What are the three contamination classes defined in ISO 8573-1 for compressed air?

ISO 8573-1 defines purity classes independently for three contamination types: solid particles (Class 0–9), water (Class 0–9, measuring both pressure dewpoint and liquid water content), and oil (Class 0–4, measuring total oil aerosol and vapour). Each class is specified independently — a system may be Class 1 for particles and Class 3 for water depending on the balance of filtration and drying equipment installed. Class 0 in each category means the user specifies purity more stringent than Class 1. Class 1 for particles requires ≤20,000 particles/m3 ≥0.1–0.5 µm (equivalent to ISO 8573-1:2010 concentration limits); Class 1 for water requires pressure dewpoint ≤−70°C; Class 1 for oil requires ≤0.01 mg/m3. Point-of-use requirements for instrument air, pneumatic controls, and breathing air are specified using these three-digit class references.

Why does pressure dewpoint matter more than atmospheric dewpoint in compressed air quality?

Atmospheric dewpoint is the temperature at which water vapour condenses at ambient pressure (1 bar absolute). When air is compressed to operating pressure (7–10 bar g for typical industrial systems), the partial pressure of water vapour increases proportionally — at 7 bar g (8 bar absolute), the partial pressure of water is 8× higher than at atmospheric pressure. This means air that is comfortably above dew point at atmospheric conditions may be saturated or supersaturated at line pressure, causing liquid water to form in distribution piping. Pressure dewpoint (PDP) — the dewpoint at line pressure — is the operationally relevant parameter. ISO 8573-1 specifies purity classes in terms of PDP: Class 4 = +3°C PDP (condensation possible above 3°C pipe temperature), Class 2 = −40°C PDP (no condensation above −40°C pipe temperature), Class 1 = −70°C PDP (ultra-dry, pharmaceutical and semiconductor grade).

How does a refrigerant dryer differ from a desiccant dryer in compressed air drying performance?

Refrigerant dryers (also called refrigerated dryers) cool compressed air to 2–10°C using a refrigeration circuit, condensing and draining water vapor to achieve a pressure dewpoint of typically +2 to +7°C PDP. This is sufficient for most general industrial pneumatic applications (ISO 8573-1 Class 4 for water). Desiccant dryers use activated alumina, silica gel, or molecular sieve adsorbent to adsorb water vapor from compressed air, achieving PDP of −20°C to −70°C depending on adsorbent type and regeneration method. Refrigerant dryers are lower energy (no regeneration energy) but cannot achieve PDP below approximately 0°C. Desiccant dryers achieve the ultra-dry performance required for instrument air (ISO Class 2 or Class 1) but require 15–25% of compressed air flow for heated regeneration (purge air loss) and have higher capital cost.

What contamination sources exist inside a compressed air system independent of the compressor inlet?

Contamination sources within the compressed air distribution system include: (1) Compressor oil carry-over — lubricant from rotary screw or piston compressors enters the air stream as aerosol or vapor; typical carry-over from well-maintained compressors is 3–5 mg/m3 oil, reduced to <0.01 mg/m3 by coalescing separator and activated carbon filter. (2) Pipe scale and rust — carbon steel distribution piping corrodes at moisture condensation points, releasing rust particles (Fe2O3) and iron corrosion flakes in the 10–500 µm range. (3) Polymer particles from hose and fitting degradation — compressed air hose liners and O-ring seals shed particles as they degrade, appearing downstream as black rubber particles. (4) Biological contamination — liquid water in distribution lines at above-ambient temperature zones creates conditions for bacterial growth; biofilms shed as particulate contamination. Each source requires targeted filtration upstream of sensitive point-of-use applications.

What point-of-use filtration is required for compressed air used in pneumatic food packaging applications?

