Drain the Tank

Understanding the cause, effect, and solution to contaminated air systems.


A critical yet often overlooked part of daily vehicle maintenance is ensuring that the compressed air system is providing clean, dry air to each air driven component on the vehicle. Air dryers are important to the process but even the very best, if yet to be invented, air dryer will pass water and oil to the rest of the system. A simple fix is to drain the air system daily. This should be a required action. Relying on the air dryer alone to provide clean, dry air will be a losing proposition in the long run. Air systems with excess moisture and oil will create problems with air dependent parts within the system as well air compressors and air dryers.

Where it All Begins

The process begins with the air compressor building up air pressure, sending the compressed air down the line. Compressors are lubricated with and cooled by the engine which means oil and coolant will contaminate the air right away. Compressing air also creates heat which leads to condensation as the compressed air cools. Water and oil vapor are naturally occurring aspects of compressing air. But water and oil are not good for any component of a vehicle that relies on clean, dry air to function properly. The next step is to remove the contaminates from the air.

Dry the Air

The compressed air travels to the air dryer where it passes over a desiccant bed. The air dryer lowers the humidity of the air and therefore the dew point. Frequent brake application causes the compressor to cycle more often, saturating the desiccant with water. Compressors can also pass a significant amount of oil to the air dryer. Desiccant fouled with water and oil will allow more oil and water to move to the next step in the process.

Oil and Water Don’t Mix

The supply (wet) tank is the first stop for compressed air after the air dryer. As the air continues to cool, the water and oil vapor become liquid. This creates an emulsion which, if not removed from the system, will impact the performance and function of any component on the truck that relies on clean, dry air. Air from the wet tank is delivered to the rest of the air system.

Trouble Down the Line

Today’s trucks employ complex pneumatic systems which contain many valves and seals that are highly sensitive to water, oil, rust and dirt. Passing dirty air down the line can impact the performance and useful life of these controls as well as other air reliant components such as fan clutches, suspensions, automatic tire inflation systems, and seats to name a few.


A Vicious Cycle

An overloaded air dryer will pass quite a bit of water to the wet tank. If this water is allowed to build up the compressor will cycle more frequently. If a compressor is cycling more than should its useful life will be reduced. A vicious cycle is created whereby the compressor cycles too often, creating more water and oil vapor which overloads the desiccant allowing more water and sludge to buildup in the air system.

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. If manually draining the air system daily is not feasible a fleet may want to invest in drain valves that will automatically remove the water, oil, dirt and rust that forms in the wet tank.

The cost of passing contaminated air through the air system can be difficult to determine. But 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.


Erik Johnson is Sales Manager for Expello Air Products the manufacturer of the Expello Automatic Drain Valve which has been providing clean, dry air for compressed air systems for 40 years. He can be reached at ejohnson@expelloairproducts.com or visit www.expelloairproducts.com