Many control and closed-loop processes in commercial vehicles, for instance in brake systems, work with compressed air. However, air contains moisture, which can damage piping and containers, contaminate lubricants and freeze at low temperatures.

Air driers are used to assure failure-free operation of safety systems. They remove moisture from the air, prevent corrosion in pipes and containers and guard against premature wear and malfunctions. To guarantee the operational safety of air brake systems, it is imperative that air driers are replaced according to the manufacturer's guidelines.

MAHLE air-drying cartridges operate independently of temperature. They contain a drying agent that is regenerated under system control. At the same time, contaminations from the pre-filter and condensed water are removed. Our air-drying cartridges are resistant to corrosion from the inside and outside, completely sealed and, of course, pressure stable.

Safe and dry

Structure of a compressed air brake

1. Air compressor

2. Pressure regulator

3. AIr dryer

4. Separate air tank

5. Compressed air tank

6. Brake cylinder

Air dryer cartridges – the guarantee for safe braking in HGVS
More than 2/ 3 of all goods are transported on roads in about 42 million lorries all over Europe. And the trend is upwards. Anybody involved in the transport business will know that breakdowns often have far reaching consequences, of which the financial ones may be of least concern. In order to increase operational safety of the commonly used compressed-air brake systems, air driers have been employed for some time. It can be recognized easily whether a commercial vehicle is fitted with such a system: two conspicuous tanks can be seen between the front and rear axles and some valves, tubes . . . and something that looks like an oversized spin-on filter: the air dryer.

How brakes on trucks work
While passenger cars are normally fitted with hydraulic brakes (using brake fluid), lorries and trailers are equipped with compressed-air brakes. Frequently, a mixed system is used (hydraulic brakes activated by compressed air) in vehicles up to a total weight of 10 tons. From 10 tons upward, mainly compressed-air brakes are used: here the pistons in the wheel cylinders are moved by compressed air. The braking power therefore no longer depends on the power that the driver applies to the brake pedal. The brake is merely controlled via the displacement of the pedal. The compressed-air brake in the trailer works in principle in the same way as in the truck. The required compressed air is provided by the truck via two tube connections. The connection that provides a permanent supply of air to the trailer is marked red and the brake connection that controls the braking operation in the trailer is marked yellow. As mentioned earlier, the air drier is an important safety feature for this type of system.

Why are air dryers needed?
Air contains water vapour. Moist air in an air brake system is especially dangerous, as this can lead to corrosion of important functional parts. In winter it may also block important functions due to freezing. This affects the braking performance – and may lead to total failure. In early compressed-air brake systems it was necessary to drain the air tanks regularly via manually or automatically operated valves. The invention of the air dryer has provided advantages. The background of this ingenuous invention: instead of removing moisture from the system afterwards, it is prevented from entering in the first place. The air dryer removes moisture from the intake air to such an extent that the air cannot release any moisture even when heated up strongly.

This is how an air dryer works

The air from the compressor is cooled initially at the pressure regulator and at the air dryer. Part of the moisture is already lost due to condensation. The air is then fed via a drying agent in the actual air dryer. These granules bind water vapour – the higher the pressure and the lower the temperature, the stronger the effect.

What happens when the air dryer is full?

The granules cannot absorb unlimited amounts of moisture. After the switch-off pressure is reached, the air generated by the compressor is stored in a small tank and afterwards fed back, under low pressure through the drying agent and is then released into the open air. Dissipating the absorbed moisture to the outside regenerates the drying agent.

How long does an air dryer last?

The air dryer is still subject to wear, when the granular material (molecular sieve, in technical jargon) is regenerated regularly. The change intervals stipulated by the vehicle manufacturers indicate when air dryers should be replaced. And from the current catalogue it can be seen, which cartridge from the MAHLE Filter and Knecht range is the optimal replacement for the spent cartridge.

Fully receptive

Under severe pressure:
Pneumatics in commercial vehicles In medium- and heavy-duty commercial vehicles with gross vehicle weights upwards of around 7.5 tons, a number of components are operated pneumatically. A compressor—driven by the vehicle’s engine—takes in clean air via the air filter and compresses it to approximately 11 bar of pressure. The compressed air is stored in several compressed air vessels on the towing vehicle and trailer. All loads are supplied from these vessels.

