Replacing the cartridge
Replacing the cartridge is normally straightforward, but the actual procedure varies widely with the construction of the dryer. Regardless, the first step is to drain all air from the system so it will be safe to begin disassembling parts that normally hold pressure. On the Bendix AD-9 air dryer, one of the workhorses of the industry, you must remove eight bolts (two of which fasten the unit in place) so the outer shell can be lifted off the base. Then unscrew the cartridge with a strap wrench, change its two O-rings and reassemble the unit. On the Bendix AD-IP, removing a through-bolt from underneath the dryer base allows the cartridge (which slightly resembles a spin-on oil filter) to be simply lifted off. On the MeritorWABCO System Saver 1200 and the CR Turbo-2000 and Turbo-3000 air dryers, the cartridge is almost exactly like a spin-on oil filter and is spun off and on similarly. In the case of the CR units, an O-ring needs to be replaced, too.
On all units, also wipe out any accumulated oil and moisture (a few units have an integral sump to help collect them).
Look up manuals on the Web or see your dealer or distributor for printed copies.
In the case of cartridges, you might want to forgo cheaper aftermarket designs for OEM parts, Canale says. Though the aftermarket parts are less expensive than the $30-40 OEM parts, the makers may substitute plastic for the metal used in all the ArvinMeritor cartridges designed for U.S. applications. Canale says this material isn’t strong enough to withstand the normal limits on air dryer heat and can fail, allowing beads to clog your system – a mess that’s difficult and costly to cure.
If you’ve had good service in the past, but more recently are having to replace the cartridge often due to oil fouling, it’s probably time to replace the air compressor. If it’s worn, you’ll probably also hear it running more of the time even though the brake system is relatively free of air leaks.
MeritorWABCO 1200P and 1800P air dryers use their own external purge air tank so air won’t be bled out of the brake system during the purge cycle. If you find you’re not getting the obvious air blast when the air compressor cycles off and the dryer should be going into the purge cycle, check the line between dryer and tank. Look for kinks, internal clogging from the goo that forms in the brake system, and any other blockage.
Leslie Kern mentioned a different situation – heavy brake system use. This is often the case in vocations like dump operations where the driver is constantly accelerating and braking. CR recently introduced a special oil separator that can easily be retrofitted for trucks in these high duty cycle applications to prolong desiccant life. The separator would also be helpful if your air compressor is starting to show signs of wear (some oil blowby), but still keeps the brake system pressurized. She says it is ideal for equipment equipped with ABS systems, as the “pristine” air helps keep contamination from “preventing the system from functioning properly.”
The CR separator can easily be inserted under the desiccant cartridge after de-pressurizing the brake system, removing the eight attaching bolts and then pulling off the bolt-on cap. It requires no maintenance.
For most trucks that are well maintained and have the right unit, properly installed, replacing the cartridge at the right time will keep your dryer working right and your brake system protected from moisture.
How Air Dryers Work
Air dryers contain a cartridge filled with something called a “desiccant” in the form of small beads. The beads are made of a material that attracts moisture very strongly. Air flows through the dryer on its trip from the compressor to the wet tank during the on cycle of the compressor.
Oil is first separated by a simple, mechanical separator – a strainer made of beads. It drips off the oil separator and settles in a sump below. Then, as the air passes through and over the desiccant bed, moisture clings to the outside surfaces of the beads in this attraction process, called “adsorption.” This leaves the air so dry that any remaining moisture stays completely vaporized.
The desiccant beads hold only a few ounces of moisture. But the adsorption process is almost completely reversible. Get the moisture back off, and they can be used over and over again. So, to keep the dryer working continuously, it goes through a “purge cycle” each time the air compressor shuts off.
The dryer also holds some compressed air in spaces called the “purge volume.” The air compressor is periodically shut off by the air pressure governor, which is operated by the air pressure in the system. When shutoff occurs, the air pressure opens a device called a purge valve. Then the pressurized air from the purge volume flows backward through the desiccant and oil separator and out through the purge valve at the bottom of the dryer. As the pressure drops to what is normal outside, the water is evaporated off the beads, and they are left dry. The oil is also blown out of the sump. This is the blast of air you hear each time the air compressor cycles off.