The compressed air system you choose is important – and it sets the tone for everything that follows.
How smoothly it runs. And how much it costs you, month after month.
One of the basic differences between piston compressors comes down to compression stages.
With a single-stage unit, the air is compressed to its final pressure in one go.
This typically achieves a maximum of 10 bar. A two-stage compressor divides the process into two steps with intermediate cooling. This allows for pressures up to 30 bar and improves energy efficiency.
Design Air (Scotland) Ltd provides this guide to assist in selecting the correct Atlas Copco solution for your facility.
What is the Fundamental Mechanical Difference?
The difference is defined by how many times the air is compressed before entering the receiver tank.
Single-Stage Mechanics
- Air is drawn into a cylinder.
- The piston compresses the air in a single stroke.
- The air is discharged directly into the receiver tank.
- Identification: Cylinders are typically the same size. There is no intercooler tube.
Two-Stage Mechanics
- Air is drawn into a large Low-Pressure (LP) cylinder.
- The air is compressed to an intermediate pressure (approx. 3–4 bar).
- The air is routed through a finned intercooler to bring the temperature down.
- Once cooled and denser, it’s fed into a smaller high-pressure (HP) cylinder.
- The HP piston compresses it to the final pressure (11–30 bar).
- Identification: The unit features one large cylinder and one smaller cylinder connected by a cooling pipe.
The Compression Concept
Consider filling a bicycle tyre versus a high-pressure racing tyre.
A standard pump works efficiently up to a certain pressure. As resistance builds, more effort is required. A two-stage system splits this effort. It uses a large volume stroke to start, followed by a high-pressure stroke to finish. This method is more efficient for higher pressures.
How Does Thermodynamics Affect Efficiency?
Compression generates heat. In a single-stage unit, the temperature can rise to between 130°C and 200°C in a single adiabatic stroke.
The Role of the Intercooler
There’s an intercooler between the stages. Its role is simple but important – cool the air before it reaches the next cylinder.
- Density: Cooling the air decreases its specific volume.
- Reduced Work: The second piston requires less energy to compress the denser air.
- Efficiency: This results in 15–20% more CFM (Cubic Feet per Minute) per kW compared to a single-stage unit.
Clearance Volume
Single-stage units experience re-expansion losses.
High-pressure air remaining in the cylinder must expand before new air enters. Two-stage units compress to a lower intermediate pressure first. This minimizes losses and improves volumetric efficiency.
Comparison: Single-Stage vs Two-Stage Performance
The table below sets out the key operational differences between the two designs.
| Feature | Single-Stage Compressor | Two-Stage Compressor |
| Max Pressure | Up to 10 bar (145 PSI) | 15–30 bar (217–435 PSI) |
| Typical RPM | High (1200+ RPM) | Low (700–900 RPM) |
| Noise Level | High (High pitch) | Lower (Low rumble) |
| Discharge Temp | High (>130°C) | Moderate (80–100°C) |
| Efficiency | Lower CFM per kW | 15–20% Higher CFM per kW |
| Duty Cycle | Intermittent (50–60%) | Continuous (up to 100% on Industrial models) |
| Valve Life | Shorter (Heat exposure) | Longer (Cooler operation) |
| Primary Use | DIY, Light Workshop | Industrial, Heavy Automotive |
What Pressure Can a Single-Stage Compressor Reach?
Single-stage units are mechanically limited. Typically, this is to approximately 10 bar (145 PSI).
As you operate at higher pressures, you’ll increase thermal stress.
Typical Applications
- Framing nailers
- Small spray guns
- Dusting guns
- Intermittent workshop tasks
Carbonisation Risks
Continuous operation at 9–10 bar generates significant heat.
This degrades lubricating oil. Carbon deposits may form on the valves. Leaking valves extend running times, increasing heat generation and wear.
When is a Two-Stage Compressor Required?
Two-stage technology is necessary when pressure demands exceed 14 bar or for heavy duty cycles.
High Pressure Applications
- Truck Tyre Inflation: Once you’re dealing with commercial HGV tyres – often well beyond 130 PSI – a two-stage compressor stops being a nice-to-have and becomes the more dependable way to do it safely.
- Laser Cutting: The quality of the cut depends on pressure staying steady at a high level.
- Garage Lifts: With air-over-hydraulic setups, pressures of around 150 PSI – and sometimes more – are part of normal operation.
- Nitrogen Generation: Purity and efficiency usually depend on higher pressures – typically above 13 bar.
Booster Compressors
For applications requiring 30 to 40 bar (such as PET bottle blowing), an Atlas Copco LB Booster acts as a secondary compression stage. This boosts standard plant air to the required higher pressure.
