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Air Dryers



Diesel powered coaches typically use an engine mounted air compressor to supply air to both the brakes as well as the air-ride suspension. While these systems are widely varied and complex, there is one common thread between them - They all hate moisture in the air system. Unfortunately an air compressor doesn't "make" nor does it "subtract" anything. It merely takes what ever is fed to it, compresses it, and sends it on it's way, albeit at a higher pressure. What goes in, must go out.

Unless you are RVing in the Sahara Desert, moisture is a fact of life. If it's in the air to start with, it'll pass through the compressor and into the system. This moisture can freeze up in low temperature conditions and it doesn't do much good to the internal components of the suspension and brake systems. Plus, it's never perfectly clean and the compressor can add a bit of oil to the air as it seeps past the compressor's piston rings during normal operation. When this oil mixes with moisture you get a real gunky gray mess that tends to rot things out.

Air Dryer Operation:

So the secret is to remove those contaminants and moisture from the system, keeping it clean and freeze-proof. That's where the Air Dryer comes in. While many people think of it as a filter, it is in fact a multi-task device that performs a number of functions that are all related to providing clean, dry air to the system.

The air dryer is found between the air compressor's output and the primary air tank's input. The air enters the dryer and uses cyclonic action to spin off the heavy stuff and then goes through a coalescing filter element to remove any final moisture or oils from the air. It then passes the clean air down the line and deposits the sediment in the bottom of the air dryer. During normal operation the air compressor will pump air until the governed air pressure, which is usually around 125 PSI, is reached. At that point the air compressor's governor will send an air trigger, or signal, to the compressor head. This signal will cause the unloader valves to open and the compressor will stop pumping air, even though it is still turning. This is called the unloaded position. As soon as the air pressure drops below the cut-in threshold the unloader valves release and the compressor starts pumping again. The line from the governor is fitted with a tee. In addition to feeding the compressor unloader valves, another line runs down to the air dryer. Whenever the compressor governor reaches it's peak pressure it will momentarily operate the air dryer's dump valve. A loud burst will be heard from the air dryer and all of the accumulated moisture and contaminants will be expelled out the bottom of the air dryer via it's dump valve. This keeps the dryer from accumulated a build up of moisture and gunk. If the temperature drops below a given set point, there is a thermostaticaly controlled heating element that will activate to keep the bowl area of the air dryer warm enough so that the moisture doesn't freeze in there during cold weather operation.

Air Tanks:

Checking your air tanks is considered a daily task. In earlier times vehicles did not have air dryers so air tanks needed to be manually drained each morning. It is still important to check your tank drains on a regular basis but the reason for doing so has changed. If the air dryer is doing it's job you should see very little, if any, moisture coming out the tank drain valves. However, the filters in the air dryers do not last forever and they do require eventual service. By checking what comes out of the tank drains you will be given a good warning as to when it's time to service your dryer. Service intervals have increased over the years and the 18 month interval of the Haldex-Midland PureAir dryer is now up to 36 months on the newer PurEst dryer. Still, you never know when a premature failure can occur. These are average "expected" lifetimes. If you run your RV in the bayou area you'll go through filters faster than if you live in the Mohave.

The following links will let you access the various documents relating to air dryers:


PureAir Air Dryer Manual

Purest Air Dryer Manual


Submitted by Mark Quasius - 2/08/06

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