BIZOMAT MAKEOS · Pneumatics
Compressed-Air Main Sizing (Bore, Pressure Drop, Velocity, Receiver)
What is the smallest standard pipe bore for this compressed-air main within my pressure-drop and velocity limits, and how big a receiver covers the peak demand?
Sizing of a compressed-air main: velocity, Reynolds number, friction factor and pressure drop for a bore (as the compressed-air pipe drop tool), the smallest standard bore that keeps both the drop and the velocity within your limits (searched through common nominal bores), the check of the bore you chose, and the receiver volume to cover a peak demand above the compressor output over a pressure band (as the air receiver tool).
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Flow (free air) | m³/min | Flow of free air the main carries at peak. |
| Line pressure | bar | Gauge pressure in the main. |
| Air temperature | °C | Air temperature in the main. |
| Equivalent length (pipe plus fittings) | m | Pipe length plus the equivalent length of bends, tees and valves. |
| Pipe roughness | mm | Absolute roughness of the pipe bore. |
| Bore chosen | mm | Inside diameter of the pipe you intend to use. |
| Largest pressure drop | % of line pressure | Largest pressure drop you allow along this main (often 1–5% of line pressure). |
| Largest air velocity | m/s | Largest air velocity you allow in the main (about 6–10 m/s for mains). |
| Receiver | ||
| Peak demand above compressor output (free air) | m³/min | How far the peak demand exceeds compressor output. |
| Duration of the peak | min | How long the peak lasts. |
| Pressure when the peak starts | bar | Receiver pressure when the peak begins. |
| Least pressure allowed | bar | Least pressure the users can accept. |
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.
What this means: Flow (free air)
- What it is
- The peak air consumption this main carries, expressed as free air at atmospheric conditions, the way compressor output and tool consumption are quoted.
- Units and format
- Enter a number in m³/min: cubic metres per minute. The form accepts 0.000001 m³/min to 10000 m³/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 m³/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Air velocity in the main rises in direct proportion to flow.
- Get it right
- Use free air, not the compressed volume at line pressure.
What this means: Line pressure
- What it is
- The gauge pressure at the start of the main. It sets how far the free air is compressed, and so its actual velocity and density in the pipe.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 50 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For the same free-air flow, higher pressure lowers the actual velocity and the pressure drop.
- Get it right
- Gauge pressure; the tool adds atmospheric pressure itself.
What this means: Air temperature
- What it is
- The temperature of the compressed air in the main, used for its density and actual volume flow.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -20 °C to 80 °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.
What this means: Equivalent length (pipe plus fittings)
- What it is
- The straight pipe length of the main plus the equivalent length of its bends, tees and valves, taken from fitting tables or the pipe supplier.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 10000 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
- Pressure drop is directly proportional to the equivalent length.
- Get it right
- Leaving out the fittings underestimates the pressure drop.
What this means: Pipe roughness
- What it is
- The absolute roughness of the pipe bore, the average height of its surface irregularities, from the pipe maker or a roughness table for the material.
- 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.
What this means: Bore chosen
- What it is
- The inside diameter of the pipe you intend to use; it is checked against both the pressure drop and the velocity limits.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Inside bore, not nominal size or outside diameter.
What this means: Largest pressure drop
- What it is
- The largest pressure loss you accept from the start to the end of this main, as a share of the line pressure.
- Units and format
- Enter a number in % of line pressure: % of line pressure. The form accepts 0.1 % of line pressure to 20 % of line pressure. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 % of line pressure as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The tool applies this share to the absolute line pressure (gauge plus atmospheric), not to the gauge pressure.
What this means: Largest air velocity
- What it is
- The highest actual air velocity you accept in the main, a limit that keeps noise, pressure loss and carry-over of water down.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0.000001 m/s to 40 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Receiver
What this means: Peak demand above compressor output (free air)
- What it is
- How much the peak demand exceeds what the compressors deliver, as free air. The receiver must supply this difference while the peak lasts.
- 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 direct proportion.
What this means: Duration of the peak
- What it is
- How long the peak demand lasts before consumption falls back within compressor output.
- Units and format
- Enter a number in min: minutes. The form accepts 0.000001 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 direct proportion.
- Get it right
- Minutes, not seconds.
What this means: Pressure when the peak starts
- What it is
- The receiver pressure when the peak begins, normally the pressure at which the compressors are still loaded or cut out.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 50 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the same basis (gauge or absolute) as the least pressure; only the difference between them counts.
What this means: Least pressure allowed
- What it is
- The lowest receiver pressure the air users can still work with at the end of the peak.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 50 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6.5 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A narrower band between the two pressures needs a larger receiver.
- Get it right
- It must be below the starting pressure.
Method and formulas
- Actual flow and density from free air at line pressure and temperature; Re and Colebrook friction factor; Δp = f (L ÷ d) ρ v² ÷ 2 (as the compressed-air pipe drop tool).
- Search the common bores 15, 20, 25, 32, 40, 50, 65, 80, 100, 125, 150, 200, 250 and 300 mm for the first that meets both limits.
- Receiver V = t × deficit × p_atm ÷ (p₁ − p₂) (as the air receiver tool).
- Example: 5 m³/min in 50 mm runs at 5.4581 m/s with 0.05902 bar drop; the smallest bore that passes is 50 mm; the receiver needs 1.351 m³.
Limitations
- Equivalent length must include fittings; long branch lines need their own check.
- Steady flow; compressor control band and dryer drops are not included.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a new compressed-air ring main or header
- Check an existing line after adding machines
- Size a receiver for a peak demand
Who it is for
- Plant engineers
- Maintenance engineers
- Energy managers
What you get
- Velocity and pressure drop in the chosen bore
- Smallest standard bore that meets the limits
- Receiver volume for the peak
Interpreting the result
- A ring main halves the flow path and lets a smaller bore meet the same drop.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3036–3041, 3024–3027. 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
Receivers and compressor control
In Compressed Air Plant by the Numbers
What it covers: Size CA-1's receiver from its cycle time, compare load and unload, modulation and variable speed control at 70 per cent load, store air for peaks, and catch a receiver valved out of service.
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Open the Compressed-Air Main Sizing (Bore, Pressure Drop, Velocity, Receiver) calculator