- Describe, with the aid of a sketch, a three-stage air compressor system, suitable for compressed air diving equipment. (SCUBA).
(10)
Description (Written Answer – 8 marks)
A three-stage air compressor system is used to compress atmospheric air to high pressure (typically up to 200–300 bar) suitable for filling SCUBA cylinders.
The air is compressed in three successive stages, with intercoolers, separators, and filters fitted between each stage to remove heat, moisture, and oil vapour.
1. First Stage (Low-Pressure Stage)
- Function: Draws in atmospheric air through an intake filter and compresses it to about 3–5 bar.
- Cooling: The air becomes hot during compression and passes through an intercooler where it is cooled before entering the second stage.
- Moisture Removal: A moisture separator removes condensed water.
2. Second Stage (Intermediate Stage)
- Function: Compresses the cooled air from the first stage to about 25–35 bar.
- Cooling: The air again passes through a second intercooler to reduce temperature before entering the third stage.
- Moisture/Oil Removal: Another separator/filter is fitted to remove water and oil droplets.
3. Third Stage (High-Pressure Stage)
- Function: The final compression raises pressure to the required charging pressure (200–300 bar).
- Aftercooler: Air passes through an aftercooler to reduce temperature and condense any remaining moisture.
- Final Filter Tower: The cooled air enters a chemical purification tower, which removes oil vapour, odour, and CO₂, ensuring breathing air purity.
- Delivery: The purified high-pressure air passes through a non-return valve to a storage cylinder or filling manifold.
4. Lubrication and Drive
- The compressor is usually crankshaft-driven by an electric motor or diesel engine.
- Lubrication is by splash or forced-feed system using special synthetic compressor oil compatible with breathing-air service.
5. Safety Features
- Pressure relief valves fitted at each stage to prevent overpressure.
- Automatic drains on separators to remove accumulated water and oil.
- Final pressure gauge and non-return valve at the outlet.
Sketch (2 marks)
Below is a text-based representation you can reproduce neatly in your exam:
┌──────────────────────────────────────────────┐
│ Air Outlet (200–300 bar) │
│ ↓ │
│ ┌───────────────────────┐ │
│ │ Chemical Filter Tower│ ← Removes oil vapour, CO₂, odour
│ └───────────────────────┘
│ ↑
│ Aftercooler + Separator
│ ↑
│ ┌────────────────────┐
│ │ 3rd Stage (HP) │
│ └────────────────────┘
│ ↑
│ Intercooler + Separator
│ ↑
│ ┌────────────────────┐
│ │ 2nd Stage (Med) │
│ └────────────────────┘
│ ↑
│ Intercooler + Separator
│ ↑
│ ┌────────────────────┐
│ │ 1st Stage (LP) │
│ └────────────────────┘
│ ↑
│ Air Intake Filter
└──────────────────────────────────────────────┘
Motor Drive / Crankshaft