Contaminants attack equipment air systems in two ways: condensation of moisture from the air, and small amounts of oil and soot that escape from the air compressor. These contaminants enter the air system in both liquid and vapor forms, which are produced by the heat that is generated during compression.
The reservoir catches most contaminants; nevertheless, some pass through to the brake system and contaminate the valves. Draining the reservoir reduces the amount of contamination, but it doesn't eliminate it, and the resulting moisture can cause the brakes to freeze up.
Brake air dryers reduce these type of problems.
"We are able to take moisture out of the air stream going into the air tank," says Jon Canale, senior project engineer, Meritor WABCO. "We can generate a dew point reduction to that air that flows over the desiccant bed. As things cool down (for example, when a machine is climbing a mountain and the weather changes), the air dryer keeps you from dropping out liquid water in your air system. That liquid water can freeze up on the valves and cause problems with the operation of the air-brake system."
Freeze up occurs, generally when a machine works hard all day and the next morning, he says, "things don't work, because that moisture has been accumulating in the system and has frozen up your brake valves."
The brake air dryer reduces many of these problems by providing clean, dry air to the brake system. The dryer can be installed between the compressor and the wet reservoir to help remove moisture from the compressed air.
The majority of air dryers contain a high moisture-absorbent desiccant and an oil filter. Others are hollow with baffles that are designed to help separate moisture from the air. Both types use pressure to flush contaminants out of the desiccant bed. A heater element, which keeps the moisture from freezing when equipment is operating in cold climates, is located in the purge valve.
Bill Underwood, CEM, senior mechanical engineer with Virginia's Department of Transportation, says that the desiccant cartridge and filtration system provide the clean, dry air. Different models of air dryers contain different amounts of desiccant.
"The models with the larger quantity of desiccant are a little more involved when it comes to changing the cartridge," he says.
Canale says Meritor WABCO offers two different types of dryers, one for haul-type applications, and another for heavy-duty construction and refuse applications, which has 50 percent more desiccant.
"It is meant for heavier applications where the air compressors are running for a longer period of time between charge cycles, or when you run into situations where you're running longer than you normally would," Canale says. "In the heavier duty applications, where you are working the compressor harder, there is a tendency to pump a little more oil at the same time. The oil gradually degrades the desiccant, so having more desiccant gives you more time between maintenance intervals."
Air compressors reduce the volume of air, but they don't reduce the amount of water, says Fred Hoffman, engineering manager for air treatment at Bendix. When heat is added, the air traveling down the discharge line and into the dryer is moist and warm.
"If there were no air dryer and the discharge line went into the first reservoir, a significant amount of moisture would accumulate when the temperature drops," says Hoffman. "The reason is that the air temperature has everything to do with the amount of moisture that the air can hold. It goes from being very moist and hot to being cooled in the first reservoir."
The air dryer's job, he says, is to take that very moist air at high temperature, cool it down, and physically lower the dew point. Dew point is the temperature at which water vapor in air starts to condense, so the lower the dew point the better. Lower dew point means there is less water vapor available to accumulate in the air system.
In addition to removing moisture and oil, the air dryer removes carbon as well. "During compression, the compressor will entrain a certain amount of oil," Hoffman says. "And because temperatures are fairly high, it produces carbon in the discharge line. The air dryer prevents that carbon from getting downstream."
Hoffman says desiccant levels don't necessarily affect performance and durability. Bendix has found ways of improving brake air dryer performance by adding extra features to the process, such as skin cooling, he says. Changes in the desiccant have occurred as well.
"Skin cooling means that when the air comes in the skin covering of the air dryer, instead of sending it directly to the desiccant bed, we send it on a different path on the outside diameter," Hoffman says. "The cartridge itself is actually helping to cool that incoming air stream before it gets to the desiccant. The cooler we can maintain that air as it goes into the desiccant, the more effective the desiccants will be."
Another feature of Bendix driers has to do with what Hoffman calls the "basic philosophy" of the purged air. "We save some of the first air that comes through the dryer, which is the driest," he says. "When the dryer goes into the purge mode, we have some of the cleanest, driest air available for the purging. It's more efficient."
Location of the air dryer on the truck is also important, says Randy Petresh, vice president, technical service, Haldex Commercial Vehicle Systems. "It has to have enough discharge line length between the dryer and the compressor to give the dryer a reasonable chance to operate properly," he says.
The inlet temperature should be lower than 150 degrees as it goes into the dryer, according to Petresh. "The desiccant beds inside the dryer start losing their efficiency after 150 degrees. The closer the dryer is to the engine, or the shorter the discharge line, the hotter that air is going to be."