Food contact compressed air applications require ISO 8573-1 Class 1:2:1 or better at point of use — Class 1 particles (≤20,000 particles/m3 ≥0.1 µm), Class 2 water (PDP ≤−40°C), Class 1 oil (≤0.01 mg/m3). The rationale for these stringent specifications is that compressed air contacting food or food contact surfaces can introduce particles, oil residues, or water that compromise food safety and organoleptic quality. Activated carbon adsorbers at point of use reduce oil vapour to <0.003 mg/m3. High-efficiency sterile-grade filters (removal rating ≥0.01 µm at 99.9999% efficiency) are specified for direct food contact applications per EC Regulation 1935/2004 and relevant national food safety legislation. Oil-free compressors eliminate the oil aerosol source but do not eliminate particulate or moisture contamination from the compression process.

How is compressed air oil contamination quantified, and what are the limits for instrument air?

Compressed air oil contamination is measured as total oil concentration (aerosol plus vapor) at the point of use, expressed in mg/m3 at the measurement pressure and temperature. ISO 8573-3:2010 defines the sampling and analysis procedures for oil aerosol measurement. Oil vapor measurement uses ASTM D3796 or equivalent activated carbon adsorption tubes followed by solvent extraction and gravimetric or gas chromatographic analysis. Instrument air for pneumatic control systems (typically ISO 8573-1 Class 1 for oil: ≤0.01 mg/m3) is specified at this limit because oil contamination of pneumatic valve actuators causes stiction (valve sticking), delayed response, and position error — critical failures in process control applications where valve position accuracy must be within ±1% of commanded position.

What filter element types are required in a typical compressed air treatment train?

A standard compressed air treatment train uses three filter stages in sequence: (1) Pre-filter (bulk liquid and coarse particle separator): removes liquid water droplets and particles above 3–5 µm from compressed air before the aftercooler and dryer; typically a cyclone separator combined with a coalescing element. (2) Coalescing filter (general purpose): removes sub-micron oil aerosol droplets and fine particles; typically rated 0.1 µm removal at 99.9%, achieving ISO Class 2 for oil (≤0.1 mg/m3). (3) Activated carbon adsorber: removes oil vapor and odours not captured by coalescing filtration; rated by breakthrough time at specified concentration and temperature. A fourth stage — sterile membrane filter 0.01 µm — is added for pharmaceutical or food contact applications. All filter elements have finite service life: coalescing elements are changed when pressure drop increases 0.1 bar above initial, carbon adsorbers are changed on a time or concentration breakthrough basis.

What causes compressed air filters to exhibit higher oil concentration downstream than upstream?

Downstream oil concentration exceeding upstream concentration indicates filter element saturation and liquid oil re-entrainment. Coalescing filter elements saturate when the accumulated oil holding capacity is exceeded — the liquid oil film in the media becomes unstable and is re-entrained by the high-velocity airstream as fine droplets. This occurs when: the filter element has exceeded its service life; the system is operating at flow rates above the rated capacity of the coalescing element (velocity too high for stable coalescence); or the filter has experienced liquid flooding — a sudden slug of liquid water or oil (from a downstream cooler or separator draining) overwhelms the element capacity. Filter pressure drop is an inadequate indicator of saturation for coalescing elements — a saturated element may still show acceptable pressure drop while re-entraining oil. Oil downstream monitoring is required to detect saturation events.

What maintenance procedure applies to compressed air receiver tanks to prevent internal corrosion?

Compressed air receiver tanks accumulate liquid water at the base from intercooler and aftercooler drainage, creating an electrolytic corrosion environment. Maintenance requirements: (1) Manual or automatic drain valves must discharge accumulated liquid daily — automatic timed drain valves set to open for 5–30 seconds every 2–4 hours during production. (2) Annual internal inspection per relevant pressure vessel codes (PED 2014/68/EU in Europe, ASME Section VIII in USA) to measure wall corrosion rate and verify structural integrity. (3) pH measurement of drain water — pH below 5.5 indicates acidic condensate requiring investigation of upstream contamination (SO2 or NOx absorbed from industrial atmosphere). (4) Five-year pressure test or inspection per pressure vessel registration requirements. Receivers without drainage management develop corrosion pits that eventually perforate — catastrophic receiver failure presents a significant safety hazard.

