People are normally interested
in the amount of air that is moved. This is most commonly expressed
as a Volumetric Flow Rate. Typical units of measurement are:
-
m3/h metres cubed per hour
cfm cubic feet per minute
l/s litres per second
Resistance to airflow
Air needs to be pushed or pulled to move it from
one point to another. When moving any object, force is required
to overcome the resistance to movement. Moving air requires a force
to overcome the resistance through a duct or tube. The resistance
to flow is due to the viscosity of air and the friction of each
molecule passing over one another. Typical Units of measurement:
-
mBar millibar
Pa Pascals
InWg Inches Water Gauge
Fan performance
Fan performance is presented on a graph
called a fan curve. Volume flow is on the X axis and pressure is
on the Y axis .It shows how much volume of air is moved against
resistance to flow, or pressure development of the fan. As you increase
the resistance to flow, the volume output of the fan decreases.
System resistance
The system be it a tube, black box or a
room has a resistance to airflow, along it or through it. This resistance
to airflow is expressed as a pressure. The value of this resistance
to flow increases with increase in volume. The system resistance
curve is a square law. Pressure changes in relation to air volume
to the power of two.
P is proportional to V²
For example twice the volume is 2² which = four time the pressure.
Half the volume 0.5² = 0.25 of the pressure.
This can be plotted on a fan curve and where the system resistance
crosses the fan curve will be the duty point of the system with
that fan fitted.
Noise
A fan is a good generator of noise! We hear noise
by sensing pressure variation on our ear drums. When a fan operates
the noise generated from various parts of the fan is transmitted
by air molecules bouncing against each other and spreading out as
small pressure waves.
Sound power and sound pressure
- Sound can be expressed in two ways. Sound power, how many watts
of energy is converted into sound, or sound pressure where the pressure
of the sound waves are measured in Pascal's.
Decibels dB - Both sound power
and sound pressure are expressed in units of measurement called
decibels. The measurements are on a logarithmic scale as the ear
is sensitive to noise in a logarithmic fashion.
‘A’ Weighting -
Noise figures are commonly expressed as A weighted. This is a method
to provide a figure which gives a sound level which is perceived
by the human ear.