He recommends the inlet to the dryer be mounted under the cab, "or some place like that to get as far away from the compressor as you can.
"If you do get too close, it won't blow up or anything like that, but the desiccant efficiency goes down," he says. "The trick is to remove the contaminants, the oil, and the moisture but do it so that you do not contaminate the desiccant bed to the point that you significantly reduce life."
Haldex's solution is to have a more efficient dryer. "We have a multistage cartridge that has several different desiccant bead sizes inside the dryer to optimize the efficiency without contaminating it," Petresh says.
A significant factor of how well a dryer performs is its maintenance, says Hoffman, citing two important points to remember. First, he says, is to make certain the dryer is receiving the proper voltage.
"Sometimes a wire will corrode or it will get torn off, or a fuse will blow," he says. "Then the dryer is not getting the heat that it needs to prevent freeze-up in the purge valve assembly."
The second point is for fleet or shop managers to determine if the dryer is doing its job or not. "The best way to tell," he says, "is to drain the reservoir periodically and find out if there is moisture in it. A small amount of oil is normal, but if you are getting significant amounts of water, then it might be time to change the cartridge on the dryer."
Canale at Meritor WABCO agrees. "There is nothing wrong with having a little bit of oil in the system," he says. "But gradually that oil from the air compressor will eventually degrade the performance of the desiccant. At some point, you'll have to change your cartridge. The ease of doing that is a key consideration."
Hoffman recommends fleet managers read through the suppliers' service data sheets. The one thing that absolutely should not be done to an air-brake system, he says, is to dump alcohol into the system. "When you put moisture into the system — whether it's alcohol or water — it tends to degrade the performance of the air-brake valves."
Erle Potter, CEM, state equipment manager for Virginia DOT, says VDOT has about 3,500 units equipped with brake air dryers. He advises matching the dryer to the cubic feet per minute requirements of the air-brake system and with the air pressure of the system. "Also select one that is rated to remove the greatest proportion of moisture," he says.
Consider the amount of maintenance required. "Minimum maintenance is the key," Potter says. "Some require more maintenance than others. Most all have a filter of some type, and some filters have desiccant that traps moisture. That filter has to be changed frequently when it becomes saturated. Another type ejects the excess moisture automatically. Others have heaters to get rid of the moisture without manual intervention."
Petresh advises fleet professionals to make sure the air dryer matches the compressor capacity. "There are a range of compressor sizes starting at 12 or 13 cfm and going up to 20 or 30 cfm," he says. "Some dryers are not capable of handling that whole range and certainly not capable of handling the higher range. That's one thing a fleet manager needs to take a look at."
Bendix technical marketing manager Eric Weese warns against knock-off products that have begun to appear in the U.S. market from overseas. "They sometimes use the same parts number, for instance, that we do," he says. "This leads the equipment manager to believe it is a Bendix product when it isn't. Use of these knock-off products will void the fleet-manager's warranty.
"We've done tests on desiccant on these look-alike components, and at the end of the test we see how poor the desiccant performs," Weese says. "Often times on a knock-off dryer, the desiccant will break down, sending dust into valves and components downstream."
Canale also ties choice to the compressor. "[A manager] should probably have a good idea of what his air compressor set up is on the engine," he says. "You need to know how large it is, whether it has turboboosted air tanks to the compressor, meaning the compressor breathes the same turbocharged air that the engine does, or is it naturally aspirated air compression that we are seeing a lot of nowadays. That means it's just breathing air after the air cleaner and it's not boosted by the engine."
Canale and Hoffman both suggest that fleet professionals also consider product support.
One development that has had an impact on brake air dryers, according to Canale, has been the changes necessary to reduce engine emissions.
"With 2007, we're going to different engine emissions and some of these engine emission packages have an impact on what air dryers you should use," Canale says. "In some cases, you might need a turbo cutoff valve. Even though some of these variations have existed for years, you might see the mix change because of the difference in emissions packages and the air compressors installed."
"We're getting more and more demand on the air compressor," he says. "That puts more and more demand on the air dryer. You can deal with this in two ways: Make things bigger, or make them smarter. We're looking at both."
Petresh says Haldex will add electronics to its air-dryer design, perhaps before the end of the year.
"Electronic controls will be integrated into the dryer so that it can talk better to the compressor," he says. "We have a brand new generation of air dryers now that has been in the field for testing during the past two years. We expect to have them on the market before the end of the year."
With tighter-emission engines running hotter and temperatures under the hood climbing to higher levels, "the task of keeping that inlet temperature down becomes more difficult," Petresh says. "All this affects the compressor and all are pumped downstream. Air dryers are not simple pieces of equipment as people might think. They are more complex and involved than people would know."