Calculating Air Demand
It is recommended to select a compressor that exceeds the CFM requirements of your tools by 25%.
| Tool | Average Consumption (CFM) | Recommended Compressor |
| Brad Nailer | 0.5 – 1.5 | Small Single-Stage |
| Impact Wrench (1/2″) | 4 – 5 | Single-Stage (3 HP) |
| DA Sander | 10 – 20 (Continuous) | Two-Stage (5.5 HP+) |
| Sandblaster | 20+ | Large Two-Stage or Screw |
How Atlas Copco Ranges Differ
Atlas Copco separates piston technology into professional and industrial tiers.
Automan Series (Professional Workshop)
- AF/AH Series: Direct-drive units with aluminium blocks. Designed for mobile or light use.
- AC Series: Belt-driven units. Models AC 21–31 are single-stage (10 bar). Models AC 40–200 are two-stage (11–15 bar) with cast iron blocks.
L-Series (Industrial Heavy Duty)
- LE Series: Single-stage industrial. Direct-drive motor eliminates belt transmission losses.
- LT Series: Two-stage industrial. Capable of 15, 20, or 30 bar.
- Valve Technology: The LE/LT series utilises stainless steel reed valves. These offer a lifespan of up to 16,000 hours, compared to the 2,000–4,000 hour standard for carbon steel valves.
- Duty Cycle: The LE/LT range is engineered for continuous operation.
How PSSR 2000 Regulations Affect Selection
The compressor you choose has a direct bearing on your responsibilities under the Pressure Systems Safety Regulations (PSSR) 2000.
The “Relevant Fluid” & Threshold
The key trigger is stored energy. At 250 bar-litres, you cross a line. Beyond that level of stored energy, the system is treated as containing a relevant fluid – and it’s also where extra regulatory controls come into play.
Formula: Pressure (bar) × Volume (litres) = Bar-Litres
Scenario A: Exempt (Single-Stage)
- Equipment: Portable compressor, 24-litre tank @ 8 bar.
- Calculation: 24 × 8 = 192 Bar-Litres.
- Result: Exempt from Written Scheme of Examination (WSE).
Scenario B: Non-Exempt (Two-Stage)
- Equipment: Workshop compressor, 270-litre tank @ 11 bar.
- Calculation: 270 × 11 = 2970 Bar-Litres.
- Result: Non-Exempt. A WSE must be prepared by a competent person before operation.
Design Air provides full PSSR compliance services. We draft the Written Scheme and conduct statutory inspections.
Maintenance and Cost Considerations
Two-stage units often provide a lower Total Cost of Ownership (TCO) for commercial users.
Energy Costs
Two-stage compressors produce more air per kW. Energy savings over a 5-year period often offset the higher initial purchase price.
Moisture & Slugging
- Single-Stage: High heat evaporates some condensate, but it reforms in the tank. Short cycling can cause oil emulsification.
- Two-Stage: Intercoolers remove water efficiently. However, neglected interstage drains can lead to “slugging,” where water enters the HP cylinder.
- Solution: Adhere to maintenance requirements and install automatic drains to protect the system.
Is Your Power Supply Compatible?
Single Phase (230V)
This is often the bottleneck.
A standard plug tops out at 13 amps. Hardwiring gives you more breathing room – typically up to about 32 amps – but it doesn’t change the ceiling by much.
In normal operation, you’re effectively capped at around 3 HP (2.2 kW) before reliability starts to drop off.
Three Phase (400V)
Once you move into two-stage territory, three phase becomes the norm. Most machines rated at 5.5 HP (4 kW) and above rely on it for smoother torque and a lower current draw, especially under continuous load.
Note: Don’t assume it’s available. Always confirm three-phase supply before committing to a 5.5 HP compressor.
Why Choose Atlas Copco?
While brands like Ingersoll Rand and ABAC offer options for beginners, Atlas Copco’s main goal is to lower the Total Cost of Ownership.
- Valve Tech: Stainless steel valves offer extended service life.
- Direct Drive: The LE/LT series eliminates belt maintenance.
- Parts Availability: Design Air stocks genuine parts to ensure uptime.
Summary
If a compressor is running regularly or pushing higher pressures, a two-stage setup usually makes more sense. It stays cooler, copes better with the load, and holds up longer over time.
Single-stage machines still have their place. They suit shorter, intermittent jobs – or jobs where pressure requirements are modest.
Design Air (Scotland) Ltd serves Airdrie, Glasgow, Edinburgh, Dundee, Stirling, Fife, Perth, and the Central Belt. Our engineers size systems based on CFM demand and duty cycle to ensure efficiency.
To assess the efficiency of your current system, book a compressed air energy audit with our team.
