The right oil for an air compressor follows the compressor type. A piston compressor in normal duty uses mineral oil in the manual's grade (ISO VG: the viscosity, or resistance to flow, at 40 °C). In a screw compressor, the base follows the manual's change interval. Under ISO 6743-3, changes up to 2,000 hours call for mineral oil; 2,000 to 4,000 hours, highly refined mineral or semi-synthetic; above 4,000 hours, semi-synthetic or synthetic oil such as PAO (polyalphaolefin, a base oil made by synthesis).
In this machine the oil does the job of engine oil and radiator coolant in a car at once: it lubricates the bearings and rotors, seals the gap between the two screws and carries away the heat of compression. So it has to withstand hot, humid air without oxidizing, and it has to release water easily.
Which oil goes in each air compressor?
Start with the compressor type and the grade the manual asks for. In a rotary compressor, compression comes from a rotating part: two meshed screws or, in a vane compressor, a rotor with blades that slide against the housing. The ISO VG scale is the same one used for types of hydraulic oil, but the formulation changes: compressor oil is made to circulate mixed with hot air.
| If your plant's compressor is… | Use | Why |
|---|---|---|
| Piston or rotary (screw or vane), when the manual calls for ISO VG 100 or 150, in normal duty (air and discharge pressure within what the manual foresees), compressing air, nitrogen, carbon dioxide or helium | Compstar M 100 or M 150 (mineral, ISO VG 100 and 150) | Refined mineral oil with additives against oxidation, wear and corrosion, which repels water and forms little foam |
| Screw or piston, when the manual calls for ISO VG 46 and semi-synthetic oil | Compstar SS 46 (semi-synthetic, ISO VG 46) | A blend of synthetic and mineral base oil, with fast water separation |
| Screw, when the manual calls for ISO VG 46 and synthetic oil | Compstar S 46 (synthetic PAO, ISO VG 46) | The PAO base resists heat and oxidation better than a semi-synthetic one, which suits longer change intervals and hotter operation |
| Refrigeration or industrial air conditioning, with refrigerants such as ammonia and carbon dioxide | Siberian line (naphthenic) | Stays fluid at low temperature, is paraffin-free and moisture-free, and tolerates refrigeration gases. Newer fluorinated refrigerants (HFC) usually call for synthetic oil; confirm the refrigerant with the Kelpen Oil Technical Team |
The Compstar line is indicated for air and process gases (the gases compressed for use in production, such as nitrogen and carbon dioxide). For refrigeration, where the oil circulates together with the refrigerant gas, the indication is the Siberian line. The oil-injected vane compressor follows the screw logic: the international classification standard (ISO 6743-3) puts both in the same family. All options are at compressor oils.
The grade comes from the manual. Oil thicker than indicated raises energy consumption and is slow to circulate at start-up; thinner oil seals the gap between the rotors less well.
Why does an air compressor need a specific oil?
Because in a compressor the oil works inside the hot air being compressed. In an oil-injected screw compressor, the path is this:
Air enters through the filter and is squeezed between two screw-shaped rotors that turn meshed together. Compressing heats the air.
The oil injected into the chamber seals the gap between the rotors and the housing, absorbs heat and lubricates, and leaves together with the air: both exit the screw element (the assembly of the two rotors and the housing) hot and mixed.
In the separator vessel, most of the oil falls by gravity and the separator element, a fiber filter, catches the fine mist, so the air goes on to the line carrying only a small oil residue.
The oil at the bottom passes through the thermostatic valve, the cooler and the filter and is injected again. The valve keeps the oil warm enough that moisture in the air does not condense inside the circuit.
On every cycle the oil meets oxygen, heat and moisture, which leads to oxidation: the reaction with oxygen that darkens the oil, raises acidity and forms varnish (a hard, dark film) and sludge (a pasty deposit).
Varnish and sludge foul the cooler, which pushes the oil temperature up and speeds oxidation. On the separator element, the dirt raises the pressure drop, which leads to higher energy use and more oil passing into the air line. By the Arrhenius rule of thumb, an approximation of reaction chemistry, every 10 °C of extra temperature cuts oil life by about half.
