Free air delivery is the number that matters
Flow is quoted as free air delivery: the volume of air the machine delivers, measured at inlet conditions, per unit time. FAD is not the same as displacement. Displacement describes how much volume the rotors sweep; FAD describes what comes out of the outlet after internal losses have been taken. A datasheet that quotes only displacement is telling you about the machine, not about your plant.
Pressure is a working point, not a maximum
Flow and pressure are linked. A machine rated at a given flow at 7 bar will deliver less flow at 10 bar, because compressing to a higher pressure takes more work from the same motor. When you compare two machines, compare them at the pressure you will actually run, not at whichever pressure flatters the brochure.
| Datasheet item | What to check |
|---|---|
| Flow (m3/min) | Quoted as FAD, and at which pressure |
| Working pressure (bar) | The pressure at which that flow is delivered - not the relief valve setting |
| Motor power (kW) | Shaft power available; compare specific power instead of kW alone |
| Specific power (kW per m3/min) | The only fair efficiency comparison between machines of different size |
| Ambient limit | Usually stated as a maximum; capacity falls as ambient rises |
| Cooling | Air or water cooled, and the water requirement if applicable |
| Noise | Measured at what distance and in what acoustic condition |
| Connection | Outlet size, and whether a receiver and dryer are included |
Why ambient changes the number
Compressor ratings assume an ambient temperature, commonly around 45 C for industrial screw packages. A machine installed in a room that reaches 50 C will not deliver its datasheet flow, and the dryer downstream will be working outside its design point as well. The datasheet is a promise about a condition, so the condition belongs in the comparison.
Power: kW, not horsepower
Horsepower figures are often rounded and sometimes refer to a motor frame rather than delivered shaft power. Work in kW, and divide by flow at your working pressure to get specific power. That single ratio makes two machines of different brands, sizes and ages comparable.
What a good enquiry contains
Required flow in m3/min at the working pressure, the pressure itself, voltage and frequency, ambient conditions, air quality requirement, and whether the machine will run continuously or intermittently. With those, a quotation becomes a specification rather than a guess.
Send us your working pressure, required flow and site conditions and we will propose a configuration. Contact OPPAIR.