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The Inner Workings of Truck Air Dryers

The Inner Workings of Truck Air Dryers

Global suspension system manufacturer Hendrickson explains what’s really going on inside the inner workings of truck air dryers.

For any truck air dryer system to work, the pneumatic truck and trailer air supply must be completely clean and dry. Air naturally contains a certain amount of moisture – this is where your air dryer comes in.

If left unchecked, this moisture will cause corrosion and damage to pneumatic components, such as brake components, height control valves, Hendrickson lift axle control valves and TIREMAAX tyre pressure control systems.

It is the dryer’s job to remove as much of this moisture as possible before it passes into the rest of the pneumatic system.

Compressing air requires a lot of energy, some of which can become heat and can reach up to 150°C after leaving the compressor. It is important therefore to reduce the air temperature, otherwise it becomes very difficult to remove any moisture.

The process of the dryer drying cycle. (Image: Hendrickson/Supplied)

Initial cooling occurs in the outlet pipe from the compressor, but most dryers also have an ‘aftercooling’ function to allow the compressed air to cool further. Some even use external cooling fins to improve cooling efficiency.

Before removing any moisture, it is important to remove any oil vapour that may have been discharged by the compressor.

Most dryer designs use mesh, baffles to do this, or alternatively, a series of passages as an initial stage to separate any oil mist, so that it does not contaminate the desiccant.

The main feature of the dryer is this desiccant, which is quite different to the silica gel desiccant found in packaged goods. It is usually made up of special ceramic beads.

These beads are designed to condense moisture onto their outer surface as the air passes by them. The water, now in liquid form, adheres to the beads until the purge cycle.

The science behind the dryer purge cycle. (Image: Hendrickson/Supplied)

Dryers use a purge cycle to force air, in reverse, down past the beads and out the purge valve. When air pressure is applied to the purge control port it will open the purge valve, which releases air through the vent at the bottom of the dryer body.

The purge air carries the liquid water away from the desiccant, along with any accumulated oil, dispelling it from the dryer. This ensures that dryer is regularly cleared of waste matter, which would otherwise choke it.

A delivery check valve is integrated into the system to ensure only air within the dryer is released. There are purge volume areas in the dryer to ensure that there is sufficient air volume and flow to clear the dryer of condensed water and oil.

Eventually the desiccant beads will lose efficiency and require replacement. Therefore, dryer cartridge replacement must be carried out at recommended manufacturer replacement intervals or if water starts accumulating in the tanks.

Systems with high air requirements benefit from using twin cartridge dryers. These twin cartridge systems enable high air flow without any reduction in air quality.


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