BIZOMAT MAKEOS · Pneumatics
Compressed-Air System Sizing
How big a compressor, receiver and main pipe does this plant need, and what do the leaks cost?
Sizing a compressed-air system from the average demand: compressor free-air delivery with leakage and growth allowances, theoretical compression power with an overall efficiency (the same method as the compressor power tool), receiver volume to cover a peak deficit within the pressure band, the main pipe bore that keeps the pressure drop within a limit (Darcy–Weisbach, as the air line tool), and the annual energy cost of leakage.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Demand | ||
| Average demand of the users | m³/min FAD | Average free-air demand of all users together (after diversity). |
| Leakage allowance | % | Air lost to leaks, as a share of demand (often 20–30% in unsurveyed plants). |
| Future growth allowance | % | Allowance for future users. |
| Peak demand above compressor output | m³/min | Short-term demand above what the compressor supplies. |
| Duration of the peak | min | How long that peak lasts. |
| Pressure | ||
| Working pressure | bar g | Pressure needed at the users. |
| Receiver upper pressure | bar g | Pressure at which the compressor unloads. |
| Receiver lower pressure | bar g | Lowest acceptable receiver pressure. |
| Main pipe | ||
| Main pipe length (incl. fitting allowance) | m | Length of the main pipe run, plus an allowance for fittings. |
| Allowable pressure drop | bar | Largest pressure drop you accept along the main. |
| Air temperature | °C | Temperature of the air in the main. |
| Pipe roughness | mm | Pipe roughness. |
| Energy | ||
| Compressor overall efficiency | % | Overall efficiency relating theoretical adiabatic power to shaft power. |
| Running hours per year | h | Hours the system is pressurised each year. |
| Electricity price | $/kWh | Cost of electricity. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Demand
What this means: Average demand of the users
- What it is
- Average free-air demand of all the air users together, after allowing for the fact that they do not all run at once.
- Units and format
- Enter a number in m³/min FAD: m³/min FAD. The form accepts 0.000001 m³/min FAD to 10000 m³/min FAD. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 m³/min FAD as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Compressor capacity, power and pipe size rise with demand.
- Get it right
- Use free air delivery (FAD): volume at intake conditions, not compressed volume.
What this means: Leakage allowance
- What it is
- Air lost through leaks, as a percentage of the users’ demand, added on top of the demand.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises compressor capacity and the annual cost of leaks.
- Get it right
- A leak survey gives the real figure; type a percentage.
What this means: Future growth allowance
- What it is
- Extra capacity, as a percentage, kept for users you expect to add in future.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 200%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Compressor capacity, power and pipe size rise with it.
- Get it right
- It is applied after leakage, so the two allowances compound.
What this means: Peak demand above compressor output
- What it is
- The free-air flow by which a short peak in demand exceeds what the compressor delivers. The receiver must supply this difference.
- Units and format
- Enter a number in m³/min: cubic metres per minute. The form accepts 0 m³/min to 10000 m³/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 m³/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Receiver volume rises in proportion.
- Get it right
- Enter it as free air, like the demand, not as compressed volume.
What this means: Duration of the peak
- What it is
- How long the peak demand above compressor output lasts.
- Units and format
- Enter a number in min: minutes. The form accepts 0 min to 600 min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Receiver volume rises in proportion.
- Get it right
- Enter minutes, not seconds.
Pressure
What this means: Working pressure
- What it is
- Gauge pressure the users need at the end of the main. It sets the air density for the pipe sizing and is compared with the receiver lower pressure.
- Units and format
- Enter a number in bar g: bar g. The form accepts 0.000001 bar g to 50 bar g. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7 bar g as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Gauge pressure (bar g) is above atmospheric; do not add atmospheric pressure.
What this means: Receiver upper pressure
- What it is
- Gauge pressure at which the compressor unloads or stops, the top of the receiver’s pressure band. Compressor power is calculated to this pressure.
- Units and format
- Enter a number in bar g: bar g. The form accepts 0.000001 bar g to 50 bar g. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 bar g as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher upper pressure raises power but widens the band, reducing receiver volume.
