If you want to know how to calculate an air compressor about Nm3/min converted to m3/min, fuel consumption, and and air tank fill time, you’re in the right place.

How to do calculate Nm3/min to m3/min

Firstly, let’s figure out what is mean of Nm³/min. The definition of the standard state of air is: pressure 101.325Kpa and temperature is 0°C, humidity is 0%. In the standard state, the density of air is 1.293kg/m³. If use it as the unit of air capacity of air compressor and dryer, it should be written as Nm³/min. This is so-called “standard cube”. Definition of M3/min is under 20°C.

According to the “Ideal Gas State Equation”

Pv = nRT  The pressure, amount of substance, and constants are the same in both states; the only difference is the temperature (thermodynamic temperature K).

So we get V1/T1=V2/T2

Assuming V1, V2 is the standard flow, and V2, T2 is the cubic meter

Then, V₁：V₂=T₁：T₂

V₁：V₂=273：293

V₁≈1.07V₂

1Nm³/min≈1.07m³/min

Common conversion of flow units of air compressor:

  • 1CFM =0.02832 M3/min; ②1M3/min=35.311CFM; ③ CFM — ft3/min, cubic feet per minute;

How do calculate fuel consumption of an air compressor?

Example: an air compressor is 250KW, 8kgf/m3, capacity is 40m3/min, oil content is 3PPM, then what is the fuel consumption when running 1K hours?

Fuel consumption per cubic meter

3*1.2=3.6mg/m3

For produce 40m3 air, the fuel consumption

40*3.6/1000=0.144g

Running 1k hours fuel consumption

1000*60*0.144=8640g=8.6kg

Density of lubricating oil is 0.8

So fuel consumption is

8.64/0.8=10.8L

The oil consumption listed above is merely theoretical. It is higher than this (the oil filter keeps getting worse). For instance, a 40m3 air compressor will use at least 40 liters (two barrels) of oil if its calculations are based on 4,000 hours. For every maintenance, a 40m3 air compressor typically uses 10 to 12 barrels (18 liters per barrel) of oil, or around 20% of the total oil used.

How to calculate the air production on the plateau?

We know that the air gets thinner at high altitudes; then how’s the air production of an air compressor when it installed on the plateau?

According to V1/V2=R1/R2

V1=air supply in plain, V2= air supply in plateau

R1= compression ratio of the plain; R₂ = compression ratio of the plateau;

Example: a compressor 110kw, pressure 8bar, capacity is 20m3/min, calculate the air production at an altitude of 2000 meters (assume pressure will be 0.85 bar). For actual calculation, please refer to the atmospheric pressure table corresponding to the altitude.

According to the formula V1/V2=R1/R2

R₁=9/1=9

R₂=(8+0.85)/0.85=10.4

V₂=20×9/10.4=17.3m³/min

The answer is 17.3m3/min,

This means that air production will be much decreased of this air compressor when it employed in plateau regions.

How to calculate air receiver tank fill time? 

Although this calculation is full of uncertainty factors, First, the two gas states volume conversion; second, some parameters are hard to get, like temperature; and third, it is hard to control the air compressor running.

Example, a 10m3/min air compressor, pressure is 8bar, how long to fill up a 2m3 air receiver tank?

An easy way to estimate the time

p₁V₁=p₂V₂, The change in gas volume is actually the compression ratio.

so：t=V₁/（V₂/R）min

（mark 1 is the volume of air tank，2 is the volume flow of air compressor）

t=2m³/（10m³/9）min=1.8min

to fill up a 2m3 tank need 1.8 minutes.

Some more complex formulas to calculate accurate time:

Q0: Compressor volume flow without condensate m3/min

Vk－volume of air tank m3；

τ－fill up time min.

px1－Compressor suction pressure MPa；

Tx1－Compressor suction temperature K；

pk1－Gas pressure in the gas tank at the beginning of inflation MPa；

pk2－The gas pressure in the gas tank after the charging is terminated and the thermal equilibrium is reached MPa；

Tk1－Gas temperature in the gas tank at the beginning of charging K；

Tk2－The temperature of the gas in the gas tank after the charging is terminated and the thermal equilibrium is reached K；

Tk －Gas temperature in gas tank K

How to calculate the air consumption of pneumatic tools

According to Q＝0.5ψ×K1×K2×K3×Qmax  m³/min, we can calculate the pneumatic tools air consumption in the compressed air system

Qmax－the actual maximum amount of air that is needed

ψ－factor of utilization

It accounts for the fact that not all pneumatic equipment will be utilized simultaneously.  The empirical value of the coefficient is 0.95 to 0.65. Pneumatic equipment is generally less used simultaneously and has a smaller value the more of it there is. On the other hand, the higher the value. Two pieces of equipment require 0.95, four pieces require 0.9, six pieces require 0.85, eight pieces require 0.8, and ten or more require 0.65.

K1: Leakage coefficient, value range 1.2～1.5

K2: Spare factor, value range 1.2～1.6

K3: Uneven coefficient

It is intended to guarantee optimal utilization and account for the irregularities in the gas source system’s average gas consumption computation. Its value is chosen to be between 1.2 and 1.4.

How to calculate the gas volume difference when the gas volume is insufficient?

Due to the increase air consumption, devices will supply air insufficiently. To keep the rated working pressure, how much air compressor is required?

Qa: The air compressor flow rate required by the system under actual conditions

Qo: Volume flow of the original air compressor

Pa: The pressure that can be achieved under actual conditions MPa:

Po: Working pressure that can be achieved by original use MPa

ΔQ: Volume flow need to be increased (m3/min).

Example: air compressor 10m3/min, pressure at 8bar. Due to devices increase, working pressure is now is 5bar.

According to the formula, △Q=10*(0.8-0.5)/(0.5+0.1013）

≈4.99m³/min

So, we need an air capacity of 4.99m3/min; the pressure is an 8 bar air compressor