on the way down, and the test has known repeatability limits.
Two ISO VG 68 ammonia lubricants can carry identical published pour points of −42 °C and differ by more than 25 % in measured viscosity at −35 °C. Across a group of commonly used products, the ranking by pour point does not match the ranking by measured low-temperature viscosity.
The comparable measurement is scanning Brookfield viscosity under ASTM D5133, which reports actual centipoise as the sample cools. It matters because a lubricant that thickens in a low-temperature vessel drains slowly or not at all, so it accumulates exactly where it does the most damage to heat transfer.
NEXT publishes measured Brookfield data rather than pour point alone when comparing ammonia grades.
What actually happens when lubricant accumulates in an ammonia evaporator?
It insulates the heat transfer surface, and the cost appears as compressor run hours rather than as an oil problem.
Because ammonia has very low miscibility with mineral and PAO lubricants — and because the lubricant is denser than liquid ammonia — anything escaping the separator settles as a separate layer at the bottom of vessels and evaporators rather than circulating back.
Published work on R717 in an 8 mm tube shows tube-side heat transfer coefficients falling at oil concentrations between 0.1 % and 1 %, with the effect strongest in annular flow. Earlier work recorded a 30 % reduction in heat transfer coefficient from an oil film. It does not take much accumulation before a plant is either losing capacity or running colder suction temperatures to compensate.
The controls are separator performance, reliable draining of oil pots and low points, and a lubricant fluid enough at low temperature to actually drain when the valve is opened.
Can polyol ester (POE) be used with ammonia?
No. This is one of the few absolutes in ammonia compressors.
Ammonia attacks the ester bond and depolymerizes POE, producing solids, viscous residues and sludge that foul heat transfer surfaces and can block orifices and expansion devices.
It matters beyond product selection in one specific case: in an ammonia/CO₂ cascade the CO₂ side commonly runs POE, and the CO₂ circuit normally sits at the higher pressure — so a cascade heat exchanger leak carries ester contamination toward the ammonia circuit rather than away from it. That belongs on the design review checklist, not only on the lubricant order.
What changes when converting from a naphthenic oil?
The seals, and it catches people out because it has nothing to do with chemical compatibility.
Naphthenic oils swell elastomers. A plant that has run one for years has a seal population that has been swollen for that entire time. Moving directly to a highly refined hydrotreated mineral or PAO can shrink those seals and produce leaks on a system that was tight the week before.
The answer is a seal-conditioned grade for the transition rather than a different chemistry. NEXT 717-68-SC carries seal conditioners for exactly this case, and it is the grade NEXT routes every naphthenic replacement to.
Expect a second effect at the same time: modern hydrotreated base stocks will lift deposits the naphthenic oil left behind. That is desirable over the life of the system and a filter problem in the first weeks, so monitor oil and suction filters closely after the change.
Can another brand's ammonia compressor oil be replaced without flushing?
In most cases within the same chemistry family, yes — and NEXT holds documentation rather than an opinion on it.
NEXT tests its ammonia grades against widely used competitor products at 80/20, 50/50 and 20/80 mixture ratios, covering chemical compatibility, physical properties and foaming tendency. Products that pass receive a formal substitution statement, which confirms that the NEXT product can be used for top-up without draining the system, that performance will be equivalent or better, and that no equipment or operating condition changes are required.
Statements currently exist for ten commonly used ammonia lubricants across hydrotreated mineral, naphthenic and PAO chemistries.
Two limits apply. Naphthenic products route to the seal-conditioned grade for the reason above. And an ammonia-miscible PAG for direct-expansion duty is not compatible with hydrocarbon lubricants in either direction — that conversion requires a full drain, flush and filter change.
Ask for the statement covering the product currently in the machine before planning the changeover.