Compressed air can contain dust, water, rust, oil aerosols and hydrocarbon vapours that damage pneumatic equipment and affect product quality. Learn how to choose the right air compressor filters, understand the differences between pre-filters, coalescing filters and activated carbon filters, and minimise pressure drops and operating costs.
Compressed air can contain dust, water, rust, oil aerosols and hydrocarbon vapours. These contaminants accelerate the wear of pneumatic tools, disrupt valves, cylinders and automation systems, and can directly affect the quality of paint finishes. Correctly selected air compressor filters protect the pipework and connected equipment, but a single filter element cannot always remove every type of contamination.
Filter selection must reflect the application. An impact wrench requires different air quality from a CNC machining centre or paint spraying station. Compressed air treatment should therefore be carried out in stages: separating larger particles and condensate, removing oil aerosols and, in demanding processes, adsorbing oil vapours.
It is important to distinguish intake filters, which protect compressors from environmental dust, from inline filters, which clean air after compression. Oil filters and internal air-oil separators in oil-lubricated compressors perform different functions. Suitable CORMAK air filters, oil filters and separators must be matched to the specific compressor model and do not automatically replace downstream air treatment.
Pre-filter
A pre-filter removes larger solid particles such as dust, rust, metal swarf and deposits released from pipework. It protects finer elements against premature blockage and helps maintain a stable airflow.
In basic workshop systems, it may be combined with a condensate separator. Such an arrangement can be sufficient for blow-off work, less demanding air tools and routine servicing, provided the equipment specifications allow it. It will not normally deliver the purity required for paint spraying, precision automation or processes sensitive to oil and moisture.
An inline pre-filter must not be confused with a compressor intake filter. For example, the CORMAK THEOR 20 / THEOR 20 COMPACT air filter protects the compressor on the intake side; it is not a universal downstream filter for a compressed air network.
Coalescing filter
A coalescing filter captures fine droplets of water and oil suspended in compressed air. The droplets combine into larger drops, collect in the lower part of the housing and are discharged through a condensate drain. This reduces liquid aerosols entering the downstream system.
These filters are used before CNC machines, pneumatic cylinders, valves and equipment requiring cleaner air. However, a coalescing filter does not replace a dryer. Water vapour can pass through the element and condense later in colder sections of the pipework.
Where dry air is required, a refrigerated or desiccant dryer should be selected according to the specified pressure dew point. Suitable CORMAK refrigerated air dryers can work alongside correctly positioned pre-filters and fine filters.
The internal oil separator is also important in oil-lubricated compressors. For example, the CORMAK LUFT 1500 / THEOR 15-10 separator filter separates oil inside the compressor and should not be confused with an inline coalescing filter installed before air consumers.
Activated carbon filter
Activated carbon adsorbs oil vapours, selected organic compounds and odour-causing substances. It is used in paint shops, electronics manufacturing and finishing processes where even small quantities of hydrocarbon vapours can impair the product.
A suitable coalescing filter and drying system should be installed upstream of the carbon element. Liquid oil and excessive moisture can quickly reduce its adsorption capacity. Some installations also require a final particulate filter.
Activated carbon has limited adsorption capacity. It does not protect against every gas and must not be regarded as a complete breathing-air purification system. Breathing-air applications require dedicated equipment and quality monitoring.
How do you choose a filtration system?
For basic workshop tasks, condensate separation and pre-filtration may be enough, depending on tool requirements. For CNC machinery and pneumatic automation, finer aerosol filtration and drying should be considered. Paint spraying generally requires multi-stage treatment, including the removal of oil vapours.
First establish the required air quality class, for example using ISO 8573-1, which classifies solid particles, water and oil separately. Only then should the individual components be selected. Adding more filters does not automatically guarantee better air quality.
| Application | Example arrangement | Main criterion |
|---|---|---|
| Workshop | Condensate separator and pre-filter | Tool requirements and moisture |
| CNC and automation | Pre-filter, coalescing filter and dryer | Air purity and pressure dew point |
| Paint shop | Separation, drying, fine coalescing filtration and activated carbon | No unwanted water, aerosols or oil vapours |
These configurations are examples rather than universal designs. The correct order, filter grades and purity requirements must follow manufacturers' instructions and the specifications of the connected equipment.
Flow capacity and pressure drop
A filter must handle the maximum actual airflow in the installation. Consider simultaneous demand, future expansion, operating pressure and temperature. Catalogue flow ratings may require correction to the real operating conditions.
An undersized filter creates excessive pressure drop. The compressor may then need a higher pressure setpoint to maintain the required pressure at the equipment, increasing energy consumption and potentially causing unstable machine operation. Contaminated elements further increase flow resistance.
Before purchasing, check rated flow, pressure and temperature ranges, connection size, filtration grade, permissible pressure drop and condensate discharge arrangements. Allow a reasonable capacity margin in accordance with the manufacturer's documentation.
Where should filters be installed?
Main filters are installed at suitable points after the compressor and cooling equipment, often near the receiver or air dryer. There is no universal sequence: some dryers require upstream filtration, while others also need downstream filters. Manufacturer instructions and system design determine the arrangement.
An additional final filter can be installed immediately before a particularly sensitive air consumer. This is useful with long pipework that may release rust or particles, or where condensate might form again.
One example of air treatment equipment is the CORMAK IZBERG N30S refrigerated dryer with a nominal flow of 3800 l/min. Its capacity must be matched to actual conditions and overall system requirements, not simply the compressor's rated output.
When should filter elements be replaced?
A contaminated filter element increases differential pressure and can reduce filtration performance. Inspection and replacement should follow the manufacturer's schedule and differential pressure indicators, where fitted.
Dusty environments, high condensate levels, intensive compressor operation and corroded pipework can require more frequent replacement. Do not wait until the element is completely blocked. Activated carbon may reach its adsorption limit without a significant increase in pressure drop.
Before servicing, isolate and safely depressurise the filter housing in accordance with the manual. Condensate drains must also be checked and seals replaced as specified in the maintenance procedure.
Summary
Selecting compressor filters starts with defining the required compressed air quality. Pre-filters remove larger particles, coalescing filters separate fine liquid aerosols, and activated carbon adsorbs oil vapours and certain gaseous contaminants. A suitable air dryer controls water vapour.
Filters must be matched to flow, pressure, temperature, contaminants and equipment requirements. A well-designed system reduces breakdowns, supports stable production and delivers compressed air of the required quality without unnecessary energy losses.
