Clean, dry, compressed air systems will function better and longer than systems that are fouled with water, oil, dirt and rust. Protecting the vehicle’s air system from contamination pays off in the long run.
By Geoff Selby

The compressed air system on a heavy-duty truck is typically specified based on the size and requirements of the brake system on the vehicle. Therefore, the performance of the air brakes is also the primary driver of the maintenance of the air system. As important as optimum performance of the air brake system is, there are many other systems on the truck that require the use of compressed air. Keeping all of these systems working optimally means that maintaining a system of clean, dry compressed air on the vehicle has never been more important than it is today.

Clean, dry, compressed air systems will function better and longer than systems that are fouled with water, oil, dirt and rust. Image courtesy of D&D Instruments.

Why Is There Water in my Air System?
It is a fact of physics that all compressed air systems generate water. The air that we breathe at atmospheric pressure contains molecules of mostly Nitrogen (78 percent), and Oxygen (20 percent). The rest of the air is made up of Carbon Dioxide, water vapor and other gases. If you think back to your high school science class, you might remember that there is a relationship between the air pressure, the volume of the container and the temperature of the air.
Specifically, as the pressure of the air increases, the temperature will increase as well. So, the act of compressing the molecules of air into a container generates heat. When this hot air begins to cool, the water vapor within it condenses. This condensed water byproduct then flows through the air system and creates a number of problems.

The Effect of Air System Moisture
One of the chief problems with water in the compressed air system is that it can freeze. In cold weather climates, a vehicle that has been sitting overnight has condensed water that than can form an ice chunk. This ice can effectively plug the air system lines preventing the braking system from reaching the minimum operating pressure. More than an annoyance, this is a common cause of unplanned downtime among fleets which can be expensive.
Other vehicle systems on waste hauling trucks also rely on compressed air including:
• Automated Manual Transmissions (AMT) with pneumatic shifters
• Fan clutches
• Air ride suspensions
• Automatic tire inflation systems
• Emission control systems

All of these components will perform at a suboptimal level without clean air. In addition, any of them can fail due to wet, dirty air.
AMTs pose a particular problem. Fleets specifying trucks using Automated Manual Transmissions have become more frequent over the course of the last decade. Over-the-road fleets have been ordering AMTs on 90 percent of their truck purchases, and vocational fleets have spec’d them on 50 percent of new vehicle purchases.1 Many AMTs use compressed air to shift gears. Fleets using pneumatically-shifting AMTs should be particularly careful about maintenance of their air systems.

According to Eaton, a major producer of AMTs, wet, dirty air can be catastrophic to the system. Moisture in the system can result in harsh shifting, or even a complete loss of shifting.2 When this happens it is a road call, a tow and an expensive replacement.

Drying the Air
After the hot, moist air leaves the air compressor, it typically flows into a system designed to remove the moisture and contaminants. Depending on the setup on the truck, any of these components, or a combination of them may exist:
• Expansion (Ping) Tank—This tank is designed to condense and trap moisture prior to the air dryer
• A Pre-cooler—This component is used to cool the air causing condensation, then expel the condensation out the bottom
• Air Dryer—Uses a desiccant bed to remove moisture from the air

The effect of these systems is to lower the humidity level and the dew point of the air and, therefore, the risk of contaminants in the compressed air system. This effect can be degraded over time due to driving and road conditions, as well as maintenance concerns.

Trouble Down the Line
Waste haulers, in particular, can face a variety of issues. Residential collection routes have frequent brake applications which causes the air compressor to run almost continuously. This high frequency use can cause a vicious cycle where an overloaded air dryer will pass more moisture onto the wet tank. If this water is allowed to build up, the compressor will cycle even more frequently. This water in the tanks is what can cause problems to the vehicle components.

Keep it Clean (and Dry)
Planned air compressor and air dryer maintenance as well as daily draining of the air system are key to ensuring the cleanest, driest air possible. Daily draining of the air system is often overlooked by tired and overworked drivers. Maintenance personnel typically have more pressing matters to deal with. But this is a step that cannot be neglected.
The cost of passing contaminated air through the air system can be difficult to determine. The possible failure of a major component like an AMT makes a compelling case for vigilant maintenance. However, even the hassle of one road call is enough to make the case for daily air system draining. Premature air system component failure and denied warranty claims can also be costly. Clean, dry, compressed air systems will function better and longer than systems that are fouled with water, oil, dirt and rust. Protecting the vehicle’s air system from contamination pays off in the long run. | WA

Geoff Selby is General Manager at D&D Instruments (Minneapolis, MN) that manufactures the Expello line. He can be reached at (612) 255-1155, e-mail or visit

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