Why is an air dryer needed in an air brake system?
- To heat the air in the system
- To reduce the amount of moisture in the system ✓
- To expand the life of the air tubes
- To cool the air in the system
Answer: B. To reduce the amount of moisture in the system
An air dryer is needed to reduce the amount of moisture that gets into the air brake system.
Why this is the answer
The handbook places the air dryer between the air compressor and the supply tank. Air from the compressor passes through it, cools, and goes through drying material, so moisture is removed before the air reaches the supply tank. The passages also say moisture in the brake system can freeze in cold temperatures and may cause brake failure, and too much moisture and oil in the tanks can reduce air volume and may cause brake failure. The dryer's stated job is moisture reduction.
Why the other answers are wrong
- A. To heat the air in the system — Heating the air is not the dryer's role; the handbook describes it as removing moisture, and it says the air cools as it passes through the dryer.
- C. To expand the life of the air tubes — The passages do not say an air dryer is there to make air tubes last longer; they tie it to keeping moisture out of the system.
- D. To cool the air in the system — The air does cool as it passes through the dryer, but that is part of the drying process, not the reason the handbook gives for having a dryer; the stated job is reducing moisture.
Remember it
Think "dryer means drier air": it takes moisture out before the air reaches the supply tank.
Official handbook: Official MTO Air Brake Handbook, Air Supply Subsystem – Air dryer