Short answer: Five datasheet numbers decide the running cost — FAD at a stated pressure, specific power, ambient basis, motor class and turndown — and a 4% FAD gap at the same 75 kW is worth about 1,440 USD a year (per 6,000 h/yr · 0.08 USD/kWh · 75 kW machine rated 13.0 m³/min at 7 bar).
By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.
Most datasheets we are asked to compare are not wrong. They are incomplete in the same three places, and the missing lines are exactly the ones that decide what the machine costs to run. In the field we have measured units that matched their nameplate power and missed their quoted flow by nearly a tenth, and the buyer had no way to see it because the sheet never said at which pressure the flow was measured. This is the reading method we use before quoting a replacement, written so a procurement team can apply it to any brand.
Which five numbers on a datasheet actually decide running cost?
FAD at a stated pressure, specific power, the ambient basis of the rating, motor efficiency class and turndown ratio. Those five lines explain most of the 8–10% spread between competing quotes at the same kW. Flow without a pressure is not a specification; power without a flow is not an efficiency; and an ambient limit without a derating curve is a claim rather than a number.
| Datasheet line | What it should state | Why it decides cost |
|---|---|---|
| FAD | m³/min or CFM at a named pressure | flow falls as pressure rises on the same machine |
| Specific power | kW per m³/min, or kWh/m³ | the only fair comparison between two brands |
| Ambient basis | rated at 25°C, 40°C or 46°C, plus derating | a 45°C plant room derates a 25°C rating |
| Motor | IE3, IE4 or IE5, and whether PM | part-load efficiency differs sharply |
| Turndown | minimum stable flow, or VSD range | decides how much time the unit spends unloaded |
What does FAD at a stated pressure tell you that “CFM” alone does not?
It tells you what the machine delivers under the condition you will actually run it. A screw compressor quoted at 13.0 m³/min at 7 bar will not deliver 13.0 m³/min at 10 bar — the same airend moves less air against a higher pressure, and the motor reaches its limit sooner. When a sheet prints a single CFM figure with no pressure beside it, the number is unverifiable, and comparing it with a properly stated figure is comparing two different measurements.
How much does a 4% FAD difference cost over six years?
More than the price difference between most competing quotes. Two 75 kW machines, both honest about their ratings:
- Machine A: 13.0 m³/min at 7 bar → specific power 75 ÷ 13.0 = 5.77 kW per m³/min.
- Machine B: 12.5 m³/min at 7 bar → 75 ÷ 12.5 = 6.00 kW per m³/min.
- Same duty of 12.5 m³/min: A draws 12.5 × 5.77 = 72.1 kW; B draws 75 kW.
- Annual difference: 2.9 kW × 6,000 h = 17,400 kWh → about 1,392 USD/yr at 0.08 USD/kWh.
- Six years: roughly 8,350 USD (per 6,000 h/yr · 0.08 USD/kWh · 75 kW · 7 bar).
That is the arithmetic behind the 1,440 USD figure in the short answer, rounded for a 3 kW gap. It is also why we publish specific power rather than a headline flow number.
Why does the same 75 kW rating deliver different flow on two datasheets?
Because the rating is a motor limit, not an output. Airend displacement, rotor profile, drive arrangement, cooler capacity and the pressure the manufacturer chose to rate at all move the flow for the same shaft power. Two 75 kW machines can differ by 8–10% in delivered flow at the same pressure, and both sheets can be truthful. The one that states the pressure and the specific power is giving you the means to compare; the one that states neither is asking you to trust a number.
What does the ambient rating hide about a 45°C plant room?
The basis. A unit rated at 25°C ambient loses capacity and motor margin as intake temperature climbs, and the loss is not linear — the derating curve, not the headline, is the real specification. Our PMV range is rated for full output at 50°C ambient, which matters in tropical and desert plants; if a competing sheet quotes capacity without an ambient basis, ask for the curve at your summer intake temperature before you compare the two.
How do you check specific power without a test report?
Run a pump-up test with a known receiver and a stopwatch. Close the outlet, note the pressure, and time the rise across a known band:
- Receiver volume: 1,500 L = 1.5 m³.
- Pressure rise: 6 to 8 bar = 2 bar.
- Time: 60 seconds.
- Free air delivered: 1.5 × 2 ÷ 1 ≈ 3 m³ in 60 s → 3 m³/min (per 1.5 m³ receiver · 2 bar rise · 60 s).
Repeat it at two pressure bands and you have a flow figure you measured yourself. Pair it with a kW reading from the controller or a clamp meter and you have specific power — no laboratory, no vendor data.
What did a measured flow figure change in a real plant?
Our Vietnamese case is a good example of why the numbers need measuring rather than reading. The plant — an iron fabrication works in Ho Chi Minh City, 15,000 m², more than 200 employees, 16-hour days — was running an ageing fixed-speed unit with pressure swinging ±0.5 bar and air electricity above 3,200 USD a month. We audited the installation before quoting, and the replacement 75 kW PMV unit was commissioned at 12 m³/min at 8 bar into a 1,500 L receiver: 75 ÷ 12 = 6.25 kW per m³/min, which is the figure we now use as the comparison point for that plant’s second line. The full write-up is in the Vietnam PMV case study.
