Dry Me a River
by Jim Park
Air dryers can hardly be considered optional these days. ABS systems, to name only one component, demand clean, dry air for proper operation – the modulating valves just won’t function properly if there’s moisture present. The days of getting by with air systems full of sludge are behind us. Demands on air systems have never been higher either, particularly here in Canada. We run up to eight axles as opposed to five, many on air, and all craving their share of wind.
It’s no longer a matter of just preventing winter freeze-ups. While that remains an issue, it’s worth noting that air’s ability to hold moisture increases with temperature. With high summer temperatures in humid environments, removing moisture is critical. Moisture can affect the operation of the transmission, fan clutch, VGT and EGR actuators, ABS, cab and suspension leveling valves, etc. The most effective method is an air dryer.
Installed between the air compressor and air reservoirs, the dryer filters and dries compressed air before it enters the vehicle’s air system. It contains a replaceable filter/desiccant kit which does the filtering and drying. After the compressor loading cycle charges the system, the dryer forces air back through the desiccant (small, moisture-absorbing beads) where accumulated moisture is removed. Moisture-laden air and other contaminants are then carried to a purge valve where they’re expelled, and the dryer is then ready for the next cycle.
Some air dryers include a small heating element to keep moisture that has collected in the purge valve from freezing; others contain filters mounted upstream of the dryer inlet to trap oil and carbon from the compressor before they enter the dryer.
Built to Suit
Air dryers are not a one-size-fits-all component. Demand on the dryer varies with the number of air-consuming components. Generally, the more air a vehicle uses, the higher the capacity of an air dryer should be. Air-dryer manufacturers provide application charts that include vocations, the number of axles, compressor duty cycles, etc., and recommendations for each application.
According to the Technology & Maintenance Council (TMC), duty cycle - the percentage of time a vehicle’s compressor is pumping air relative to total engine operation time - is a critical variable when selecting an air dryer, and it varies widely with vocation. A linehaul application, for example, typically has a compressor duty-cycle of 10% or less, while a severe-service application can have a duty-cycle of 60% or more.
It’s worth mentioning that a truck originally spec’d for tandem/tandem operation that’s now hauling a seven-axle garbage scow may need an air dryer upgrade. There are four more brake chambers to feed and possibly four more air springs to keep inflated. If the trailer is equipped with an automatic tire-inflation system, demand could be well beyond what it was originally spec’d for.
Time for Refit?
If you’re considering a refit, remember that correct installation is critical. Air-inlet temperature is an important factor, and the length of the line from the compressor to the dryer should be what the manufacturer recommends to allow for proper cooling. Ideally, the inlet line should be straight, gently sloping toward the dryer, and free of kinks or dips where moisture might collect and freeze. Avoid mounting the dryer close to a heat source such as an exhaust pipe. Generally, inlet-air temperature should be no greater than 15O degrees.
In recent years, innovations in air-dryer design have brought modular systems to the market that feature spin-on desiccant cartridges for quick, easy replacement, and two-cartridge systems – like the Bendix EverFlow model – for very-high-demand applications where one is in the charge mode while the other one is in the purge or regeneration mode.
Meritor WABCO’s System Saver 1200 features a regeneration system that draws air back from the brake system. The 1200P uses a separate purge tank, and for heavier duty applications, the 1800 and 1800P models have higher-capacity desiccant cartridges.
Haldex now offers a system that consolidates brake-circuit functions into the air dryer. ModulAIR combines air treatment, intelligent control, and air distribution in a single platform, reducing the total number of air-system components.
The Turbo-3000 from SKF (formerly Chicago Rawhide) uses an efficient new formula to match compressed air to a compact desiccant package. It’s designed for on-highway use and 15% duty-cycle loads, and features a spin-on cartridge and a patented internal valve system that provides full turbo-boost protection.
For the full rundown on these products, check out these air-dryer supplier websites: