Why is an air dryer needed in an air brake system?
- To expand the life of the air tubes
- To heat the air in the system
- To reduce the amount of moisture in the system ✓
- To cool the air in the system
Answer: C. To reduce the amount of moisture in the system
An air dryer is needed to reduce the amount of moisture in the system.
Why this is the answer
The handbook says air dryers sit in the compressor discharge line before the first reservoir and are designed to remove water vapour, oil mist and carbon particles from the air. That directly matches reducing moisture. The integrated dryer description also says it purges moisture and contaminants from the cartridge when the compressor unloads. So the dryer's job is cleaning and drying the air, not changing its temperature or protecting the tubes.
Why the other answers are wrong
- A. To expand the life of the air tubes — The passages describe removing water vapour, oil mist and carbon particles; they do not say a dryer is there to expand the life of air tubes.
- B. To heat the air in the system — The dryer handles warm, moist air from the compressor, but the handbook does not say it heats the air; its stated job is removing moisture and contaminants.
- D. To cool the air in the system — Cool metallic surfaces are mentioned as where some water vapour condenses, but the dryer is not described as a device for cooling the air.
Remember it
Think dry air: the dryer takes water vapour out before the air reaches the first reservoir.