With a foundation in basics of physics, the next step is to learn how air compressor measurements—work, power, and flow—are measured. This information is useful when determining the appropriate size and power you need for a particular application.
In this article, we will explain the basics of measuring work, power and volume rate of flow.
When selecting a compressor, the two most important outputs are typically:
Flow, how much air is available over time (capacity)
Pressure, the force level the system can deliver
Work and power explain the energy required to produce that compressed air, and they support efficiency comparisons.
Mechanical work may be defined as the product of a force and the distance over which the force operates on a body. Exactly as for heat, mechanical work is energy that is transferred from one body to another. The difference is that it is now a matter of force instead of temperature.
An illustration of this is gas in a cylinder being compressed by a moving piston. Compression takes place as a result of a force moving the piston. Energy is thereby transferred from the piston to the enclosed gas. This energy transfer is work in the thermodynamic sense of the word. The result of work can have many forms, such as changes in the potential energy, the kinetic energy or the thermal energy.
The mechanical work associated with changes in the volume of a gas mixture is one of the most important processes in engineering thermodynamics.
The SI unit for work is the joule:
1 J = 1 N·m = 1 W·s
Power is the rate at which work is performed, work per unit time. It describes how quickly energy is transferred.
The SI unit for power is the watt:
1 W = 1 J/s
For example, the power or energy flow to a drive shaft on a compressor is numerically similar to the heat emitted from the system plus the heat applied to the compressed gas.
Compressor capacity is often discussed as “flow rate”. In practice, flow can be expressed as mass flow or volumetric flow, and the chosen form affects how comparisons should be made.
Flow rate can be measured using a mass flow meter. For gases, volumetric flow feels intuitive, but volume changes with inlet temperature and pressure. That is why flow statements should always include the conditions at which they apply.
The volumetric flow rate of a system is a measure of the volume of fluid flowing per unit of time. It may be calculated as the product of the cross-sectional area of the flow and the average flow velocity, often expressed as:
l/s (common on compressor datasheets)
cfm (common in some regions)
The SI unit for volume rate of flow is m3/s
However, when buying a compressor you will typically find the capacity of the compressor, expressed in liters/second (l/s). This is the FAD or free air delivery of the compressor.
Free air refers to air at the compressor inlet conditions, typically close to ambient temperature and pressure. Delivery refers to the air that actually leaves the compressor outlet.
Delivered air is not always identical to intake air because some air can leak internally between inlet and outlet. When FAD is determined by measuring flow at the outlet, it captures what is delivered, then converts it back to “free air” using inlet conditions.
FAD is used to:
Compare different compressors
Match compressor capacity with the air consumption of tools
Unless stated otherwise, the FAD of a compressor or tool (as listed on spec sheets) is measured at reference inlet conditions:
20°C
1 bar
0% RH
The air mass that fits into the swept volume of a compressor element varies with air density, which changes the amount of flow effectively obtained at the outlet.
Air density depends on:
Temperature
Pressure
That is why the measured outlet mass flow is divided by the inlet air density. This cancels out the density effect.
Even when density is accounted for, temperature and pressure can still affect measured results:
Temperature effects on leakage
The size of gaps between parts changes with temperature, causing more or less leakage.
Inlet pressure effects on compression
A change in inlet pressure can cause overcompression or undercompression, which changes the resulting outlet flow rate.
Because of these secondary effects, it is important to compare compressors at the same conditions, generally (but not necessarily) the reference conditions defined in ISO 1217:2009. In other sectors or regions, different reference conditions can be used.
When the flow is defined as a “normal flow”, it is considered a flow with a certain reference condition.
For example, Nm3/min is a unit of flow of 1 m3 of gas per minute at a pressure of 1 atmosphere and a reference temperature, often 0°C (32°F) or 20°C (68°F).
The relation between the two volume rates of flow is shown in the original equation (note that the simplified formula does not account for humidity).
Where:
qFAD = Free Air Delivery (FAD) in l/s (actual flow rate at outlet conditions)
qN = Normal flow rate in Nl/s (flow rate at standard conditions)
TFAD = Standard inlet temperature (20°C / 68°F)
TN = Normal reference temperature (0°C / 32°F)
PN = Normal reference pressure (1.013 bar(a) / 101.3 kPa)
PFAD = Standard inlet pressure (1.00 bar(a) / 100 kPa)
Atlas Copco expert comment:
Engineers and industrial buyers rely on qN for benchmarking, while qFAD is crucial for actual system design and operation. What is the difference between qFAD and qN?
While it appears to be a volume flow rate, FAD can be thought of as a mass flow rate expressed in terms of volume. This is because for fixed conditions, the density of the air flow is constant and hence the mass flow is constant and known.
Question: What does an FAD of 39 l/s for a compressor working at 10 bar(e) mean? How long does it take to fill a 39 l tank at a pressure of 10 bar(e)?
You can treat FAD as a mass flow rate.
10 bar(e) equals 11 bar(a).
The total mass of 39 l of air at 11 bar(a) is 11 times the mass of 39 l of air at ambient conditions.
Call the mass of 39 l at ambient conditions one unit of mass.
If the tank starts filled with ambient air, it already contains 1 unit of mass, so you need 10 more units to reach the final pressure.
Since the compressor delivers 1 unit of mass per second, it takes 10 seconds to deliver the required mass.
Is important to undesrtand the difference between bar(a) and bar(e).
The SER is a measure for efficiency, expressed as the amount of energy that is required to deliver 1 liter FAD at a certain pressure. This gives a value in Joules/liter (J/l).
For example:
If a compressor consumes 35 kW and delivers 100 l/s, then:
SER = 35,000 J/s ÷ 100 l/s = 350 J/l
Specifying your compressed air system by flow and pressure – not kW or horsepower – is the best way to match its performance to your needs. Compressor sizing should match your business requirements more precisely than just going by kW rating.
| Term | What it means | Typical unit(s) | Where it is used |
|---|---|---|---|
| Work | Energy transferred by force (compression work adds energy to air) | J (joule) | Theory and thermodynamics, not usually a headline datasheet value |
| Power | Work per time, how fast energy is transferred | W, kW, hp | Motor sizing, energy input, efficiency calculations |
| Flow rate | Air delivered over time (must state conditions) | l/s, m³/h, cfm, m³/s | Main sizing input alongside pressure |
| FAD (Free Air Delivery) | Delivered air expressed as equivalent volume at inlet (free air) conditions | l/s (often), m³/h | Comparing compressor capacity and matching to demand, with stated inlet/reference conditions |
| Normal flow (qN) | Flow referenced to defined “normal” conditions | Nl/s, Nm³/min | Benchmarking and comparison across systems when normal conditions are consistent |
| SER | Energy per delivered liter of FAD at a stated pressure | J/l | Efficiency comparison at a defined operating point |
Understanding mechanical work, power, and flow helps you invest in equipment that fits your application. Equipment that is too large or too small risks inefficiency.
When selecting a compressor, the key specifications to evaluate are flow and pressure.
Different applications require different combinations of flow and pressure. Pneumatic tools may need higher flow for continuous operation, while applications such as lifting or clamping rely more on pressure.
It is also important to assess whether your operations demand continuous air delivery or if air is needed intermittently. This helps determine the right compressor size and whether a fixed speed or variable speed drive (VSD) machine is more suitable.When selecting a compressor, the key specifications to evaluate are flow and pressure.
Flow is the volume of air available over time, expressed in liters per second (l/s), cubic feet per minute (cfm), or cubic meters per hour (m³/h).
Pressure is expressed in bar or pounds per square inch (psi) and represents the force of the compressed air.
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