BIZOMAT MAKEOS · Pneumatics
Compressed-Air Blow-Off Consumption (Nozzles, Air, Power and Cost)
How much compressed air do these blow-off nozzles or open pipes use, what share of the compressor is that, and what does it cost each year?
Consumption of open blow-off nozzles or pipe ends: jet velocity, choked flow and free-air flow through each opening (as the escaping air velocity tool), total flow for the nozzles at their duty, share of compressor capacity, compressor power and yearly energy cost from your specific power, and the saving from changing to engineered nozzles of a known flow.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Supply pressure (gauge) | bar | Gauge pressure behind the openings. |
| Outside pressure (absolute) | bar | Absolute pressure outside (atmosphere). |
| Air temperature | °C | Air temperature. |
| Ratio of specific heats | Ratio of specific heats (1.4 for air). | |
| Opening diameter | mm | Bore of each open pipe or nozzle orifice. |
| Discharge coefficient | Discharge coefficient (about 0.6–0.65 for drilled holes and pipe ends). | |
| Number of openings | Number of openings blowing. | |
| Share of time blowing | % | Share of running time the air flows. |
| Compressor specific power | kW per m³/min | Compressor input power per m³/min of free air. |
| Compressor capacity | m³/min | Air delivered by the compressors, as free air. |
| Running hours per year | h | Hours per year the line runs. |
| Electricity price | $/kWh | Electricity price. |
| Flow of an engineered nozzle at this pressure | L/min | Consumption (as free air) of the engineered nozzle you would fit instead (from its data sheet; 0 if none). |
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: Supply pressure (gauge)
- What it is
- The gauge pressure of the air just behind the open pipe ends or nozzles while they blow.
- 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 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Flow through a choked opening rises in proportion to absolute supply pressure.
- Get it right
- Gauge pressure at the opening, which is often lower than compressor pressure.
What this means: Outside pressure (absolute)
- What it is
- The absolute pressure of the surroundings the air blows into, normally the local atmosphere.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.5 bar to 2 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.01325 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Absolute pressure, unlike the supply pressure.
What this means: Air temperature
- What it is
- The temperature of the air arriving at the openings.
- 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 21 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
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. It sets the choking condition and the flow formula.
- Units and format
- Enter a plain number with no unit. The form accepts 1.1 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.
What this means: Opening diameter
- What it is
- The bore of each open pipe end or nozzle orifice the air escapes through.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Flow and cost rise with the square of the opening diameter.
What this means: Discharge coefficient
- What it is
- The ratio of actual to ideal flow through the opening, which allows for the jet contracting as it leaves; from the nozzle maker or orifice data.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.65 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Number of openings
- What it is
- The number of openings or nozzles blowing on this supply.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Total flow and cost rise in direct proportion.
What this means: Share of time blowing
- What it is
- The share of running time the air actually flows through the openings.
- 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 50% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Average flow, power and cost rise in direct proportion.
What this means: Compressor specific power
- What it is
- The compressor input power needed to deliver one cubic metre per minute of free air, from the compressor data sheet or a measured audit.
- Units and format
- Enter a number in kW per m³/min: kW per m³/min. The form accepts 0.000001 kW per m³/min to 30 kW per m³/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6.5 kW per m³/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Compressor capacity
- What it is
- The total air the compressors deliver, as free air, used to show what share of it the openings consume.
- 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.
What this means: Running hours per year
- What it is
- The number of hours a year the line runs with air supplied to the openings.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 8784 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.
What this means: Electricity price
- What it is
- What you pay for each kilowatt-hour of electricity, from your energy bill or contract.
- 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.
What this means: Flow of an engineered nozzle at this pressure
- What it is
- The free-air consumption of one engineered nozzle you would fit in place of each opening, at this supply pressure, from its data sheet.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0 L/min to 100000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 = no replacement nozzle; the saving is then the whole current cost.
Method and formulas
- Choked when p₂ ÷ p₁ < [2 ÷ (k + 1)]^(k/(k−1)); jet velocity sonic, flow ṁ = C_d A p₁ √(k ÷ R T) [2 ÷ (k + 1)]^((k+1)/(2(k−1))), as free air (as the escaping air velocity tool).
- Average flow = per opening × openings × duty; power = flow (m³/min) × specific power; cost = power × hours × price.
- Saving = (flow now − engineered flow × openings × duty) × specific power × hours × price.
- Example: each 2 mm opening blows 168.16 L/min; 6 at 50% duty use 5.0448% of the compressor and cost 3,279.1 a year.
Limitations
- Open discharge to atmosphere; long or small supply hoses lower the pressure at the opening.
- Engineered nozzle flow must be taken at the same pressure.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Cost the open pipes used for blow-off on a line
- Justify engineered air nozzles
- Check that blow-off does not overload the compressor
Who it is for
- Energy managers
- Maintenance engineers
- Production engineers
What you get
- Jet velocity and flow per opening
- Total flow, share of capacity, power and cost
- Saving with engineered nozzles
Interpreting the result
- Lowering the blow-off pressure or switching air off between parts often saves more than any nozzle change.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3037–3038, 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
Set the pressure and stop the waste
In Compressed Air Plant by the Numbers
What it covers: Price each 100 kPa of CA-1's pressure, see how leaks and open blow-offs grow with it, find the pressure lost before the air leaves the room, and fix a low-pressure complaint at its cause.
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.
Open the Compressed-Air Blow-Off Consumption (Nozzles, Air, Power and Cost) calculator