Compressibility

All gases are compressible but this can generally be neglected for fan systems where the pressure above atmospheric is less than 2.5 kPa.

It may be noted for example that water is about 100 times more elastic than steel and about 0.012 times as elastic as air. Com­pressibility is very temperature dependant and slightly pressure dependant. Any values used must related to the operating con­ditions. Classically, compressibility is expressed in terms of the bulk modulus defined by the relationship:

1

Compressibility = — Equ 2.4

. Ap

K =p— =

Ar

Ap

Av

Where:

K =

Bulk modulus of the gas (N/m2)

P =

Pressure (Pa)

P =

Density of the gas (kg/m3)

V =

Volume of gas (m3)

A =

Change of magnitude

The change in volume due to a change in pressure can be cal­culated directly from the definition:

VAp

IT

Equ 2.6

Where the minus sign indicates that the volume decreases with increasing pressure.

Posted in Fans Ventilation A Practical Guide


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