AD-IS and DRM Air Dryers

Scrap · March 31, 2016

So it is looking like a bad time of the year to be an air governor. Way back in 02 Freightliner started using the DRM - Dryer Reservoir Module - which was a big long 2 circuit air tank with a little mini purge tank welded on the end and an AD-IS dryer o-ringed on the end of that. It was enabled by the final death of the Holset QE compressor in 02, solved a whole lot of air charge problems that trucks had in the 90's, and completely changed the way air systems work. KW adopted the dryer and purge tank part of it somewhere around 05 and PB copied it shortly later. I see that a few of you might have one and it works so different than the standard 3 circuit system that almost all posts are on that I thought I'd bring it up.

When this dryer is used there is no longer a supply tank (ie: wet tank), there is no pressure protected check valve (ie: SC-PR) and there are no regular check or pressure relief valves in any of the tanks. It is also not plumbed with 5/8 hose. The dryer has pressure protection valves integrated into it and you'll find two 3/8 lines coming from the dryer to each of your tanks. All of your air accessories also plumb to a pressure protected port, and accessories you add should 'home run' to here as well. The FMVSS min volume still applies so you do not lose any volume with the loss of a supply tank, but it does its circuit protection at the dryer instead of at the tanks. The advantage you get here is that if you lose a circuit then the dryer can isolate it and you still get compressor pumpage to what is left working. You'll notice when the truck builds air from 0 that it'll do the primary alone up to 103, then the secondary up to 105, then they'll go to cutout together. Cutout is also different in that most all of these were 135psi systems.

The thing about this dryer is that the governor is integral to the dryer. It is gasketed and check valved in place. You cannot use a standard off the shelf governor and you cannot move it somewhere else. If you want to carry a spare then you have to order the governor, check valve, and o-ring lube kit and carry that along with you. So if your truck builds air differently then everyone else and you find your governor hooked onto your dryer then you probably have one of these.

http://www.bendixvrc.com/itemDisplay.asp?documentID=5684

noteven · March 31, 2016

Scrap - could you comment on the importance of changing dessicant cartridges and how frequently this should be done - especially on high mileage compressors?

Thanks

Scrap · April 5, 2016

You probably remember the 90's when bunches of trucks used to die of AD-IP cancer.... Luckily they've gotten pretty good at making cartridges now but the same thing can happen when they are left on for too long. They need their dry air to regenerate with, but the oil gets in the way, then they don't make dry purge air, then they start to saturate, then they turn the beads to mush, then the white mush starts to work its way into things and then you've got a mess on your hands. This is getting extreme, but even if you are religious about pulling tank cords and change your cartridge at the first sign of moisture it can still take almost 3 weeks to get things dried up again.

Now add in some mileage and add in some 90s trucks where the Holset was made to pass oil, and add in some coke in the discharge line, which then restricts things, which then elevates the dryer intake temp, and add in a layer of mud insulating the purge volume, and the dryer can't make its temp drop, can't hit its dewpoint, and again things go on a downward spiral. Add in a more regional use where the truck uses a lot more air than it ever did as a longhauler and the dryers got a lot harder life. I know you guys have talked industrial dryers and temps and dewpoints and all that stuff in the past and these really aren't any different.

I know they say to change the dessicant every 2-3 years, but there are a ton of guys out there that wash and grease their truck every Sunday and service their dryer (ie: change dessicant, purge valve and put a test light on the heater plug) every fall.......

Jack Mayer · April 5, 2016

I change mine every two years - sometimes before. They are not costly and the downside of it failing is a mess, as Scrap said. My truck is in humid air a lot, so I'm guessing it works a little harder....

On all "new to you" trucks it is one of the first things to do.....IMO. After changing you want to monitor the tanks and make sure you are not getting stuff out of them. Really, with a properly working cartridge you should get "little-to-nothing" from the tank.