How to size an air compressor: flow, pressure and storage
Sizing a compressor properly is not a matter of picking a power rating in kW: it means calculating the real flow, the right pressure and suitable storage. A 4-step method, with a worked example.
The four questions to ask first
Sizing a compressor is not about picking a power rating in kW. It is about answering four questions, in this order: what flow do I need, at what pressure, with what air quality, and under what operating mode? The power (kW) follows from the answers; it is never the starting point.
Poor sizing is expensive either way. Undersized, the compressor runs flat out all the time, pressure drops at every peak and the process falters. Oversized, it starts far too often, wastes energy and wears out prematurely.
Power in kW is an output, not an input. Always start from the real flow and pressure.
Step 1: the real flow (m³/min)
Flow is the most important parameter, and the one most often misjudged. Start from the sum of the consumption of every point of use (tools, machines, processes), expressed in m³/min (for reference: 1 m³/min ≈ 16.7 l/s).
But the points of use never all draw air at the same time. A diversity (or simultaneity) factor is applied to reflect the share actually in use at any given moment, then allowances are added for leaks and future growth.
- List every consumer with its rated flow (nameplate or data sheet)
- Estimate its actual utilisation rate (a point of use in service 30% of the time does not count as 100%)
- Apply the overall diversity factor to the whole installed base
- Add the leak allowance, then the growth margin
| Gross sum of consumption | Reference 100% |
|---|---|
| Diversity factor | ×0.6 to 0.85 depending on the number of points of use |
| Leak allowance (new network) | +10% (up to +25% on an existing network) |
| Growth margin over 5-7 years | +20 to +30% |
With many small tools, the diversity factor falls towards 0.5. With 2-3 large machines running together, it rises to 0.9. When in doubt, measure: a data logger left for 7 days on the existing compressor gives the real flow, better than any theoretical calculation.
Step 2: the right pressure (and not one bar more)
The pressure set on the compressor is not the pressure needed at the point of use. Every pressure drop along the chain has to be added to it: filters, dryer, pipework.
Above all, every extra bar of pressure costs roughly 6 to 7% in energy. Setting a compressor to 8 bar "to be on the safe side" when the process needs 6.5 bar means paying around 10% more for electricity, for nothing, for the life of the machine.
| Pressure required at the point of use | e.g. 6.5 bar |
|---|---|
| Filtration (per stage) | +0.1 to 0.3 bar |
| Dryer (refrigerant / adsorption) | +0.2 to 0.4 bar |
| Well-designed network | +0.1 to 0.3 bar (poorly designed: up to 1 bar) |
| → Compressor set point | e.g. 7.2 to 7.5 bar |
Before raising the compressor pressure, look for the pressure drop first: a clogged filter or an undersized network is often "compensated for" by raising the pressure, which is the most expensive fix of all. Lowering the set point by 1 bar is one of the quickest energy savings to obtain.
Step 3: how much storage in the air receiver?
The air receiver has two roles: absorbing short demand peaks without letting the pressure drop, and spacing out the compressor's load/unload cycles to reduce wear.
Beware of a common trap: a receiver does not store much energy. A 1,000-litre receiver on a network consuming 10 m³/min provides only about 6 seconds of autonomy at full flow. It smooths peaks; it does not replace flow.
- Fixed speed: a generous receiver to limit starts (frequent cycling = wear). A common order of magnitude: 1 to 3 m³ per 10 m³/min
- VSD: the receiver can be 2 to 3 times smaller, as the drive absorbs variations continuously
- Highly pulsed demand (cylinders, intermittent blow-off): oversize the receiver, or even add a second one as close as possible to the large consumer
- Always place the receiver after the compressor; a receiver upstream of the dryer helps condense part of the water
The right receiver volume depends on the maximum number of starts per hour the motor allows. Let the integrator calculate it from your profile: this is a point where approximation is paid for in premature failures.
Step 4: the operating mode (fixed speed, VSD or two-stage)
Once flow and pressure are known, the operating mode is chosen according to your load profile over time. It is the operating mode that determines the energy bill over 10 years.
- Stable 80-100% load, running continuously (24/7): fixed speed, or two-stage to save energy on large energy-intensive sites
- Fluctuating 40-70% load with peaks: permanent magnet VSD, where it saves the most
- Highly variable or seasonal load (10-90%): a VSD is essential, otherwise you pay for air that is never used
- Large continuous site: two-stage on the base load (around 10% less energy than a standard screw), VSD on the swings
A single large compressor is rarely optimal. Two machines (a base-load unit plus a VSD trim unit) or a cascade handle variations better and safeguard production continuity.
The corrections everyone forgets
- Altitude: the air drawn in is thinner, about 1% less mass per 100 m. At 1,000 m, the same flow in m³/min delivers about 10% less air by mass (in Nm³) than at sea level
- Compressor room temperature: above 40 °C, most compressors are derated. Plan for ventilation or a margin
- Air quality (ISO 8573-1): it does not change the compressor's flow, but it dictates the downstream treatment (dryer, filters) and its pressure drops, to be included in step 2
- Heat recovery: up to 84% of the absorbed electricity can be recovered (hot water, workshop heating), to be planned for as soon as the compressor room is laid out
Worked example: a machine shop
Take a workshop with 8 pneumatic workstations (screwdrivers, grinders) at 0.6 m³/min each, 2 test benches at 1.2 m³/min and a paint booth at 2 m³/min.
Gross sum: (8 × 0.6) + (2 × 1.2) + 2 = 9.2 m³/min. Diversity factor 0.7 (the stations are rarely all in use) → 6.4 m³/min. Leak allowance +15% → 7.4 m³/min. Growth margin +25% → ~9.2 m³/min target flow.
Pressure: painting requires 6.5 bar at the point of use; +0.3 (filters) +0.2 (refrigerant dryer) +0.2 (network) → set point ~7.2 bar. Fluctuating load profile (intermittent painting) → permanent magnet VSD, receiver of about 1 m³. Result: a VSD screw compressor of around 45-55 kW delivering ~9 m³/min at 7.5 bar, a refrigerant dryer and a 1,000 L receiver.
This calculation takes 30 minutes with the right data. Our online selector does it in 5 questions and suggests 3 suitable models, or our team validates it with real measurements taken on your site.
The questions we get asked
How do I calculate the compressed air flow I need?
What network pressure should I aim for?
Should I choose a variable speed or a fixed speed compressor?
What size of air receiver do I need?
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