The pressurised components in commercial vehicles include:

  1. The passenger doors of buses (during opening and closing)
  2. The pneumatic spring bellows on the entrance side of buses (during lateral lowering, referred to as “kneeling”, to allow passengers to enter and exit comfortably)
  3. The pneumatic suspension of the vehicle and the trailer
  4. The height adjustment of trucks during loading and unloading
  5. The airsprung driver’s seat
  6. The clutch (with pneumatic clutch support)
  7. The transmission (with pneumatic shifting of the automatic transmission)
  8. The exhaust gas treatment system (by pneumatic dosing of urea)
  9. The external connections for tools (impact screwdriver, tyre pressure gauge, etc.)

The air dryer—smart design with intrinsic values
The housing of the air dryer consists of a thick-walled steel cartridge and an end plate with a screw-on thread. The core—a container filled with desiccant—is located in the interior of the air dryer. The desiccant is composed of granules measuring approximately 1 to 3 mm made of a very open-pored material. Given the large number of small, regular pores, channels and cavities, the active inner surface is enormous. One gram of desiccant has an active surface of more than 1,000 square metres—making the available surface area of one kilogramme of desiccant, which is the amount normally used in trucks, approximately 1,000,000 square metres. This corresponds to the surface area of 200 football pitches.

The chemical and structural properties of the granules allow water vapour from the air to deposit on their surface. This is referred to as adsorption (in contrast to absorption, see below). The majority of the granules’ surface is saturated after a compressor operating time of approximately 3 to 5 minutes—and must therefore be regenerated. For this purpose, the delivery of the compressor is interrupted, compressed air from the separate regeneration air vessel (see Fig. 4 in the diagram “Design of a pneumatic brake”) is blown in the opposite direction through the air dryer and discharged to the outside. The water vapour adsorbed by the desiccant is removed by relieving the pressure in the air dryer, carried away by the air flow and discharged to the outside, together with the regeneration air, through a silencer.

Regeneration—also beneficial for the compressor
Regular regeneration of the air dryer is required for proper operation. At the same time, the operating pause resulting from the regeneration phase gives the compressor the opportunity to cool down. With uninterrupted operation, the compressor would run too hot, causing the oil content in the compressed air to rise drastically—and, as a result of overheating, the oil in the air would also chemically decompose. The pneumatic system is therefore designed to operate the compressor for no more than two thirds of the time... while the remaining time is used to prevent overheating.

Pressure fault in the system?
What happens when there is a leak in the pneumatic system? Potential causes include a defective brake cylinder on the towing unit or trailer, a leaking hose coupling on the compressed air supply of the trailer, a rupture on a pneumatic spring bellows or a blockage on the vent hose. At times, a leak is simply due to human error—for example, if the driver has forgotten to close the valve after venting. In all these instances, the compressor reaches the necessary system pressure much too late (or never). If the system pressure is not reached, the control unit does not receive a signal to regenerate the air dryer. As a result, the air dryer is not regenerated often enough or not at all. When the desiccant is saturated, i.e. when its surface is fully covered with a layer of moisture, water condenses in the dryer and floods the granules. Instead of adsorption, absorption takes place: all the pores of the granules become filled with water. Moisture and condensation water thus penetrate further into the pneumatic system—with severe consequences. After all, a large number of components, such as directional control valves, brake cylinders or gearbox parts, are made of aluminium and steel. If water finds its way to these parts, corrosion is inevitable. Moving parts, such as electromagnetic valves, can malfunction. And the desiccant suffers too: the excessively high water content makes it very heavy, the desiccant clumps—and the vibrations that develop during driving as well as the pulsations of the compressor literally crush the granules and grind them into fine powder. The ensuing fine dust escapes from the air dryer, mixed with condensation water, and contaminates the pneumatic system. This abrasive granule powder destroys all mechanical pneumatic components.

Inspections and maintenance—protection for the pneumatic system
Inside the pneumatic system, the air dryer is an important, safety-relevant component. Defects in the pneumatic system also always affect the air dryer, render it inoperational and destroy it. Regular inspections of the vehicle’s (and trailer’s) pneumatic system for leakage, observing warnings from the on-board electronics system, and maintaining and replacing the air dryer at regular intervals are critical for the safety of the vehicle and for preserving its value.

MAHLE air dryers are available for value preserving repairs. MAHLE Aftermarket is also active when it comes to the heart of the compressed-air system—the compressor— and is continuously expanding the recently launched basic range of MAHLE air compressors. In addition, we offer spare parts and repair kits for a large number of compressors, which enable the replacement of pistons, valves and seals in air compressors with recycled or refurbished parts.