Compressed Air Basics
Compressed air is the most expensive utility in industrial plants. Learn the fundamentals, why it costs so much, and the key principles of efficient compressed air systems.
Why Compressed Air is Expensive
Compressed air is often called the most expensive energy source in a plant. Only 10 to 20% of the electrical energy input to a compressor reaches the point of use as compressed air energy. The remaining 80 to 90% is lost as heat.
Despite this, compressed air is essential. It powers pneumatic tools, actuators, valves, instruments, and processes where electricity would be dangerous or impractical. The goal is not to eliminate compressed air but to use it efficiently.
Key Concepts
- Pressure: Measured in bar or psi. Higher pressure requires more energy. Only generate the pressure you need. Typical plant air: 6 to 8 bar.
- Flow rate: Measured in litres per second (L/s) or cubic feet per minute (CFM). Determines compressor size. Oversized compressors waste energy.
- Free Air Delivery (FAD): The actual volume of air delivered by the compressor, referenced to atmospheric conditions. This is the true capacity rating.
- Dew point: The temperature at which moisture condenses from the compressed air. Lower dew point means drier air. Refrigerated dryers achieve +3 degrees C; desiccant dryers achieve -40 degrees C.
- Specific energy: kWh per cubic metre of free air delivered. The key efficiency metric. Typical: 0.08 to 0.15 kWh per cubic metre.
The Cost of Compressed Air
Cost calculation
A 100 kW compressor running 8,000 hours per year at $0.15/kWh costs $120,000 per year in electricity alone. If 20% of that air is wasted through leaks, that is $24,000 per year wasted. Fixing leaks is the highest-ROI maintenance activity in any compressed air system.
Efficiency Opportunities
- Fix leaks. A 3mm leak at 7 bar wastes about $2,000 per year. Most plants have 10 to 30% leakage.
- Reduce pressure. Every 1 bar reduction saves about 7% in compressor energy.
- Turn off compressors when not needed. Idling compressors still consume 20 to 40% of full-load power.
- Recover heat. Compressor heat can be used for space heating, water heating, or process heating.
- Use variable speed compressors for variable loads. VSD compressors match output to demand.
- Eliminate inappropriate uses. Using compressed air for cleaning or cooling is extremely wasteful.
Tip: Start with a leak survey. Ultrasonic leak detectors can find leaks you can't hear in a noisy plant. Tag and repair every leak, then re-survey quarterly. Most leak repair programs pay for themselves in 3 to 6 months.