Lubricated Compressors

Lubricated piston compressors use oil to lubricate the cylinder, piston, and other internal components. They are the most common and economical type.

Operation

    ┌─────────────────┐
    │     Valves      │
    │  ┌───┐ ┌───┐    │
    ├──┴───┴─┴───┴────┤
    │░░░░░░░░░░░░░░░░░│ ← Oil film
    │                 │    on walls
    │    ═══════      │ ← Piston with rings
    │    ║     ║      │
    │    ║     ║      │
    └────╨─────╨──────┘
         │     │
    ┌────┴─────┴──────┐
    │  ≈≈≈≈≈≈≈≈≈≈≈≈≈  │ ← Oil in crankcase
    │  ≈≈≈≈≈≈≈≈≈≈≈≈≈  │
    └─────────────────┘

Oil serves several functions:

1. Lubricates piston, rings, and cylinder
2. Seals the gap between rings and cylinder
3. Cools internal components
4. Protects against corrosion

Lubrication System

Splash Lubrication

Common in small compressors:

    ┌─────────────┐
    │   Cylinder  │
    │      ↑      │
    │      │      │
    └──────┼──────┘
           │
    ┌──────┼──────┐
    │  ↗   │   ↖  │
    │≈≈≈≈≈≈≈≈≈≈≈≈≈│ ← Oil splashed
    │≈≈Crankcase≈≈│    by crankshaft
    └─────────────┘

• Crankshaft splashes oil as it rotates
• Simple and economical
• For small compressors (< 5 HP)

Pressure Lubrication

For larger compressors:

    ┌─────────────┐
    │   Cylinder  │ ← Pressurized oil
    │      ↑      │
    │      │      │
    └──────┼──────┘
           │
    ┌──────┼──────┐
    │      │      │
    │  ═══►├═══►  │ ← Oil pump
    │  Crankcase  │
    └─────────────┘

• Oil pump supplies lubricant
• Better lubrication
• For medium and large compressors

Compressor Oil

Oil Requirements

Property	Importance
Correct viscosity	Adequate lubrication
High flash point	Prevent combustion
Oxidation resistance	Long service life
Low carbon formation	Avoid deposits
Demulsibility	Separate water easily

Oil Types

Type	Characteristics	Application
Mineral	Economical, basic	General use
Semi-synthetic	Cost/performance balance	Moderate use
Synthetic	Longer life, better performance	Intensive use
Food grade	Food grade (NSF H1)	Food industry

Recommended Viscosity

Ambient Temperature	SAE Viscosity
< 32°F (0°C)	SAE 10W
32-77°F (0-25°C)	SAE 20 or 30
> 77°F (25°C)	SAE 30 or 40

Don't Use Motor Oil

Automotive motor oil contains detergents that can damage compressor valves. Always use oil specifically designed for compressors.

Oil Carryover

An inevitable problem in lubricated compressors is oil carryover:

    Air + oil
         ↓
    ┌─────────────┐
    │    Oil      │ ← Separates most
    │  Separator  │    of the oil
    └──────┬──────┘
           ↓
    Air with traces
    of oil
    (3-10 ppm typical)

Carryover Amount

Compressor Type	Typical Carryover
With basic separator	10-25 ppm
With good separator	3-10 ppm
With coalescing filter	< 1 ppm

Effects of Oil in Air

Application	Acceptable Oil	Solution
Pneumatic tools	Yes, beneficial	None
Automotive painting	No	Coalescing filters
Instrumentation	No	Filters + dryer
Food	No	Oil-free or special filtration

Lubrication System Maintenance

Daily Checks

• Oil level (between min/max marks)
• Oil color (not black or milky)
• Visible leaks

Oil Change

Usage Type	Change Interval
Light (< 4 h/day)	Every 500-1000 hours
Moderate (4-8 h/day)	Every 250-500 hours
Intensive (> 8 h/day)	Every 150-250 hours

Change Procedure

1. Run compressor to warm up oil
2. Shut down and depressurize
3. Drain hot oil through bottom plug
4. Replace oil filter (if applicable)
5. Add new oil to correct level
6. Check level after running 5 minutes

Advantages and Disadvantages

Advantages

Advantage	Description
Lower cost	More economical than oil-free
Higher efficiency	Oil improves sealing
Less wear	Lubrication protects components
Longer lifespan	Parts last longer
Cooler operation	Oil dissipates heat

Disadvantages

Disadvantage	Description
Oil in air	Requires filtration for clean air
Oil maintenance	Regular changes needed
Contamination	Risk if wrong oil is used
Not for all applications	Limited in food, medical

When to Choose Lubricated

Lubricated compressors are ideal when:

• Air is for pneumatic tools (oil is beneficial)
• Initial cost is an important factor
• Air can be filtered before final use
• No strict air quality requirements exist

Downstream Filtration

With proper filtration (high-efficiency coalescing filters), a lubricated compressor can provide air quality comparable to oil-free at lower cost.