4 Appendix I — Glossary
34
Meritor WABCO Maintenance Manual 34 (Revised 03-16)
4 Appendix I — Glossary
Definitions
Basic Air System/Air Dryer Terms
Air Compressor A device that pumps air to and builds air pressure in an air system.
Air Dryer A device that cools, filters, and dries the air delivered by an air compressor.
Air Governor A device that controls the operation of the air compressor by constantly monitoring air pressure in the
supply tank of the air system. The air governor initiates the compressor load cycle when “cut-in”
pressure is realized, and initiates the compressor unload cycle when the “cut-out” pressure is reached.
The air governor also controls the air dryer by sending an air signal (at the beginning of the compressor
unload cycle) to the control port of the air dryer, initiating the purge cycle. When this air signal is
removed by the governor (at the beginning of the compressor load cycle), the purge valve closes and the
drying cycle begins.
Compressor Load Cycle The time during which the air compressor is building air pressure in an air system.
Compressor Unload Cycle The time during which the air compressor is idling and is not building air pressure in an air system.
Cut-In Pressure The pressure level in the air system supply tank which triggers the compressor load cycle.
Cut-Out Pressure The pressure level in the air system supply tank which triggers the compressor unload cycle.
Dedicated Purge Tank A separate air tank used exclusively for holding air used in an air drying cycle. This tank eliminates the
need for a regeneration valve. Optimum mounting location for the dedicated purge tank is ABOVE the air
dryer.
Desiccant A granular substance that has a high affinity for water and is used to retain moisture from the air stream
flowing through the air dryer cartridge.
Discharge Line — Unloaded
Compressor
An unloader or air discharge line used to dump unused air to atmosphere once system has reached
cut-out pressure.
Drying Cycle The time during which the air dryer cools, filters, and removes moisture from the air delivered by the air
compressor. The drying cycle begins and ends the same as the compressor load cycle.
Purge The initial blast of air (decompression) from the air dryer purge valve at the beginning of the unload
cycle of the air compressor.
Purge Cycle The time during which the air dryer is undergoing purge and regeneration. This cycle starts at the
beginning of the compressor unload cycle and normally ends well before the beginning of the
compressor load cycle.
Regeneration The mild backflow of air through the air dryer and out the purge valve that begins immediately after the
purge and lasts normally 10 to 25 seconds. This backflow of air from the air system and through the air
dryer removes moisture from the desiccant cartridge and readies the air dryer for the next compressor
load cycle.
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