Aerodynamics

Aerodynamics is fundamental to understanding centrifugal compressor behavior. The phenomena of surge and choke define the operational limits of these machines.

Characteristic Curve

Each centrifugal compressor has a characteristic curve relating pressure and flow:

Pressure (bar)
    │
  9 ┤         ╭────╮
    │        ╱      ╲
  8 ┤       ╱        ╲
    │      ╱          ╲
  7 ┤     ●────────────●────────●
    │   Surge        Design    Choke
  6 ┤    │              │         │
    │    │              │         │
  5 ┤    │              │         │
    │    │              │         │
    └────┴──────────────┴─────────┴──── Flow (CFM)
        500           1500        2500

Surge

Surge is the most critical phenomenon in centrifugal compressors. It is an unstable condition that can destroy the compressor.

What is Surge?

When air flow drops below a critical minimum while discharge pressure is high, air flows in reverse momentarily:

Normal Operation:        Surge:

Inlet → Impeller → Outlet    Inlet ← Impeller ← Outlet
                                    (reverse flow)

Surge Consequences

Effect	Severity	Consequence
Violent vibrations	Critical	Structural damage
Explosive noise (bang-bang)	High	Hearing damage
Bearing damage	Critical	Mechanical failure
Impeller fatigue	High	Cracks, rupture
Temperature increase	Medium	Seal degradation
Total destruction	Extreme	Equipment loss

The Surge Line

Pressure
    │
    │    ╔═══════════════╗
    │   ╔╝  SURGE ZONE   ║
    │  ╔╝   (danger)     ║
    │ ╔╝                 ║
    │╔╝   Surge ─────────╫────── Design point
    │║     Line          ║            ●
    │║                   ║
    │║   SAFE ZONE       ║
    │║   (operation)     ║
    │╚═══════════════════╝
    └────────────────────────── Flow
         Low           High

• Surge line: The minimum flow limit for each pressure
• Safe zone: To the right of the surge line
• Surge zone: To the left - operation prohibited

Surge Causes

Cause	Mechanism	Prevention
Low demand	Little air consumption	Anti-surge control
Closed valves	Discharge restriction	Interlock
Clogged filters	Suction restriction	Maintenance
Sudden changes	Fast transients	Load ramp
Control failure	Sensor/actuator	Redundancy

Choke (Stonewall)

Choke or stonewall is the opposite limit to surge: it occurs when flow is too high.

What is Choke?

When air velocity reaches the speed of sound at some point in the compressor (typically in the diffuser throat), flow becomes "blocked":

Air velocity in diffuser:

Normal:          Choke:
   ▓▓            ▓▓▓▓▓▓▓▓
   ▓▓            ▓▓▓▓▓▓▓▓  ← Mach 1.0
   ▓▓            ▓▓▓▓▓▓▓▓
   ▓▓            ▓▓▓▓▓▓▓▓
  ────           ────────
  Subsonic      Sonic (blocked)

Choke Characteristics

Aspect	Description
Maximum flow	Cannot increase further
Pressure	Drops drastically
Efficiency	Very low
Damage	Less than surge, but inefficient
Noise	High, whistling

Surge vs Choke Comparison

Characteristic	Surge	Choke
Flow	Very low	Very high
Pressure	High	Low (drops)
Danger	Extreme	Moderate
Potential damage	Destruction	Inefficiency
Correction	Immediate required	Reduce demand
Indicator	Noise, vibration	Pressure drop

Operating Map

The complete operating map shows all zones:

Pressure
    │
    │  SURGE                           CHOKE
    │    ║                               ║
 10 ┤    ║    100% speed ──────────     ║
    │    ║   ╱                    ╲     ║
  9 ┤    ║  ╱  90% speed ─────────  ╲    ║
    │    ║ ╱  ╱                  ╲  ╲   ║
  8 ┤    ║╱  ╱  80% speed ───────  ╲  ╲  ║
    │    ╳  ╱  ╱                ╲  ╲  ╲ ║
  7 ┤   ╱║ ╱  ╱                  ╲  ╲  ╲║
    │  ╱ ║╱  ╱      SAFE          ╲  ╲  ╳
  6 ┤ ╱  ╳  ╱       ZONE           ╲  ╲╱║
    │╱  ╱║ ╱                        ╲ ╱ ║
  5 ┤  ╱ ║╱                          ╳  ║
    │ ╱  ╳                          ╱║  ║
    └────╫──────────────────────────╫───── Flow
        500                        2500

Surge Margin

Control systems maintain a safety margin from the surge line:

Pressure
    │
    │         Surge       Control
    │          Line        Line
    │           │             │
    │           │←── 10% ───→│
    │           │   margin    │
    │           │             │    ● Operation
    │           │             │
    └───────────────────────────── Flow

Margin	Application
10%	Well-controlled systems
15%	Typical applications
20%	Highly variable loads
25%+	Critical processes

Factors Affecting the Curve

Inlet Conditions

Factor	Effect on Surge	Effect on Capacity
High temperature	Surge at lower flow	Lower capacity
Altitude	Surge at lower flow	Lower capacity
High humidity	Minor effect	Minor effect
Dirty filter	Surge at lower flow	Lower capacity

Altitude Correction

Altitude (m)	Correction Factor
0 (sea level)	1.00
500	0.94
1000	0.89
1500	0.83
2000	0.78
3000	0.69

High Altitude Installations

At high altitudes, compressor capacity is significantly reduced. A 1000 CFM compressor at sea level only delivers ~780 CFM at 2000 meters.