Oil-Free Compressors

Oil-free compressors guarantee 100% oil-free air in the compression chamber, essential for critical applications.

Why Oil-Free?

In a lubricated compressor, although the separator removes >99.9% of oil, a residue always remains (<3 ppm). For some applications, this is unacceptable:

Industry	Oil risk
Pharmaceutical	Product contamination
Food & Beverage	Food safety
Electronics	Semiconductor defects
Hospitals	Oxygen contamination
Laboratories	Incorrect results
Premium painting	Finish defects

Types of Oil-Free Compressors

1. Dry Screw

Rotors operate with precise clearances without contact:

    ┌────────────────────────────────────┐
    │         No oil                     │
    │    ╭───╮   ╭───╮   ╭───╮          │
    │   ╱     ╲_╱     ╲_╱     ╲         │
    │   ╲_   _╱ ╲_   _╱ ╲_   _╱         │
    │     ╰─╯     ╰─╯     ╰─╯           │
    │   ↕ 0.002-0.004" clearance        │
    │    ╭───╮   ╭───╮   ╭───╮          │
    │   ╱     ╲_╱     ╲_╱     ╲         │
    │   ╲_   _╱ ╲_   _╱ ╲_   _╱         │
    │     ╰─╯     ╰─╯     ╰─╯           │
    └────────────────────────────────────┘

Characteristics:

• Clearances of 0.002-0.006" (0.05-0.15 mm)
• Special coatings (PTFE, ceramic)
• External timing gears
• Typically two stages

Rotor coatings:

Coating	Properties
PTFE	Low friction coefficient
Ceramic	High wear resistance
Nickel-carbide	Extreme hardness

2. Water-Injected Screw

Uses water instead of oil for sealing and cooling:

Advantages:

• Nearly isothermal compression
• Single stage for 100-145 psi (7-10 bar)
• Water is incompressible (better sealing)
• Very low discharge temperature (~105°F / 40°C)

Disadvantages:

• Water treatment system required
• Corrosion risk
• Higher maintenance

3. Class 0 Oil-Free

ISO 8573-1 Class 0 certification guarantees:

"Air contains no oil from any source, including ambient"

This goes beyond the compressor - requires:

• Oil-free compressor
• Inlet air filtration
• Compressed air treatment
• Continuous monitoring

Two-Stage Design

Dry screw oil-free compressors typically use two stages with intercooler:

                    Intercooler
                        │
    Stage 1  ──────────┴────────── Stage 2
    (low P)                        (high P)
       │                               │
    15→45 psi                      45→115 psi
       │                               │
    High temp.      Cooling         Moderate temp.
    (~350°F)        (~105°F)         (~320°F)

Why Two Stages?

Without oil cooling, compression temperature is much higher:

Type	Discharge temp. (single stage @ 115 psi)
Lubricated	175-210°F (80-100°C)
Oil-free dry	480-570°F (250-300°C)

Temperatures >400°F (200°C) would cause:

• Coating damage
• Rotor deformation
• Component degradation

Solution: Split compression into two stages with intercooling.

Comparison: Lubricated vs. Oil-Free

Aspect	Lubricated	Oil-Free (dry)
Oil in air	<3 ppm	0 ppm
Initial cost	$	$$-$$$
Efficiency (full load)	100% base	85-95%
Efficiency (part load)	Variable	Lower
Discharge temperature	175-210°F	300-350°F (post 2nd stage)
Noise	65-75 dB	70-80 dB
Maintenance	Oil + separator	Bearings + gears
Airend life	40,000-100,000 h	25,000-50,000 h
Stages (100-115 psi)	1	2

ISO 8573-1 Certification

Oil Classes

Class	Maximum content
0	User defined (stricter than 1)
1	0.01 mg/m³
2	0.1 mg/m³
3	1 mg/m³
4	5 mg/m³

Class 0 vs. Oil-Free

An oil-free compressor doesn't automatically guarantee Class 0. Inlet air may contain oil vapors from environment (vehicles, other compressors, nearby industry).

For Class 0, you need:

• Oil-free compressor
• Inlet filtration with activated carbon
• Possibly additional discharge filtration

Applications

Application	Class required	Reason
Semiconductor production	0 or 1	Microscopic defects
Pharmaceutical packaging	0 or 1	Product contamination
Fermentation	1	Culture death
Medical instruments	1	Sterility
PET bottle blowing	1 or 2	Food contact
Breathing air	1	Human health
Automotive painting	1 or 2	Finish quality

Cost Considerations

Lifecycle Cost

Component	Lubricated	Oil-Free
Initial equipment	$100K	$150-200K
Energy (10 years)	$500K	$550-600K
Maintenance (10 years)	$50K	$80K
Total	$650K	$780-880K

Example for 100 HP compressor

When Is It Worth It?

Oil-free is justified when:

1. Process demands oil-free air
2. Contamination cost > additional cost
3. Regulations require it
4. Reputational risk is high

Alternative

For some applications, a lubricated compressor with activated carbon filtration can achieve Class 1 at lower cost. However, it cannot achieve Class 0.