- Get it right
- Gauge pressure; it must be higher than the lower receiver pressure.
What this means: Receiver lower pressure
- What it is
- Lowest gauge pressure the receiver may fall to during a peak, the bottom of its pressure band.
- Units and format
- Enter a number in bar g: bar g. The form accepts 0.000001 bar g to 50 bar g. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6.5 bar g as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower limit widens the band and reduces receiver volume.
- Get it right
- Keep it at or above the working pressure, or users will be starved during peaks.
Main pipe
What this means: Main pipe length (incl. fitting allowance)
- What it is
- Length of the main pipe run from the compressor room to the furthest user, plus an equivalent length allowance for bends, tees and valves.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 100000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer main needs a larger bore for the same pressure drop.
- Get it right
- Add the equivalent length of fittings to the straight length.
What this means: Allowable pressure drop
- What it is
- Largest pressure loss you accept along the main between compressor and users. The tool finds the smallest bore that meets it.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 5 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.1 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A tighter limit needs a larger pipe bore.
- Get it right
- This is a pressure difference, so gauge or absolute does not matter.
What this means: Air temperature
- What it is
- Temperature of the compressed air flowing in the main, used for its density and actual flow volume.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -20 °C to 100 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the air temperature after the aftercooler, not the compressor discharge temperature.
What this means: Pipe roughness
- What it is
- Absolute roughness of the main pipe’s inner wall, used in the friction factor for the pressure-drop calculation.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 5 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.045 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from pipe data for the pipe material; corroded steel is rougher than new.
Energy
What this means: Compressor overall efficiency
- What it is
- Overall efficiency relating the theoretical single-stage adiabatic power to the actual shaft power of the compressor.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 20% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 75% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shaft power and leak cost rise as efficiency falls.
- Get it right
- Compressor makers publish specific power; work back from it if you can. Type the percentage.
What this means: Running hours per year
- What it is
- Hours per year the system is pressurised, during which leaks waste air whether or not production is running.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 8760 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4000 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Annual leak cost rises in proportion.
- Get it right
- Include nights and weekends if the system stays pressurised.
What this means: Electricity price
- What it is
- Price paid for one kilowatt-hour of electricity, used for the annual cost of leaks.
- Units and format
- Enter a number in $/kWh: money per kilowatt-hour. The form accepts $0/kWh to $10/kWh. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $0.25/kWh as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Annual leak cost rises in proportion.
- Get it right
- Use the average delivered price including network charges, from your electricity bills.
Method and formulas
- Compressor FAD = Q × (1 + leak) × (1 + growth).
- Power = single-stage adiabatic k = 1.4 from 1.013 bar to (p₁ + 1.013) bar ÷ efficiency.
- Receiver V = deficit × time × 1.013 ÷ (p₁ − p₂).
- Main bore found so the Darcy–Weisbach drop with Colebrook friction at working pressure equals the limit.
- Leak cost = leak flow × specific power × hours × price.
- Example: 5 m³/min of demand needs a 6.9 m³/min compressor of about 47.156 kW, a 1,351 L receiver and a 50.922 mm main.
Limitations
- Single-stage power estimate; real compressors publish their own specific power.
- Pipe bore is the minimum internal bore; choose the next larger standard pipe.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Plan a compressor, receiver and ring main for a workshop
- Show the energy cost of leaks to justify a leak survey
- Check a plant after adding air users
Who it is for
- Plant engineers
- Compressed-air engineers
- Maintenance managers
What you get
- Compressor capacity and power
- Receiver volume
- Main pipe bore and leak cost
Interpreting the result
- Fixing leaks is usually the cheapest capacity increase available.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3030–3037, 3421–3425. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Free air, pressure and mass flow
In Compressed Air Plant by the Numbers
What it covers: Work out what a cubic metre of compressed air means, convert free air delivery to mass and actual volume, state pressure ratios in absolute terms and meet the CA-1 compressor room.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.