How does an air preparation unit pneumatic filter work?

The pneumatic air filter in an air treatment system is a key component for processing compressed air.

The air treatment system includes components such as the air filter, pressure regulator, lubricator, air dryer, oil-water separator, air receiver tank, safety valve and drain valve.These devices together provide filtering, pressure regulation, drying, and monitoring functions to maintain the quality of compressed air, optimize machine performance, and extend the service life of system components.

This article introduces compressed air filtration and treatment, the working principle of a pneumatic air filter, its types, and how to choose the appropriate air treatment components.

What is a pneumatic air filter?

The compressor air filter and dryer are pneumatic components that maintain compressed air quality.As the name implies, the filter is used to remove particles and moisture from the compressed air, while the dryer performs a deeper level of air drying.

Structure and Working Principle of a Pneumatic Air Filter

Filter Body
The main structure of the filter, designed to withstand air pressure and contain the filter element, drainage system, and other components.

Filter Element
The core component that removes solid contaminants and part of the liquid water and oil from the air.Different types of filters mainly differ by the type of filter element used—coarse, fine, or ultra-fine—corresponding to different types of pneumatic air filters.

Drain Cup
Collects water droplets, oil, and impurities separated during the filtration process.

Drain System
Discharges the liquid accumulated at the bottom of the drain cup.

Seals & Fittings
Prevent air leakage and ensure airtightness.

Filtration Process

When compressed air enters through the inlet, it passes through a deflector that creates a swirling airflow.Because water has a higher density than compressed air, the water droplets are thrown toward the bowl wall by centrifugal force.
These droplets then flow down to the bottom of the bowl and can be discharged either manually or automatically.

After leaving the regulator, the compressed air passes through the filter.Depending on the filter precision, solid particles larger than a certain size are retained.

Fokca provides two standard filtration grades: 40 μm and 5 μm.Using a 40 μm filter can produce compressed air that meets Fokca’s recommended minimum quality level of 7:4:4.

Fine Filter

If a higher compressed air quality is required, finer filters must be used.

In such cases, filtration should be carried out in multiple stages.For example, if the application requires that only particles smaller than 1 μm remain in the compressed air, larger particles should first be captured in stages, such as 40 μm and 5 μm filters.
Otherwise, all particles larger than 1 μm would accumulate on the ultra-fine filter, quickly saturating it and causing a high pressure drop.Therefore, filters are often equipped with a differential pressure indicator to monitor filter saturation.
When the pressure difference becomes too high, the indicator changes from green to red.

High Flow

Filters inevitably increase resistance to compressed air, thereby reducing throughput or flow rate.To minimize this effect, “high flow” filters can be used.These type of Pneumatic filter elements have a larger filtration area, which reduces the impact on flow rate—though it requires more installation space.

Activated Carbon Filter

To achieve the highest quality level by removing oil and aerosols, an activated carbon filter can be used.Oil molecules adhere to the activated carbon surface through adsorption at the molecular level,using an activated carbon filter, the oil content can be reduced to 0.003 mg/m³.