Air treatment units (FRL)2023-09-20T12:03:43+02:00

Air preparation units (FRL)

  • Air treatment units (FRL)
  • elementy przygotowania powietrza FRL

Air treatment units (FRL) – general information

FRL – preparation of air as a working medium in pneumatics.

Atmospheric air is polluted with gases, vapour, particles of various chemical compounds but also with dust of various shapes, structure and density usually swirling in the air. The composition and concentration of pollutants in the atmospheric air depends on the location and time of the year. All of these pollutants enter pneumatic systems enriched with impurities brought in during compression, unless they are removed from the air before and after the compression. In pneumatics, compressed air is prepared in special devices (air preparation units – FRL) which:

  • remove impurities from air (F-filter),
  • regulate pressure to the required level (R-regulator),
  • lubricate the air (L-lubricator).

In addition, a shut-off start-up valve is usually installed before FRL units whereas a soft-start valve after FRL units (before a final device).

Filtered air should not contain:

  • water in the form of drops – water vapours are allowable as long as the dew-point (the condensation temperature of the air at a given pressure and humidity) is about 5 ÷ 10°C below the lowest temperature in the installation, and the relative humidity at the lowest working temperature does not exceed 80% (relative humidity is expressed in %, 0-dry air without water vapour, 100% saturated with water vapour – its cooling causes immediate condensation),
  • oil and other fluids in the form of drops,
  • mechanical pollution greater than 50 µm

  • A filter is a device that does not remove water in the form of steam from the air (for this purpose use a refrigeration or adsorption dryer which transmits air through a tank with adsorbent which captures the moisture).
  • A filter insert should be replaced every half a year on average (depending on the extent of air pollution).
  • – A filter with an insert of 5 µm filtration degree should be installed before an accurate filter (depurator). It removes
    biggest particles so that sudden clogging of accurate filters is avoided.

Regulating valves (regulators)

  • The range of regulating valve output should be carefully selected – the required working pressure must be within
    the range but as close to the maximum valve output pressure as possible. Then the regulator operates closer
    to its set point value and has smaller hysteresis.
  • Care must be taken to provide only clean air to the regulating valve inlet. Possible impurities can cause problems
    with closing of the valve and continuous air venting through secondary venting.
  • Some regulating valves are equipped with an air vent working continuously, which is not its disadvantage (it
    allows to maintain the required value of output pressure more precisely).
  • the required pressure in majority of regulators should be set “from the bottom”.


  • Most of pneumatic actuators (e.g. all drives-servo motors MW-…) can work without lubrication (without an external lubricator). If a lubricator is used, its tank must be refilled with oil periodically because oil mist rinses factory-applied grease off.
  • At assembly and start-up of the lubricator, a needle which adjusts the amount of the oil mist must be closed and then opened to the required dosage of oil.
  • The distance between the lubricator and actuators should not exceed 7 meters.


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Shut-off and soft star valves

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FAQ – Air preparation units

What does FRL stand for?2023-04-24T14:43:47+02:00

The abbreviation FRL stands for a compressed air preparation unit consisting of:

F – filter – removes contaminating particles and moisture (condensate) from a compressed air system;

R – regulator – reduces and maintains pressure at a required level;

L – lubricator – injects a lubricant (oil) into an application that requires lubrication.

When should a filter element be replaced?2023-04-24T14:44:28+02:00

The condition of a filter element should be verified based on the quality of the compressed air. The contaminated (dirty) filter element can cause pressure, flow and performance drop. The filter element should be replaced every six months according to the manufacturer’s recommendations to ensure the correct operation of the pneumatic system.

What is a filtration level and how is it described?2023-04-24T15:03:47+02:00

Depending on the application, compressed air filters are equipped with filter elements of different gradations.

A filtration level, expressed in micrometres (µm), determines the size of particles above which the particles are retained by the filter.

The most common levels of filtration:

Pre-filtration (coarse) – 5µm or 20µm filter element

Fine filtration – 0.01 µm filter element

When should a lubricator be used?2023-04-24T15:05:51+02:00

A lubricator is recommended for:

– operation in a damp and dusty environment,

– operation of actuators at speeds > 1 m/s,

– valve switching frequency > 3 Hz.

How to choose the right pressure-reducing valve?2023-04-24T15:06:39+02:00

When selecting a pressure reducing valve, apart from the supply pressure and operating temperature, it is necessary to take into account the required flow rate which is related to the size of a connection. Whereas the control range should be matched to the required operating pressure (a pressure reducing valve maintains the set value more accurately and has a smaller hysteresis).

Additional information:

Video: Air preparations unit FR+L

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