The resistance of a ventilating system is caused by:
a
the loss of energy at the point of entry of the air due to a sudden increase in air velocity from practically zero to the velocity along the duct.
b
the friction between the air and the side surface of the duct.
c
changes of cross-sectional area of the duct, where there are expansions and contractions, or changes of shape (say from square to oblong section). Expansions, contractions and changes of size or shape should be made by gradual taper sections, not abruptly, ideally 15° included in angle.
d
changes of direction, such as bends and Tee-junctions are large wasters of energy. Changes of direction should be by easy bends and well-rounded corners, not by sharp elbows, unless fitted with guide vanes.
The loss of pressure due to all of these sources, known as the system resistance, is for practical purposes proportional to the square of the velocity at the point of loss. Therefore, for a fixed system, it may be said that the pressure required to pass a given volume of air through the system will vary as the (volume flow rate)² i.e. P °C Q².
Therefore, if it is required to double the air flow through a system, the fan must be capable of providing twice the volume flow rate at four times the original pressure! AND EIGHT TIMES THE FAN MOTOR POWER!
If a specified duty requirement does not exactly match the available fan performance, it is advisable to superimpose a system resistance curve onto the fan performance curve to confirm the final anticipated duty. Data points for plotting the system resistance curve may be derived from the following formula:
P2 = P1 × (Q2/Q1)2
P1, Q1 = Specified system pressure and volume flow.
P2, Q2 = New values of pressure and volume flow to be plotted.
(Simply choose a new value for Q2 and calculate the corresponding new value for P2. Repeat the procedure until there are enough points to plot the curve - three will usually suffice).
Square law
Resistance Varies as the Square of the Velocity P °C Q²
As velocity varies directly as volume, we can say that Resistance varies as the square of the volume. The equation then becomes:
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