Two-Stage Compressors

Two-stage screw compressors use two compression units in series to achieve higher pressures or greater efficiency.

Why Two Stages?

Single Stage Limitations

A single compression stage has practical limits:

Factor	Typical limit (1 stage)
Maximum pressure	190-220 psi (13-15 bar)
Compression ratio	8-10:1
Discharge temperature	210°F (100°C) lubricated
Efficiency	Decreases >145 psi (10 bar)

Two-Stage Advantages

              SINGLE STAGE                    TWO STAGE

    15 psi ═══════════════> 145 psi    15 psi ══> 50 psi ══> 145 psi
              CR = 10                     CR₁=3.5    CR₂=2.9

    More heat generated                 Less total heat
    Less efficient                      More efficient

Benefits:

1. Higher efficiency: 10-15% better than single stage at >145 psi
2. Lower temperature: Intercooling between stages
3. Higher pressure: Up to 360 psi (25 bar) or more
4. Less stress: Each stage works less

Configurations

Two-Stage Lubricated

┌────────────────────────────────────────────────────────────────┐
│                                                                │
│   Inlet       ┌──────────┐           ┌──────────┐    Discharge│
│   ──────────→ │ Stage 1  │ ────────→ │ Stage 2  │ ──────────→ │
│    15 psi     │ (low P)  │  45-60 psi│ (high P) │  145-360 psi│
│               └────┬─────┘           └────┬─────┘             │
│                    │                      │                   │
│                    ↓                      ↓                   │
│               Common oil             Common oil               │
│               (single system)                                 │
│                                                               │
└────────────────────────────────────────────────────────────────┘

Lubricated two-stage characteristics:

• Shared oil system
• Single separator tank
• Oil cooling in both stages
• Controlled temperature

Two-Stage Oil-Free

┌────────────────────────────────────────────────────────────────┐
│                                                                │
│   Inlet       ┌──────────┐  ┌────────┐  ┌──────────┐  Discharge│
│   ──────────→ │ Stage 1  │─→│Inter-  │─→│ Stage 2  │────────→ │
│    15 psi     │ (low P)  │  │cooler  │  │ (high P) │  115-145  │
│               └──────────┘  └────────┘  └──────────┘   psi    │
│                    │                          │               │
│                 ~350°F        ~95°F        ~320°F             │
│                                                               │
└────────────────────────────────────────────────────────────────┘

Oil-free two-stage characteristics:

• Essential intercooler (reduces temperature)
• Each stage has own gears
• Independent synchronization
• Greater complexity

Compression Ratio Calculations

Optimal Distribution

For maximum efficiency, compression ratios should be equal in each stage:

$$CR_{optimal} = \sqrt[n]{CR_{total}}$$

Where $n$ = number of stages.

Example for 145 psi (CR total ≈ 10):

$$CR_{stage} = \sqrt{10} = 3.16$$

• Stage 1: 14.7 psi → 46.5 psi
• Stage 2: 46.5 psi → 147 psi

Intermediate Pressure

$$P_{intermediate} = \sqrt{P_{suction} \times P_{discharge}}$$

Example:

$$P_{int} = \sqrt{14.7 \times 147} = 46.5 \text{ psia}$$

Intercooler

Function

The intercooler between stages:

1. Reduces temperature of air before second stage
2. Condenses water (cooler air = less moisture)
3. Increases density of air (more efficiency in stage 2)
4. Protects second stage from excessive temperatures

Design

Type	Application
Air-air	Most common, simple
Water-air	High capacity, superior cooling

Typical temperatures:

Point	Temperature
Stage 1 outlet	285-350°F (140-180°C) oil-free
Intercooler outlet	95-115°F (35-45°C)
Stage 2 inlet	95-115°F (35-45°C)
Stage 2 outlet	285-350°F (140-180°C) oil-free

Energy Efficiency

Comparison

Configuration	Specific power @ 145 psi
Single stage lubricated	22-25 kW/100 CFM
Two stage lubricated	18-22 kW/100 CFM
Single stage oil-free	25-30 kW/100 CFM
Two stage oil-free	22-25 kW/100 CFM

Energy Savings

Two-stage vs. single-stage savings increase with pressure:

Pressure	Typical savings
100 psi (7 bar)	3-5%
145 psi (10 bar)	8-12%
190 psi (13 bar)	12-18%
290 psi (20 bar)	20-30%

$$\text{Annual savings} = kW_{saved} \times \text{hours} \times \$/kWh$$

Applications

When to Use Two Stages

Application	Recommendation
Pressure >190 psi (13 bar)	Two stage required
Pressure 145-190 psi	Two stage recommended
Continuous high pressure operation	Two stage (efficiency)
Breathing air (2900+ psi)	Multiple stages
PET blowing (580 psi)	Two stage + booster

Typical Applications

Industry	Pressure	Type
PET blowing	360-580 psi	Two stage + booster
CO₂ laser	190-290 psi	Two stage
Fiber optics	220-290 psi	Two stage oil-free
Textile (looms)	145-190 psi	Two stage lubricated
Natural gas (CNG)	2900-3600 psi	Multi-stage