Compressor Flow and Capacity Calculator (Nm³/h, FAD)

The difference between Nm³/h and m³/h

Air is compressible, so the amount of air in a cubic metre depends on pressure and temperature. Flows are therefore quoted at a reference condition. A normal cubic metre (Nm³) is the volume at 0 °C and 1.01325 bar absolute. The ideal gas relation converts a flow at actual conditions to the reference condition:

Qn = Q × (p / pn) × (Tn / T)

Pressures are absolute and temperatures in kelvin. For example, a compressor drawing in 10 m³/min (600 m³/h) at 20 °C and 1 bar absolute delivers 600 × 0.9196 = 552 Nm³/h.

Free air delivery (FAD)

The flow in compressor catalogues is usually free air delivery: the volume of the delivered air referred to inlet conditions. The actual volume in the pressurised line is far smaller, roughly one eighth at 8 bar absolute.

Pressure ratio and volumetric efficiency

The pressure ratio is the absolute discharge pressure over the absolute suction pressure. In a multistage machine the ratio per stage is the overall ratio taken to the root of the number of stages. In a reciprocating compressor the air trapped in the clearance re-expands and displaces fresh charge: ηv = 1 − c × ((p2 / p1)^(1/n) − 1). With 5 percent clearance and a ratio of 8 the efficiency is about 80 percent.

Receiver fill time

Raising a receiver of volume V from pressure pb to ps takes V × (ps − pb) / patm of free air. Taking a 500 litre receiver from 0 to 8 bar needs 4 cubic metres of air, which a 1 m³/min compressor delivers in 4 minutes.

Limits

The calculation is for dry air as an ideal gas. Humidity, high pressure and real machine losses have to be assessed separately. The definition of a standard cubic metre varies between sources, so always confirm the reference.