FAD and reference condition conversion tool

What the conversion tool calculates

Compressor brochures, flow meters, dryer data sheets and process specifications quote flow in many forms: cfm, m³/min, m³/h, L/s, FAD, ACFM, SCFM and Nm³/h. The unit is only half of the definition. A cubic metre of air at 1 bar absolute and 20 °C contains a different mass of air from a cubic metre at 1.013 bar and 0 °C, and very much less than a cubic metre at 7 bar gauge in the pipe.

The tool answers four practical questions:

• What is this flow equivalent to at another reference condition?
• Are two quoted compressor capacities actually comparable?
• What assumptions about atmospheric pressure, temperature and humidity were made?
• What actual volume flows through a pipe, dryer or filter at working pressure and temperature?

It does not change how much air a machine delivers. It only describes the same quantity of air on a different basis.

Flow bases you will meet

Because “standard” has no single meaning, the tool never assumes one silently. The user selects or enters the reference pressure, temperature and humidity for both source and target.

Method: preserve the dry-air mass flow

At ordinary compressed-air pressures, the ideal-gas law is accurate enough for conversion. For a fixed mass flow of dry air:

Q₂ = Q₁ × (P₁,dry ÷ P₂,dry) × (T₂ ÷ T₁)

• Q – volumetric flow in any consistent unit
• P,dry – absolute pressure minus water-vapour partial pressure (bar a)
• T – absolute temperature in kelvin (°C + 273.15)

If humidity is not known, the tool uses total absolute pressure and labels the result as a dry-basis approximation. Two errors cause most bad conversions: using gauge pressure instead of absolute pressure, and using °C instead of kelvin. The tool converts both internally and shows the absolute values used.

Water-vapour partial pressure is relative humidity multiplied by the saturation pressure at that temperature. At 35 °C saturation pressure is about 0.056 bar, so humid monsoon air at 35 °C and 60% RH carries roughly 0.034 bar of water vapour. On a 1 bar inlet that is a correction of about 3% – small, but enough to matter in an acceptance test.

Unit factors used

Unit factors convert units only. A flow in cfm FAD remains FAD after conversion to m³/min; it becomes Nm³/min only after the pressure and temperature correction.

Worked example 1: FAD to normal flow

A screw compressor is rated at 10 m³/min FAD, referred to 1.0 bar absolute and 20 °C, dry. What is that in Nm³/h?

1. Pressure ratio: 1.0 ÷ 1.01325 = 0.9869
2. Temperature ratio: 273.15 ÷ 293.15 = 0.9318
3. Normal flow: 10 × 0.9869 × 0.9318 = 9.20 Nm³/min
4. Hourly: 9.20 × 60 = about 552 Nm³/h

The same machine is therefore “10 m³/min” and “552 Nm³/h” at once. A process engineer who specifies 600 Nm³/h and receives a 10 m³/min FAD offer is about 8% short, even though the numbers look generous at first glance.

Worked example 2: FAD to actual flow in the pipe

The same 10 m³/min FAD is delivered into a header at 7 bar(g) and 35 °C at a site near Hyderabad, where local atmospheric pressure is around 0.95 bar because of altitude.

1. Absolute line pressure: 7 + 0.95 = 7.95 bar a
2. Pressure ratio: 1.0 ÷ 7.95 = 0.1258
3. Temperature ratio: 308.15 ÷ 293.15 = 1.0512
4. Actual flow: 10 × 0.1258 × 1.0512 = about 1.32 actual m³/min

This actual flow – not the FAD – sets pipe velocity, pressure drop and the size of the dryer and filters at working pressure. Use it in the pipe sizing calculator.

Why site conditions in India matter

Ratings are made at the maker’s reference conditions; your plant does not run at them. Three local effects are worth checking:

• Altitude. Hyderabad and much of the Deccan plateau sit at roughly 500 m, so atmospheric pressure is a few percent below sea level. A compressor’s FAD in volume terms changes little, but the mass of air it delivers falls with inlet density.
• Heat. Inlet air at 45 °C in May is less dense than air at 20 °C. Where the compressor room is poorly ventilated, inlet temperature can be well above outdoor temperature, reducing mass delivered and raising discharge temperature.
• Humidity. Monsoon air carries more water vapour, displacing dry air and loading the aftercooler and dryer with condensate.

For true comparisons of mass-based demand – process consumption, flow-meter reconciliation, dryer load – convert everything to one declared basis first.

