Compressed air is the most expensive utility in most factories, and almost nobody meters it. It arrives through a pipe like water, it feels free at the point of use, and the bill is buried somewhere in the site electricity account. It is not free — as a working figure, compressed air costs about eight times what the same work would cost delivered electrically.
Where the energy goes
Roughly 90% of the electrical energy going into a compressor leaves as heat. Of what remains, more is lost to pressure drop, leaks and the inefficiency of the tool itself, so somewhere between 10 and 15% of the input reaches the work. That is not a badly-run plant — that is a well-run one, because the losses are mostly thermodynamic rather than avoidable.
Leaks are the biggest single item
An untreated plant typically leaks 20–30% of everything it produces, and a single 3 mm hole at 7 bar costs several thousand pounds a year in electricity. Leaks are also the cheapest thing to fix: an ultrasonic detector, a walk round during a quiet shift, and a tag list. The reason they persist is that nobody owns them — a leak is not anybody's fault and it does not stop production.
Pressure is the cheapest lever nobody pulls
Every 1 bar of system pressure costs roughly 7% of compressor power. Most plants run higher than they need because one machine somewhere wants 7 bar and it is easier to raise the whole system than to fix that one drop. Fitting a booster or a larger line for the single demanding machine and dropping the rest of the site by a bar is often the best-paying change available. Higher pressure also increases leak flow, so the saving compounds.
Intercooling, and why big machines are multi-stage
Compression heats air, and hot air is harder to compress. Splitting the job into stages with a cooler between them moves the process toward the isothermal ideal and saves roughly 10–15% of shaft power for a typical 8:1 ratio. The other reason is temperature: a single 8:1 stage discharges well above 200 °C, which is hard on mineral lubricants and on everything downstream. Check both on the Compressed Air tool.
The heat is worth recovering
Since 90% of the input becomes heat, and it comes off at a useful 60–90 °C, recovering it for space heating or process hot water is often the single largest saving available on the whole system. It is also unglamorous and frequently ignored.
Do not use air where something else will do
Blowing, cooling, drying, moving light parts, stirring — these are the jobs where compressed air gets used because there is a pipe nearby, and where a fan or blower does the same work for a fraction of the energy. Open blowing through an unregulated pipe is the worst case of all, and replacing it with engineered nozzles typically cuts that consumption by half.
What to measure
Specific power — kW per m³/min of free air delivered — is the single number that lets you compare compressors and track degradation. Then pressure dew point, to confirm the dryer is doing its job, and system pressure at the furthest point of use rather than at the compressor. Everything else follows from those three.