An open blow gun or blow-off nozzle uses, as a rough guide, around 150 to 250 l/min while it is blowing, at about 6 bar. That is often more than the rest of the machine tool put together. To cut it, work in this order: blowing time, pressure at the nozzle, the nozzle itself and leaks, before you touch the compressor.
Why blow-off weighs so much
A machine tool uses air in bursts: a tool change, a clamp, a door. A nozzle flows continuously through an orifice. Its flow depends on the orifice diameter and the upstream pressure. As our page on the compressor for CNC machining shows, a machining centre uses about 50 to 150 l/min on average for all its functions. One nozzle left open all the time can therefore double a machine's consumption.
Step-by-step calculation
Take a blow-off nozzle on a CNC lathe, rated at 200 l/min by the nozzle maker, blowing for half of each cycle, 8 hours a day.
- Blowing time per day: 8 h × 60 min × 0.5 = 240 minutes.
- Volume per day: 200 × 240 = 48,000 L, or 48 m³ of air.
- Three machines fitted the same way: 3 × 48 = 144 m³ per day.
- As an average flow: 3 × 200 × 0.5 = 300 l/min. That is more than a quarter of the output of a K 75 (1.1 m³/min at 8 bar), just for blowing.
To put a figure on that volume, our article on the cost of a m³ of compressed air and the compressor electricity cost calculator link it to your bill. If the nozzle data sheet gives the flow in CFM or Nm³/h, the flow and pressure converter turns it into l/min.
Five ways to use less air
- Blow only when needed. A solenoid valve driven by the program, with a timer, blows at the right moment and only for as long as it is useful. Halving blowing time halves the 48 m³ in the example.
- Lower the pressure at the nozzle. A dedicated regulator on the blow-off line is enough. Flow through an orifice is roughly proportional to the absolute upstream pressure. Going from 6 to 4 bar, that is from 7 to 5 bar absolute, cuts the flow by about 30% (5 ÷ 7 = 0.71). Check that the chips still clear properly.
- Pick the right nozzle. Air amplifying nozzles draw in ambient air around the jet: the blast stays strong with less compressed air. A flat nozzle forms an air knife better suited to clearing a surface. Each nozzle's flow is given by its maker.
- Shut off when idle. A nozzle left open during a break, a setup or overnight blows for nothing. A valve that closes the line when the machine stops solves it.
- Track down leaks. Kinked blow gun hoses, worn quick couplings: the leak cost calculator puts a number on them and our article on compressed air leaks shows how to find them.
For some long or very fine chips, extraction or a conveyor does a better job than air anyway.
What about the compressor?
Redo the calculation after these changes. Choose the compressor on the reduced demand, not the old one.
- Blow-off remains significant and varies a lot: a variable speed compressor follows it instead of switching between load and idle. See variable speed or fixed speed.
- Shop with a few machines: the K 75, 4-in-1, 1.1 m³/min at 8 bar with built-in dryer and filtration.
- Heavy blow-off on several machines: the K 110, 1.7 m³/min at 8 bar, or the S 110 at the same flow if air treatment is already in place.
Do not raise the compressor pressure to blow harder: every extra bar costs electricity and makes the whole network leak more.
Blow-off air quality
Blow-off air hits the part directly. Wet air leaves rust marks on freshly machined steel, and oily air stains parts before inspection or painting. A refrigerated dryer and filtration after the compressor cover most cases. See water in compressed air.
To size your installation after these savings, use the configurator or send us your readings with a quote request.
For your trade: Compressor for CNC machining and machining centres.