Air Compressor FAD to Standard Converter
This world-class engineering tool converts FAD (Free Air Delivery) to Standard flow rates like SCFM, Nm³/hr, and Sm³/min. It applies rigorous thermodynamic corrections for inlet pressure, temperature, and relative humidity, adhering to ISO 1217 Annex C and ASME PTC 9 standards.
Conversion Results
Psychrometric Volume Change
Calculating...
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The Ultimate Guide to Compressor Flow Physics
Introduction: Why Flow Conversion is Critical
In the world of pneumatics and industrial gases, confusion over flow rate definitions is the number one cause of system under-performance. An engineer might specify a compressor for 1000 CFM, but if they mean Standard CFM (SCFM) and the vendor delivers 1000 CFM (FAD) at a hot, humid site like Mumbai or Houston, the actual mass of air delivered could be 15% less than required.
This guide explains the physics behind Free Air Delivery (FAD), Standard Flow, and the rigorous math used in this calculator (based on ISO 1217 and ASME standards).
1. Understanding FAD (Free Air Delivery)
Free Air Delivery (FAD) is the volume of air drawn into the compressor from the atmosphere, measured at the intake conditions. It is not the volume of compressed air coming out of the pipe; it is the volume of "loose" air the machine swallows.
Because air is compressible, 1 cubic meter of air at 5°C is much denser (contains more oxygen and nitrogen molecules) than 1 cubic meter of air at 45°C. Therefore, a compressor rated for 500 CFM FAD will deliver a different mass of air in winter than in summer.
The Common Mistake
Never assume "CFM" means "SCFM". If a catalog says "500 CFM," it almost always means FAD. If you need 500 SCFM for your tools, and you buy a 500 CFM (FAD) compressor for a hot summer operation (40°C), you might only get 430 SCFM of usable air. Always convert FAD to Standard Flow to compare apples to apples.
2. The "Standard" Confusion: Definitions of Normal vs. Standard
There is no single global definition of "Standard Air." This is where most errors occur. This calculator allows you to select your specific reference, but you must know what your downstream equipment requires.
| Term | Unit | Pressure | Temperature | Humidity (RH) | Typical Use |
|---|---|---|---|---|---|
| Normal | Nm³/hr | 1.01325 bar (1 atm) | 0°C (273.15 K) | 0% (Dry) | Europe, Science, ISO standards (DIN 1343) |
| Standard (ASME) | SCFM | 14.7 psi | 68°F (20°C) | 0% (Dry) | USA, CAGI, Pneumatic Tools |
| Standard (Gas) | SCFM | 14.696 psi | 60°F (15.6°C) | 0% (Dry) | Oil & Gas, API 618 |
| ISO 1217 (FAD) | m³/min | 1 bar | 20°C | 0% (Dry) | Compressor Rating Plate |
3. The Physics: General Gas Law & Humidity
To convert between these states, we use the General Gas Law, but with a critical modification for humidity. Air is a mixture of dry gases (Nitrogen, Oxygen, Argon) and Water Vapor. The "Standard" flow is almost always defined as Dry Air. Therefore, we must subtract the volume occupied by water vapor from the FAD volume.
The Governing Equation
The relationship used in this tool is derived from the Ideal Gas Law ($PV=nRT$) combined with Dalton's Law of Partial Pressures:
$$Q_{std} = Q_{FAD} \times \left( \frac{P_{inlet} - P_{vapor}}{P_{std}} \right) \times \left( \frac{T_{std}}{T_{inlet}} \right)$$
Where:
- $Q_{std}$ = Standard Flow Rate (Dry)
- $Q_{FAD}$ = Free Air Delivery (Wet intake)
- $P_{inlet}$ = Absolute Inlet Pressure
- $P_{vapor}$ = Partial Pressure of Water Vapor ($RH \times P_{sat}$)
- $T_{std}, T_{inlet}$ = Absolute Temperatures (Kelvin)
4. Deep Dive: The Impact of Humidity ($P_{vapor}$)
Water vapor takes up space. In a hot, humid environment (e.g., 35°C, 90% RH), water vapor exerts a partial pressure of nearly 5.6 kPa (0.056 bar). If your atmospheric pressure is 1.0 bar, only 0.944 bar is actual air. The rest is water.
When this air is compressed and cooled in the aftercooler, most of that water condenses into liquid and is drained away. That volume is lost. The compressor did the work to suck it in and compress it, but it doesn't contribute to the flow downstream. This is why humidity correction is vital for accurate sizing.
This calculator uses the Tetens Equation to calculate saturation vapor pressure ($P_{sat}$) with high precision:
$$P_{sat} = 610.78 \times \exp\left(\frac{17.27 \times T}{T + 237.3}\right) \text{ (Pascals)}$$
5. Altitude Derating (The Pressure Effect)
Altitude kills compressor performance. As you go higher, atmospheric pressure ($P_{inlet}$) drops. At 2000 meters, atmospheric pressure is only about 0.8 bar.
Looking at the governing equation above, if $P_{inlet}$ drops from 1.0 to 0.8, your mass flow drops by 20% directly. A compressor that delivers 500 SCFM at sea level will only deliver ~400 SCFM at 2000 meters. This tool captures that effect automatically when you input the correct absolute inlet pressure.
6. ISO 1217 Annex C: The Testing Standard
Best Practice: Ask for Annex C Data
When buying a compressor, always ask for the datasheet according to ISO 1217 Annex C. This standard strictly defines how flow is measured (at the discharge terminal) and corrected back to intake conditions. It accounts for losses inside the package (intake filters, internal piping, cooling fans). The tolerance for flow is typically +/- 4% or +/- 5% depending on the machine size.
7. Practical Sizing Strategy
When designing an instrument air system, follow this workflow:
- Sum the Consumers: Add up the SCFM/Nm³/hr requirements of all pneumatic tools and instruments.
- Determine Worst-Case Ambient: Identify the maximum summer temperature and humidity (e.g., 45°C, 80% RH) and the minimum pressure (if at altitude).
- Back-Calculate to FAD: Use this tool to convert your required SCFM into the FAD required at those worst-case conditions.
- Add Margin: Add 10-15% for leaks, wear, and future expansion.
- Select Compressor: Pick a machine with a rated FAD higher than your calculated value.
Energy Implications
While high inlet temperature reduces mass flow, it also reduces the power required to compress the air (less mass = less work). However, the specific energy (kW/100 cfm) usually worsens slightly. Conversely, cold intake air increases mass flow and increases motor power draw. In extremely cold climates, motors must be sized to handle this "dense air" overload.