Where the difference goes
In a piston compressor, filling is affected by factors such as re-expansion of air in the clearance volume, valve flow, intake heating and leakage. Operating conditions change as discharge pressure rises. A single fixed percentage ratio between intake and delivered volume cannot therefore be assigned to every machine.
What to ask for in a quotation
Look for FAD delivery at the pressure you actually need. In addition to instantaneous delivery, check the permitted duty cycle. An air receiver helps with peaks, but cannot replace insufficient long-term compressor capacity under continuous demand.
What to check in practice
- Request outlet flow and pressure stated together.
- Check whether the figure covers the complete supplied package.
- Check tool consumption while running, not just the shift average.
Example scenario
An example machine has an intake capacity of 400 L/min and documented delivery of 280 L/min at 7 bar(g). It cannot continuously support a consumer requiring 320 L/min at the same reference conditions. A larger receiver only extends the time before pressure falls.
What to watch out for
- Calculating the required reserve from intake capacity.
- Confusing volumetric “capacity” in L/min with motor power in kW.
- Assuming identical delivery at every pressure setting.
Can delivery be checked by filling a receiver?
Approximately, yes, if volume, pressures, time and temperatures are known and there are no other consumers. Heating during filling distorts the result, however. For comparing quotations, documented measurement under specified conditions is preferable.