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Formulae 1 to 4 can be applied to any fan provided the diameter does not change. If the diameter does not change the fans must be
geometrically similar. The units of density, fan rotational speed and diameter are not important as long as both are the same.
In formulae 5 to 8 the units shown in the nomenclature must be used to satisfy the formulae.

Nomenclature for symbols used in this page :
qv = volume flow of air, m3/sec
n = rotational speed of fan
d = diameter of fan
p = pressure developed by the fan
ρ = density of air, kg/m3
PR = power absorbed by the fan, kW
B = barometric pressure (millibars)
T = absolute temperature, K (K = °C + 273)
ptF = fan total pressure, Pa
psF = fan static pressure, Pa
pdF = fan dynamic/velocity pressure, Pa
pd = system dynamic/velocity pressure, Pa
V = velocity of air, m/sec
PWL = sound power level
Formulae 1 to 4 can be applied to any fan provided the diameter does not change. If the diameter does not change the fans must be
geometrically similar. The units of density, fan rotational speed and diameter are not important as long as both are the same.
In formulae 5 to 8 the units shown in the nomenclature must be used to satisfy the formulae.

Symbol Definition
Area m2 square metre
Density ρ density of air kg/m2
Dimensions mm millimetre
cm centimetre
m metre
�� square
Ø diameter
Electricity Amps amperes
D.I.P. dust and ignition proof motor
Ex d flame-proof motor
Ex e explosion-proof motor
Ex n non-sparking motor
kW kilowatt
ph phase
Ø phase
Power PR impeller absorbed power, kW
Pressure kg/m2 kilogram per square metre
Pa pascal
kPa kilopascals
pdF fan dynamic/velocity pressure
psF fan static pressure
ptF fan total pressure
Sound dB decibel
dB(A) decibel, A weighted
dBW sound power level, Lw
Hz frequency, Hertz
Lp sound pressure level, dB
Lw sound power level, dB
NC noise criteria
NR noise rating
PNC Preferred Noise Criterion
pW picowatt
μpa micropascal
Speed of rotation r/s revolutions per second
rev/sec revolutions per second
Temperature °C degrees celcius
K Kelvin, absolute temp.
Time sec or s seconds
Velocity m/s metres per second
Volumetric flow L/s litres per second
m3/s cubic metres per second
qv volume flow
Volume m3 cubic metres
Weights wt weight
kg kilogram