BIZOMAT MAKEOS · Pneumatics
Compressed Air Pipe Pressure Drop
What pressure drop does this compressed air flow cause in this pipe?
Pressure drop in a compressed air line from free-air flow, line pressure, pipe size and length, using the Darcy–Weisbach method with air density at line pressure and a Colebrook friction factor.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Air flow (free air delivery) | m³/min | Flow of free air (at inlet conditions) through the pipe. |
| Pipe inside diameter | mm | Inside diameter of the pipe. |
| Equivalent pipe length (including fittings) | m | Straight length plus equivalent length of fittings and valves. |
| Line pressure (gauge) | bar | Pressure at the start of the pipe. |
| Air temperature | °C | Temperature of the compressed air. |
| Pipe roughness | mm | Roughness: about 0.045 mm for steel, 0.0015 mm for aluminium or plastic air pipe. |
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: Air flow (free air delivery)
- What it is
- The air flow through the pipe expressed as free air delivery, the volume at intake conditions, as compressor and tool ratings usually state it.
- Units and format
- Enter a number in m³/min: cubic metres per minute. The form accepts 0.000001 m³/min to 100000 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
- Pressure drop rises roughly with the square of flow, so doubling the flow far more than doubles the drop.
- Get it right
- Use free air flow, not the compressed volume; the tool converts it to the actual volume at line pressure.
What this means: Pipe inside diameter
- What it is
- The inside diameter (bore) of the air pipe.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Pressure drop falls very steeply with a larger bore, so a size up makes a large difference.
- Get it right
- Use the real bore, not the nominal pipe size or outside diameter.
What this means: Equivalent pipe length (including fittings)
- What it is
- The length of straight pipe plus an allowance for fittings, valves and bends expressed as extra length of pipe.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 1000000 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 rises in direct proportion to equivalent length.
- Get it right
- Add the equivalent length of each fitting from the fitting maker’s data; leaving fittings out understates the drop.
What this means: Line pressure (gauge)
- What it is
- The gauge pressure in the line at the start of the pipe run, as read on a pressure gauge.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 100 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
- Higher line pressure compresses the air, lowering its velocity and the pressure drop.
- Get it right
- Enter gauge pressure; the tool adds atmospheric pressure to get absolute pressure.
What this means: Air temperature
- What it is
- The temperature of the compressed air in the pipe, used with the pressure to work out its density.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -40 °C to 200 °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
- Enter degrees Celsius; the tool converts to kelvin.
What this means: Pipe roughness
- What it is
- The average height of the roughness on the pipe’s inside wall, which affects the friction factor in turbulent flow.
- 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 the pipe maker or a fluid-flow reference for the pipe material and condition.
Method and formulas
- Line density ρ = 1.204 × (p_abs ÷ 1.01325) × (293.15 ÷ T); actual flow = free air flow × (1.01325 ÷ p_abs) × (T ÷ 293.15).
- Re = ρ v d ÷ μ (μ ≈ 1.81 × 10⁻⁵ Pa·s); Colebrook friction factor.
- Δp = f (L ÷ d) ρ v² ÷ 2.
- Example: 5 m³/min at 7 bar g through 100 m of 40 mm pipe drops about 0.18438 bar.
Limitations
- Valid while the drop is a small share of the line pressure (under about 10%); otherwise calculate in sections.
- Equivalent lengths of fittings must be estimated from fittings data.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size compressed air mains and drops
- Check pressure at a remote user
- Find savings from a larger main
Who it is for
- Maintenance engineers
- Energy and utilities managers
- Design engineers
What you get
- Air velocity and Reynolds number
- Friction factor
- Pressure drop and percentage of line pressure
Interpreting the result
- A common design aim is under 0.1–0.3 bar drop from compressor to the furthest user.
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
Size the mains and lay them out
In Compressed Air Plant by the Numbers
What it covers: Size CA-1's main and branches from actual volume and a pressure budget, compare dead-end and ring layouts, choose suitable pipe materials and avoid a branch too small for the line it feeds.
Method explained here · BIZOMAT original book
Supply clean, dry air at the right pressure
What it covers: Trace pressure drop from compressor to machine, fix the restrictions before raising pressure, size a receiver for peak demand, and dry, filter and drain the air so valves and cylinders keep working.
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