Why Compressed Air Becomes Contaminated
Before discussing purification solutions, it is necessary to identify the sources of contaminants, which helps us understand why a complete after-treatment system is indispensable.
1. Natural contaminants in the atmosphere
Compressed air originates from ambient air, which is not perfectly clean. It generally contains dust, water vapor, pollen, microorganisms and various fine particles. When ambient air enters the compressor, contaminants become concentrated during compression, degrading compressed air quality.
2. Contamination generated during compressor operation
For oil-injected screw air compressors, lubricating oil forms oil mist in the process of lubrication, sealing and cooling. Even after passing through the oil-air separator, a small amount of oil will still enter the compressed air. In addition, wear debris and metal particles generated from long-term equipment operation may also mix into the air system.
It should be noted that an oil-free compressor does not inherently deliver compressed air of high cleanliness grade. Moisture, particulate matter in the intake air and contaminants introduced by the air transmission pipeline still need to be further removed via drying and filtration equipment.
3. Secondary contamination arising from air storage and transportation
After leaving the compressor, compressed air flows through air receivers, pipelines and various valves. If water vapor in the air is not removed in a timely manner, condensation will form inside pipelines, further triggering corrosion, bacterial growth and particle detachment, resulting in so-called secondary contamination.
Therefore, a compressed air purification system must address contaminants generated during compression and prevent continuous accumulation of pollutants throughout the entire transmission route.
Understanding ISO 8573-1 Air Quality Classes (Oil, Water & Particulates)
The globally recognized standard is ISO 8573-1(corresponding Chinese standard GB/T 13277.1-2023). This standard defines purity classes for three categories of contaminants: solid particulates, water and oil. A smaller class number indicates higher air cleanliness.
The three indicators are rated independently. For example, a system marked ISO 8573-1 Class 1.2.1 means Class 1 for particulates, Class 2 for water, and Class 1 for oil. Overall air quality cannot be judged based on a single indicator alone.
Key limits for the three contaminant categories:
Water (Pressure Dew Point): Class 1 ≤ -70°C; Class 2 ≤ -40°C; Class 3 ≤ -20°C; Class 4 ≤ +3°C; Class 5 ≤ +7°C. A lower pressure dew point means less residual water vapor and lower risk of condensation in terminal pipelines. (Class 0 is also specified in the standard, with stricter requirements than Class 1; specific parameters shall be negotiated between the air user and equipment supplier.)
Oil (Total Oil Content): Class 1 ≤ 0.01 mg/m³; Class 2 ≤ 0.1 mg/m³; Class 3 ≤ 1 mg/m³; Class 4 ≤ 5 mg/m³; Class 5 ≤ 25 mg/m³. Consistent with other indicators, Class 0 applies to working conditions requiring performance exceeding the published standard limits, with parameters agreed by both parties.
Solid Particulates: For higher cleanliness grades (Class 1 to 5), the standard adopts particle counting for different particle size ranges instead of mass concentration metrics. For instance, Class 1 requires no more than 20,000 particles/m³ within the 0.1–0.5 μm size range. Smaller class numbers represent stricter requirements. The lowest grade (Class 6) still uses mass concentration (≤5 mg/m³) without particle counting.
How to Choose the Right Air Purity Class Without Over-Specifying
After learning about ISO 8573-1 purity classes, many customers raise a question: does higher class always equal better performance?
In fact, there is no universal "optimal purification solution" for all enterprises. The compressed air purification system shall be configured comprehensively based on production processes, required air quality and investment costs, instead of blindly pursuing the highest grade.
The following factors require priority consideration:
1. Target air quality requirements. Requirements for particulates, moisture and oil vary widely across industries. The target ISO class shall be defined according to actual process conditions rather than arbitrarily upgrading specifications.
2. Air flow rate and working pressure. After-treatment equipment must match the exhaust capacity of the air compressor. Undersized filters or dryers will create excessive pressure drop and compromise purification efficiency.
3. Pressure dew point requirements. For general industrial production where only pipeline condensation prevention is needed, a refrigerated air dryer normally meets demands. Industries including electronics, lithium batteries and pharmaceuticals frequently require desiccant air dryers to achieve lower pressure dew points.
4. Operating costs. Higher filtration precision and lower pressure dew points do not guarantee economic viability. Rational system design balances equipment investment, energy consumption and maintenance costs on the premise of meeting air quality targets, achieving optimal full-lifecycle cost.
