Oil-Injected Compressors (Lubricated)

Oil-injected screw compressors, also called "oil-flooded" or "lubricated," are the most common type in industrial applications.

Oil Injection Principle

Oil is injected directly into the compression chamber, mixing with air throughout the process.

        SUCTION                    DISCHARGE
           ↓                          ↓
    ┌──────────────────────────────────────┐
    │      ░░░░░░░░░░░░░░░░░░░░░░░░░      │
    │    ░░   Air + Oil mixed         ░░   │
    │      ░░░░░░░░░░░░░░░░░░░░░░░░░      │
    └──────────────────────────────────────┘
                      ↑
              Oil injection point

Functions of Oil

Oil serves three critical functions simultaneously:

1. Lubrication

• Reduces friction between rotors and housing
• Prevents metal-to-metal contact
• Extends bearing life

2. Sealing

• Seals clearances between lobes
• Seals between rotors and housing
• Increases volumetric efficiency

$$\eta_{vol} = \frac{FAD}{V_{displacement} \times RPM}$$

Without sealing oil, internal leakage would significantly reduce efficiency.

3. Cooling

Oil absorbs compression heat:

$$Q = \dot{m}_{oil} \times C_p \times \Delta T$$

Where:

• $Q$ = heat removed
• $\dot{m}_{oil}$ = oil mass flow
• $C_p$ = oil specific heat
• $\Delta T$ = temperature rise

Result: Compression approaches an isothermal process, more efficient than adiabatic compression.

Lubricant Types

Mineral Oils

Property	Typical value
Base	Refined petroleum
Service life	2,000-4,000 hours
Viscosity	ISO VG 32-68
Cost	$ (low)

Advantages:

• Economical
• Widely available
• Compatible with most seals

Disadvantages:

• Short service life
• Degrades at high temperatures
• Forms varnish and sludge

Synthetic Oils

Type	Base	Service life	Application
PAO	Polyalphaolefin	8,000 h	High temperature
Ester	Diester/Polyester	8,000 h	High performance
PAG	Polyglycol	8,000 h	Special applications
Silicone	Silicone	10,000+ h	Extreme conditions

Advantages:

• Longer service life (2-4x mineral)
• Better thermal stability
• Less deposit formation
• Better low-temperature protection

Disadvantages:

• Higher initial cost
• Incompatibility between types (don't mix)

Food Grade Oils

For industries with risk of incidental food contact:

Certification	Meaning
NSF H1	Incidental contact permitted
NSF H2	No food contact
Kosher/Halal	Religious certifications

Compatibility

NEVER mix different oil types. Completely flush system before changing types.

Oil Quantity

Oil/Air Ratio

The amount of injected oil is critical:

Parameter	Typical value
Oil/air ratio	1:50 to 1:100 (volume)
Oil flow	0.5-1.3 gal/min per 100 CFM
Oil at discharge	2,000-5,000 ppm (pre-separation)
Residual oil	<3 ppm (post-separation)

Operating Temperatures

Temperature Profile

Inlet: 68-95°F (20-35°C)
     ↓
Injection: 120-140°F (50-60°C) ←── Oil from cooler
     ↓
Compression: 160-195°F (70-90°C) (with oil cooling)
     ↓
Discharge: 175-210°F (80-100°C)
     ↓
Aftercooler: 85-115°F (30-45°C) (20-30°F above ambient)

Temperature Limits

Parameter	Normal	Alarm	Shutdown
Air discharge	175-200°F	212°F	220-230°F
Oil at injection	120-150°F	160°F	-
Oil in tank	160-185°F	195°F	200°F

High Temperature

Continuous operation above 212°F (100°C) causes:

• Rapid oil degradation
• Varnish formation on rotors
• Seal and bearing damage
• Reduced airend life

Oil Carryover

Separation Process

The goal is to reduce oil in air from ~3,000 ppm to <3 ppm:

Stage	Method	Efficiency
Primary	Impact/gravity	90-95%
Secondary	Coalescence	99.9%+
Total	Combined	99.99%+

Factors That Increase Carryover

1. Saturated separator - Clogged element
2. High oil level - Foaming
3. Unload operation - Low air velocity
4. Degraded oil - Poor coalescence
5. Clogged return line - Accumulated oil

Scavenge Line (Return Line)

Oil accumulating at separator element bottom must return:

Separator element
        │
        │ ← Accumulated oil
        ↓
   Return tube
        │
        ↓
Injection (before airend)

If clogged:

• Oil accumulates in element
• Oil carryover increases dramatically
• Oil passes into air system

Advantages vs. Oil-Free

Aspect	Lubricated	Advantage
Initial cost	30-50% lower	Lubricated
Energy efficiency	5-15% better	Lubricated
Maintenance	Simpler	Lubricated
Operating temperature	Lower	Lubricated
Noise	Similar	-
Airend lifespan	Longer	Lubricated

Typical Applications

Industry	Viability	Notes
General manufacturing	✅ Ideal	Most applications
Automotive	✅ Ideal	Painting requires filtration
Textile	✅ Ideal	High volume, low cost
Food (non-contact)	✅ Viable	With proper filtration
Food (contact)	⚠️ Requires	Food grade oil + filters
Pharmaceutical	❌ Not recommended	Prefer oil-free
Electronics	❌ Not recommended	Prefer oil-free