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| Illustration 1 | g00486029 |
Two air dryers are located on the truck. The air dryers are located in front of the left front wheel. Air flows from the air compressor to the air dryers. The air dryers remove the contaminants and moisture from the air system.
When the air compressor governor reaches the cutout pressure, the air compressor stops producing compressed air. The control valve will still cycle the air dryers through the purge cycle until the air pressure is below the pressure setting of the control valve.
A relief valve is located on the air dryers in order to protect the air system if the air compressor governor malfunctions. A heating element in the bottom of the air dryers prevents moisture in the dryers from freezing in cold weather.
The Charge Cycle of the Air Dryers
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| Illustration 2 | g01106265 |
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(1) Air dryer
(2) Check valve (3) Orifice (4) Desiccant cartridge (5) Purge volume (6) Purge orifice (7) Oil separator filter (8) Control port (9) Air supply port (10) Check valve (11) Purge valve (12) Exhaust (13) Heater (14) Sump (15) Delivery port | |
When the air compressor is loaded the air that is under pressure is sent from the air compressor through air supply port (9) into air dryer (1). When the air enters air dryer (1), the velocity (speed) of the air decreases and the air is cooled. Much of the oil and water in the air is condensed. The oil and water then drops into sump (14). The air flows to the bottom of air dryer (1) and the direction of the air is reversed. This allows more oil and water to drop into sump (14).
The air flows through oil separator filter (7) that is between the outer shell and the inner shell of the cartridge. Oil separator filter (7) removes water in the liquid form as well as more oil and foreign materials. The air is still saturated with water.
The air that still has water vapor is cooled as the air exits oil separator filter (7). The air continues upward out of oil separator filter (7).
When the air reaches the top of the cartridge the air reverses the direction of flow. The air enters the desiccant drying bed (6). The air flows through desiccant cartridge (4) and the air becomes progressively dryer as the water vapor adheres to the desiccant material. This process is called adsorption. Desiccant cartridge (4) uses the process of adsorption typically in order to remove most of the water vapor from pressurized air.
The majority of the dry air exits the top of desiccant cartridge (4) through check valve (2) in order to fill purge volume (5). Some air will also exit through orifice (3) next to check valve (2).
Dry air flows out of purge volume (5) through check valve (10) and out delivery port (15) to the first reservoir of the air system.
The air dryer will remain in the charge cycle until the pressure for the air system builds up to the governor cutout setting.
The Purge Cycle of the Air Dryers
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| Illustration 3 | g01106272 |
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(1) Air dryer
(2) Check valve (3) Orifice (4) Desiccant cartridge (5) Purge volume (6) Purge orifice (7) Oil separator filter (8) Control port (9) Air supply port (10) Check valve (11) Purge valve (12) Exhaust (13) Heater (14) Sump (15) Delivery port | |
For the purge cycle (regeneration), the air flow through desiccant bed (6) of air dryer (1) is in reverse of the charge cycle.
When air pressure from the air compressor increases to the high setting of the air compressor governor, air goes through the governor and into the area below the unloader pistons in the cylinder block of the air compressor. The unloader pistons go up and the unloader plungers move up. This will hold the inlet valves off the valve seats.
As the inlet valves are held off the valve seats, air goes through the unloader line and into control port (8). The purge piston moves in response to the air pressure that causes purge valve (11) to open to the atmosphere and cutoff valve for the turbocharger to close off the supply of air from the compressor. The cutoff valve for the turbocharger is used to prevent the loss of air pressure from the engine turbocharger through air dryer (1) when the compressor inlet is connected to the engine turbocharger. The sudden opening of purge valve (11) permits the air pressure that is in air dryer (1) to exhaust the condensation and foreign material that has gathered in sump (14). The exhaust air goes past purge valve (11) to the atmosphere. The sudden decompression (decrease in air pressure) of the drying bed of desiccant cartridge (4) removes moisture from the pores (cavities) of the micro-crystalline particles that make up the drying bed. After the rapid decompression that is caused by the purge valve opening, air goes from purge volume (5) through purge orifice (3) into desiccant cartridge (4). As the air passes through purge orifice (3), the air expands and the air will go through the drying bed in the reverse direction. The air will then go through purge valve (11) and exhaust to the atmosphere.
The air in purge volume (5) is dried as the air moves through desiccant bed (6) during the charge cycle. As air moves back into desiccant bed (6) the air becomes super dry air once the air expands to atmospheric pressure through purge orifice (3). This super dry air is very efficient in removal of water when the air reverses flow through desiccant bed (6).
If the purge cycle time is greater than the time that is needed to drain the purge volume to the atmosphere, no other action takes place. Check valve (11) in delivery port (12) will keep the air in the braking system from returning into air dryer (1).
The purge cycle will only occur for a few seconds. A audible burst of air is exhausted at the end of the purge cycle.
The actual action of desiccant drying bed (6) begins as dry air flows from purge volume (5) through orifice (3) and into desiccant bed (6). Pressurized air from purge volume (5) expands after passing through orifice (3). The air pressure is lowered and the volume increases. The flow of dry air through desiccant bed (6) reactivates the desiccant material by removing the water vapor that is adhering to the material. Generally, 15 to 30 seconds are required for the entire purge volume of a standard air dryer to flow through the desiccant drying bed.
Check valve (10) prevents air pressure in the air system from returning to air dryer (1) during the purge cycle. After the 30 second purge cycle is complete, air dryer (1) is ready for the next charge cycle to begin.
