# Calculation to convert Nl/min to l/min

Questions & AnswersCalculation to convert Nl/min to l/min
Cas Staff asked 4 years ago

I need to convert from Nl/min to l/min and I have read your explanation in “Converting Nl/min to l/min“, but I don’t understand one thing.

First let me make an introduction and then I will do the question.

The question is about the way to give the flow in the compressors. I think that when you give the flow, you have to say the flow and also the pressure (correct me please if I am wrong). For example: 10 l/min at 7 bar, 10 l/min at 5 bar… If you say 10 Nl/min it means 10 l/min in normal conditions, so it means 10 l/min at 1 bar (absolute). Is it like this?

After that explanation I don’t understand when you say: “50 Nl/min at 7 bar”. Is it ok? I think you mean that you are going to convert from 50 Nl/min to l/min at 7 bar, don’t you?

I am asking that because in our factory our compressors’ flow is given at 7 bar but the air consumption of the ejectors we want to set up is given in Nl/s. We want to know if we have enough air to install them or not, so we have to do the conversion in order to be able to compare them.

Regards,

Cas Staff answered 4 years ago

Hi,

What you say is absolutely right. If you say “10 Nl/min it means 10 l/min in normal conditions, so it means 10 l/min at 1 bar (absolute).”

And when I say “50 Nl/min at 7 bar”, what I mean to say is this:
50 Nl/min is the flow/capacity. (50 l/min at normal conditions). But the system is 7 bar. So yes, the l/min at 7 bar will be about 7 times lower.

About your situation: are you sure the compressors capacity is given in l/s at 7 bar (not Nl/sec). Very often, even compressor manufacturers use the wrong terminology.

If the word “FAD” (free air delivery) is used, that’s (almost) the same as Nl/min.

For example: Capacity FAD: 99 l/s — means: 99 l/s is sucked in and compressed to (for example 7 bar). So the output flow will be about 7 times smaller.

To be sure, compare your compressor to these examples:

– 15 kW air compressor running at 7.5 bar, gives 155 m3/h FAD
– 22 kW air compressor running at 7.5 bar, gives 217 m3/h FAD

Hope this explain it more ðŸ™‚ It can be really confusing sometimes, even for me writing this right now ðŸ™‚

I highly recommend to check out my free course “Compressed Air Basics” – all this is and more is explained!

Cas

Hi Cas,

Thank you very much for your answer. It is everything very clear.

You are right, my compressor capacity is given in free air delivery. It is a KAESER CSG-2 of 75 KW and the capacity is 11.76 m3/min. Comparing to your references about the m3/h FAD of 15 KW and 22KW compressors, it is clear that in my case the flow is given in m3/min Free Air Delivery. You say that m3/min FAD is practically the same as Nm3/min, so my compressor capacity finally is 11.76 Nm3/min.

I have also been looking for data of this compressor on the internet and in the following link it says that the flow is free air delivery (2nd line): http://en.kaeser.co.th/Current_Affairs/press-A-CSG-2.asp

I was confused because the manual says that the flow is given at 7 bar, but now I suppose that it means the working pressure of the compressor to give that flow is 7 bar. In a similar way as you said, it will be:

– 75kW air compressor running at 7 bar, gives 11.76 m3/min FAD.

Thanks because you have solved my problem about the calculation. I didnâ€™t imagine that the mistake was in the capacity data, I thought that the mistake was in my way to do the conversion. Now my problem is that we donâ€™t have enough air in the factory, but this is another issue.

Regards,

IÃ±igo

Good evening, the calculation is not right.
You have to consider a ratio of absolute pressures.
Nm3 is 1,01325 barA (101325 PaA)
7 bar(g) means in fact 8,01325 barA (7 x 1 + 1,01325)
And so 1 m3 @ 7 bar is equal to 7,90 Nm3

1 x (1,01325 + 7 ) / 1,01325

We can also consider the Absolute temperature in calculation.
P0*V0/T0 = P1*V1/T1
T = t +273,15 in Â°K
t is in Â°C

Hope it can help

Regards