How this shows up in your plant: oil darkening ahead of schedule, discharge temperature rising until the compressor trips, milky oil in the sight glass (a sign of mixed-in water), the separator element replaced early, and oil showing up in the drains and the air line, the familiar symptom of a compressor throwing oil into the system.
Figure 1. Oil-injected screw compressor: the air (blue) is compressed between the rotors together with the oil (amber), the two separate in the vessel, and the oil returns through the thermostatic valve, the cooler and the filter. While the oil is cold, the thermostatic valve sends the flow through the bypass (dashed), without passing through the cooler.
In a splash-lubricated piston compressor, the connecting rod (the rod that links the crankshaft, the shaft that moves the pistons, to each piston) dips into the oil in the crankcase, the bottom part of the machine, and splashes oil onto the parts and the cylinder wall. Some of that oil rises with the air and passes through the discharge valves, the hottest parts of the compressor. It oxidizes there and forms carbon deposits on the valves and the discharge piping, and that hot deposit can catch fire. That is why ISO 6743-3 states that the aim of the classification is to reduce the risk of fire and explosion in stationary air compressors as far as possible.
What is the specification of air compressor oil?
The specification has three parts: the standard class, the viscosity grade and the change interval. The two reference standards separate the oils like this:
DIN 51506, the German standard for air compressor oils, was written mainly for piston compressors and separates the classes by the air temperature at discharge: VBL up to 140 °C, VCL up to 160 °C and VDL up to 220 °C, the latter with higher resistance to aging.
ISO 6743-3, the international classification, separates by machine type. For the piston compressor and the drip-lubricated vane compressor: DAA for normal duty and DAB for severe duty, with hotter air and higher pressure. For the screw and the oil-injected vane: DAG, DAH and DAJ, according to the change interval (up to 2,000 hours, from 2,000 to 4,000 hours and above 4,000 hours).
The ISO VG grade comes from the compressor manual. It gives the viscosity at 40 °C in cSt (centistokes, the unit of kinematic viscosity), with a tolerance of 10% either way under ISO 3448.
Which oil for a screw compressor?
The change interval the compressor manual allows defines the oil base. For a screw compressor in ISO VG 46, Kelpen Oil has Compstar SS 46, semi-synthetic, and Compstar S 46, synthetic PAO; the base that resists heat better suits longer intervals and hotter operation, always according to the compressor manual. When the manual calls for ISO VG 100 or 150, the option is Compstar M 100 or M 150, mineral. ISO 6743-3 organizes the classes by change interval, in the hour ranges given at the start of this article; in the 2,000 to 4,000 hour range, the indicated mineral oil is the highly refined one with a special formulation.
The difference comes from the base. PAO is built from small molecules, all of the same type, which gives chains of uniform length, without the sulfur and unstable compounds of petroleum. With an antioxidant, it oxidizes more slowly and changes less in viscosity with temperature. That change is measured by the viscosity index: the higher it is, the less the viscosity changes. In the classification of the American Petroleum Institute (API), which divides base oils into five groups by composition and refining, conventional mineral oils of Groups I and II have a viscosity index below 120; PAO forms a group of its own, Group IV, with a higher index than theirs.
At every change of base, drain the reservoir, the cooler and the lines: old oxidized oil left in the circuit shortens the life of the new oil. The indicated interval holds for the manual's operating condition; temperature above it, air with a lot of dust or a lot of moisture call for oil analysis before the due date.
Does an oil-free air compressor use oil?
It does, outside the compression chamber. In an oil-free compressor the air is compressed without contact with lubricant, and the oil stays in the bearings (the supports of the shafts), the gears and the crankcase, separated from the air. For this case ISO 6743-3 indicates a gear, bearing and transmission lubricant, and the choice follows the logic of oil for an industrial gear reducer: viscosity and additives by the load and the housing temperature, with the compressor manual as the reference.
Oil-free compression appears where the air touches the product, such as food, beverages, pharmaceuticals and electronics. The lubricated compressor delivers air with a small oil residue: the separator element retains almost all of it, and line filters catch what passes.
How does Kelpen Oil check compressor oil?
With the same tests that tell whether the oil in use is still fit for service. At the in-house laboratory in Arujá (SP), no production batch is released without laboratory approval.
