Hi,

Nl/min means “normal liters per minute”. It is the amount of air that flows through a pipe, but calculated back to ‘normal’ conditions.

Normal conditions being:

0 degrees Celsius

1 atmosphere (1.01325 bar)

**The quick and dirty method:**

Divide the Normal liters per minute (Nl/min) by the pressure plus 1 to get the l/min through the pipe.

50 Nl/min at 7 bar = 50/8 = 6.25 l/min compressed air flow through the pipe.

**The exact method, using the ‘ideal gas law’:**

*P V =n R T*

Since the amount of molecules (n) stay the same, we can say:

V1 = ( P2 / P1 * T1 / T2 ) * V2.

V1 = normal liters per minute

V2 = liters per minute

P1 = pressure at normal conditions

P2 = actual pressure

T1 = temperature at normal conditions

T2 = actual temperature of air

The P1/P2 part compensates for the difference in pressure.

The T1/T2 part compensates for the difference in temperature.

In the calculation to normal conditions, we need to use absolute pressure (not relative) and absolute temperature (Degrees Kelvin: 0 degrees Celsius is 273.15 degrees Kelvin).

**Example 1:**

50 l/min at 7 bar, 25 degrees Celsius, to Nl/min

50 Lpm x (810 kPa/101.325 kPa) x 273.15 / (273.15 + 25) = 367 NL/min

**Example 2:**

500 Nl/min to actual compressed air flow at 7 bar and 20 degrees Celsius.

(7 bar relative = 8 bar absolute = 8 * 101.325 = 810 kPa)

500 Nl/min = (810 kPa/101.325 kPa) x 273.15/(273.15 + 20) * V2

We can calculate that V2 = 67.2 l/min

Answer: there flows 67.2 liters of compressed air thruogh the pipe, at 7 bar and 20 degrees Celsius.

**Remember:**

liters/min is what you will measure using a flow meter. There are flow meters that can also show Nl/min (it wil need a built-in pressure and temp sensor also for this).

Nl/min is what is usually indicated on compressors and also on pneumatic devices.

I hope all of this made sense, it can become quite messy when typing math in plain text.

**If you want to learn more about this, and other compressed air basics, I highly recommend that you join the (free) “Industrial Compressed Air Systems” 7-day email course.**

Please let me know if you have more questions.

Dear Danial,

You have taken 8 bar as actual pressure while the actual pressure at site should have been 1 bar for Pakistan before operating the compressor. 8 bar can only come if the compressor is operational. We are just calculating the effect of conditions here.

Actual conditions (40C, 1 bar)and Normal conditions (0 C, 1.013 bar).

How is that possible a compressor is upgraded in actual conditions than normal conditions???

Qazi

Let assume my nozzle in 3.8 mm and the air velocity is 4 m/s. Can we directly calculate the air volume as pi(1/4) ( 0.0038) square * 4 m/s?

Hello, I have one question.

If we have some pipe and inside passes air with pressure 5MPa (for example). If we measured the flow air inside (pressure before flow meter = pressure after flow meter) , this is L/min (volume per time).

But if we measured the flow when the air go to the atmosphere (come from compressor, passes through flow meter and go to atmosphere) this is normal liters per minute (in this case NL/min approximately equal L/min)

Is this true?

Best Regards,

Krassimir