Air dryers

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Ambient atmosphere always includes a certain amount of water vapour. This
moisture can lead to corrosion of components; in addition, the
life-time lubrication of pneumatic components suffers under an excessively high
moisture content.


Air humidity

The maximum amount of water which air can absorb is temperature dependent.
For example, at 20°C, a maximum of 17 g water can be absorbed into 1 m³ air. After cooling the air down to 5°C, only approx. 7 g/m³ are absorbed – the remaining 10 g would be precipitated as condensate.
This fact has a positive and a negative effect. On the one hand it is possible to cool air down in order to dry it; on the other hand condensate is inadvertently precipitated in tubes during the transportation of air through tube sections with varying ambient temperatures.
The diagram provides an impression of the temperature sensitivity of the maximum amount of water in the air.


Wartung Wassergehalt.png

Dew point

In a normal ambient atmosphere, the amount of water contained in the air (absolute air humidity) generally lies below the maximum possible.
If the air is then cooled, a point is reached at which the amount of water contained in the air at that moment is at the same time the maximum possible amount. The relative air humidity is then 100% (absolute air humidity = maximum humidity). This point is called the dew point. If the air were to be cooled down further, water would be precipitated as condensate.
When using uncompressed air, meaning under atmospheric conditions, we talk about the atmospheric dew point or just the dew point.
However, for pneumatic applications compressed air is used, and therefore the term pressure dew point is used to differentiate between the two.


Condensate

When air is compressed, water is also precipitated as condensate. This is because air can be compressed, but water is hardly compressible. Therefore, when compressing air we press water out of it, through the same principle as a fruit press.
After compression, the relative air humidity is 100%.
Both processes for removing water in the form of condensate - the cooling and the compression of air - are used to dry compressed air.


Air dryer methods

To dry air therefore means to reduce the available air humidity by separating water from the air in the form of condensate. Apart from the two methods of compressing and cooling air, there are also the possibilities of diffusion and adhesion.


Membrane air dryers

Membrane air dryers work through diffusion


Wartung Membrantrockner.png
Membrane air dryer MS

The air dryer consists of a large number of hollow fibres which are parallel with one another lengthways. The air flows through these fibres in a lengthwise direction. The structure and the material of these fibres means that water can penetrate the side walls quicker than air.
Using dried air which has already passed through the same air dryer, a flow of air is produced outside the fibres that is drier than the air within the fibres. This concentration gradient leads to the diffusion of the water from the inside of the fibres to the outside. The humid air is then channelled away.



Waertung Funktionsskizze Membrantrockner.png

Process characteristics
Prefiltration to less than 0.01 μm required
Minor pressure loss in the air dryer
Very compact
Maintenance-free and low-energy
No formation of condensate, as this is "flushed" using the drying flow of air in the atmosphere


Adsorption dryers

The adsorption dryer PDAD works according to the principle of adhesion

Wartung Adsoptionstrockner.png


Adsorption dryer PDAD

The compressed air flows through a drying agent, and during transport the water is deposited by means of adhesive forces.
With this method of drying, two reservoirs are always used - one in which the air is dried and a second in which the drying agent is regenerated.
After a specified time, the air flow is redirected from one reservoir to the other.


Wartung Funktionsskizze Adsorptionstrockner.png

This process is possible because, within this short time span, the humidity is only deposited on the surface of the drying agent.
Process characteristics
Simple design, no external power
For smaller systems and flows
Regenerated air is removed from the main flow and is then no longer
available


Links

Wikipedia explanation of adhesion