What is the difference between ISO 8573-1 Class specifications for particles in two revisions (1991 and 2010)?

ISO 8573-1:1991 specified particle classes using particle size thresholds of ≥1 µm, ≥5 µm, and ≥40 µm. The 2010 revision changed thresholds to ≥0.1–0.5 µm, ≥0.5–1 µm, and ≥1–5 µm, reflecting improved particle counting technology capability at sub-micron sizes. Class 1 in the 1991 version (≤100,000 particles/m3 ≥1 µm) is approximately equivalent to Class 3 in the 2010 version for sub-micron particles. Systems specified against the 1991 version cannot be directly compared to 2010 specifications without clarifying which edition applies. When specifying compressed air purity for new installations, ISO 8573-1:2010 should be referenced explicitly, and the three-digit format (e.g., "ISO 8573-1:2010 Class 1:2:1") should include the standard year to prevent ambiguity between editions.

Knowledge Center›Engineering›Compressed Air Purity

Engineering · 10 min

Compressed Air Purity

Q: What point-of-use filtration is required for compressed air used in pneumatic food packaging applications?

Food contact compressed air applications require ISO 8573-1 Class 1:2:1 or better at point of use — Class 1 particles (≤20,000 particles/m3 ≥0.1 µm), Class 2 water (PDP ≤−40°C), Class 1 oil (≤0.01 mg/m3). The rationale for these stringent specifications is that compressed air contacting food or food contact surfaces can introduce particles, oil residues, or water that compromise food safety and organoleptic quality. Activated carbon adsorbers at point of use reduce oil vapour to <0.003 mg/m3. High-efficiency sterile-grade filters (removal rating ≥0.01 µm at 99.9999% efficiency) are specified for direct food contact applications per EC Regulation 1935/2004 and relevant national food safety legislation. Oil-free compressors eliminate the oil aerosol source but do not eliminate particulate or moisture contamination from the compression process.

Q: How is compressed air oil contamination quantified, and what are the limits for instrument air?

Compressed air oil contamination is measured as total oil concentration (aerosol plus vapor) at the point of use, expressed in mg/m3 at the measurement pressure and temperature. ISO 8573-3:2010 defines the sampling and analysis procedures for oil aerosol measurement. Oil vapor measurement uses ASTM D3796 or equivalent activated carbon adsorption tubes followed by solvent extraction and gravimetric or gas chromatographic analysis. Instrument air for pneumatic control systems (typically ISO 8573-1 Class 1 for oil: ≤0.01 mg/m3) is specified at this limit because oil contamination of pneumatic valve actuators causes stiction (valve sticking), delayed response, and position error — critical failures in process control applications where valve position accuracy must be within ±1% of commanded position.

Q: What filter element types are required in a typical compressed air treatment train?

A standard compressed air treatment train uses three filter stages in sequence: (1) Pre-filter (bulk liquid and coarse particle separator): removes liquid water droplets and particles above 3–5 µm from compressed air before the aftercooler and dryer; typically a cyclone separator combined with a coalescing element. (2) Coalescing filter (general purpose): removes sub-micron oil aerosol droplets and fine particles; typically rated 0.1 µm removal at 99.9%, achieving ISO Class 2 for oil (≤0.1 mg/m3). (3) Activated carbon adsorber: removes oil vapor and odours not captured by coalescing filtration; rated by breakthrough time at specified concentration and temperature. A fourth stage — sterile membrane filter 0.01 µm — is added for pharmaceutical or food contact applications. All filter elements have finite service life: coalescing elements are changed when pressure drop increases 0.1 bar above initial, carbon adsorbers are changed on a time or concentration breakthrough basis.

Q: What causes compressed air filters to exhibit higher oil concentration downstream than upstream?

