789D and 789D XQ Off-Highway Truck Air System and Brakes: Air Dryer

Location of Air Dryer

The air dryer is located under the cab and in front of the left front wheel. Air flows from the air compressor to the air dryer. The air dryer will remove the contaminants and moisture from the air system.
When the air compressor governor reaches the cutout pressure. The air compressor governor sends an air pressure signal to the purge valve in the bottom of the air dryer. The purge valve opens. Air pressure that is trapped in the air dryer is exhausted through the desiccant, an oil filter, and the purge valve.
A relief valve is located on the air dryer in order to protect the air system if the air compressor governor malfunctions. A heating element in the bottom of the air dryer prevents moisture in the dryer from freezing in cold weather.

The Charge Cycle of an Air Dryer

(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, 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 will then drop into sump (14). The air flows to the bottom of air dryer (1). Then the direction of the air is reversed allowing 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.
Most 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 an Air Dryer

(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. The unloader pistons go up and the unloader plungers move up and holds 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. The cutoff valve for the turbocharger closes 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 in air dryer (1) to exhaust the condensation and foreign material 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 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. An 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 will keep the moisture in sump (6) from freezing. The thermostat will operate between 10 °C (50 °F) and 29 °C (85 °F).
When the pressure in the air system drops below the governor pressure setting. The governor will be activated 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.

Changing the Desiccant

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.

Glossary of Terms

Adsorption - The collection of molecules (small particles) from a gas or substance (foreign material) on 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

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789D and 789D XQ Off-Highway Truck Air System and Brakes: Air Dryer 2013-01-25T20:26:00-08:00 Rating: 4.5 Diposkan Oleh: Unknown