Basics of Duct Design
Basics of Duct Design
Basics of Duct Design
Duct Sizing Methods Hence the pressure loss through the branch path is:
There is no single duct sizing method that will inherently give 1/2p x ku x Vu2 = 1/2p x kB x VB2
the best duct design. Whilst most people are aware of the (where p = density of air = 1.2 kg/m2)
constant pressure gradient, constant velocity and static
Designers should also be aware that for a number of fittings,
regain methods, there is a further method known as the
notably bends, the published data must be corrected for the
balanced pressure drop method and yet another more recent
angle of turn of the bend and also there is a Reynolds
method known as the T-Method Optimisation.
Number correction. This can give a significant increase in
The balanced pressure drop method is described in the AIRAH pressure drop at higher velocities (Refer clause 6-30 of DA3)
Application Manual DA3 and involves sizing the duct layout
Fitting Interaction
using the constant pressure gradient or static regain method,
Another important consideration with fitting losses is that
determining the index run (the path with the greatest
fittings in close proximity can have a higher (and in some
pressure drop) and then reducing the duct sizes in all other
cases lower) combined pressure loss. Whilst it is reasonable
paths (without exceeding velocity limits) such that the out of
to say that fittings should not be located close together,
balance pressure drop in each path is minimised. The
particularly in an S configuration, in practice this often
objective of this method is to achieve a more nearly balanced
cannot be avoided, eg when ducts have to drop under beams.
system thereby reducing noise and making the system more
Clauses 6-40 to 6-120 of DA3 discuss the effects of fitting
easily balanced when commissioned.
interaction and also the effects of poorly configured fan
The T-Method Optimisation optimises the duct design on the layouts.
basis of system capital cost and the present worth of energy.
Duct Attenuation
It is described in detail in the ASHRAE Fundamentals
Published data on lined duct attenuation is generally very
Handbook. The author is not aware of this method being
sparse. Much of the data is for only a limited set of sizes
used in Australia.
making interpolation for intermediate sizes extremely
Fitting Losses difficult. Duct attenuation is not linear, ie if you keep
increasing the length of duct, the attenuation does not keep
Whilst fitting losses can be allowed for by allowing an
increasing in proportion. This is because of self-generated
equivalent length, more reliable and comprehensive data is
noise in the duct. Suppliers attempt to account for this by
available in the form of loss coefficients (k). Care must be
publishing attenuation for different lengths of duct. Thus we
taken when using this data however because different texts
get the anomalous situation where two lengths of two metre
base loss coefficients on different velocities in the fitting eg.
duct either side of a transition gives (apparently), a higher
the branch path pressure loss for a divided flow fitting can be
attenuation to that of a four meter length of straight duct.
expressed as a k factor based on the branch duct velocity or
based on the main or upstream duct velocity. These different To determine the attenuation accurately, account must
loss coefficients are related by: therefore be taken of self-generated noise in the duct. The
same applies to fittings. The noise level in a duct system
ku = kB.(VB /Vu)2
does not progressively decrease away from the fan until it
Where: reaches zero. There is a lower limit caused by self-generated
noise.
ku = the loss coefficient based on the upstream velocity
Self-Generated Noise
kB = the loss coefficient based on the branch velocity
Self-generated noise is generally proportional to velocity to
Vu = the upstream duct velocity the sixth power (pressure is proportional to the square of
VB = the branch duct velocity velocity), the duct cross sectional area, a characteristic
dimension in the case of fittings and the frequency. The