Downstream oil concentration exceeding upstream concentration indicates filter element saturation and liquid oil re-entrainment. Coalescing filter elements saturate when the accumulated oil holding capacity is exceeded — the liquid oil film in the media becomes unstable and is re-entrained by the high-velocity airstream as fine droplets. This occurs when: the filter element has exceeded its service life; the system is operating at flow rates above the rated capacity of the coalescing element (velocity too high for stable coalescence); or the filter has experienced liquid flooding — a sudden slug of liquid water or oil (from a downstream cooler or separator draining) overwhelms the element capacity. Filter pressure drop is an inadequate indicator of saturation for coalescing elements — a saturated element may still show acceptable pressure drop while re-entraining oil. Oil downstream monitoring is required to detect saturation events.

Q: What maintenance procedure applies to compressed air receiver tanks to prevent internal corrosion?

Compressed air receiver tanks accumulate liquid water at the base from intercooler and aftercooler drainage, creating an electrolytic corrosion environment. Maintenance requirements: (1) Manual or automatic drain valves must discharge accumulated liquid daily — automatic timed drain valves set to open for 5–30 seconds every 2–4 hours during production. (2) Annual internal inspection per relevant pressure vessel codes (PED 2014/68/EU in Europe, ASME Section VIII in USA) to measure wall corrosion rate and verify structural integrity. (3) pH measurement of drain water — pH below 5.5 indicates acidic condensate requiring investigation of upstream contamination (SO2 or NOx absorbed from industrial atmosphere). (4) Five-year pressure test or inspection per pressure vessel registration requirements. Receivers without drainage management develop corrosion pits that eventually perforate — catastrophic receiver failure presents a significant safety hazard.

Q: What is the difference between ISO 8573-1 Class specifications for particles in two revisions (1991 and 2010)?

ISO 8573-1:1991 specified particle classes using particle size thresholds of ≥1 µm, ≥5 µm, and ≥40 µm. The 2010 revision changed thresholds to ≥0.1–0.5 µm, ≥0.5–1 µm, and ≥1–5 µm, reflecting improved particle counting technology capability at sub-micron sizes. Class 1 in the 1991 version (≤100,000 particles/m3 ≥1 µm) is approximately equivalent to Class 3 in the 2010 version for sub-micron particles. Systems specified against the 1991 version cannot be directly compared to 2010 specifications without clarifying which edition applies. When specifying compressed air purity for new installations, ISO 8573-1:2010 should be referenced explicitly, and the three-digit format (e.g., "ISO 8573-1:2010 Class 1:2:1") should include the standard year to prevent ambiguity between editions.

ISO 8573-1 Quality Classes, Contamination Measurement, and System Design

Compressed air contains particles, liquid water, water vapour, and oil aerosols and vapour introduced during compression and distribution. ISO 8573-1 defines quality classes for each contaminant type, providing a standardised specification language for compressed air systems. Achieving the correct purity class requires understanding contamination sources, measurement methods, and the filtration and drying technologies that control each contaminant type.

≤0.01 mg/m³

ISO 8573-1 Class 1 oil

≤ −70°C

Class 1 pressure dew point

Zero permitted

Class 1 particles (≥5 µm)

+2°C to +10°C

Refrigerant dryer PDP

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ISO 8573-1 Quality Classes

ISO 8573-1 defines quality classes 0 through 6 for three contamination categories independently: solid particles (by count and size), moisture (by pressure dew point, °C), and total oil (aerosol plus vapour, mg/m³). Class 0 is specified by the user and is more stringent than Class 1 — it is not a defined numerical value in the standard. Class 1 is the highest standard-defined purity; Class 6 is the least stringent. A compressed air quality specification written as ISO 8573-1:2010 [2:4:2] means Class 2 for particles, Class 4 for moisture, and Class 2 for oil. Each class must be specified independently because different treatment stages address each contamination type — a highly efficient particulate filter provides no benefit for dew point, and a refrigerant dryer reduces moisture but does not remove oil aerosols.

