Truck Air Dryers: Frequently Asked Questions

we take a look at some frequently asked questions regarding air dryers and their usage - and why making a change might be the right decision for your business.

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September 2, 2022 at 4:50:00 PM PDT September 2, 2022 at 4:50:00 PM PDTnd, September 2, 2022 at 4:50:00 PM PDT

Air dryers have been used since time immemorial on commercial heavy-duty trucks to help filter out water and oil vapour. But as time has marched on, so have advancements in technology that have served to highlight some of the factors that might cause an air dryer to malfunction.

Why does oil get beyond the air dryer and into brake valves?

It's science. When the compressed air temperature cools, the gaseous vapours revert to oil. These oil droplets may then go on to contaminate the air valves downstream of the dryer. Further complicating matters, most valves are lubricated by the manufacturer with specific lubricants designed to provide longevity. Compressor oil tends to wash away these lubricants, sending them through the air system where they pick up additional contaminants. In some cases, the lubricant washed out by the compressor oil is essential for the valve to function properly.


Finally, when the discharge temperature of the air compressor becomes too high, the lubricating oil breaks down into a vapour which then floats on the air stream and coats the desiccant pellets. There are several reasons why the temperatures would become excessive, including leaks within the air brake system, a malfunctioning governor valve, and leakage from the purge valve due to oil contamination. Just like humans, being overworked like this will cause us to shut down. So perhaps it's time to give your air compressors (and yourself) a break by not making them work beyond their specific duty.


Why do the air tanks have water in them even with an air dryer installed?

Over time, desiccant beads in the air dryer will become coated with carbon and oil, preventing them from absorbing water vapour and moisture. There are other reasons, too, such as leakage from the purge valve and the air compressor being too small for the vehicle it is installed in. Don't cause yourself problems that could easily be prevented - install the proper air dryer for the type of vehicle you are operating.


Why do air dryer purge valves constantly leak air?

Contaminated purge valves leak due to carbon and oil particles, preventing them from sealing properly.


Why must I frequently replace desiccant canisters?

The lifespan of the desiccant is determined by the amount of contamination filtered from the air system. Desiccant canisters generally need to be replaced when large amounts of water are found in the air tanks.


Why does build-up to maximum air pressure take longer than usual?

The more the desiccant has been saturated with carbon and oil, the longer it takes to reach maximum air pressure. A leaky purge valve and a blocked discharged compressor air line are other common causes.


What causes premature air compressor breakdown?

If you've been reading this entire post, you can probably already guess the answer. An air dryer saturated with contamination causes restrictions, leading to greater demand and workload for the compressor. This leads to a premature breakdown.

What causes carbon build-up in the discharge line of the air compressor?

High air temperature and pressure turn oil vapour into carbon deposits. The combination of carbon particles and oil droplets saturates the desiccant in the dryer, restricting its airflow. 


How can an air dryer malfunction cause a vehicle breakdown?

If working properly, the dryer should cycle on and off to exhaust impurities out of the system. However, carbon deposits in the purge valve of the dryer could cause it to vent into the atmosphere continuously. This, in turn, can cause air starvation to the brake system, which will trigger the spring brakes.