How does a low-pressure datasheet differ for process industries?
The pressure basis changes everything about how the numbers should be read. A low-pressure air compressor for the textile / ceramic / glass / cement industry is specified at 3–5 bar, and its flow at 3 bar cannot be compared with an 8 bar machine’s flow — the compression work is different, so the same motor produces far more volume at the lower pressure. On our low-pressure series, 15–300 HP at 3–5 bar, the datasheet states flow at each pressure point rather than one headline figure, because a textile plant sizing for weaving and a cement plant sizing for conveying will read different rows.
Which datasheet claims are worth asking for in writing?
Four, in this order: the FAD test standard and the pressure it was measured at, the specific power at your duty point, the derating curve against ambient temperature, and the sound pressure level with the measurement distance. ISO 1217 is the acceptance-test reference for displacement compressors, and a supplier who can name the standard and the test pressure is usually also the one whose numbers survive a site check.
How do you compare three quotes on one page?
Rebuild all three into the same five columns, and convert anything that is not in those units. Our CFM to kW and FAD worksheet does that conversion step by step, and the standards guide covers which certificate a quote should carry for which market. A quote that cannot fill the specific-power column is not a cheaper quote; it is an unfinished one.
Key takeaways
- Flow without a pressure is not a specification — insist on FAD at the pressure you will run.
- A 4% FAD gap at 75 kW is worth roughly 1,400 USD a year (per 6,000 h/yr · 0.08 USD/kWh), and about 8,350 USD over six years.
- Ask for the derating curve, not the ambient headline: a 25°C rating behaves differently in a 45°C plant room.
- A pump-up test gives you flow in ten minutes — 1.5 m³ receiver, 2 bar rise, 60 seconds, about 3 m³/min.
- Low-pressure sheets must be read at 3–5 bar, never against an 8 bar machine’s flow.
Frequently asked questions
What is a good specific power for a 75 kW screw compressor?
Typically around 5.8–6.3 kW per m³/min at 7 bar for a modern PMV unit, which is roughly 0.097–0.105 kWh/m³. Anything above 6.5 kW per m³/min at that size deserves a question about the airend and the rating pressure.
Why do two datasheets quote different FAD for the same kW?
Because the motor rating is a limit, not an output. Airend size, rotor profile, drive type, cooler capacity and the pressure the maker chose to rate at all change the flow that the same shaft power produces.
Does a VSD compressor’s datasheet flow apply at part load?
No — it applies at the rated point. A VSD unit’s advantage is the turndown range below that point, so ask for the minimum stable flow and the part-load specific power curve rather than the single rated figure.
How accurate is a pump-up test for checking FAD?
Within roughly 5–8% on a tight system, which is enough to catch a 4% datasheet gap but not enough to replace a calibrated flow meter. Close every outlet, stabilise temperature first, and repeat the test twice.
Can I compare a 3 bar low-pressure unit with an 8 bar machine on flow alone?
No. A low-pressure air compressor for the textile / ceramic / glass / cement industry delivers its volume at 3–5 bar, and the compression work per cubic metre is lower, so the flow figures are not interchangeable with an 8 bar machine’s.
Sources and standards
- ISO 1217:2009 — displacement compressor acceptance testing; the standard behind any FAD figure worth quoting.
- ISO 11011:2013 — Compressed air — Energy efficiency — Assessment; the method behind specific power and annual cost comparisons.
- ISO 8573-1:2010 — air purity classes, which belong on the same sheet as flow and pressure once the air reaches a process.
- ISO 9001:2015 and CE marking — held by AirSpace Machinery Co., Ltd.; certificate scans on our downloads page.
- Our published references — the PMV screw series, the low-pressure series and the Vietnam 75 kW PMV case study.
About the author
Johnny Wayne (Wei Zhuang) is General Manager of AirSpace Machinery Co., Ltd., the Shanghai manufacturer behind ChinaCompressor.org. The company builds oil-injected and Class 0 oil-free screw compressors from 2 to 540 HP (1.5–400 kW), an oil-free scroll range of 2–51 HP, a diesel portable range of 50–300 HP and the low-pressure series at 15–300 HP for 3–5 bar duty, from a 4,000 m² production facility with 20 years of engineering history. Products carry CE marking and ISO 9001:2015 certification, and we sell through distributors rather than around them.
Send two competing datasheets to sales@chinacompressor.org and we will put them into the same five columns, including the lines the other sheet left out.
⚡ Is Your Factory Bleeding Cash?
Most fixed-speed compressors waste $3,600/year in "Unload Tax." Test your waste level in 30 seconds.
RUN THE ROI TEST →Ready to See How Much Your Facility Can Save?
Fill out the form below for a Free Energy Assessment.
