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FFFF A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M DDDD E P A R TM E NE P A R TM E NE P A R TM E NE P A R TM E N O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
LECTURE-11
FAN AND DUCT SYSTEM-2
Fr iction Factor for Ducts:
The value of friction factor, for smooth ducts may be obtained by using the
following expressions:
1. For laminar flow, the friction factor
� � ����
…………………………………15
2. For turbulent flow, the friction factor
� � �.�
���.��…………………………………16
In case of rough pipes or ducts the friction factor depends upon the roughness
factor (e/D), where (e) is the absolute roughness of the surface and (D) is the
diameter of duct. The friction factor for rough pipes or ducts is:
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FFFF A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M DDDD E PA R TM E NE PA R TM E NE PA R TM E NE PA R TM E N O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
� �
.����������� ��
�…………………………………17
Fr iction Chart for Circular Ducts:
From equations 11 and 13 the frictional pressure loss for circular ducts
�� � ������
���
� ����
�� (N/m2)……………………..…………….18
The frictional pressure loss for circular ducts (mm of water ) for various
velocities (m/s) and duct diameter (m) may be obtained directly from the friction
chart. In these charts, the vertical ordinates represent volume flow rate of air and the
.horizontal ordinates represent frictional pressure loss in mm of water per unit length
of the circular duct (Pf/L).
Dynamic Losses in Ducts:
The dynamic losses are caused due to the change in direction or magnitude of
velocity of the fluid in the duct. The change in direction of velocity occurs at bends
and elbows. The dynamic pressure loss,
�� � ��� � �� ��.��
��
…………………………(19)
Where (C) is the dynamic loss coefficient.
The dynamic pressure loss expressed in terms of an additional equivalent
length (Le) of the duct is given by:
�� � ���
�� � ���
� �
�.���
� in mm of water……………(20)
FFFF A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M DDDD E PA R TM E NE PA R TM E NE PA R TM E NE PA R TM E N O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
From equations 19 and 20, the relationship between the dynamic loss
coefficient (C) and equivalent additional length is
� � ���
�! �� � � �
……………………………….…….…(21)
Duct Design:
The object of duct design is to determine the dimensions of all ducts in the
given system. The ducts should carry the necessary volume of conditioned air from
the fan outlet to the conditioned space with minimum frictional and dynamic losses.
The area changes must be gradual where possible and limited to not more than
20o for diverging area and 60o for convergent area. For rectangular ducts the
aspect ratio of 4 and less is desirable but it should not be greater than 8 in any
case.
The velocities in the ducts must be high enough to reduce the size of the ducts
but it should be low enough to reduce the noise and pressure losses to economize
power requirement as table (1)
Designation
Recommended Velocities in m/s
Residences School, theater and
public building Industrial building
Outdoor air intakes 2.5 2.5 2.5
Filters 1.25 1.5 1.75
Heating coil 2.25 2.5 3
Air washers 2.5 2.5 2.5
Fan Outlet 5-8 6.6-10 8-12
Main Duct 3.3-5 5-6.6 5.8-9.1
Branch Duct 3 3-4.5 4-5
Branch Risers 2.5 3-3.5 4
FFFF A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M DDDD E PA R TM E NE PA R TM E NE PA R TM E NE PA R TM E N O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
Methods for Determination of Duct Size:
The following three methods for determination of duct size are important as:
1. Velocity Reduction Method
The various steps involved in this method are:
i. Select suitable velocities in the main and branch ducts
ii. Find the diameters of main and branch ducts from airflow rates and velocities
for circular ducts. For rectangular ducts, find the cross-sectional area from flow
rate and velocity, and then by fixing the aspect ratio, find the two sides of the
rectangular duct.
iii. From the velocities and duct dimensions obtained in the previous step, find the
frictional pressure drop for main and branch ducts using friction chart or
equation.
iv. From the duct layout, dimensions and airflow rates, find the dynamic pressure
losses for all the bends and fittings.
v. Select a fan that can provide sufficient Flow for the index run
vi. Balancing dampers have to be installed in each run. The damper in the index
run is left completely open, while the other dampers are throttled to reduce the
flow rate to the required design values.
