Air prep

How does an air preparation unit work?

In the blog 'What is an air preparation unit?' we explained that an air supply in a pneumatic system is necessary to maintain the compressed air quality and to to control the machine centrally. This is extends the life of the components and optimises the operation of the machine in terms of reliability, availability, efficiency (energy consumption), safety and accuracy. In this blog we will discuss the components that are required to achieve this.

Filters in air treatment maintain compressed air quality

In order to maintain compressed air quality, air treatment has various filters and dryers. The filters are there to remove particles and moisture from the compressed air, the dryer supports deeper drying of the compressed air.

Filter regulator unit

The first filtration step is achieved in the filter regulator unit. In addition to regulating the compressed air pressure, this unit is equipped with a condensate bowl and a filter located within the housing.

As part of the pressure control, the compressed air is passed through a baffle [1] that creates a vortex. Since the density of moisture is higher than that of compressed air, the moisture will hit the bowl wall [2] due to the centrifugal force of the vortex. This moisture then flows into the base of the bowl and can be drained manually or automatically.

air prep

When the compressed air leaves the regulator, it passes through a filter. Depending on the fineness of the filter, the solid particles above a certain size are trapped. Festo has two standard filtration levels40 and 5 µm. With the 40 micron filter, compressed air can be produced that meets the minimum quality class 7:4:4 recommended by Festo in accordance with ISO 8573-1:2010.

Fine filter

Where compressed air quality needs to be higher than this standard, it is necessary to use a filter with a higher filter fineness. In this case, filtration should be carried out in several stages. For example, if an application requires that only particles smaller than 1 µm remain in the compressed air, then it makes sense to first capture the larger particles in, for example, the 40 and 5 µm stages. If this is not done, all particles larger than 1 µm will remain on this very fine filter, which will quickly lead to saturation and an accompanying high pressure drop. The saturation of the filter can be monitored cases by applying a differential pressure indicator to the filter. When the differential pressure becomes too high, it turns from green to red.

High flow

Filters inevitably increase the resistance and thus reduce the compressed air throughput or flow. To minimise this impact, it is possible to choose 'High Flow' filters. These elements have a larger filter surface, which reduces the impact on the flow. Of course, this requires more installation space.

Activated carbon filter

To specifically filter oil and aerosols to the highest quality level, an activated carbon filter can be used. Oil adheres to the carbon on a molecular level based on adsorption. Oil can be removed down to 0.003 mg/m3 with this.

Membrane dryer

Finally, in the context of compressed air quality, we neeed to mention membrane dryers. These are used when the filters discussed above cannot sufficiently reduce the moisture content, and thus the pressure dew point. The filter reduces the pressure dew point to 3 °C as standard. This means that compressed air above this temperature cannot condense and therefore cannot cause any moisture to form in the pipe.

Critical applications often require a lower dew point, which can be achieved by using a membrane dryer. These dryers create a small pressure difference between two chambers separated by a moisture absorbing membrane. Because of the pressure difference, compressed air is pressed through this membrane, leaving behind any moisture. In this way, the pressure dew point of the compressed air can be lowered by at least 15 °C. If this is still not sufficient, there is also the option of using a double adsorption dryer.

Sensors in the air supply system

For central control of the pneumatics, the air supply has various sensors that monitor different parameters.

Pressure sensors

The two important parameters with regard to compressed air are pressure and flow (also called throughput or volume flow). Pressure sensors have the advantage that no flow is required to perform the measurement. This means that practically every air supply can be equipped with a pressure sensor. Various options are possible, from a simple teach-in sensor with LED indication for adjustable pressure levels to multi-coloured readable LCD variants with programming functionality.

Sensors for flow measurement are installed in the flow direction and are therefore only available as a separate module. For reliable measurement, it is also important that the sensor is in a laminar air flow. This can be achieved by placing a component in front of the flow sensor that does not cause any disturbances or eddies. The flow sensor can only be assembled in this way if a manifold block (without a specific function) is installed on the incoming side.


Finally, an air preparation system contains various valves to control the relevant application centrally.

Pressure regulator

One of the most important valves is the pressure regulator. With this valve the working pressure is regulated by simply turning a knob. With the help of a pressure gauge (manometer), or pressure sensor, the pressure can be monitored and adjusted if necessary. The operation is shown in the figure: By adjusting the knob, the force on the diaphragm [2] is adjusted via the main spring [1]. The regulating piston [4] moves downwards along with the diaphragm, releasing the lower sealing seat [6]. If the pressure at the working port [5] exceeds the set working pressure, the air can escape through another sealing seat in the diaphragm (secondary venting).

pressure regulator

Safety valve

To guarantee the safety of people, machines and the environment, an air supply also has a so-called safety valve. This combines various functions:

  • Quick exhaust/dump valve; Due to a relatively large exhaust port the system can be quickly brought to atmospheric pressure and the system pressure is depressurised.
  • Slow start-up; With the slow start function, the system can be gradually brought to pressure. This prevents damage caused by actuators ‘jumping’ into the reset position as they start from a pre-exhausted state. The speed of pressurisation is adjustable.
  • Redundant operation; In line with machine safety guidelines, redundant valves are available. In these, the coils and position sensors are duplicated in order to be able to cope with possible product failures.

Switch-on/shut-off valve

In addition to regulating the working pressure, the option of a central on/off valve is one of the most desirable features on an air supply. With an on/off valve, the main pressure can be shut off, whereby the pressure on the work side is simultaneously vented [3] to make the system pressure-free. The switching is done manually by means of a rotary knob [1]. A LOTO (Lock Out Tag Out) is also fitted in the knob [2] for safe machine maintenance. In addition to a manual version, the switching valves are also available as an electrical version that can be switched for example from a PLC,.


July 2022