Scroll Compressors

Scroll compressors are positive displacement compressors that use two interleaved spirals to compress air. They are known for their smooth, quiet, and efficient operation.

Operating Principle

The scroll compressor uses two identical spirals (scrolls):

1. Fixed scroll - Remains stationary, mounted in the housing
2. Orbiting scroll - Moves in an orbital pattern without rotating

Top view of the process:

    ┌─────────────────┐
    │   ╭───╮         │
    │  ╭╯   ╰─╮       │  1. Air enters from
    │  │  ●   │       │     the outside
    │  ╰─╮   ╭╯       │
    │    ╰───╯        │  2. Air pockets
    └─────────────────┘     form

    ┌─────────────────┐
    │    ╭───╮        │
    │   ╭╯ ○ ╰╮       │  3. Pockets move
    │   │     │       │     toward center
    │   ╰╮   ╭╯       │
    │    ╰───╯        │  4. Volume decreases
    └─────────────────┘     = compression

    ┌─────────────────┐
    │     ╭─╮         │
    │    ╭╯●╰╮        │  5. Compressed air
    │    ╰───╯        │     exits from center
    └─────────────────┘

Compression Cycle

Phase	Description
1. Intake	Air enters at the periphery of the spirals
2. Sealing	Orbiting scroll creates sealed air pockets
3. Compression	Pockets move toward center, reducing volume
4. Discharge	Compressed air exits through central port

Continuous Compression

Unlike piston compressors, scroll has multiple compression pockets at different stages simultaneously. This results in nearly continuous flow with minimal pulsations.

Main Components

    ┌─────────────────────────────────┐
    │         Upper housing           │
    │  ┌───────────────────────────┐  │
    │  │       Fixed scroll        │  │
    │  │  ┌─────────────────────┐  │  │
    │  │  │   Orbiting scroll   │  │  │
    │  │  │     ┌───────┐       │  │  │
    │  │  │     │Discharge│     │  │  │
    │  │  └─────┴───────┴───────┘  │  │
    │  └───────────────────────────┘  │
    │         Oldham coupling         │
    │  ┌───────────────────────────┐  │
    │  │    Eccentric crankshaft   │  │
    │  └───────────────────────────┘  │
    │            Motor                │
    └─────────────────────────────────┘

Component	Function
Fixed scroll	Mounted in housing, forms compression chamber
Orbiting scroll	Generates orbital motion to compress
Oldham coupling	Prevents rotation of orbiting scroll
Eccentric crankshaft	Generates orbital motion
Bearings	Support radial and axial loads
Tip seals	Seal between scrolls

Scroll Geometry

Scrolls follow a mathematically defined involute curve:

Parameter	Description
Base radius	Defines scroll size
Pitch	Distance between turns
Height	Determines displacement
Number of turns	Affects compression ratio

Typical compression ratio: 3:1 to 5:1 (fixed, determined by geometry)

Advantages

Feature	Description
Quiet operation	5-15 dB quieter than piston
Smooth flow	Minimal air pulsations
Few moving parts	Only one part in orbital motion
High efficiency	80-85% isentropic efficiency
Reliability	Less wear, longer lifespan
Compact	Small and lightweight design
100% duty cycle	Can operate continuously
Low maintenance	Long service intervals
Low vibration	No reciprocating motion

Disadvantages

Feature	Description
Limited capacity	Generally up to 15-50 HP per module
Limited pressure	Typically up to 145 PSI (10 bar)
Fixed ratio	Not adjustable like screw
Limited control	Primarily on/off
Liquid sensitive	Cannot tolerate liquid carryover
Initial cost	Higher than equivalent piston

Comparison with Other Compressors

Aspect	Scroll	Piston	Screw
Noise	50-65 dB	70-90 dB	60-75 dB
Pulsations	Minimal	High	Low
Duty cycle	100%	50-60%	100%
Maintenance	Low	Medium-High	Medium
Full load efficiency	80-85%	70-80%	75-85%
HP range	1.5-50	0.5-500+	5-500+
Vibration	Very low	High	Low
Moving parts	1	10+	2

Typical Specifications

Parameter	Typical Range
Power per module	1.5 - 15 HP (1 - 11 kW)
Flow per module	3 - 60 CFM (5 - 100 m³/h)
Maximum pressure	145 PSI (10 bar)
Noise level	50 - 65 dB(A)
Ambient temperature	5 - 45°C (41 - 113°F)
Speed	2,900 - 3,500 RPM
Compression ratio	3:1 - 5:1
Scroll life	40,000 - 80,000 hours

Technology Origin

Scroll technology was patented in 1905 by Léon Creux, but wasn't practical until the 1970s-80s when manufacturing techniques allowed the necessary precision.

Today it dominates the market for:

• Residential and commercial air conditioning
• Commercial refrigeration
• Heat pumps
• Small-medium capacity compressed air