Purge valve (11) will remain open after the purge cycle is complete. Purge valve (11) will not close until the pressure for the air system is reduced.
An electric heater and a thermostat keeps the moisture in sump (6) from freezing. The thermostat will operate between
When the pressure in the air system drops below the governor pressure setting, the governor will be activated and the unloader pistons and the unloader plunger will no longer hold the inlet valves of the air compressor off the valve seats. The air compressor will again start sending compressed air through air supply line (8) and the charge cycle begins.
- Check for moisture in the air system by opening the drain cocks or valves on the air tanks. If moisture is present, the desiccant may require replacement. The following conditions may also cause water to accumulate and these conditions should be considered.
- An outside source of air has been used in order to charge the air system. This air did not pass through the drying bed.
- The amount of air that is being used is exceptionally high. This may be due to the demands for accessory air or some unusual requirements for air. This demand does not allow time for the loading and unloading of the compressor in a normal fashion. Check for high leakage in the air system.
- The air dryer has been installed in a system that has been previously without an air dryer. This type of system will be saturated with moisture and the system may take several weeks of operation in order to dry.
- The location of the air dryer is too close to the air compressor.
- In areas with more than a 30 degree range of temperature that occurs in one day, small amounts of water can accumulate in the air system due to condensation. Under these conditions, the presence of a small amount of moisture is normal.
Note: A small amount of oil in the system may be normal and the oil should not be a reason to consider replacing the desiccant. A desiccant that is strained with oil will work adequately.
- Check mounting bolts for tightness.
- Visually check for physical damage to the air dryer such as broken lines, missing parts, and bad electrical connections.
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| Illustration 4 | g00486029 |
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Air Dryer | |
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The air lines to and from the air dryer must be at atmospheric pressure. If the air lines are not at atmospheric pressure, personal injury could result. Release the air pressure from the air system completely before performing maintenance. |
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To prevent personal injury, stop the engine and drain the air reservoir before performing maintenance on the air dryer. |
- Test the check valve that is in the outlet port by building the air system to the governor cutout and observing the air gauge that is installed in the reservoir. A rapid loss of pressure could indicate a failed check valve in the outlet port. This can be confirmed by purging the air system, by removing the check valve assembly from the end cover and by applying air pressure to the unit. Apply a soap solution to the exhaust side of the check valve. Leakage should not exceed a one inch bubble in one second.
- Check for excessive leakage around the purge valve. While the compressor is loaded, apply a soap solution to the exhaust port of the housing of the purge valve and observe the leakage. The leakage should not exceed a one inch bubble in one second. If the leakage is excessive service the purge valve.
- Close all drain cocks on the reservoir. Build up the system pressure to the governor cutout and note the escape of air by the purge valve. Pump the service brakes in order to reduce the air system pressure to the governor cut-in. After the pressure is reduced, the air system should build up to full pressure. Then in about 30 seconds, the air system should have a purge cycle.
- Check the operation of the safety valve by pulling the exposed stem while the compressor is loaded. There must be an exhaust of air while the stem is held. When the stem is released, the valve should reseat.
- Check all the lines and the fittings that lead in and out of the air dryer for leakage and integrity.
- Check the operation of the heater and thermostat assembly during cold weather operation.
- With the ignition or engine control ON, check for power at the air dryer with a voltmeter or test light. If there is no voltage, look for a blown fuse, broken wire, or corrosion in the vehicle wiring harness.
- Turn off the ignition switch and cool the end cover assembly to a temperature below
4 °C (40 °F) . With an ohmmeter, check the resistance between the electrical pins in the female connector or the electrical terminal and the aluminum of the end cover. Table 1 shows the resistance reading that should be found for the air dryer that is being checked. If the resistance of the heater is higher than the maximum resistance, replace the heater or heater assembly.Show/hide tableTable 1 Resistance of Heater Assembly Air Dryer Group
Part NumberVoltage Resistance 101-8352 24 6.8 Ω to 9.0 Ω 166-7416 24 6.8 Ω to 9.0 Ω - Heat the end cover assembly to a temperature higher than
32 °C (90 °F) and check the resistance. The resistance should be greater than 1,000 ohms. If the resistance reading is over 1,000 ohms the heater and the thermostat are operating correctly. If the resistance reading is below 1,000 ohms the heater and/or the thermostat should be replaced.
The procedure for removing the desiccant from the air dryer is described in the Disassembly and Assembly, "Air Dryer - Disassemble". The procedure for installing the new desiccant is in the Disassembly and Assembly, "Air Dryer - Assemble" section.
Adsorption - The collection of molecules (small particles) from a gas or substance (foreign material) on very small particles that look like crystals
Air Compressor - The air compressor is a component in the system that increases the pressure in the air system.
Ambient Temperature - Temperature of the air outside the reservoir
Condensation - Condensation is moisture in the air that has been changed into water.
Contaminants - Dirt, water, and air
Desiccant - The desiccant is a material for drying in air dryers. The desiccant removes moisture from the air system.
Desorption - Desorption is the removal of molecules of a gas from the openings of the crystal.
Dew Point - Dew point is the temperature when water will begin to condense from a mixture of water vapor and air.
Dew Point Depression - The difference between the ambient temperature and the dew point in the air reservoir
Loaded Cycle - The air compressor is compressing air.
Purge Valve - The purge valve provides a way to remove the moisture and foreign materials from the air dryer that was removed from the air.
Purge Volume - Storage area in the air dryer for air that has passed through the drying bed
Regeneration - The procedure that is used to dry the desiccant