Which oil your screw compressor needs is written on the machine itself: the manual, the oil label on the door and the discharge temperature under load. How to read all three and decide when to change the oil.
"Which oil goes in this compressor?" comes up most often when a machine is outside the warranty period, when the engineer who used to look after it has left, or when the last drum in the store is empty and the previous supplier is slow to deliver. The answer is not a product name. It is written in three places on the machine itself: the operating manual, the oil label on the cabinet door, and the discharge temperature under load. Read all three and you know which oil type and which viscosity grade the machine needs, and why two compressors of the same rating in the same compressor room can need two different oils.
In an oil-flooded screw compressor the oil is injected straight into the airend. It does three jobs at the same time: it lubricates the two rotors and their rolling bearings, it seals the clearance between the rotors so compressed air cannot leak back, and it absorbs most of the heat of compression and carries it to the cooler. The oil is then separated from the air in the oil separator and separator element and returns to the airend. One liter of oil makes this trip thousands of times a day at the machine's discharge temperature, in contact with oxygen, moisture and dust drawn in with the intake air. That is why screw compressor oil is a product in its own right, not a hydraulic oil or engine oil used as a stopgap, and why the two properties that decide its service life are oxidation stability and the ability to shed water and release air.
Every screw compressor manual has a lubrication section, and it normally states three things: the oil type (screw compressor oil, mineral or synthetic base oil), the ISO VG viscosity grade, and the change interval in hours or months with the conditions it applies to. Most oil-flooded screw compressors in the 15–160 kW range are specified for ISO VG 46; some models call for VG 32 or VG 68, in particular machines in very hot compressor rooms and older designs. If the manual has gone missing, ask the dealer who sold the machine for an electronic copy against the model and serial number. The wrong viscosity grade is the single most common error when oil is changed without the manual.
The third item, the change interval, is the one most often misread. The hours in the manual are tied to a reference condition of temperature, humidity and load. Manuals usually add a sentence like "shorten the interval under severe operating conditions", and a compressor room at 40 °C in the Vietnamese hot season is a severe condition by the machine manufacturer's definition.
The dealer or the maintenance team usually sticks a label on the machine with the oil name, the fill date and the running hours at the last fill. Once a machine has been through several oil changes this label matters more than the manual, because it tells you which base oil is in the sump right now. The base oil dictates how a changeover is done. Mineral oils and PAO or PAO/ester oils can be swapped for one another after a complete drain. Polyglycol (PAG) oils do not mix with either of those and need a full flush whenever you change to or from them. If the label does not state the base oil, the product name on it can be looked up in that oil maker's TDS. If there is neither a label nor a log, an oil sample sent to a laboratory will identify the base oil before you touch anything.
Next to the label, the machine log should record three figures at every oil change: total running hours, loaded hours, and the discharge temperature under load at the time of the change. With those three figures you can compare the real oil life from one change to the next and from one machine to another.
The oil and air temperature at the airend discharge, read from the controller while the machine is running loaded, is the single parameter that most decides which oil to use. The manual gives the normal range and the alarm threshold for each model; compare your reading with that range, not with the machine next to it. A machine that runs steadily within its normal range in a well-ventilated room with an even load does fine on a high-quality mineral compressor oil, with the interval per the TDS and periodic oil analysis. A machine that runs hotter than normal per its manual, sits in a humid room, runs three shifts non-stop, or belongs to a plant that wants fewer shutdowns, gains from a synthetic oil, because a synthetic oxidizes far more slowly at the same temperature. Food plants whose compressed air can touch the product have one further requirement: NSF H1 registration.
| Operating condition | Oil type that usually fits | What else to do |
|---|---|---|
| Discharge within the normal range per the manual, well-ventilated room, even load | High-quality mineral compressor oil in the correct ISO VG grade | Sample at 2,000 hours to set the change interval; 4,000 hours per the TDS (CPI-9370-46), extended only with oil analysis |
| Continuous running, discharge hotter than normal, longer interval wanted | PAO or PAO/ester synthetic oil | Replace the separator element and oil filter at the same time; sample 500 hours after the changeover |
| Humid room, high intake-air humidity, discharge hotter than normal per the manual | PAG/ester synthetic oil | Flush thoroughly when coming from mineral or PAO; never top up with a different oil |
| Compressed air may contact food or pharmaceutical product | NSF H1 registered compressor oil (usually PAO/ester) | File the TDS and the NSF registration number in the audit records |
For a machine running at the reference condition, changing by the hours in the manual is enough. For machines in Vietnam the safer method is to pull a sample at 2,000 hours and decide on three parameters: viscosity against new oil, acid number (TAN) and water content. A viscosity shift of more than 10 % against new oil, a TAN climbing fast between two samples or beyond the laboratory's limit for that product, or water above the laboratory's limit means change now, whatever the hour meter says. The reverse also holds: a cool-running machine on a high-quality mineral oil can use the full 4,000 hours per the TDS (CPI-9370-46), and goes beyond that only when oil analysis allows. How to read each parameter is covered in Reading an oil analysis report: the 6 parameters that matter.
Hop Tien is the authorized distributor of Lubrizol Fluid Engineering (LFE) in Vietnam and carries the CPI range of compressor oils: CPI-9370 hydrocracked mineral oil for machines running within the normal temperature range in their manual, CPI-4601 PAO synthetic for machines that need long intervals, CPI-1522 PAG/ester for machines running hot and humid, NSF H1 registered CPI-4600-F and CPI-4265 for food plants, and the CPI COMP CLEAN II cleaning oil for machines that already carry varnish. A Hop Tien engineer visits the machine, goes through the manual and the oil label with the maintenance team, recommends the viscosity grade and oil type from the measured discharge temperature, and sets up an oil sampling schedule per machine. Results on 22–200 kW screw compressors at plastics, cement, textile, packaging and food plants in Vietnam are in the Case studies section.
A dedicated oil for oil-flooded screw compressors, in the type and viscosity grade the manual specifies (most 15–160 kW machines take ISO VG 46), on a mineral or synthetic base oil depending on the discharge temperature and the plant's requirements.
Not recommended. Screw compressor oil is formulated for oxidizing conditions at high discharge temperature in the presence of oxygen and moisture, and for water separation and air release. Other oils tend to form varnish and foam and plug the separator element.
By the manual under reference conditions. In Vietnam, sample at 2,000 hours and decide on viscosity, TAN and water. Synthetic oil usually gives a longer interval than mineral oil at the same temperature.
Mineral to PAO or PAO/ester: a complete drain plus new separator element and filter is enough if the machine has no deposits. Mineral to PAG/ester or the reverse: a full flush, because PAG does not mix with mineral oil or PAO. Flush a varnished machine with cleaning oil first.
This article is general guidance; the specific conditions of each machine must be assessed by an engineer.