Accordingly, engineering design of compressed air purification solutions generally integrates air flow, pressure, ambient temperature, target pressure dew point and terminal process requirements, rather than simply adding filters or upgrading purification equipment.
Purification Solutions: How to Obtain Clean and Safe Compressed Air
Achieving standard-compliant compressed air normally requires a complete after-treatment system instead of individual equipment. The sequence of each treatment stage directly affects purification performance and cannot be rearranged arbitrarily.
1. Primary Treatment: Air Receiver + Automatic Drain Trap
This is the sedimentation stage. The large-volume tank reduces airflow velocity to separate liquid water and oil mist, which are drained promptly to reduce the load on downstream equipment.
2. Pre-Filtration (Protect Downstream Equipment)
Pre-filters are installed upstream of dryers to remove coarse dust and liquid condensate. If a desiccant dryer is deployed downstream, high-efficiency coalescing oil removal filters must be added on the basis of pre-filtration to control oil mist below 0.01 mg/m³ (Class 1 oil standard). This step is critical: once desiccant inside desiccant dryers is contaminated by oil, its adsorption capacity will drop sharply or even fail. Therefore, oil must be removed completely before air enters the dryer to protect the desiccant.
3. Deep Drying (Moisture Control)
Select refrigerated air dryers or desiccant air dryers according to target pressure dew point. A refrigerated air dryer delivers a standard pressure dew point of approximately +3°C, suitable for most general industrial applications. Stringent working conditions requiring a pressure dew point of -40°C or below require desiccant air dryers.
4. Post Particulate Filtration
- Downstream of desiccant dryers: particulate filters are mandatory to intercept fine desiccant dust shed from molecular sieves or silica gel and prevent dust from entering downstream pipelines.
- Downstream of refrigerated dryers: although no desiccant dusting occurs, general particulate filters are still recommended before air supply terminals to capture pipeline scale, rust contaminants and residual condensate droplets.
5. Special Oil-Free Requirements (Activated Carbon Filtration)
For processes demanding absolute oil-free air (e.g., food production, breathing air), activated carbon filters shall be installed at the end of the full filtration chain. Activated carbon removes oil vapor that cannot be captured by coalescing filters and further upgrades oil cleanliness grade. Activated carbon media generates fine carbon powder, so a terminal particulate filter must be fitted after the activated carbon filter. This step is particularly vital for such high-risk application scenarios.
Specific project solutions shall be subject to customer process requirements or third-party test reports. We will calculate the applicable purification grade based on on-site working conditions and recommend reasonable combinations of drying equipment and filters.
Purification Systems Require Continuous Maintenance, Not One-Time Installation
Many enterprises assume that purification equipment can operate stably for a long time once installed. In reality, after-treatment systems require regular maintenance.
Filter elements continuously trap particulates and oil mist during operation. As runtime accumulates, differential pressure across filter elements rises and filtration efficiency declines. Without timely replacement, energy consumption of air compressors will increase, and contaminants may penetrate filter elements and enter downstream equipment.
For desiccant dryers, desiccant gradually ages after prolonged moisture adsorption. Deteriorated regeneration performance leads to elevated pressure dew point. For refrigerated dryers, regular inspection of refrigerant circuits, automatic drain traps and condenser operating status is required to sustain stable dehumidification capacity.
Enterprises are advised to formulate scientific maintenance schedules combining equipment runtime, differential pressure variation and compressed air quality test results. Regular testing of pressure dew point, oil content and particulate concentration ensures stable long-term operation of the entire purification system.
Value of Purification Solutions Goes Beyond Compliance
A well-designed purification system delivers far more benefits than merely enabling compressed air to meet standards.
Ensure operational safety: Dry and clean air effectively prevents pipeline freezing in winter, reduces unexpected air leakage caused by corrosion, and lowers operational risks from equipment downtime.
Extend equipment service life: Removal of abrasive particulates and corrosive moisture significantly cuts maintenance frequency for pneumatic components and valves, generating substantial maintenance cost savings.
Safeguard product quality: Prevent contamination of every production batch by secondary pollutants in compressed air. For coating, electronics, food and pharmaceutical industries, this is essential to protect brand reputation.
As a professional supplier of compressed air purification equipment, we help enterprises resolve core compressed air quality challenges with optimally configured solutions. Whether designing purification systems for new projects or upgrading existing systems, we provide full-process services covering equipment selection, layout configuration and long-term maintenance recommendations.
