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Laws of compressed air

Basic law, air flow principles, pressure drop, flow rate performance.

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Basic law

One of the most significant gas laws - Marriott and Gay-Lussac law states:

P x V = a x T
P : absolute pressure (Pa)
V : volume (ft3 m3)
T: absolute temperature (K)
a : constant

This relation is used within the compressor : constant air volume is pumped from the compressor chamber, and the volume decreases. This decrease causes an increase in both the pressure and the temperature of the air.


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Air flow calculation

Flow is equivalent to the quantity of compressed air conveyed in a given section per unit of time.

Q = A1 x V1 = A2 x V2
Q: flow (cfm)
A: flow section (ft²)
V: speed (ft/min)

The international system of flow is cubic meters / second (m3/s), but we generally use l/s, m3/h or cfm. This varies according to several factors, and, in particular, to the air pressure and the length/ID of the pipe, which conveys the compressed air.

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Pressure drop calculation

When compressed air flows in a straight pipe, the flow can depend on two factors: the laminar rate or the rate of turbulence, according to the value of the Reynolds Number " R ".
The Reynolds number is a dimensionless ratio between inertia and friction in a flowing medium.


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Compressed air in the system is determined by the rate of turbulence

Pressure drop in a compressed air system is a critical factor. Pressure drop is caused by friction of the compressed air flowing against the inside of the pipe and through valves, tees, elbows and other components that make up a complete compressed air pipe system.
Pressure drop can be affected by pipe size, type of pipes used, the number and type of valves, couplings, and bends in the system.

Turbulence caused by friction, reduces the volume of compressed air conveyed through the pipe. Furthermore, the surface of the internal pipe walls becomes irregular.
These factors, combined with flow, create pressure drop - resulting from friction caused by the dynamics of airflow within the pipe. Pressure drop values are shown as dP and are stated in psi or bar.

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Flow rate performance for a defined pressure drop

Values for one metre air pipe system.

To convert Nm3/h to CFM, please use the coefficient of 0.588
To convert CFM to Nm3/h, please use the coefficient of 1.699

Nomogram with values in CFM and PSI\ft


Example: Diameter 25 mm, flow 100 Nm3/h
Pressure : 8 bar D p/m = 0.003
Hence, for a 30m air pipe system
D p = 0.003 x 30 = 0.09 bar and conversely