Multi-Unit Systems

For capacities greater than a single scroll module (typically 15-50 HP), multi-unit systems in parallel are used. This architecture offers unique advantages over a single large compressor.

Concept

┌────────────────────────────────────────────────────────┐
│                  MULTI-SCROLL SYSTEM                   │
│                                                        │
│   ┌─────┐   ┌─────┐   ┌─────┐   ┌─────┐   ┌─────┐    │
│   │ S1  │   │ S2  │   │ S3  │   │ S4  │   │ S5  │    │
│   │7.5HP│   │7.5HP│   │7.5HP│   │7.5HP│   │7.5HP│    │
│   └──┬──┘   └──┬──┘   └──┬──┘   └──┬──┘   └──┬──┘    │
│      │         │         │         │         │        │
│      └────┬────┴────┬────┴────┬────┴────┬────┘        │
│           │         │         │         │             │
│           ▼         ▼         ▼         ▼             │
│   ┌───────────────────────────────────────────┐       │
│   │              Common manifold               │       │
│   └─────────────────────┬─────────────────────┘       │
│                         │                              │
│                         ▼                              │
│                    Output: 37.5 HP total               │
└────────────────────────────────────────────────────────┘

Available Configurations

By Number of Modules

Modules	Typical Total HP	Typical CFM	Application
2	10-30 HP	30-100 CFM	Small industrial
3	15-45 HP	45-150 CFM	Medium industrial
4	20-60 HP	60-200 CFM	Industrial
6	30-90 HP	90-300 CFM	High demand
8	40-120 HP	120-400 CFM	Maximum capacity

Manufacturers and Models

Manufacturer	Series	Modules	Max HP
Atlas Copco	SF+	1-4	30 HP
Ingersoll Rand	Sierra	2-8	60 HP
Kaeser	Dental/Lab	1-4	20 HP
BOGE	EO	1-6	45 HP
Anest Iwata	SLP	1-4	30 HP

Operating Modes

Sequential (Cascade)

Modules start one by one based on demand:

Demand:      0%   20%   40%   60%   80%  100%
            ─────────────────────────────────
Module 1:   OFF   ON    ON    ON    ON    ON
Module 2:   OFF  OFF    ON    ON    ON    ON
Module 3:   OFF  OFF   OFF    ON    ON    ON
Module 4:   OFF  OFF   OFF   OFF    ON    ON
Module 5:   OFF  OFF   OFF   OFF   OFF    ON

Advantages:

Simple to implement
Low control cost
Efficient at partial loads

Lead Rotation

The "lead" module changes periodically to equalize wear:

Week 1: S1 → S2 → S3 → S4 → S5
Week 2: S2 → S3 → S4 → S5 → S1
Week 3: S3 → S4 → S5 → S1 → S2
...

Benefit: Uniform service life across all modules.

Hour Equalization

Controller starts the module with fewest operating hours:

Module	Hours	Priority
S3	4,521	1 (starts first)
S1	4,890	2
S5	5,012	3
S2	5,234	4
S4	5,456	5 (starts last)

Master Controllers

Basic Functions

Function	Description
Sequencing	Start/stop order
Monitoring	Pressure, temperature, status
Alarms	Faults, maintenance
Rotation	Lead change

Advanced Functions

Function	Description
Energy optimization	Optimal point selection
Demand prediction	Anticipates changes
Remote communication	Modbus, Ethernet, IoT
History	Event and data logging
Predictive maintenance	Trends and alerts

Multi-Unit System Advantages

Redundancy

4-module system:
                                    Capacity
Normal:     [ON] [ON] [ON] [ON]     100%
1 fails:    [ON] [ON] [ON] [XX]      75%
2 fail:     [ON] [ON] [XX] [XX]      50%

Never a total shutdown as long as at least one module works.

Partial Load Efficiency

Load	1 Large Compressor	4 Modules
100%	Optimal	Optimal
75%	Modulating (less efficient)	3 modules ON (optimal)
50%	Load/unload	2 modules ON (optimal)
25%	Very inefficient	1 module ON (optimal)

Maintenance Without Shutdown

Rotating maintenance:

Day 1: [MAINT] [ON]  [ON]  [ON]   Capacity: 75%
Day 2: [ON]  [MAINT] [ON]  [ON]   Capacity: 75%
Day 3: [ON]  [ON]  [MAINT] [ON]   Capacity: 75%
Day 4: [ON]  [ON]  [ON]  [MAINT]  Capacity: 75%

Scalability

Year 1: [S1] [S2] [  ] [  ]   Demand: 50%
Year 2: [S1] [S2] [S3] [  ]   Demand: 75%
Year 3: [S1] [S2] [S3] [S4]   Demand: 100%

Add modules as demand grows.

Installation

Typical Configuration

        Inlet (each module)
              │
    ┌─────────┼─────────┐
    │         │         │
    ▼         ▼         ▼
┌──────┐ ┌──────┐ ┌──────┐
│Filter│ │Filter│ │Filter│
└──┬───┘ └──┬───┘ └──┬───┘
   │        │        │
┌──┴───┐ ┌──┴───┐ ┌──┴───┐
│Scroll│ │Scroll│ │Scroll│
│  1   │ │  2   │ │  3   │
└──┬───┘ └──┬───┘ └──┬───┘
   │        │        │
   └────────┼────────┘
            │
   ┌────────┴────────┐
   │   Check valves  │
   └────────┬────────┘
            │
   ┌────────┴────────┐
   │    Manifold     │
   └────────┬────────┘
            │
            ▼
        To dryer

Check Valves

Critical to prevent backflow when a module is off:

Type	Characteristic
Disc check	Economical, general use
Spring check	Faster closing
Non-slam check	Minimal pressure loss

Space and Ventilation

Modules	Minimum Space	Ventilation
2	3 × 2 m	200 CFM
4	4 × 3 m	400 CFM
6	5 × 4 m	600 CFM
8	6 × 5 m	800 CFM

Economic Comparison

Example: 40 HP Required

Factor	1 × 40 HP Screw	4 × 10 HP Scroll
Initial cost	$25,000	$28,000
Redundancy	None	75%
Efficiency 50% load	70%	85%
Noise	72 dB	62 dB
Maintenance/year	$3,000	$2,000
Flexibility	Low	High

10-Year Total Cost

Factor	Screw	Multi-Scroll
Equipment	$25,000	$28,000
Energy*	$120,000	$105,000
Maintenance	$30,000	$20,000
Downtime**	$10,000	$2,000
Total	$185,000	$155,000

*Assuming typical variable operation **Estimated cost of unscheduled shutdowns

Concept​

Available Configurations​

By Number of Modules​

Manufacturers and Models​

Operating Modes​

Sequential (Cascade)​

Lead Rotation​

Hour Equalization​

Master Controllers​

Basic Functions​

Advanced Functions​

Multi-Unit System Advantages​

Redundancy​

Partial Load Efficiency​

Maintenance Without Shutdown​

Scalability​

Installation​

Typical Configuration​

Check Valves​

Space and Ventilation​

Economic Comparison​

Example: 40 HP Required​

10-Year Total Cost​