9. DUCT DESIGN.pdf

DESIGN OF AN AIR
CONDITIONING DUCTS
Md. Osman Ali
Lecturer
Dept. of ME
DUET, Gazipur
ME-3521
MECHANICALEQUIPMENT

IMPORTANCE OF DUCT DESIGN
ME-3501
MECHANICAL EQUIPMENT
Dhaka University of Engineering & Technology, Gazipur-1707
In an AHU, air is transmitted through various ducts and other
components with the help of fans. Since the fan motor consumes a
large amount of power, and the duct system occupies considerable
building space, the design of air transmission system is an important
step in the complete design of air conditioning systems. In the end the
success of any air conditioning system depends on the design of
individual components as well as a good matching between them
under all conditions. In order to design the system for transmission of
air, it is important to understand the fundamentals of fluid (air) flow
through ducts.
The purpose of the duct design is to select suitable dimensions of duct
for each run and then to select a fan, which can provide the required
supply airflow rate to each conditioned zone.

DUCT
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DUCT: It is simply a path to distribute and manage the air flow in the
required zones/areas.
 The channels for supplying and returning air are known as ducts.
 Duct shape may be rectangular, square and circular.
 Duct materials can be of the following types-
1. G.I Sheet
2. Aluminium
3. Stainless Steel
 Aspect ratio: It the ratio of duct width to height.

CHIEF REQUIREMENTS
ME-3501
The chief requirements of an air conditioning duct system are:
1. It should convey specified rates of air flow to prescribed locations
2. It should be economical in combined initial cost, fan operating cost and
cost of building space
3. It should not transmit or generate objectionable noise
Factors affecting duct design:
1. Equipment losses
2. Air velocity
3. Duct Material
4. Duct size and shape
5. Duct length
6. Noise level
7. Appearance

GENERAL RULES FOR DUCT DESIGN
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1. Air should be conveyed as directly as possible to save space, power and
material
2. Sudden changes in directions should be avoided. When not possible to
avoid sudden changes, turning vanes should be used to reduce pressure loss
3. Diverging sections should be gradual. Angle of divergence ≤ 20o
4. Aspect ratio should be as close to 1.0 as possible. Normally, it should not
exceed 4
5. Air velocities should be within permissible limits to reduce noise and
vibration
6. Duct material should be as smooth as possible to reduce frictional losses

CLASSIFICATION OF DUCT SYSTEMS
ME-3501
Ducts are classified based on the load on duct due to air pressure and
turbulence. The classification varies from application to application, such as
for residences, commercial systems, industrial systems etc. For example, one
such classification is given below:
Low pressure systems: Velocity ≤ 10 m/s, static pressure ≤ 5 cm H2O (g)
Medium pressure systems: Velocity ≤ 10 m/s, static pressure ≤ 15 cm
H2O (g)
High pressure systems: Velocity > 10 m/s, static pressure 15<Ps ≤ 25 cm
H2O (g)
High velocities in the ducts results in:
1. Smaller ducts and hence, lower initial cost and lower space requirement
2. Higher pressure drop and hence larger fan power consumption
3. Increased noise and hence a need for noise attenuation

CLASSIFICATION OF DUCT SYSTEMS
ME-3501
Recommended air velocities depend mainly on the application and the
noise criteria.
Typical recommended velocities are:
 Residences: 3 m/s to 5 m/s
 Theatres: 4 to 6.5 m/s
 Restaurants: 7.5 m/s to 10 m/s
If nothing is specified, then a velocity of 5 to 8 m/s is used for main
ducts and a velocity of 4 to 6 m/s is used for the branches.
The allowable air velocities can be as high as 30 m/s in ships and
aircrafts to reduce the space requirement.

FAN
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To overcome the fluid friction and the resulting head, a fan is required
in air conditioning systems. When a fan is introduced into the duct
through which air is flowing, then the static and total pressures at the
section where the fan is located rise. This rise is called as Fan Total
Pressure (FTP). Then the required power input to the fan is given by:
FTP should be such that it overcomes the pressure drop of air as it
flows through the duct and the air finally enters the conditioned space
with sufficient momentum so that a good air distribution can be
obtained in the conditioned space.

PRESSURE LOSS IN DUCTS
ME-3501
As air flows through a duct its total pressure drops in the direction of
flow. The pressure drop is due to:
1. Fluid friction
2. Momentum change due to change of direction and/or velocity
 The pressure drop due to friction is known as frictional pressure
drop or friction loss, Δpf.
 The pressure drop due to momentum change is known as
momentum pressure drop or dynamic loss, Δpd.
 Thus the total pressure drop Δpt is given by

EVALUATION OF PRESSURE DROP IN DUCTS
ME-3501

DYNAMIC LOSSES IN DUCTS
ME-3501
Dynamic pressure loss takes place whenever there is a change in either the velocity or
direction of airflow due to the use of a variety of bends and fittings in air conditioning
ducts. Some of the commonly used fittings are: enlargements, contractions,
elbows, branches, dampers etc. The dynamic loss is proportional to square of
velocity. Hence these are expressed as:

DUCT DESIGN METHODS
ME-3501
 Index run:
The run with the highest pressure drop is called as the index run.
 The following methods are most commonly used for
simpler lay-outs such as the one shown in Fig.
1. Velocity method
2. Equal Friction Method
3. Static Regain method
Equal friction method is simple and
is most widely used conventional
method. This method usually yields
a better design than the velocity
method.

EQUAL FRICTION METHOD
ME-3501
In this method the frictional pressure drop per unit length in the main
and branch ducts (Δpf/L) are kept same, i.e.
The equivalent diameter of the main duct (A) is obtained from the
selected value of (Δpf/L) and the airflow rate.

ME-3501

RECTANGULAR DUCTS
Even though circular ducts require the least material for a given flow rate and
allowable pressure drop, rectangular ducts are generally preferred in practice as
they fit easily into the building construction thus occupying less space, and they
are also easy to fabricate. The ratio of the two sides ‘a’ and ‘b’ of the
rectangle (a/b) is called as aspect ratio of the duct. Since square ducts
with aspect ratio 1.0 come close in performance to a circular duct, it is preferable
to use an aspect ratio as close to unity as possible for best performance.
A rectangular duct is said to be equivalent to a circular duct, if the volumetric flow rate .Qair
and frictional pressure drop per unit length (ΔPf/L) are same for both.
Equating these two parameters for a rectangular duct and an equivalent circular duct, it can be
shown that the equivalent diameter is given by:
ME-3501
a
b

MATHEMATICAL PROBLEM
ME-3501
Find the dimensions of a rectangular duct of aspect ratio (1:2) when 0.2
m^3/s of air flows through it. The allowable frictional pressure drop is 3
Pa/m.
Solution:

ME-3501
The following figure shows a typical duct layout. Design the duct system
using a) Velocity method, and b) Equal friction method. Take the velocity of air in the
main duct (A) as 8 m/s for both the methods. Assume a dynamic loss coefficient
of 0.3 for up stream to downstream and 0.8 for upstream to branch and
for the elbow. The dynamic loss coefficients for the outlets may be taken as
1.0. Find the FTP required for each case and the amount of dampering required

ME-3501

9. DUCT DESIGN.pdf

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