User-defined, more stringent than Class 1

Class 0

≤0.01 mg/m³ (total oil)

Class 1 oil content

ISO 8573-1 [2:4:2] = particles:moisture:oil

Notation example

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Particle Quality Classes

ISO 8573-1 particle classes are defined by the maximum number of particles per cubic metre at specified size ranges. Class 1 (particles): ≤20,000 particles/m³ at 0.1–0.5 µm; ≤400 at 0.5–1 µm; ≤10 at 1–5 µm; no particles ≥5 µm. Class 2: same ≤400,000 at 0.1–0.5 µm; ≤6,000 at 0.5–1 µm; ≤100 at 1–5 µm; no particles ≥5 µm. Class 3: no limit for <1 µm; ≤1,000 at 1–5 µm; no particles ≥5 µm. Classes 4 and 5 allow particles ≥5 µm in increasing quantities. Class 6 allows particle mass concentration up to 10 mg/m³ without size distribution limits. For most industrial pneumatic actuator and tool applications, particle Class 3 or 4 is adequate. Instrument air (analytical instruments, pressure transmitters, control valves) requires particle Class 1 or 2 to prevent instrument blockage and measurement error.

Zero permitted

Class 1 (≥5 µm particles)

≤1,000 particles/m³

Class 3 (1–5 µm)

Class 1 or 2

Instrument air class

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Moisture Classes and Pressure Dew Point

Moisture in compressed air is expressed as pressure dew point (PDP) — the temperature at which condensation occurs at the system operating pressure. A PDP of −40°C at 7 bar means the air contains so little moisture that condensation will not form until the temperature drops to −40°C, even at 7 bar line pressure. ISO 8573-1 moisture classes: Class 1: PDP ≤ −70°C; Class 2: ≤ −40°C; Class 3: ≤ −20°C; Class 4: ≤ +3°C; Class 5: ≤ +7°C; Class 6: ≤ +10°C. Refrigerant dryers achieve PDP between +2°C and +10°C (Classes 4–6). Adsorption (desiccant) dryers achieve PDP between −20°C and −70°C (Classes 1–3). The conversion from PDP to water content in g/m³ at a reference condition allows calculation of absolute humidity: at −40°C PDP and 7 bar, the water content is approximately 0.003 g/m³ — compared to 1.4 g/m³ at +7°C PDP.

≤ −70°C

Class 1 PDP

+2°C to +10°C (Classes 4–6)

Refrigerant dryer PDP range

−20°C to −70°C (Classes 1–3)

Desiccant dryer PDP range

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Oil Content Classes and Measurement

Oil in compressed air originates from lubricant carryover from oil-lubricated compressors (in the form of aerosol and vapour) and from atmospheric hydrocarbon vapours that concentrate during compression. ISO 8573-1 total oil classes (aerosol + liquid + vapour, mg/m³): Class 1: ≤0.01 mg/m³; Class 2: ≤0.1 mg/m³; Class 3: ≤1 mg/m³; Class 4: ≤5 mg/m³. Oil-free compressors (Class 0 equipment per ISO 8573-7) still require oil removal filtration for Class 1 or Class 2 applications because atmospheric hydrocarbon vapours drawn into the compressor inlet concentrate to measurable levels at the outlet. ISO 8573-2 defines the test method for aerosol oil measurement: impaction on a membrane filter with gravimetric analysis. ISO 8573-5 defines the vapour oil measurement method using activated carbon adsorption tubes. Total oil = aerosol (ISO 8573-2 method) + oil vapour (ISO 8573-5 method). Coalescing filters remove aerosol oil to Class 1; activated carbon adsorption removes vapour oil — both stages are required to achieve verified Class 1 compliance.

