Compressed air is recognized as the fourth most important source of energy in industry, after electricity, natural gas, and water. Its significance extends beyond the provision of mechanical power; the quality of compressed air directly affects equipment performance, product quality, and the service life of industrial systems.
Ambient air inherently contains solid particles, water vapor, and oil aerosols. During the compression process, the concentration of these contaminants increases significantly. If not properly treated, contaminated compressed air can lead to corrosion, blockage of pneumatic components, premature equipment failure, and reduced operational efficiency. Consequently, standardization of compressed air quality in accordance with ISO 8573-1 is essential for reliable and safe industrial operation.
Key Parameters of ISO 8573-1
The ISO 8573-1 standard classifies compressed air quality based on three primary parameters:
1. Solid Particles
These include dust, soot, pipe corrosion residues, and other suspended contaminants that may cause abrasion, clogging of pneumatic valves, and malfunction of sensitive equipment.
2. Water Content
The presence of water vapor or liquid droplets in compressed air can result in corrosion, microbial growth, and freezing at low temperatures. The principal metric used to assess moisture content is the Pressure Dew Point (PDP).
3. Oil Content
Oil contamination consists of liquid oil (aerosols) and oil vapor originating from oil-lubricated compressors or environmental sources. In sensitive industries, oil contamination can severely compromise product integrity and process safety.
ISO 8573-1 Compressed Air Quality Classes
| Quality Class |
Solid Particles (µm / m³) |
Pressure Dew Point (°C) |
Oil Content (mg/m³) |
| 0 |
As specified by the user |
As specified by the user |
As specified by the user |
| 1 |
≤ 0.1 / ≤ 20,000 |
≤ −70 |
≤ 0.01 |
| 2 |
≤ 1 / ≤ 400,000 |
≤ −40 |
≤ 0.1 |
| 3 |
≤ 5 / ≤ 90,000 |
≤ −20 |
≤ 1 |
| 4 |
≤ 15 / — |
≤ +3 |
≤ 5 |
| 5 |
≤ 40 / — |
≤ +7 |
≤ 25 |
| 6 |
— |
≤ +10 |
— |
Screw Compressors and Their Relation to Compressed Air Quality
Oil-Injected Screw Compressors
Oil-injected screw compressors require multi-stage air treatment systems to achieve higher compressed air quality classes. These systems typically include:
Such configurations enable compliance with stringent ISO 8573-1 quality requirements.
Oil-Free Screw Compressors
Although oil-free compressors eliminate direct oil contact within the compression chamber, filtration and drying remain necessary to remove ambient particulates and moisture. Appropriate air treatment ensures stable operation and protects downstream equipment.
Role of the Compressed Air Receiver
Beyond energy storage, the compressed air receiver plays a critical role in:
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Facilitating sedimentation of solid particles
-
Separating a portion of moisture and oil
-
Reducing pressure fluctuations and peak demand loads
Proper receiver design must consider compressor capacity, consumption patterns, and required air quality class.
Substituting Oil-Free Compressors with Oil-Injected Systems
In many industries—including metalworking, mining, general manufacturing, and certain food-processing applications (subject to appropriate filtration and hygiene requirements)—oil-injected screw compressors combined with advanced filtration and drying systems can achieve air quality equivalent to near Class 0
This approach offers a significant reduction in initial capital investment compared to oil-free compressor systems while maintaining comparable air quality standards.