Compressor Sizing Calculator
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Air Compressor Products
From small workshop piston compressors to large oil-free screw compressors for pharmaceutical plants — specifications, selection criteria, and what to look for from suppliers.
Buyer's guide — key specifications
To specify an air compressor: (1) FAD (Free Air Delivery) in l/s or m³/min — the actual volume of atmospheric air compressed per minute (not the displacement); (2) Working pressure in bar(g) — match to your distribution system design pressure; (3) Power in kW — drives energy cost and electrical supply requirements; (4) Specific energy in kW per l/s FAD — lower is more efficient (target <0.1 kW/l/s for rotary screw at 7 bar); (5) Oil-injected vs oil-free — oil-free required for food, pharma, electronics; (6) Fixed speed vs VSD — VSD saves 20–35% energy if demand varies >20%.
Rotary Screw Compressors
The dominant technology for continuous industrial compressed air. Oil-injected rotary screw compressors: 5–500 kW, 6–13 bar working pressure. Continuous duty, low maintenance, quiet operation. Best for: manufacturing, food processing (with downstream treatment), automotive, general industry.
| Size range | Typical FAD (l/s at 7 bar) | Motor power (kW) | Specific energy (kW/l/s) | Noise (dBA) | Typical application |
|---|---|---|---|---|---|
| Small (7.5–15 kW) | 20–45 l/s | 7.5–15 | 0.10–0.12 | 62–68 | Small workshops, single-machine tools, garages |
| Medium (15–55 kW) | 45–160 l/s | 15–55 | 0.09–0.11 | 65–72 | SME manufacturing, multi-machine shops |
| Large (55–160 kW) | 160–500 l/s | 55–160 | 0.085–0.10 | 68–75 | Large manufacturing, automotive, food processing |
| Very large (>160 kW) | 500+ l/s | 160–500+ | 0.08–0.095 | 72–80 | Large industrial plants, multiple users |
Oil-Free Rotary Screw Compressors
Oil-free compressors (Class 0 to ISO 8573-1) for applications where oil contamination is unacceptable. Typically 20–50% more expensive than oil-injected; higher specific energy; critical for: pharmaceutical manufacturing, food and beverage (direct contact air), electronics, medical devices. Available as dry screw or water-injected screw. Leading suppliers: Atlas Copco (ZR/ZT series), Ingersoll Rand, Kaeser, Sullair.
Reciprocating (Piston) Compressors
Piston compressors: lower initial cost than rotary screw for the same power; suited to intermittent duty and high pressure (>15 bar). Not suited to continuous 100% duty — typically 60–70% max duty cycle. Best for: workshops, tyre inflation, pneumatic tools at low duty, high-pressure applications (breathing air, PET bottle blowing, CNG).
| Type | Power range | Pressure range | Max duty cycle | Key characteristics |
|---|---|---|---|---|
| Single-stage, single-cylinder | 0.37–2.2 kW | Up to 10 bar | 50% | Low cost; workshop and tyre service; noisy |
| Single-stage, twin-cylinder | 1.5–7.5 kW | Up to 10 bar | 60% | Small workshop; better balance than single |
| Two-stage, belt-drive | 3–30 kW | Up to 15 bar | 70% | Industrial workshop; better efficiency; with aftercooler |
| Two-stage, direct-drive | 15–75 kW | Up to 16 bar | 75% | Industrial; better efficiency; lower maintenance than belt |
| High-pressure multistage | 7.5–200 kW | 20–400 bar | 75% | PET blowing, breathing air, CNG, special gas |
Air Treatment — Dryers, Filters, Receivers
| Product | Function | Specification to match | Key selection parameter |
|---|---|---|---|
| Refrigerated air dryer | Removes water vapour — achieves pressure dewpoint +3°C to +10°C | FAD of compressor; inlet temperature; ambient temperature | PDP (pressure dewpoint). Target: +3°C PDP for typical indoor use |
| Desiccant (adsorption) dryer | Achieves very low dewpoints: −20°C to −70°C PDP | FAD; inlet air quality; required PDP | −40°C PDP standard; −70°C for instrument air. Purge loss: 15–20% FAD |
| Coalescing filter (pre-filter) | Removes oil aerosols and water droplets. Stages: 3 µm / 1 µm / 0.01 µm | FAD; maximum pressure; oil content | Residual oil content (mg/m³) — ISO 8573-1 Class 1 = 0.01 mg/m³ |
| Activated carbon filter | Removes oil vapour and odour — final stage for breathing air or food contact | FAD; inlet oil vapour concentration | Required for ISO 8573-1 Class 1 oil vapour specification |
| Air receiver (wet side) | Pulsation damping; moisture dropout; storage buffer before dryer | Compressor FAD; system working pressure | Volume (litres) = compressor FAD (l/s) × 5–10 seconds |
| Air receiver (dry side) | Storage to handle demand peaks and allow compressor load cycling | System demand variability; compressor cycle time target | Larger = longer cycle time = less compressor starts per hour |