BIZOMAT MAKEOS · Pneumatics
Air Compressor Power
What shaft power does it take to compress this air flow to this pressure?
Theoretical isothermal and adiabatic power to compress a flow of free air in one or more intercooled stages, and the shaft power with an overall efficiency.
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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 air measured at inlet conditions (free air delivery). |
| Discharge pressure (gauge) | bar | Discharge pressure at the compressor outlet. |
| Inlet pressure (absolute) | bar | Absolute inlet pressure (1.013 bar at sea level; lower at altitude or with inlet filter loss). |
| Compression stages (intercooled) | stages | Number of stages with perfect intercooling and equal pressure ratios. |
| Ratio of specific heats | 1.4 for air. | |
| Overall efficiency (adiabatic to shaft) | % | Ratio of theoretical adiabatic power to shaft power, from the compressor maker (often 65–85%). |
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 volume of air the compressor delivers each minute, expressed at its inlet conditions, also called free air delivery.
- 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 10 m³/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- All powers rise in direct proportion.
- Get it right
- Use free air delivery, not the compressed volume at outlet pressure.
What this means: Discharge pressure (gauge)
- What it is
- The pressure at the compressor outlet, measured as gauge pressure.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 1000 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
- A higher discharge pressure raises power.
- Get it right
- Enter gauge pressure from the outlet gauge; the tool adds standard atmospheric pressure itself.
What this means: Inlet pressure (absolute)
- What it is
- The absolute pressure of the air entering the compressor: local atmospheric pressure, reduced by altitude and inlet filter loss.
- 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 1.013 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower inlet pressure raises the pressure ratio for the same discharge.
- Get it right
- Enter absolute pressure, not gauge. Note that the discharge pressure is converted using standard atmosphere.
What this means: Compression stages (intercooled)
- What it is
- The number of compression stages with intercooling between them, assumed to share the pressure ratio equally.
- Units and format
- Enter a number in stages: stages. The form accepts 1 stages to 6 stages. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 stages as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More intercooled stages lower the adiabatic power.
- Get it right
- Count stages in series, not compressors in parallel.
What this means: Ratio of specific heats
- What it is
- The ratio of the gas’s specific heat at constant pressure to that at constant volume, a property of the gas being compressed.
- Units and format
- Enter a plain number with no unit. The form accepts 1.01 to 1.67. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from gas property data for the gas compressed; it is not an efficiency.
What this means: Overall efficiency (adiabatic to shaft)
- What it is
- The theoretical adiabatic power divided by the actual shaft power, covering compressor losses, from the compressor maker’s data.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% 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
- A lower efficiency raises shaft power and specific power.
- Get it right
- Type the percentage itself; motor losses are not included in shaft power.
Method and formulas
- Isothermal: P = p₁ Q ln(p₂ ÷ p₁).
- Adiabatic with N stages: P = N × k/(k − 1) × p₁ Q [(p₂ ÷ p₁)^((k − 1)/(N k)) − 1].
- Shaft power = adiabatic ÷ efficiency; specific power = shaft kW ÷ flow (m³/min).
- Example: 10 m³/min to 7 bar g in one stage needs about 63.475 kW at the shaft.
Limitations
- Theoretical power with an overall efficiency; real compressors also lose energy at part load and in dryers.
- Moisture in the air is ignored.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate compressor motor size for a demand
- Compare single and two-stage compression
- Estimate specific power for energy studies
Who it is for
- Maintenance engineers
- Energy and utilities managers
- Design engineers
What you get
- Pressure ratio
- Isothermal power
- Adiabatic power
- Shaft power and specific power
Interpreting the result
- Compare specific power with maker data (kW per m³/min) to judge an existing compressor.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3032–3041. 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
Assist gases: oxygen, nitrogen and air
In Laser Processing by the Numbers
What it covers: What oxygen, nitrogen and compressed air each do in a laser cut, the heat from burning iron, nozzle gas flow, and supplying nitrogen from cylinders, bulk liquid or a generator.
Method explained here · BIZOMAT original book
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In Process Chillers by the Numbers
What it covers: Ideal and real compression from NIST data, isentropic efficiency, why R-134a heats less than air, motor losses, discharge temperature, and a starved evaporator.
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