Compressed air is the most expensive utility in most plants per unit of energy delivered, and typically 20 to 30 percent of it escapes through leaks. The measurement that pays for itself fastest is not the one on the compressor outlet. It is the one that lets you prove where the air is going.
Why thermal mass is the usual choice
Compressed air is measured by mass or by standard volume, not by actual volume, because the same mass occupies a different volume at different pressures. A thermal mass meter reads mass flow directly, so it needs no separate pressure and temperature compensation.
That simplicity is the whole argument. One instrument instead of three, and the reading is already in the units an energy audit needs.
The alternatives all have a catch. A vortex meter works but needs 15 to 20 diameters of straight run that compressed air headers rarely have. An orifice plate costs permanent pressure loss, which on a compressed air system you pay for twice: once at the compressor and again at the dryer. An ultrasonic clamp-on unit works but struggles with the low density and thin pipe walls typical of air headers.
Where to put the meters
Four locations, in order of what they tell you.
- After the dryer, on the main header. This is the true consumption figure, after the dryer losses and before distribution. It is the number to compare against compressor output.
- Per department or per building. This is where leak hunting starts, because it turns an abstract percentage into an allocation argument.
- On the large individual users. Any machine that consumes more than about 10 percent of total capacity is worth its own meter.
- After the compressor, before the dryer. Only if you want to quantify dryer purge loss, which can be surprisingly large on heatless desiccant dryers.
The leak test that actually works
You do not need a ultrasonic leak detector to find the big ones. You need a quiet period and a baseline.
Shut down production, leave the compressors running, and watch the header meter. Whatever flow remains is leakage. On a plant with 500 kW of compressor capacity, a 25 percent leak rate means 125 kW running continuously for nothing. At 0.10 per kWh that is roughly 100,000 per year, which is usually more than the metering project costs.
Then isolate by department, one valve at a time. The meter that shows a drop is pointing at the department with the problem.
Sizing the meter
Two things matter more than the datasheet accuracy.
Turndown. Compressed air systems swing enormously between shifts. A meter with 100:1 turndown holds accuracy through that swing; a meter with 10:1 does not. This is where thermal mass earns its place.
Insertion or inline. Below DN80, use an inline body. Above DN100, an insertion probe costs a fraction of a full-bore meter and installs through a tapping rather than a cut.
What the number is good for
Once you have per-department figures, three things become possible that were not before.
You can charge compressed air back to departments, which changes behaviour faster than any memo. You can size a new compressor on measured demand rather than on the sum of nameplate ratings. And you can prove the saving after a leak repair programme, which is what justifies the next round of funding.
A note on units
Compressed air is normally reported in Nm3/h or SCFM, meaning standard conditions. Make sure the meter is configured for the standard conditions your plant uses, because a reference temperature of 0 degrees Celsius against 20 degrees Celsius is a 7 percent difference before you have measured anything.
To check line velocities on the distribution pipework, the flow rate and pipe velocity calculator handles the conversion.
Related reading
The thermal gas mass flow meter range covers DN25 to DN300 for compressed air and process gas. For the technology comparison behind the choice, see how to choose a flow meter.