Capacity Control
Capacity control in centrifugal compressors is more complex than in positive displacement compressors due to surge. Several methods exist to adjust capacity while maintaining safe operation.
Control Methods
1. Inlet Guide Vanes (IGV)
Inlet Guide Vanes (IGV) are adjustable vanes at the compressor inlet:
IGV front view:
Open (100%) Partial (50%) Closed (minimum)
╱ ╲ ╲ ╱ ─┼─
╱ ╲ ╳ │
╱ ╲ ╱ ╲ ─┼─
─────────── ───────── ─────────
Maximum Reduced Minimum
flow flow flow
Operation:
- Vanes change the air entry angle to the impeller
- Modify the compressor characteristic curve
- Allow capacity reduction without surge
Effect on curve:
Pressure
│
│ IGV 100% ────────╮
│ ╱ ╲
│ ╱ IGV 75% ─────────╮╲
│ ╱ ╱ ╲╲
│ ╱ ╱ IGV 50% ──────────╲╲
│╱ ╱ ╱ ╲╲
│ ╱ ╱ ╲╲
└──────────────────────────────── Flow
| IGV Position | Capacity | Efficiency |
|---|---|---|
| 100% (open) | 100% | Optimal |
| 75% | 70-80% | Good |
| 50% | 45-55% | Acceptable |
| 25% | 20-30% | Reduced |
| Minimum | 15-20% | Low |
2. Discharge Valve (Butterfly)
A butterfly valve at discharge can regulate flow:
Butterfly Valve
│
Compressor ══════════════════════╪══════════> System
│
┌────┴────┐
│ ◐ Disc │
│ adjust.│
└─────────┘
Characteristics:
- Simpler control than IGV
- Less efficient (pressure loss)
- Used as secondary control
- Fast response
| Aspect | IGV | Butterfly |
|---|---|---|
| Efficiency | Higher | Lower |
| Cost | Higher | Lower |
| Complexity | Higher | Lower |
| Response | Slower | Faster |
| Control range | Greater | Limited |
3. Blow-Off Valve
The blow-off valve discharges air to atmosphere when flow approaches the surge line:
┌──→ Atmosphere (waste)
│
Compressor ══> Discharge ═════╪══> System
│
Blow-Off
Valve
Operation:
- Controller monitors flow and pressure
- When operating point approaches surge
- Valve opens and discharges air
- Flow through compressor increases
- Operating point moves away from surge
Blow-off wastes compressed air (and the energy used to compress it). It's a simple but inefficient method. Every 100 CFM of blow-off represents approximately 20-25 HP wasted.
4. Recirculation Valve
Similar to blow-off, but air recirculates to inlet:
┌────────────────────────────┐
│ Cooler │
│ ║ │
↓ ║ │
Inlet ══> Compressor ══> Discharge ═╪═> System
│
Recirculation
Valve
Advantages over blow-off:
- Doesn't lose the air
- Less noise
- Can include cooling
Disadvantages:
- Still wastes compression energy
- More complex system
- Requires cooler to prevent overheating
5. Variable Speed Drive (VSD)
Some modern compressors use variable frequency drives:
Grid ──→ VFD ──→ Motor ──→ Bull Gear ──→ Impellers
│
└── Controller
│
└── Adjusts frequency
based on demand
Effect on curve:
Pressure
│
│ 100% speed ────────╮
│ ╱ ╲
│ ╱ 90% speed ─────────╮
│ ╱ ╱ ╲
│ ╱ ╱ 80% speed ─────────╮
│╱ ╱ ╱ ╲
│ ╱ ╱ 70% speed ─────────╮
└──────────────────────────────── Flow
(Entire curve shifts, including
surge line)
| Aspect | Advantage | Disadvantage |
|---|---|---|
| Efficiency | Excellent at part load | - |
| Range | Wide (50-100%) | Limited by surge |
| Complexity | - | High in bull gear designs |
| Cost | - | Higher initial investment |
| Harmonics | - | Requires filters |
Anti-Surge Control
Typical System
┌─────────────────────────────────────────────────────────┐
│ ANTI-SURGE CONTROLLER │
│ │
│ ┌──────┐ ┌──────┐ ┌──────┐ ┌──────┐ │
│ │ FT │ │ PT │ │ TT │ │ ST │ │
│ │Flow │ │Press │ │Temp │ │Speed │ │
│ └──┬───┘ └──┬───┘ └──┬───┘ └──┬───┘ │
│ │ │ │ │ │
│ └───────────┴───────────┴───────────┘ │
│ │ │
│ ┌─────┴─────┐ │
│ │ Anti-Surge│ │
│ │ Algorithm │ │
│ └─────┬─────┘ │
│ │ │
│ ┌──────────────┼──────────────┐ │
│ ▼ ▼ ▼ │
│ ┌──────┐ ┌──────┐ ┌──────┐ │
│ │ IGV │ │Blow- │ │Recirc│ │
│ │ │ │ Off │ │ │ │
│ └──────┘ └──────┘ └──────┘ │
└─────────────────────────────────────────────────────────┘
Control Algorithms
| Algorithm | Description | Application |
|---|---|---|
| ΔP/flow² | Quadratic relationship | Industry standard |
| Predictive model | Anticipates changes | Variable loads |
| Adaptive | Adjusts parameters | Changing conditions |
| Fuzzy logic | Fuzzy rules | Complex processes |
Typical Parameters
| Parameter | Typical Value |
|---|---|
| Surge margin | 10-15% |
| Response time | less than 100 ms |
| Blow-off closing speed | less than 1 second |
| Flow measurement accuracy | +/- 1% |
| Sensor redundancy | 2 of 3 voting |
Method Combination
In practice, several methods are combined:
System demand
│
┌──────────┴──────────┐
│ │
High demand Low demand
│ │
▼ ▼
IGV open IGV partial
Max speed Reduced speed
│ │
│ ┌──────┴──────┐
│ │ │
│ Demand OK Near surge
│ │ │
│ │ ▼
│ │ Blow-off
│ │ active
▼ ▼ │
═══════════════════════════════════════
COMPRESSOR OPERATING
The optimal combination depends on the load profile. For stable loads, IGV is more efficient. For highly variable loads, VSD + blow-off offers better response.