Centrifugal Compressors

Centrifugal compressors, also called turbocompressors, are dynamic machines that use rotational velocity to accelerate air and then convert that velocity into pressure.

Operating Principle

Unlike positive displacement compressors (screw, piston), centrifugals are dynamic compressors:

1. Air enters the center of the impeller
2. The impeller spins at high speed (10,000-60,000+ RPM)
3. Centrifugal force accelerates the air outward
4. Air enters the diffuser where velocity converts to pressure
5. Air passes to the next stage or discharge

                    ┌─────────────┐
    Inlet   →  ══>  │  Impeller   │  ══>  Diffuser  ══>  Next stage
                    │  (rotating) │         ↓
                    └─────────────┘      Pressure
                         ↑
                    High velocity

Velocity vs Pressure

In a centrifugal compressor, kinetic energy (velocity) is first added to the air, then converted to pressure energy in the diffuser. This is the fundamental principle of dynamic compression.

The Bull Gear

Most industrial centrifugal compressors use a bull gear design:

                        Impeller 1
                            │
    Motor ══> Bull Gear ════╪════ Impeller 2
              (low speed)   │
                        Impeller 3

    Motor: 1,500-3,600 RPM
    Impellers: 15,000-60,000+ RPM

1. An electric motor runs at standard speed (1,500-3,600 RPM)
2. The bull gear is a large central gear
3. Small pinions mesh with the bull gear
4. Each pinion multiplies speed to each impeller
5. Impellers rotate at very high speeds

Bull Gear Design Advantages

Characteristic	Benefit
Single motor	Lower cost, simplicity
Multiple stages	Achieves high pressures
Optimized speed	Each stage can have different ratio
No complex couplings	Compact design

Multi-Stage Compression

Centrifugal compressors typically have 2 to 4 stages to reach industrial pressures (7-10 bar).

Why Multiple Stages?

Each stage has a limited compression ratio (typically 1.5:1 to 2.5:1). To achieve 7 bar, several stages are needed:

Stage 1: 1 bar → 2 bar (ratio 2:1)
   ↓ Intercooler
Stage 2: 2 bar → 4 bar (ratio 2:1)
   ↓ Intercooler
Stage 3: 4 bar → 8 bar (ratio 2:1)
   ↓ Aftercooler
Discharge: ~7.5 bar (with losses)

Comparison with Screw Compressors

Aspect	Centrifugal	Screw
Principle	Dynamic	Positive displacement
Flow	High (greater than 500 CFM typical)	Low to medium
Control	Complex (surge)	Simple
Part-load efficiency	Lower	Higher
Full-load efficiency	Higher	Lower
Oil-free	Inherent	Requires special design
Maintenance	Less frequent	More frequent
Initial cost	Higher	Lower
Service life	Very long	Long

Typical Applications

Centrifugal compressors are ideal for:

Application	Reason
Large industrial plants	High continuous volume
Petrochemical	Oil-free air, high reliability
Power generation	Instrument air, high availability
Steel mills	Massive volumes
Air separation plants	Constant flow 24/7
Large textile mills	Air looms, stable flow

Minimum Size

Centrifugal compressors are generally not economical below 200-300 HP (150-225 kW). For smaller capacities, screw compressors are more appropriate.

Typical Specifications

Parameter	Typical Range
Capacity	500 - 150,000+ CFM
Pressure	3 - 15 bar (typical 7-10 bar)
Power	200 - 50,000+ HP
Impeller speed	10,000 - 60,000+ RPM
Stages	2 - 4 (typical 3)
Isothermal efficiency	70 - 80%
Expected service life	20 - 30+ years

Major Manufacturers

Manufacturer	Series	Characteristics
Atlas Copco	ZH, ZB	Bull gear, high efficiency
Ingersoll Rand	Centac	Modular design
Cameron (Schlumberger)	TURBO-AIR	Oil and gas
MAN Energy	-	High pressure
Siemens	STC-GV	Heavy industrial
Sullair	Turbo	Industrial air