≤0.01 mg/m³

Class 1 oil content

Coalescing filter (ISO 8573-2)

Aerosol removal

Activated carbon (ISO 8573-5)

Vapour removal

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Contamination Sources During Compression

Ambient air drawn into a compressor contains atmospheric particulates (0.1–10 µm pollen, dust, soot), water vapour (humidity), and trace hydrocarbon vapours. During compression, all constituents are concentrated in proportion to the compression ratio: at 7 bar gauge (8 bar absolute), volume reduces to 1/8 and contaminant mass per volume increases 8-fold. Compressed air after-cooling (required to prevent thermal damage to downstream equipment) causes the compressed, humid air to cool rapidly — water vapour condenses into liquid water and aerosol droplets. The after-cooler and condensate separator remove the largest water fraction (typically 70–90% of the inlet water vapour at tropical ambient conditions) but leave substantial residual moisture requiring drying. Oil-lubricated compressors inject oil into the compression stage for cooling and sealing — oil carryover from the separator and coalescing filter in the compressor unit outlet is typically 1–5 mg/m³ before point-of-use treatment.

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Filtration System Design for Quality Classes

A compressed air treatment train is designed in stages, each addressing one contamination category at the conditions produced by the upstream stage. After-cooler + condensate separator: removes bulk liquid water and coarse aerosols (Stages 4–5 moisture, particle Class 4–5). General purpose coalescing filter (1 µm coalescence element): removes liquid aerosol oil to 0.1 mg/m³ and particles ≥1 µm (particle Class 2–3, oil Class 2–3). High-efficiency coalescing filter (0.01 µm coalescence element): reduces oil aerosol to 0.01 mg/m³ (oil Class 1). Activated carbon adsorber: removes oil vapour below 0.003 mg/m³ (oil Class 1). Desiccant dryer: achieves PDP ≤ −40°C (moisture Class 2). Final particulate filter (PTFE membrane or borosilicate fiber): ensures sterile or classified air downstream of all treatment stages (particle Class 1 or 2). DRYCORE™ filtration systems provide integrated particulate and coalescing elements for compressed air applications, reducing system footprint and pressure drop budget.

Oil Class 2 (0.1 mg/m³)

Coalescing filter achieves

Oil Class 1 (0.01 mg/m³)

High-efficiency coalescer

Vapour removal to full Class 1 compliance

Activated carbon adds

ENGINEERING DIAGRAMS

Compressed Air Treatment Train — ISO 8573-1 Purity Classes — Left to right sequence: ambient air inlet, compressor block with triangle pump symbol, aftercooler (sinusoidal coil symb…

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ISO 8573-1 Compressed Air Purity Classes — Three-panel reference chart for ISO 8573-1:2010. Top panel — solid particles: table showing Classes 0–5 with maximum par…

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COMMON ENGINEERING MISTAKES

Specifying ISO 8573-1 Class 0 air quality when the application only requires Class 1 or Class 2. Class 0 requires an inherently oil-free compressor — applying it to systems served by lubricated compressors forces complete system replacement rather than downstream treatment.

Measuring compressed air quality only at the dryer or filter outlet. Compressed air picks up contamination from distribution pipework — point-of-use measurements at the application connection are required to verify actual delivered quality.

Confusing pressure dew point (PDP) with atmospheric dew point. ISO 8573-1 water classes specify PDP at line pressure — a PDP of −40°C at 7 bar corresponds to approximately −27°C atmospheric dew point; the two figures are not interchangeable in specifications.

ENGINEERING REFERENCES

STANDARD

ISO 8573-1:2010, Compressed Air — Part 1: Contaminants and Purity Classes

Primary standard defining contamination classes for particles, water (dewpoint), and oil in compressed air systems.

STANDARD

ISO 8573-2:2018, Compressed Air — Part 2: Test Methods for Aerosol Oil and Particle Content

Test methods for measuring oil aerosol and particle concentration in compressed air at point-of-use sampling.

STANDARD

ISO 8573-3:2010, Compressed Air — Part 3: Test Methods for Measurement of Humidity

Test methods for pressure dewpoint measurement used to verify ISO 8573-1 water class compliance.

STANDARD

ISO 8573-7:2003, Compressed Air — Part 7: Test Methods for Viable Microbiological Contaminants

Microbiological testing standard for breathing air and pharmaceutical-grade compressed air applications.

FREQUENTLY ASKED QUESTIONS