How to interpret the result, and its limits

• A converted value is only as reliable as the stated reference conditions. Where a brochure does not state them, record that as an assumption.
• A thermal mass flow meter usually reads in normal or standard units already; check its configured reference before comparing with FAD.
• Displacement on a piston compressor plate is not FAD. Volumetric efficiency, valve condition and wear reduce delivered air, so the tool cannot convert displacement to FAD without a measured or published efficiency.
• At very high pressures, such as 35 bar (507.6 psig) borewell or booster duties, compressibility effects grow; the ideal-gas result is still a good estimate but specialist work may use a compressibility factor.
• Conversion does not test performance. To check whether a machine delivers its rated FAD, use a proper acceptance test.

How to check a quotation’s flow figure step by step

When two suppliers quote different numbers for what looks like the same duty, work through the comparison in a fixed order before deciding.

1. Identify the basis. Find whether each figure is FAD, normal, standard, actual or displacement. If a quotation simply says “cfm”, ask which.
2. Record the reference conditions. Note the stated inlet pressure, temperature and humidity, and the test standard, normally ISO 1217 for packaged compressors.
3. Check the pressure. Confirm the flow is quoted at the working pressure you need. A machine rated at 7 bar delivers less at 10 bar (145 psig), and the brochure figure for the lower pressure does not apply.
4. Convert to one basis. Restate every figure in FAD m³/min, or in Nm³/h if your process specification uses that.
5. Allow for site conditions. Consider altitude, summer inlet temperature and ventilation at your compressor room.
6. Add margin deliberately. Add allowance for leakage, future demand and dryer purge where relevant, and state it separately rather than hiding it in the conversion.

Following the same sequence every time makes comparisons transparent and makes it obvious where two offers genuinely differ.

How Reddy Compressors helps

From our office at Tarnaka, Secunderabad, we sell new and used/refurbished compressors, rent machines from 50 to 2,000 cfm at 2–30 bar, and service all major brands as an independent multi-brand specialist. Comparing offers on a common basis is a routine part of that work. When you send us competing quotations, a process specification in Nm³/h or a flow-meter reading that does not match the compressor rating, our engineers restate them on one basis, check the pressure and site conditions, and recommend a machine or rental package that genuinely meets the demand. Service teams cover Telangana and Andhra Pradesh, and we supply equipment across India.

What to send us

• The flow figure and its stated basis (FAD, Nm³/h, SCFM, actual)
• The reference pressure, temperature and humidity, if quoted
• Working pressure required at the point of use, in bar or psig
• Site location or altitude and typical summer inlet temperature
• Compressor make, model and nameplate photo, if comparing an existing machine

Frequently asked questions

What is the difference between FAD and Nm³/h?

FAD is the compressor’s delivered air restated to its inlet conditions, which data sheets commonly quote at about 1 bar absolute and 20 °C. Nm³/h refers to 1.01325 bar absolute, 0 °C and dry air. Because normal conditions are colder and slightly higher pressure, one m³ of FAD equals roughly 0.92 Nm³. Always convert before comparing a process requirement with a compressor rating.

How do I convert SCFM to FAD?

First find the reference conditions behind the SCFM figure, because publishers use different standard temperatures, pressures and humidities. Then apply the ratio of absolute pressures and the inverse ratio of absolute temperatures between the two bases. Without the stated reference, the result is only an estimate and should be labelled as one in any comparison.

Why is actual flow in the pipe so much smaller than FAD?

Compression squeezes the same mass of air into a much smaller volume. At 7 bar gauge the absolute pressure is roughly eight times atmospheric, so the actual volume is roughly one eighth of the FAD, adjusted for temperature. Pipe, dryer and filter sizing use this actual volume, which is why a pipe sized on FAD looks oversized.

Does altitude affect compressor capacity in Hyderabad?

Yes, modestly. Hyderabad sits at roughly 500 m, so atmospheric pressure and inlet air density are a few percent lower than at sea level. The volume drawn in barely changes, but the mass of air delivered falls. Hot inlet air in summer reduces it further, so good compressor-room ventilation matters as much as altitude.

Can I convert piston compressor displacement to FAD?

Not by a simple factor. Displacement is the swept volume of the cylinders, while FAD is what the machine actually delivers after volumetric losses from clearance volume, valve leakage, heating and wear. A worn machine delivers noticeably less. Use the manufacturer’s published FAD or a measured test rather than assuming a fixed percentage of displacement.

Can Reddy Compressors compare quotations from different brands?

Yes. As an independent multi-brand specialist, we can restate quotations from different makers on one flow and pressure basis, check whether stated FAD meets your demand at your site conditions, and point out missing assumptions. We also offer new, used and rental machines, so the comparison leads directly to a practical option.

Related pages

• Compressed air units and symbol dictionary
• Compressed air pipe sizing calculator
• Compressor performance acceptance worksheet
• Compressed air demand calculator
• Compressed air knowledge and calculation centre