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Air Brake Basics Part 2

Air Brake Systems

Air brake systems consist of three major subsystems:

1. Components which maintain a supply of compressed air.

2 Valves that direct and control the flow of compressed air.

3. Mechanical parts that transfer the energy of the compressed air into the mechanical force and motion necessary to decelerate the vehicle.

These subsystems are integrated into a complex web of checks and balances as shown in Figure 2-2, in which compressed air stops the vehicle. As a guard against over-pressurization, the reservoir has a high pressure relief valve called a pop-off valve that opens and releases air pressure if it was to ever reach 150 psi. If the governor is working properly, pressure will never rise enough in the system for this safety valve to blow. The one-way check-valve is one of many found in the system which allows air to flow in only one direction. This is a safety feature. In case a leak occurs, the entire air system would not be depleted.

The pressurized air leaving the compressor is approximately 300 °F and contains water and oil vapor which must be removed. If water vapor and impurities continued through the system, they could damage vital components, most importantly the valves. The air valves contain rubber o-rings that can be damaged by the acid in the oil. Impurities can cause the valves to stick and in cold weather ice can form from the water vapor and block the air flow through the hoses.

To avoid these potential problems, water and oil are removed from the air by the air dryer as shown in Figure 2-4. The air dryer is installed between the compressor and the wet tank. The dryer is usually mounted on the frame rail of the tractor. The condensation type air dryer operates by accepting the hot air from the compressor and cooling it, allowing the water and other impurities to  condense. The condensate is expelled out the purge valve whenever the air compressor unloads. In cold climates, the air dryer has an internal heater to prevent the purge valve from freezing. The air leaving the dryer is free of most of the impurities that can affect the brake system.

Most modern trucks use a desiccant air dryer. In this air dryer, the water is stripped from the air by running it through a bed of beads called the desiccant. The beads attract moisture to their surfaces and hold it there. The process called regeneration removes the moisture from the desiccant by a blast of dry air from the air tank whenever the governor opens the unloader valve. Regeneration dries thousands of desiccant beads in about 2 seconds. The water from the beads drops into a holding tank and is vented to the  atmosphere through the purge valve when the air compressor unloads.

As discussed above, the air leaving the air compressor is hot, and as it cools the water vapor condenses. Should this vapor find its way into the brake lines of valves, the brake system could be rendered inoperative

An installation option to prevent this is an alcohol evaporator. An alcohol reservoir is attached to the air compressor so that a small amount of alcohol vapor mixes with the compressed air. The water that eventually condenses out contains a small amount of alcohol which lowers the freezing point of the water, thereby preventing icing in the air lines and at the valves. Of course, this only works if the driver remembers to refill the reservoir before the water freezes. Due to improvements in air dryers, alcohol evaporators are quickly becoming obsolete.

The compressed air exits the air lifter and enters the first air tank which is called the wet tank because additional water vapor condenses on the cool tank walls.

(insert air pressure reservoir gauge picture)

(insert air dryer picture)

At least once a day the wet tank should be drained by manually opening the valve on the bottom of the tank. Air dryers do not eliminate the need to drain the wet tank.

Upon leaving the wet tank, the pressurized air divides into two separate lines, each leading to a reservoir tank (dry tank). One line leads to the rear axle reservoir, which powers the rear tractor brakes. The other leads to the front axle reservoir, which powers the front axle tractor brakes and all air operated equipment, such as the windshield wipers, gauges and horn.

These dry tanks, shown in Figure 2-5, function as storage units for the compressed air, and similar to the wet tank, have drain valves on the bottom for drainage of any oil or moisture that escaped the wet tank . These air tanks are protected by one way check valves which allow air to flow only into the tank, but never back out. This not only prevents the backwash of impurities, but also protects against air pressure loss within the system.

As a historical note, when air brake systems began to be used, there was only one wet lank and one dry tank. A second dry tank originated as an optional feature called a California tank because of a law originating in that state. This law required a backup air tank if the primary tank failed.

Eventually air brake manufacturers made the secondary tank a standard component and brake systems evolved into today’s three -tank system. To maximize available space, some tractors have multiple compartments within a single tank. One large tank is separated into smaller sections by baffles or dividers. Each section then functions as an individual tank. The two reservoir tanks form the basis of what is known as the dual circuit brake system by which modern trucks operate.

This dual circuit brake system was introduced in 1969, was required on new tractors as of 1975 and is a very important concept because it ensures that if one circuit develops a leak or failure, the other circuit will still be capable of providing a means of slowing or stopping the vehicle.

If a failure occurs in either of the reservoir tanks, a double check valve positioned between them seals off the failed tank and its air line, thus preventing the air supply of the intact tank from being depleted and preserving the remaining air pressure for use by the operational tank.

The Federal Motor Vehicle Safety Standard 121 has required the dual circuit system for all vehicles that use air brakes that have been built since 1975.


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