A specification sheet lists the compressor’s performance data, operating limits, and energy figures. It acts as the technical blueprint for the machine, providing the necessary engineering data to determine if it fits a specific application. For facility managers and engineers, reading this document accurately is essential to prevent undersizing and ensure the equipment runs efficiently under all site conditions.
The data sheet includes Free Air Delivery (FAD), pressure ratings, electrical input, and air purity standards. This guide explains how to interpret these figures to protect your investment and operational reliability.
What is the Difference Between Displacement and Free Air Delivery?
The flow rate is the primary metric for sizing, but terminology matters. The specification sheet distinguishes between Piston Displacement and Free Air Delivery (FAD).
Piston Displacement is a theoretical calculation derived from the swept volume of the piston or screw rotor geometry. It assumes 100% volumetric efficiency, ignoring internal losses, temperature changes, and intake filters. Piston compressor data sheets often display this figure to make the machine appear more powerful than it is.
Free Air Delivery (FAD) is the actual volume of compressed air delivered at the discharge terminal, measured in accordance with ISO 1217. The volume is corrected back to ambient inlet conditions.
Why this matters:
System sizing requires the FAD figure. In practice, using displacement data instead of FAD often results in an undersized compressor that cannot keep up with production demand. Always verify the FAD to ensure the machine delivers the usable air your tools require.
Understanding the relationship between volume and pressure is critical for performance. Read more about The Difference Between CFM and PSI in Air Compressors.
How Do Pressure Ratings and Control Bands Work?
Selecting the correct pressure is as important as flow. Specification sheets list Maximum Working Pressure and Rated Working Pressure.
- Maximum Working Pressure: The highest pressure the machine can reach before the safety valve opens or the unit stops.
- Rated Working Pressure: The pressure at which the manufacturer tested the machine’s efficiency.
There is a trade-off involved: higher pressure reduces Free Air Delivery. For example, a 7 bar version of a compressor normally delivers more air than a 10 bar version of the same kW rating.
Control Bands
Additionally, the spec sheet describes how the compressor maintains pressure, which affects stability at the point of use.
- Fixed-Speed (Load/Unload): The pressure changes from a load point to an unload point. This band is usually between 0.5 and 1.0 bars wide. So, the system pressure needs to be set higher than it needs to be to make up for the drop, which uses more energy.
- Variable Speed Drive (VSD): The motor speed adjusts to demand. A VSD maintains pressure within a tight band of ±0.1 bar. This reduces artificial demand and prevents over-pressurisation.
What Do the Energy Efficiency Figures Mean?
The cost of running a compressor over its lifetime is often highest for electricity. The Nominal Motor Power (e.g., 37 kW) only tells you the rating of the motor shaft, not how much power it actually draws from the mains.
To accurately assess running costs, use the Package Input Power. The motor, cooling fans, control systems, and built-in dryers all use this much energy.
The Specific Energy Requirement (SER) is the most accurate measure of how efficient something is. This tells you how many kilowatts (kW/m³/min) you need to move one cubic meter of air per minute. A lower SER indicates a more efficient machine.
ISO 1217 Testing Standards
The testing standard depends on the technology, affecting how you compare machines:
- Annex C (Fixed-Speed): Measures performance at full load only.
- Annex E (VSD): Measures performance across the turndown range, accounting for inverter losses.
VSD compressors may show a slightly higher SER at 100% load due to inverter heat losses. However, because most facilities have fluctuating demand, their efficiency improves significantly at partial load.
For assistance with these calculations, refer to our guide on Calculating an Air Compressor’s Energy Consumption. Alternatively, you can request a full compressed air audit to measure your exact usage profile.
How Are Duty Cycles and Noise Levels Measured?
Duty Cycle
The duty cycle defines the allowable run time within a specific period.
- Piston Compressors: 40%–70% duty is frequently rated. Assume that intermittent operation is necessary if the specification sheet does not specify a duty cycle. When these are used for continuous production, overheating and failure are more likely to occur.
