EXTENDED PURGE AIR DRYER STANDARD AIR DRYER
The function of the Air Dryer is to collect and removeair system contaminants in solid, liquid and vapor form be-fore they enter the brake system. It provides clean, dry airto the components of the brake system which increasesthe life of the system and reduces maintenance costs. Dailymanual draining of the reservoirs is eliminated.
The Air Dryer consists of a desiccant cartridge and adie cast aluminum end cover secured to a cylindrical steelouter shell with eight cap screws and nuts. The end covercontains a check valve assembly, a safety valve, purge valvemechanism, a cast-in heater element with a replaceablethermostat assembly and three threaded air connections.
The three air connections are identified with embossednumbers and lettering. The identification and function of eachis as follows:
Port l.D. Function/Connection CON 4 ................ Control Port
(from unloader port on governor). SUP 11 .............. Supply Port (air in). DEL 2 ................ Delivery Port (air out).
A cast-in heater element and replaceable thermostat withan external terminal are provided.
The voltage and wattage requirements of the heater andthermostat are shown with embossed numbers and lettersin the recess adjacent to the control port marked CON.
FIGURE 1 - AIR DRYER MODELS
Service - Air Dryer
OPERATION OF THE AIR DRYER
The air dryer alternates between two operational modesor cycles during operation: the charge cycle and the purgecycle. The following description of operation is separatedinto these cycles of operation.
CHARGE CYCLE (refer to Figure 2)
When the compressor is loaded (compressing air)compressed air, along with oil, oil vapor, water and watervapor flows through the compressor discharge line to thesupply port of the air dryer end cover. The flow velocity or thespeed at which the air and its contaminants travel down thedischarge line is reduced substantially as it enters the airdryer end cover baffle system behind the supply port. As airtravels through the baffle system, its direction of flow changesby 180 degrees several times, reducing the temperature,causing contaminants to condense and drop to the bottomor sump of the air dryer end cover.
After exiting the end cover baffle system, the air flows intothe desiccant cartridge. Once in the desiccant cartridge airfirst flows through an oil separator which removes water inliquid form as well as oil, oil vapor and solid contaminants.
FIGURE 2 - CHARGE CYCLE
Air exits the oil separator and enters the desiccant dryingbed. Air flowing through the column of desiccant becomesprogressively dryer as water vapor adheres to the desiccantmaterial in a process known as adsorption. The desiccantcartridge using the adsorption process typically removes95% of the water vapor from the pressurized air.
The majority of dry air exits the desiccant cartridge throughits integral single check valve to fill the purge volume betweenthe desiccant cartridge and outer shell. Some air will alsoexit the desiccant cartridge through the purge orifice adjacentto the check valve.
Dry air flows out of the purge volume through the singlecheck valve assembly and out the delivery port to the first(supply) reservoir of the air system.
The air dryer will remain in the charge cycle until air brakesystem pressure builds to the governor cutout setting.
PURGE CYCLE (refer to Figure 3)
When air brake system pressure reaches the cutout settingof the governor, the compressor unloads (air compressionstopped) and the purge cycle of the air dryer begins. The
FIGURE 3 - PURGE CYCLE
line connecting the governor unloader port to the endcover control port is pressurized when the governor unloadsthe compressor. Air pressure at the end cover controlport is also present on the purge valve piston. The purgepiston moves in response to air pressure causing the purgevalve to open to atmosphere. Contaminants in the end coversump are expelled immediately when the purge valve opens.Also, air which was flowing through the desiccant cartridgechanges direction and begins to flow toward the open purgevalve. Oil, water and solid contaminants collected by the oilseparator are removed by air flowing from the desiccant dryingbed to the open purge valve.
The initial purge and desiccant cartridge decompression lastsonly a few seconds and is evidenced by an audible burst ofair at the exhaust.
The actual reactivation of the desiccant drying bed beginsas dry air flows from the purge volume through the desiccantcartridge purge orifice and into the desiccant drying bed.Pressurized air from the purge volume expands after passingthrough the purge orifice; its pressure is lowered and itsvolume increased. The flow of dry air through the drying bedreactivates the desiccant material by removing the water
vapor adhering to it. Generally 15-30 seconds are requiredfor the entire purge volume of a standard air dryer to flow througthe desiccant drying bed.
The end cover single check valve assembly prevents airpressure in the brake system from returning to the air dryerduring the purge cycle. After the 30 second purge cycle iscomplete, the air dryer is ready for the next charge cycle tobegin.
The purge valve will remain open after the purge cycle iscomplete and will not close until air brake system pressureis reduced and the governor signals the compressor tocharge.
Important: Review the warranty policy before performingany intrusive maintenance procedures. An extended warrantymay be voided if intrusive maintenance is performed duringthis period.
Because no two vehicles operate under identical conditions,maintenance and maintenance intervals will vary. Experienceis a valuable guide in determining the best maintenanceinterval for any one particular operation.
FIGURE 4 - AIR DRYER SECTIONAL VIEW
Every 900 operating hours or 25,000 miles or every three(3) months:
1. Check for moisture in the air brake system by openingreservoirs, drain cocks, or valves and checking forpresence of water. If moisture is present, the desiccantmay require replacement; however, the followingconditions can also cause water accumulation andshould be considered before replacing the desiccant:
A. An outside air source has been used to charge thesystem. This air did not pass through the drying bed.
B. Air usage is exceptionally high and not normal for ahighway vehicle. This may be due to accessory airdemands or some unusual air requirement that doesnot allow the compressor to load and unload(compressing and non-compressing cycle) in anormal fashion. Check for high air system leakage.
C. The air dryer has been installed in a system thathas been previously used without an air dryer. Thistype system will be saturated with moisture andseveral weeks of operation may be required to dry itout.
D. Location of the air dryer is too close to the aircompressor. Refer to Locating Air Dryer OnVehicle section.
E. In areas where more than a 30 degree range oftemperature occurs in one day, small amounts ofwater can accumulate in the air brake system dueto condensation. Under these conditions, thepresence of small amounts of moisture is normaland should not be considered as an indication thatthe dryer is not performing properly.
Note: A small amount of oil in the system may be normaland should not, in itself, be considered a reason toreplace the desiccant; oil stained desiccant can functionadequately.
2. Check mounting bolts for tightness. Retorque to 270-385 inch pounds.
3. Perform the Operation & Leakage Tests listed in thispublication.
Every 10,800 hours or 300,000 miles or 36 months:
1. Rebuild the air dryer including the desiccant cartridge.
Section 4-3Note: The desiccant change interval may vary from vehicleto vehicle. Although typical desiccant cartridge life is threeyears, many will perform adequately for a longer period oftime. In order to take maximum advantage of desiccant lifeand assure that replacement occurs only when necessary,it is important that Operation & Leakage Tests be performed.
This air dryer is intended to remove moisture and othercontaminants normally found in the air brake system. Donot inject alcohol, anti-freeze, or other de-icing substancesinto or upstream of the air dryer. Alcohol is removed by thedryer, but reduces the effectiveness of the device to dry air.Use of other substanc