How to Read an Air Compressor Specification Sheet

An air compressor specification sheet defines the measurable performance of a compressor under standardised test conditions. It sets out free air delivery, operating pressure, electrical input, energy efficiency, and air quality, enabling accurate assessment of whether a machine can meet a site’s operational demand without excess energy consumption or capacity shortfall.

This guide explains how to interpret those specifications correctly. It supports clients of Anglian Compressors in selecting compressors that align with real operating conditions, regulatory requirements, and long-term efficiency objectives.

What Is the Difference Between Displacement and Free Air Delivery?

Free Air Delivery (FAD) is the only airflow figure suitable for system sizing. It is the usable compressed air output measured at the package discharge.

Displacement is a theoretical number that does not reflect real performance. It is calculated from the bore, stroke, and RPM. It assumes perfect volumetric efficiency, which never occurs in practice. In piston compressors, displacement figures are often 30–40% higher than the actual airflow.

Industrial rotary screw compressors, like the Atlas Copco GA series, list FAD. This follows the ISO 1217 standard. It measures air actually delivered to the pipe. Sizing a system based on displacement instead of FAD will leave you short of air.

How Does ISO 1217 Annex C Differ from Annex E?

Annex C tests fixed-speed machines, while Annex E tests Variable Speed Drive (VSD) machines. Annex C and Annex E data must not be directly compared without context.

Annex C: Fixed-Speed Compressors

Annex C measures performance at full load only. It tests the machine at a steady temperature. It accounts for losses from the cooler, separator, and internal piping.

Annex E: VSD Compressors

Annex E tests performance across a range of speeds. VSD compressors, like the GA VSD+, are designed to vary their output. This standard tests minimum, maximum, and intermediate speeds. It also includes energy lost in the inverter drive.

How Do I Interpret Pressure Ratings on a Spec Sheet?

Pressure sets the force of the air. Spec sheets typically list this in bar(e). The notation bar(e) refers to gauge pressure relative to the atmosphere, while bar(a) refers to absolute pressure.

Higher pressure reduces flow. A compressor needs more energy to squeeze air to a higher pressure. A 13-bar machine delivers significantly less air than a 7-bar machine of the same power.

Buying more pressure than you need wastes money. If you run a 13-bar machine at 7 bar, you lose efficiency. The air is over-compressed inside the element and then expanded. This releases wasted energy as heat.

What Does the Specific Energy Requirement (SER) Tell Me?

SER is the primary efficiency metric. It shows how much energy the compressor uses to make air.

You calculate it by dividing the power input by the air output.

SER = Package Input Power (kW) ÷ FAD (m³/min)

Nominal vs. Package Power

Nominal Motor Power refers only to the shaft output rating. Package Input Power refers to the total electricity drawn from the grid. Always use Package Input Power to check efficiency. A lower SER number is better.

How Do I Decode ISO 8573-1 Air Purity Classes?

ISO 8573-1 standards define air cleanliness. It uses three numbers to show particle, water, and oil content.

• Digit 1 (Particles): The concentration of solid particles.
• Digit 2 (Water): The Pressure Dew Point (PDP).
• Digit 3 (Oil): The total oil content.

Common Classifications

• Class 0: This ensures no oil is added during compression. It is used for critical sectors like pharmaceuticals.
• Class 4 Water: This indicates a +3°C dew point. Integrated refrigerant dryers achieve this to prevent water in indoor pipes.
• Class 1 or 2 Water: This indicates a -70°C or -40°C dew point. You need desiccant dryers to achieve this level.

See our guide to ISO 8573-1 air quality standards for more detail.

What Electrical and Installation Data Matters Most?

Installation data lists the power and physical needs of the unit. You must match this to your site infrastructure.

• Voltage: Standard UK supply is 400V 3-phase.
• Starting Method: VSD units eliminate power spikes associated with older starting methods.
• Noise: Measured at 1 metre distance per ISO 2151.
• Ambient Limits: Industrial units are typically rated for use up to 46°C.

Why Do Environmental Corrections Matter?

Spec sheets assume standard test conditions. These are usually 20°C, 1 bar pressure, and dry air.

If the ambient temperature exceeds 20°C, mass flow decreases and dryer capacity drops.

Dryer Derating

Warm air reduces dryer performance. Integrated dryers are rated for a specific temperature, usually 20–25°C. At 35°C, you may lose up to 30% of dryer capacity. The spec sheet provides correction factors to calculate this drop.

How Do Compressor Types Compare on Spec Sheets?

Different technologies present data based on their primary operational attribute.

• Piston Compressors: Defined by Displacement (theoretical volume).
• Rotary Screw: Defined by FAD (usable volume).
• Oil-Free: Defined by Class 0 (purity).

Refer to our comparison of different types of air compressors for more information.

Why is Anglian Compressors’ Verification Critical?

Verification mitigates the risk of purchasing theoretical performance that fails in reality. Testing validates the real capability on your site.

AIRScan and Demand Analysis

We use AIRScan data logging to measure your actual air use. This tracks usage over a typical week. It ensures the spec sheet you choose matches your real demand.

PSSR 2000 Compliance

Safety regulations require a Written Scheme of Examination. We verify that the pressure vessels listed on the spec sheet meet UK safety laws.

For help calculating flow, see our Air Compressor CFM resource.

Conclusion

A spec sheet defines the compressor’s limits. Accurate reading prevents undersizing and energy waste. For system verification, Anglian Compressors provides AIRScan data logging, PSSR guidance, and specification matching support across East Anglia and the Midlands. Contact us today.