For those of you who may not be familiar with it, I'd like to review how the compressed air system operates and how it gets used. Then I'd like to share some maintenance tips, because although the system requires very little attention, there are things that need to be done.
by Paul Abelson
The air compressor
Almost all our trucks today are turbo-charged. This not only makes them more fuel-efficient, it lightens the load on the air compressor. Instead of having to take in atmospheric air (a pressure of 14.7 pounds per square inch at sea level) up to 120 psi operating pressure, it gets a head start with air already at 30 to 40 psi. The air needs to be clean and dry, so it is pulled from the intake manifold between the turbocharger and the engine. Today's engine-driven air compressors can deliver anywhere from 11 to 18 cubic feet per minute at 120 psi (more than enough for almost every task). For special applications, such as pneumatically assisted unloading, high capacity (30 to 70 cfm) compressors can be specified. They may be engine-mounted or driven by a power takeoff. Being specialized, they will not be included in this discussion, although the same principles apply.
Air and water
Air can retain moisture. The quantity that can be held depends on the air temperature. As air is compressed, it heats. That heat of compression (which makes a bicycle pump warm to the touch or ignites our diesel fuel) increases air's ability to hold water vapor. Because of the variability, we don't measure moisture in the air in absolute terms, but as relative humidity. Relative refers to the percentage of what the air could hold at a given temperature is actually in the air at that temperature. If cold air and hot air both have the same amount of water vapor, the colder air will have higher relative humidity. Water that can be held in hot air could exceed cold air's ability to hold it, so it will drop out. That, by the way, is what causes it to rain or snow.By compressing (and therefore heating) air, we allow moisture to be more easily transported with the air. Allowed to remain, the moisture can condense and become liquid when the air cools. Liquids are incompressible, which as we shall soon see, can cause problems. It can also freeze, which can block lines and stop air from traveling through them. And without air, brakes won't apply or release. Unlike most materials that contract when they cool, water expands when it turns to ice. It's why ice cubes and icebergs float. By expanding, ice can generate enough force to crack valves and engine blocks. To prevent cracked valves and ruptured air lines, we need to get moisture out of the air. The moisture-laden air can be dried one of two ways: By using expansion or using a desiccant.Compressed air is stored in at least one storage tank on each vehicle (tractor, trailer, dolly), so that it is available for use when needed. The air enters the tank and expands to fill the volume. The rapid expansion cools the air, so it can no longer hold all the moisture it was carrying. The moisture precipitates and collects at the bottom of the tank. It needs to be removed at the end of each driving shift, by pulling the lanyard that opens the spitter valve. That allows the compressed air in the tank to force the water out.
To protect the air system, many trucks have two tanks, an expansion and a storage tank, or a wet and a dry tank. With a wet tank, air enters and expands, and most of the water settles out. The air (less most of the moisture) then flows to the storage tank, ready to do work. Wet tanks remove about 90 percent of moisture. Using this system, the other 10 percent is either condensed in the dry tanks or expelled through the system as it is used.
The role of desiccants
A far more efficient method is to use desiccants. A desiccant is a chemical that attracts and holds water without being changed by it. The chemical is coated onto a tiny, spherical surface. Thousands of these coated spheres are placed in a container and mounted so air can flow through. The chemical comes in contact with the moisture in the air, grabs it and holds it to its surface. Although each sphere is tiny, together they have a surface area larger than a football field. The process of holding something on a surface is called adsorption, as contrasted with absorption, which is what happens when water is held internally by a sponge.Desiccants are up to 99 percent efficient in removing water from air. Periodically (usually when the compressor unloads) valves close off the downstream air system and allow compressed air to blow through the dryer, forcing the water off the desiccant and into the atmosphere. Then the valves change and the supply process continues.Depending on size, desiccant cartridges need only be replaced every two years, unless the material becomes contaminated. According to Recommended Practice RP630 from The Maintenance Council, "If water or an oil emulsion is drained from the wet tank and the system is equipped with an air dryer, this is a clear indication that the desiccant needs to be replaced or the dryer requires servicing." As its title implies, the RP provides "Air Compressor Diagnosing for Excessive Oil Consumption."As long as the air filter is operating effectively, physical contamination should not be a problem. The most common cause of desiccant failure is oil from the compressor itself. That can have many causes. One source of problems, manufacturing defects, often becomes obvious during the early weeks of operation. To diagnose the other sources, first make sure oil consumption is, in fact, excessive. Since compressors operate mechanically, they must be lubricated. Some oil is bound to seep from rings and cylinder walls into the air. The oil mixes with water vapor and travels through the system. Over many tens of thousands of miles, it can build up on the desiccant, making drying less efficient.Air systems are becoming increasingly important on our trucks. They're not just for stopping any more. Air keeps us and our loads from feeling all the bumps. Air blows our horns, and with optional accessories, keeps our headlights clean (Sprague Cab Systems) and our tires inflated (Eaton, and P.S.I available through Meritor).Remember to drain tanks daily. This is doubly important, because water that cannot be compressed takes up an increasing amount of room when it builds up in the tanks. That's room that can't be filled with compressed air the way it's supposed to be. With water in the air tanks, you could run out of air when you need it most. And if Murphy's law applies, that will be descending the steepest downgrade on your route.Keep an eye out for oil emulsion when you drain your tanks, and listen for excessive compressor cycling. It's not hard to maintain an air system. It just takes a little thought.
Conditions that can increase the amount of oil entering the compressed air system are:
Restricted compressor air intake. If there are kinks or bends, collapsed or clogged hoses or restricted air cleaners, the compressor will suck oil up to fill the partial vacuum that is created.
Poorly filtered inlet air. Air leaks from damaged filters, damaged gaskets or fittings will let contaminants in to increase cylinder bore and ring wear, resulting in excessive oil consumption.
Restricted oil return. If drain passages are obstructed, oil will build in the compressor crankcase. In addition, excessive compressor roll or tilt from improper mounting will affect oil return.
Coolant flow. Insufficient cooling can affect compressor operation and promote oil passage. Look for heavy carbon deposits in the discharge line or fittings, or discoloration of compressor components.
Excessive duty cycles. According to RP630, "The major cause of excessive oil consumption and premature compressor failures is high compressor duty cycles (excessive pumping time) caused by air system leaks. Remember to check downstream of the tanks, to the brake chambers and the air ride-leveling valve. Excessive engine crankcase pressure. If the breather is clogged, crankcase pressure could pump oil into the compressor.
Faulty compressor. Usually other things are the cause. Do not R&R (remove and replace) the compressor unless you have exhausted all other possibilities.
The information in this article is from the Recommended Practices Manual developed by TMC members and published by The Maintenance Council. You can obtain a copy by calling 703-838-1763.
Paul Abelson serves Land Line as technical editor and freelances from his office in Lisle, IL.