Lubricated screw or oil-free compressor: Class 0 describes the air, not the machine
Most comparisons set two machines against each other. The standard, however, judges the air at the point of use. A lubricated screw followed by a treatment chain can deliver oil-free air at the point of use, at a significantly lower purchase cost; class 0 itself remains that of an oil-free machine. Here is the order in which CEZIUM selects each technology.
The two technologies in one sentence
A lubricated screw compressor injects oil into the compression chamber: the oil seals the gap between the rotors, removes the heat of compression and lubricates the bearings. It is the most widespread technology in industry, the cheapest to buy, and its heat is easy to recover, through the oil circuit.
An oil-free compressor puts no oil at all in the chamber. Three families coexist: scroll, where two interlocking spirals compress the air without contact; rotary tooth (claw), whose rotors turn with a calibrated clearance and never touch; and dry screw, whose coated rotors turn at high speed, often in two stages with intercooling.
This is where the usual comparison stops, and where it becomes misleading: it compares machines, whereas the standard measures air.
Class 0 describes the air, not the machine
ISO 8573-1 classifies compressed air at the point of use according to three contaminants: solid particles, water and total oil. Each class sets a maximum content; for oil, Class 1 corresponds to 0.01 mg/m³.
Class 0 is not one more step down the same scale: it is a category in which the content is agreed between the supplier and the user, must be more stringent than Class 1, and is verified by measurement. It therefore describes a result, not a design principle.
In practice, the trade has settled the question: Class 0 means a non-lubricated machine, and that is what a specification expects when it states it. Engineering nonetheless allows a second route, rarely mentioned: a lubricated screw followed by multi-stage filtration and an adsorption dryer delivers oil-free air, measured at acceptance. This route costs significantly less to buy, but it does not give you a certified machine. It is therefore valid for a document that asks for oil-free air, never for a document that states Class 0.
Only one situation truly separates the two: when the specification requires an oil-free machine rather than oil-free air. This is common in pharmaceuticals and electronics, where the audit covers the equipment as much as the result. The requirement is found in the document, not in the technology: check it before sizing.
Before choosing a technology, read exactly what your specification requires: compliant air, measured at acceptance, or an oil-free compressor. The two wordings do not commit you to the same budget.
Our order of preference for Class 0
CEZIUM selects the simplest oil-free technology that can deliver the required flow, then switches to a lubricated screw with a treatment chain as soon as natively oil-free machines become unreasonable. This is exactly the order used by the online diagnosis: the page and the tool say the same thing.
Our order of preference for Class 0
When a specification says Class 0, CEZIUM quotes an oil-free machine, as the trade understands it: scroll compressor up to 5 m³/min, rotary tooth compressor from 2.3 to 8.6 m³/min, dry screw or centrifugal beyond that. We also point out the alternative few suppliers mention: a lubricated screw followed by a treatment chain delivers oil-free air for a markedly lower purchase price, without carrying the Class 0 label.
In this trade, Class 0 means an oil-free machine. That is the convention, and it is what a specification expects when it writes Class 0, so that is what we quote. Technically, ISO 8573-1 classifies air at the point of use rather than the compressor, so a lubricated screw followed by multi-stage filtration and an adsorption dryer delivers oil-free air measured on acceptance. That is a genuine alternative, cheaper to buy and far better at heat recovery, but it does not give you a Class 0 certified machine. Both are worth knowing; your own documents decide.
| Rank | Technology | Catalogue flow | Power | Class 0 | Recoverable heat | Purchase cost |
|---|---|---|---|---|---|---|
| 01 | Scroll compressor | 0.3 to 4.96 m³/min | 2.2 to 45 kW | Oil-free machine | Low | Moderate |
| 02 | Rotary tooth compressor | 2.3 to 8.6 m³/min | 15 to 55 kW | Oil-free machine | Low | High |
| 03 | Dry screw compressor | 6.8 to 43 m³/min | 45 to 250 kW | Oil-free machine | Limited | Very high |
| 04 | Centrifugal compressor | 48 to 3,000 m³/min | 198 to 12,055 kW | Oil-free machine | High | Very high |
| 05 | Lubricated screw + treatment chain | 0.95 to 44 m³/min | 5.5 to 250 kW | Treated air, no label | High | Low |
Flow rates and power ratings taken from the CEZIUM catalogue, oil-free technologies and variable-speed lubricated screw. Purchase cost is given in relative ranks, at comparable flow.
