How a Rotary Screw Compressor Works
Air enters the compression chamber at the inlet port, where the two counter-rotating helical rotors mesh together. As the rotors turn, the volume between the rotor lobes decreases, compressing the trapped air. The compressed air then exits through the discharge port at the desired pressure — typically 7–10 bar (100–145 psi) for plant air systems.
Unlike reciprocating compressors, there are no valves, pistons, or crank mechanisms. This gives rotary screw units their key advantage: continuous, pulse-free airflow with low vibration and high reliability. A well-maintained rotary screw unit can run 24/7 with design lifetimes exceeding 80,000 hours.
Oil-Injected vs Oil-Free
| Feature | Oil-Injected | Oil-Free |
|---|---|---|
| Compression stages | Single stage (most) | Two stage (common) |
| Air quality | Class 1–4 (needs filtration) | Class 0 (ISO 8573-1) |
| Applications | General industrial | Pharma, food, electronics |
| Capital cost | Lower | 30–60% higher |
| Maintenance | Oil changes every 2,000–4,000 h | No oil changes; seal wear |
| Efficiency | Good | Slightly lower (2-stage) |
Preventive Maintenance Schedule
Maintenance intervals vary by manufacturer, but the following schedule applies to most oil-injected rotary screw units:
Compressor Oil Selection
Always use OEM-approved compressor oil. Standard lubricating oils are not suitable — compressor oils must resist varnish formation at high temperatures (≥90°C discharge) and be compatible with seals. Synthetic polyalphaolefin (PAO) oils extend change intervals to 4,000–8,000 hours vs 2,000 hours for mineral oils.
Common Faults and Diagnosis
| Fault | Likely cause | First check |
|---|---|---|
| High discharge temperature | Low oil level; dirty cooler; high ambient temp | Check oil level; clean oil cooler fins |
| Oil in compressed air | Worn air-oil separator; overfilling oil | Replace separator element; check oil level |
| Unit starts and stops frequently | Oversized compressor for demand; pressure differential too narrow | Review setpoint band; add air receiver |
| Not reaching pressure | Worn rotors; intake valve not opening; inlet filter blocked | Check inlet filter; test intake valve |
| Excessive noise / vibration | Worn bearings; loose coupling; belt worn | Check coupling; inspect bearings with stethoscope |
Variable Speed Drive (VSD) Units
VSD rotary screw compressors modulate motor speed to match air demand precisely. Key benefits:
- Energy savings of 15–35% vs fixed-speed units in variable-demand applications
- Reduced wear from soft starting — no star-delta current spikes
- Stable pressure control (±0.1 bar vs ±0.5 bar for fixed-speed)
- Payback typically 2–3 years in applications with >40% load variation
VSD units are not always the best choice: in constant full-load applications (>90% duty), a fixed-speed unit is simpler and cheaper to maintain. See our VSD compressor guide for a full comparison.