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Air filters: What is the purpose of a safety filter?

The Filter Guys

What is the purpose of a safety filter? Safety filter, secondary filter, inner filter; there are many names, we prefer to call it a safety filter.

A safety filter backs up the primary filter, and protects the clean air ducting while the primary is out of the housing during service. The engine should never be run with only a safety filter in place.

The safety filter is NOT a spare filter! Its purpose is to protect the engine if something goes wrong with the primary (main) filter. Until then, all it does is take up space and add a little bit of restriction.

What can go wrong with the primary filter?

It may have hidden damage from shipping that you missed when you inspected it before installation; damage from cleaning (if you carry out such an unrecommended procedure); maybe even a manufacturing glitch; mis-installation by the mechanic (but this never happens, right?); a ‘will-fit’ that doesn’t quite fit; or it’s the wrong part altogether and doesn’t fit at all into the housing (but mechanics would never try this, would they?)

All of these things happen more often than any of us like to think. We determined many years ago that a really good way to provide additional protection for large engines operating in heavy dust conditions was to always offer and for you to always use a safety filter.

There are some characteristics about the safety filter which you should be aware of, so you understand what it does and doesn’t do for your engine:

  • is smaller than the main filter, with less media.
  • is more open and less efficient for lower restriction.
  • has less contaminant-holding capacity so that it will plug rapidly if any engine-damaging dust particles get past a damaged primary filter.

A safety filter offers protection against mistakes when fitting and reduces the need to open the clean air side of the system by 66% (they usually only need changing once every three main filter changes) compared to systems without a safety filter. They are an excellent insurance against contaminants entering your engine and upon a critical failure of the main element can save your engine from serious damage.

Mann Filter Air Dryers.

The Filter Guys

Brakes are amongst the most important components of a vehicle. Pneumatic braking systems on trucks and buses must be protected from corrosion to ensure efficient operation. To achieve this, a dry, clean pneumatic operating system is critically important. The MANN-FILTER air dryer cartridge provides the optimum solution by guaranteeing the best possible protection for braking systems. Advantages of the MANN-FILTER air dryer cartridge:

  • Highest possible resistance to pressure and pulsation
  • Strength and rigidity to counteract vibration
  • Premium quality desiccant provides optimum air drying performance
  • Efficient oil and water separation and retention
  • Integrity of sealing materials assured
  • Superior corrosion protection

Air dryer cartridges with an integrated coalescence filter retain particles, dry the air in the compressed-air brake system and most effectively separate fine oil droplets. The coalescence effect combines oil droplets (aerosols) which can damage the system components into larger droplets, thus ensuring reliable system protection. MANN-FILTER air dryer cartridges contain an extremely high quality drying agent demonstrating high water resistance and mechanical stability.


  • As well as the benefits of the standard version, the air dryer cartridge with integrated coalescence filter also provides:
  • improved protection for the compressed air and brake system of commercial vehicles
  • longer service life of the system components, for example pressure regulating equipment and seals
  • lower service costs and shorter downtime
  • easy to retrofit to replace standard air dryer cartridges

The MANN-FILTER range of air brake dryers is available at Sterling Filtration Ltd.