Control Strategies

Capacity control strategies determine how a compressor adjusts its air production to match demand. The choice of strategy has direct impact on energy consumption.

Main Strategies

1. Inlet Modulation

The compressor reduces inlet flow by throttling suction:

                    Modulation
                      valve
                        │
                        ▼
    Ambient ═══════> [◐ ════] ═══════> Compressor
    air                │
                       └── Variable position
                           0-100%

Efficiency curve: See strategy comparison below.

Load	Power	Efficiency
100%	100%	Optimal
80%	90%	Good
60%	82%	Acceptable
40%	75%	Poor
20%	70%	Very poor

Energy Penalty

Modulation is the least efficient method for capacity control. At 50% load, a modulating compressor consumes ~78% of rated power. This is significant waste.

2. Load/Unload

The compressor alternates between full load and unload (idle):

Load Operation:          Unload Operation:

Inlet valve: OPEN        Inlet valve: CLOSED
Production: 100%         Production: 0%
Power: 100%              Power: 25-35%
                                  ↑
                          (parasitic losses)

Typical cycle:

Parameter	Typical Value
Power at load	100%
Power at unload	25-35%
Minimum load time	30-60 sec
Minimum unload time	10-30 sec

Efficiency by load:

System Load	Load Time	Unload Time	Average Power
100%	100%	0%	100%
75%	75%	25%	83%
50%	50%	50%	65%
25%	25%	75%	48%

3. Variable Speed (VSD)

The compressor adjusts motor speed according to demand:

                    VFD (Drive)
                         │
                         ▼
    Grid ══════> [~~~~] ══════> Motor ══════> Compressor
    60 Hz        │              │
                 └── 30-60 Hz   └── Variable RPM

Efficiency curve: See strategy comparison below.

Load	Power	Efficiency
100%	100%	Optimal
80%	78%	Excellent
60%	58%	Excellent
40%	40%	Good
20%	25%	Acceptable

4. Variable Displacement

Some screw compressors adjust physical displacement:

                    Bypass
                     valve
    ┌──────────────────┼──────────────────┐
    │                  ↓                  │
    │    Rotors ═══>[===]═══>            │
    │                  │                  │
    │        Internal recirculation       │
    └─────────────────────────────────────┘

Load	Power	Application
100%	100%	Valve closed
75%	80%	Partial bypass
50%	62%	More bypass
25%	45%	Maximum bypass

Strategy Comparison

Comparison Table

Strategy	Efficiency 50%	Cost	Complexity	Best For
Modulation	Poor (78%)	Low	Low	Never recommended
Load/Unload	Good (65%)	Low	Low	Stable load
VSD	Excellent (50%)	High	Medium	Variable load
Var. Displacement	Very good (62%)	Medium	Medium	Intermediate

Strategy Combination

In multi-compressor systems, strategies are combined:

┌─────────────────────────────────────────────────────────┐
│                    TOTAL DEMAND                          │
│                                                          │
│  ████████████████████████████████████░░░░░░░░░░░░░░░░  │
│  │              │              │              │         │
│  ▼              ▼              ▼              ▼         │
│ Comp 1        Comp 2        Comp 3        Comp 4       │
│ FS L/U        FS L/U          VSD          Standby     │
│ 100%          100%           60%            0%         │
│ (Base)        (Base)        (Trim)       (Standby)    │
└─────────────────────────────────────────────────────────┘

FS = Fixed Speed
L/U = Load/Unload
VSD = Variable Speed Drive

Combination Rules

1. Base compressors: Fixed speed, operate at 100%
2. Trim compressor: VSD, adjusts the difference
3. Only one VSD as trim: Avoid "fighting" between VSDs
4. Standby on reserve: For emergencies and maintenance

Energy Savings by Strategy

Example: 500 HP System with 60% Average Load

Configuration	Annual Power	Annual Cost*	Savings
5x100HP Modulation	2,520,000 kWh	$252,000	Base
5x100HP L/U	2,100,000 kWh	$210,000	17%
4x100HP FS + 1x100HP VSD	1,820,000 kWh	$182,000	28%
2x150HP VSD + 1x200HP FS	1,680,000 kWh	$168,000	33%

*Assuming $0.10/kWh, 8,000 hours/year

Strategy Selection

For new installations, always consider at least one VSD as "trim". The additional cost is typically recovered in 1-3 years.