- 01
Scroll compressor
Up to 5 m³/minThe first choice below 5 m³/min: oil-free by design, quiet, compact. The right format for a laboratory, a dental practice or a small production line.
See the 13 catalogue models - 02
Rotary tooth compressor
2.3 to 8.6 m³/minAbove the scroll range and up to 8.6 m³/min. The rotors never touch: no friction inside the chamber, minimal wear, TÜV-certified Class 0. Beyond that flow, several units run in parallel.
See the 14 catalogue models - 03
Dry screw compressor
7.8 to 46 m³/minThe answer at high flow when the specification says Class 0. We also state its downside: it costs a great deal to buy and lends itself poorly to heat recovery. If your documents ask for oil-free air rather than a certified machine, the previous step is worth quoting alongside.
See the 20 catalogue models - 04
Centrifugal compressor
From 48 m³/minReserved for the continuous base load of very large sites, where no screw compressor can carry the flow in a single machine.
See the 11 catalogue models - 05
Lubricated screw + treatment chain
Alternative, above 8.6 m³/minThe alternative, when your specification asks for oil-free air rather than a certified machine: multi-stage filtration and an adsorption dryer behind a lubricated screw, with acceptance measurement at the point of use. Markedly lower purchase price, and compression heat is recovered from the oil circuit, which a dry machine does poorly. Rule it out if your documents say Class 0.
See the 33 catalogue models
Technical decision criteria
| What the specification requires | Compliant air: lubricated screw + treatment is possible. An oil-free machine: scroll, rotary tooth or dry screw. |
|---|---|
| Required flow | Up to 5 m³/min: scroll. From 2.3 to 8.6: rotary tooth. Beyond that: lubricated screw + treatment chain. |
| CEZIUM catalogue range | Lubricated screw 7.5 to 250 kW (1.1 to 54 m³/min) · oil-free 2.2 to 250 kW (0.24 to 46 m³/min) |
| Heat recovery | High on the oil circuit · limited on a dry screw · low on scroll and rotary tooth |
| Noise level in the catalogue | Lubricated screw 60 to 78 dB(A) · dry screw 69 to 74 · rotary tooth 80 to 85 |
| Relative purchase cost | Lubricated screw lowest · scroll moderate · rotary tooth high · dry screw highest |
What the treatment chain costs
Choosing a lubricated screw for oil-free air does not mean installing a cheaper compressor: it is a choice of complete installation, and the treatment is an integral part of it.
The typical chain comprises the air/oil separation built into the machine, multi-stage filtration including an activated carbon stage, and an adsorption dryer that brings the pressure dew point down to -40 °C. Each of these elements has a purchase cost, a pressure drop and a cartridge replacement schedule.
That is why an honest comparison is never made between two compressors, but between two complete installations, measured at the same point of use.
- Integrated air/oil separation, residual content below 3 ppm at the machine outlet
- Multi-stage filtration, with an activated carbon stage for oil vapour
- Adsorption dryer, pressure dew point -40 °C
- Acceptance measurement at the point of use, not at the compressor outlet
- Cartridge replacement schedule, to be budgeted from the investment stage
When an oil-free machine is a must
- The specification, the audit or the end customer requires an oil-free compressor, not just compliant air
- The flow stays below 8.6 m³/min: scroll and rotary tooth cover the demand without a heavy treatment chain
- Space in the plant room is short and the installation has to fit into a single piece of equipment
- The process cannot tolerate any point of failure in the filtration, including a saturated cartridge
Dry screw remains our last resort: it is the most complex, and the most expensive to buy. We install it when it is required, and we say so when it is not.
The economics over the service life
Over the service life of a compressed air installation, energy dominates the total cost by a wide margin: the initial investment and maintenance weigh far less than the cumulative electricity bill. That is what makes a comparison on purchase price alone misleading, in both directions.
The lubricated screw starts with an investment advantage, but the treatment chain and its consumables have to be added to it. The oil-free machine starts out more expensive, and saves part of that chain. The gap is then decided by two items that most comparisons forget.
The first is heat recovery: on a lubricated screw, a large share of the absorbed energy is reused as hot water or space heating via the oil circuit. The second is operating pressure: every 0.1 bar reduction on the network cuts consumption by about 0.6 to 0.7%, which weighs more over ten years than the difference in purchase cost between two technologies.
Do not compare two compressors: compare two installations, at the same measurement point, over the same period, including recovered heat.
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