The Velocity Method Proper air flow velocities for the application
considering the environment are selected. Sizes of ducts are then given by the
continuity equation like:
A = q / v …………………………..(22)
A = duct area (m2)
q = air flow rate (m3/s)
v= air speed (m/s)
FFFF A N A N D D U CT SY STE M A N A N D D U CT SY STE M A N A N D D U CT SY STE M A N A N D D U CT SY STE M
A proper velocity will depend on the application and the environment. The
table below indicate commonly used velocity
2. Equal fr iction method
In this method the frictional pressure drop per unit length in the main
and branch ducts (∆pf/L) are kept same
Then the stepwise procedure for designing the duct system is as follows:
i. Select a suitable frictional pressure drop per unit length
the combined initial and running costs are minimized.
ii. Then the equivalent diameter of the main duct (A) is obtained from the
selected value of (∆
air flow rate in the main duct is equal to the sum total of airflow rates to
all the conditioned zones.
A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M DDDD E P A R TM E NE P A R TM E NE P A R TM E NE P A R TM E N O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
A proper velocity will depend on the application and the environment. The
table below indicate commonly used velocity limits:
In this method the frictional pressure drop per unit length in the main
/L ) are kept same,
……………………………(23)
Then the stepwise procedure for designing the duct system is as follows:
Select a suitable frictional pressure drop per unit length (∆
the combined initial and running costs are minimized.
Then the equivalent diameter of the main duct (A) is obtained from the
(∆pf/L ) and the air flow rate. As shown in Figure (1),
the main duct is equal to the sum total of airflow rates to
all the conditioned zones.
O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
A proper velocity will depend on the application and the environment. The
In this method the frictional pressure drop per unit length in the main
……………………………(23)
Then the stepwise procedure for designing the duct system is as follows:
(∆pf/L) so that
Then the equivalent diameter of the main duct (A) is obtained from the
wn in Figure (1),
the main duct is equal to the sum total of airflow rates to
FFFF A N A N D D U CT SY STE M A N A N D D U CT SY STE M A N A N D D U CT SY STE M A N A N D D U CT SY STE M
FFFF A N A N D D U CT SY STE M A N A N D D U CT SY STE M A N A N D D U CT SY STE M A N A N D D U CT SY STE M
ix. Next the dynamic pressure losses in each duct run are obtained based on
the type of bends or fittings used in that run.
x. Next the total pressure drop in each duct
the frictional and dynamic losses of that run:
xi. Next the fan is selected to suit the index run with the highest pressure
loss. Dampers are installed in all the duct runs to balance the total
pressure loss.
The major loss, or friction loss, in a circular duct in galvanized steel with
turbulent flow can for imperial units be expressed
∆p = (0.109136 q
∆p = friction (head or pressure loss) (inches water gauge/100 ft of duct)
de = equivalent duct diameter (inches)
q = air volume flow - (cfm
A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M A N A N D D U CT SYSTE M DDDD E P A R TM E NE P A R TM E NE P A R TM E NE P A R TM E N O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
……………………………(2
Next the dynamic pressure losses in each duct run are obtained based on
the type of bends or fittings used in that run.
Next the total pressure drop in each duct run is obtained by summing up
the frictional and dynamic losses of that run:
……………………………(2
Next the fan is selected to suit the index run with the highest pressure
loss. Dampers are installed in all the duct runs to balance the total
The major loss, or friction loss, in a circular duct in galvanized steel with
turbulent flow can for imperial units be expressed
p = (0.109136 q1.9) / de5.02………………….
friction (head or pressure loss) (inches water gauge/100 ft of duct)
equivalent duct diameter (inches)
(cfm - cubic feet per minute)
O F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R INO F M E CH A N ICA L E N G IN E E R IN
……………………………(27)
Next the dynamic pressure losses in each duct run are obtained based on
run is obtained by summing up
……………………………(28)
Next the fan is selected to suit the index run with the highest pressure
loss. Dampers are installed in all the duct runs to balance the total
The major loss, or friction loss, in a circular duct in galvanized steel with
…………. )29(
friction (head or pressure loss) (inches water gauge/100 ft of duct)
Recommended