- Rotary Screw Compressors: designed for 100% continuous duty, guaranteeing dependability in manufacturing settings that are open around-the-clock.
Noise Levels (ISO 2151)
ISO 2151 says that noise should be measured 1 metre away in free-field conditions.
When the sound energy goes up by 3 dB, it has doubled because the decibel scale uses logarithms. A 75 dB(A) piston compressor is much louder than a 65 dB(A) screw compressor in real life. You can often put a quieter unit on the factory floor, which saves money on piping.
How Do You Interpret Air Purity (ISO 8573-1)?
The application determines the necessary purity. The specification sheet uses ISO 8573-1, which grades contamination using three digits, to list air purity capability.
The Three Digits:
- Solid Particles: Dust and pipe scale.
- Water: Pressure Dew Point (PDP).
- Oil: Total oil content (aerosol, liquid, and vapour).
Dryer Specifications
- Integrated Refrigerant Dryers: Typically achieve Class 4 Water (+3°C PDP). This is suitable for general industrial applications where preventing liquid water in pipes is the goal.
- Desiccant Dryers: Achieves Class 2 (-40°C) or Class 1 (-70°C). This level of dryness is required for outdoor piping to prevent freezing or for sensitive pharmaceutical processes.
Oil Class
Oil-injected compressors typically achieve Class 4 Oil (< 3 mg/m³) with a standard separator. Downstream filters are required for Class 1. For guaranteed oil-free air in food or medical sectors, look for Class 0 certification, found in the Atlas Copco Z-series.
Why Do Ambient Conditions Matter?
Standard reference conditions, typically 20°C ambient temperature and 1 bar (a) inlet pressure, are used in data sheets. However, real-world conditions in Scotland vary.
Temperature Effects
High ambient temperatures reduce air density. As a result, a compressor in a 30°C plant room delivers less mass flow than the spec sheet indicates.
Dryer Correction Factors
Refrigerant dryers have a set temperature for the air that comes in (usually 20°C or 25°C). The dryer can hold a lot less if the temperature outside rises to 30°C or 35°C in the summer.
In actuality, this means that if the dryer is not the right size, moisture may seep into the system. The spec sheet lists correction factors; applying these ensures the dryer can handle the load year-round.
Limits of Operation
Always check the “Limits of Operation” section. This defines the maximum ambient temperature (often 46°C) and minimum starting temperature. This confirms the machine can survive your specific site environment.
Electrical Data and Starting Currents
Checking the electrical requirements is vital for installation planning. The UK industrial standard is 400V / 3-phase / 50Hz. Using 60Hz equipment on a 50Hz supply reduces motor speed and flow.
The spec sheet also lists the starting method, which dictates the inrush current and potential stress on your electrical supply.
| Starting Method | Typical Inrush Current | Note |
| Direct-on-Line (DOL) | 7x – 10x Nominal Current | Causes voltage dips. Common on small pistons. |
| Star-Delta | 3x Nominal Current | Standard on fixed-speed screw compressors. |
| VSD / Soft Start | 1x Nominal Current | Ramps up slowly. No current spike, reducing grid penalties. |
Compliance and Safety Regulations
Last but not least, the spec sheet is a document that shows compliance. Making sure your equipment meets these standards keeps the business safe and out of trouble with the law.
- UKCA / CE Marking: Mandatory for new machinery in the UK.
- PSSR 2000: Pressure Systems Safety Regulations require a Written Scheme of Examination for air receivers.
- ISO 1217: Confirms the flow rate is measured, not estimated.
Summary
A correct specification sheet reading prevents costly errors and ensures operational reliability. Focus on Free Air Delivery (FAD) rather than displacement to guarantee performance. Use Package Input Power for realistic energy calculations and verify that the ISO Class matches your quality needs.
If you require guidance interpreting a specification sheet or matching equipment to your demand profile, Design Air (Scotland) Ltd can review your site conditions and recommend the correct compressor service or specification.