In a compressor, water separation matters as much as oxidation. The demulsibility test (NBR 14172, the Brazilian standard, equivalent to the American standard ASTM D1401): oil and water are stirred together, and the test measures in how many minutes they separate again. Oil that releases water quickly avoids emulsion, the milky mix of oil and water that lubricates poorly and corrodes.
| What we measure | What it reveals in the compressor | Standard and equipment |
|---|---|---|
| Viscosity at 40 °C | Rising: oxidation or mixing with thicker oil. Falling: mixing with thinner oil. A change above 10% from the new oil calls for investigation | ASTM D7042, SVM 4001 viscometer |
| Acid number (TAN) | Acids formed in oxidation; a continuous rise shows the antioxidant running out | NBR 14448, Titrino Plus |
| Water by the Karl Fischer method | Water in ppm (parts per million), measured by titration (a reagent added little by little until it has reacted with all the water in the sample), before the oil turns milky | coulometric Karl Fischer |
| Demulsibility | How long the oil takes to separate from water | NBR 14172 |
| Foam | Tendency to form foam and how long it lasts | NBR 14235 |
| Infrared spectrum, the chemical fingerprint of the oil | Oxidation products and signs of mixing with another oil | ASTM E2412, Spectrum Two |
| Metals by ICP spectrometry, which identifies each metal by the light it emits | Barium, boron, calcium, phosphorus and zinc: show mixing with oil from another family, such as engine oil, rich in calcium and zinc | NBR 14786, Optima 7000 DV |
| Color | Darkening that accompanies oxidation | NBR 14483 (ASTM D1500) |
The full list is at tests and standards of the lubricant laboratory, with the 25 tests done in Arujá (SP).
Frequently asked questions
Can I use hydraulic oil in an air compressor? Only when the compressor manual allows it. Hydraulic oil is formulated and tested for the hydraulic pump and circuit; compressor oil, to work in hot air and release water. An ISO VG 68 hydraulic oil and an ISO VG 68 compressor oil have the same viscosity but different formulations.
Can I use engine oil in an air compressor? Only if the compressor manual says so. Engine oil carries detergents and dispersants, which hold water and soot mixed into the oil, and additives with calcium and zinc, which leave ash on hot parts; in a compressor, water must separate from the oil and the discharge valve must stay clean.
When should I change air compressor oil? At the interval in the compressor manual, which depends on the base. Under ISO 6743-3, in a screw compressor, mineral oil suits changes up to 2,000 hours; highly refined mineral or semi-synthetic, from 2,000 to 4,000 hours; semi-synthetic or synthetic, above 4,000 hours. Change earlier if analysis shows acidity rising faster than in previous analyses or passing the manual's limit, water above the limit given by the compressor maker or the laboratory, or viscosity more than 10% above or below that of the new oil.
What is the best oil for an air compressor? The one that meets the class and grade in the manual and withstands the temperature and running hours of your compressor. In a screw compressor that runs almost all year, synthetic PAO, which resists heat better and suits longer intervals, cuts oil changes, used-oil disposal and stoppages; in an intermittent-duty piston compressor, mineral oil in the right grade does the job.
Glossary
ISO VG: viscosity grade of the oil measured at 40 °C, in cSt, under ISO 3448.
PAO (polyalphaolefin): synthetic base oil, made by chemical synthesis, which oxidizes slowly and changes little in viscosity with temperature.
Semi-synthetic oil: a blend of synthetic and mineral base oil.
Discharge temperature: temperature of the compressed air at the outlet of the compression chamber.
Separator element: fiber filter, at the top of the vessel, that removes oil mist from the compressed air.
Acid number (TAN): amount of acid in the oil, in milligrams of potassium hydroxide per gram; it rises with oxidation.
Demulsibility: the ability of the oil to separate from water.
Naphthenic oil: mineral oil from a family that stays fluid at low temperature.
Talk to the Technical Team
The Kelpen Oil Technical Team evaluates the compressor type, the discharge temperature, the operating hours per year and the change interval you want, and confirms the right product. If the oil is already in use, the Arujá (SP) laboratory performs analysis of the oil in use, as a paid service. Save this article or send it to whoever looks after the compressor room at your company.
