Rotor Profiles

The geometry of screw compressor rotors directly impacts efficiency, reliability, and noise. Understanding rotor design helps in evaluating and comparing compressors.

Rotor Fundamentals

Male vs Female Rotors

Cross-section view:

    Male Rotor              Female Rotor
   (typically 4 lobes)     (typically 6 flutes)

        ╱──╲                   ╱╲
       ╱    ╲                 ╱  ╲
      │      │              ╱    ╲
       ╲    ╱              │      │
        ╲──╱                ╲    ╱
                            ╲╱

Component	Male Rotor	Female Rotor
Typical lobes	4	6
Role	Drives	Driven (in most designs)
Speed	Higher	Lower (4/6 ratio)
Wear	More (higher tip speed)	Less

Key Geometric Parameters

Lobe Combination (L/G ratio):

$$\text{Speed ratio} = \frac{\text{Female flutes}}{\text{Male lobes}} = \frac{6}{4} = 1.5$$

Common combinations:

• 4/6 (most common, good balance)
• 5/6 (premium efficiency)
• 5/7 (specialized applications)
• 3/5 (compact design)

Wrap Angle: The angular extent of the helix from suction to discharge.

$$\text{Wrap angle} = \frac{L}{D} \times 360°$$

Where:

• L = Rotor length
• D = Rotor diameter

Wrap Angle	Characteristic
200-250°	Standard, good efficiency
300°+	Higher compression ratio, more stages

Profile Evolution

First Generation: Symmetric Profile (SRM)

Developed by Svenska Rotor Maskiner (SRM) in 1930s-1950s.

Symmetric profile (simplified):

    ╭───╮
   ╱     ╲       Characteristics:
  │   ●   │      - Circular arcs
   ╲     ╱       - Equal male/female profile
    ╰───╯        - Moderate efficiency

• Simple to manufacture
• Established the technology
• Efficiency ~70-75%

Second Generation: Asymmetric Profile

1970s-1980s improvements.

Asymmetric profile:

    ╭───╮
   ╱     ╲       Characteristics:
  │   ●    ╲     - Different suction/discharge sides
   ╲       │     - Reduced blow-hole area
    ╰─────╯      - Better sealing

Improvement	Benefit
Reduced blow-hole	Less internal leakage
Optimized contact line	Better sealing
Efficiency	75-82%

Third Generation: Modern Profiles

1990s to present - computer-optimized designs.

Common Modern Profiles:

Profile	Developer	Features
Sigma	Kaeser	Reduced tip speed, low noise
N-profile	Atlas Copco	Optimized for oil-free
GHH	GHH-Rand	High efficiency lubricated
Holroyd	Various	Mathematical optimization

Modern optimized profile (conceptual):

    ╭─────╮
   ╱   ●   ╲     Characteristics:
  │    │    │    - Computer-generated curves
   ╲   │   ╱     - Minimized leakage paths
    ╰──┴──╯      - Optimized for specific duty

Performance Impact of Profile Design

Blow-Hole Area

The triangular area where male lobe, female flute, and housing meet.

Blow-hole (leakage path):

  Housing
  ═══════╗
         ║╲
  Male───╬──╲───Female
         ║   ╲
         ║    ╲
  ═══════╝     ╲
               ↑
          Blow-hole
          (air leaks back)

Factor	Effect
Larger blow-hole	More leakage, lower efficiency
Smaller blow-hole	Less leakage, better efficiency

Modern profiles reduce blow-hole area by 30-50% vs. original SRM.

Contact Line Length

The sealing line between rotors.

$$\text{Sealing effectiveness} \propto \text{Contact line length}$$

Longer contact line = better sealing = less leakage

Tip Speed

$$V_{tip} = \pi \times D \times N$$

Where:

• D = Rotor diameter (m)
• N = Speed (rev/s)

Tip Speed	Effect
< 30 m/s	Quiet, low wear
30-50 m/s	Standard operation
> 50 m/s	High wear, noise, heat

Rotor Construction

Materials

Application	Material	Properties
Lubricated	Cast iron	Economical, adequate wear
Lubricated (premium)	Steel alloy	Higher strength, precision
Oil-free	Stainless steel	Corrosion resistant
Oil-free	Coated aluminum	Lightweight, thermal expansion

Coatings for Oil-Free Rotors

Since oil-free rotors don't have lubricant film:

Coating	Properties
PTFE (Teflon)	Low friction, limited life
Ceramic	Hard, long-lasting
Silicon carbide	Extreme hardness
Nickel-based	Good all-around

Manufacturing Tolerances

Parameter	Typical Tolerance
Profile accuracy	±0.01 mm
Rotor diameter	±0.02 mm
Center distance	±0.01 mm
Surface finish	Ra 0.8-1.6 μm

Precision Matters

A 0.05 mm increase in clearance can reduce efficiency by 2-3%.

Clearances

Internal Clearances

Key clearance locations:

  ┌─────────────────────────────┐
  │     ╭───╮     ╭───╮        │
  │    ╱     ╲───╱     ╲       │
  │◄─►│   ●   │─│   ●   │◄─►  │ Radial clearance
  │    ╲     ╱───╲     ╱       │
  │     ╰───╯     ╰───╯        │
  │         ▲   ▲              │
  │         │   │              │
  └─────────│───│──────────────┘
            │   │
      Interlobe clearance

Clearance	Typical Value	Effect of Increase
Radial (rotor-housing)	0.05-0.15 mm	Leakage, lower efficiency
Interlobe (rotor-rotor)	0.05-0.10 mm	Leakage, contact risk
Axial (rotor-end plate)	0.05-0.15 mm	Leakage

Thermal Expansion Considerations

Clearances must account for thermal growth:

$$ΔL = α \times L \times ΔT$$

Where:

• α = Thermal expansion coefficient
• L = Original length
• ΔT = Temperature change

Oil-free compressors require larger clearances due to:

• No oil cooling
• Higher operating temperatures
• Different male/female expansion rates

Efficiency Comparison by Profile Generation

Generation	Typical Efficiency	Specific Power
1st (SRM)	70-75%	7.5-8.5 kW/100 CFM
2nd (Asymmetric)	75-82%	6.5-7.5 kW/100 CFM
3rd (Modern)	82-90%	5.5-6.5 kW/100 CFM
Premium VSD	85-92%	5.0-6.0 kW/100 CFM

When Evaluating Compressors

Ask vendors for specific power (kW per 100 CFM or per m³/min) at your operating conditions. This accounts for rotor efficiency plus motor and drive losses.

Visual Inspection Points

During maintenance, inspect rotors for:

Condition	Indicates
Scoring marks	Contamination ingestion
Pitting	Corrosion (oil quality issue)
Wear patterns	Bearing or alignment problems
Deposits	Oil degradation, cooling issues
Contact marks	Clearance loss, thermal issues