It is an important topic in refrigeration and air conditioning

1. GANDHINAGAR INSTITUTE OF
TECHNOLOGY
REFRIGERATION AND AIR CONDITIONING
ACTIVE LEARNING ASSIGNMENT
DUCT DESIGN
PREPARED BY
VADI RAKSHIT D (150123119055)
MECHANICAL 6
D3

2. 2/28/2017RAC/ALA/6D32
Contents :-
Function of Duct
Classification of Duct
Economic factors influencing duct layout
Duct design methods
dynamic losses & its determination
Requirement of air distribution system

3. Function of Duct
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 The function of duct is to transmit air from the air
handling apparatus to the space to be air
conditioned.
 To fulfil the above function, the system must be
designed within the prescribed limits of available
space, friction loss, velocity, sound level and heat
losses.
 Duct system are also used to bring cooled air to
the proper distribution points where several
rooms are to be conditioned.

4. Classification of Duct
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1) According to velocity of air :
 Low velocity duct
 High velocity duct
2) According to pressure :
 Low pressure
 Medium pressure
 High pressure
3) According to type of air :
 Supply air
 Return air
 Fresh air

5. Economic factors influencing duct
layout
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1) Heat gain or loss from the duct :
 The heat gain or loss in the supply and return
duct system is considerable.
 This occurs due to the duct passing through the
unconditioned space.
 The allowance must be made for duct heat loss
or gain in load.

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2) Aspect ratio of the duct :
 Aspect ratio is defined as the ratio of longer side of
the rectangular duct to shorter side of rectangular
duct.
 Mathematically,
Aspect ratio = a/b
b
a
 This ratio is an important factor to be considered in
the initial design of duct.

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3) Duct friction :
 If duct friction rate is higher, higher will be the
size and aspect ratio.
 This will be increase the initial and running cost.
4) Type of fittings :
 The fittings can be divided into class-A and
class-B .
 It is desirable to use fittings of class-A as
fabrication cost of class-A is 1/2.5 to that of
claas-B .

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Duct design methods
 There are mainly three methods which are
commonly used for duct design.
1) Velocity reduction method
2) Equal friction loss method
3) Static regain method

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1) Velocity reduction method :
 In this method the duct designed in such a way
that the velocity decreases as flow proceeds.
 The pressure drops are calculated for this
velocities for respective branches and main
duct.
 The duct size are determined for assumed
velocities and known quantities of air to be
supplied through the respective ducts.

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2) Equal friction loss method :
 In this method, the frictional pressure drop per
unit length of duct is maintained constant
throughout the duct system.
 The procedure is to be select a suitable velocity
in the main duct from the sound level
consideration.
 Knowing the air flow rate and the velocity in the
main duct, the size and friction loss are
determined from the friction chart.

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3) Static regain method :
 For the perfect balancing of the air duct layoiut
system, the pressure at all outlets must be
made same.
 This can be done by equalizing the pressure
losses in various branches.
This is possible if the friction loss in each run is
made equal to the pressure gain due to
reduction in velocity.
Advantages :
 It is possible to design long run as well as short
run for complete regain.
 It is sufficient to design the main duct for

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Dynamic losses in duct
1 ) Pressure losses due to Enlargement
2 )Pressure loss due to contraction
3 ) Pressure loss at inlet and outlet of a duct

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1 ) Pressure losses due to Enlargement
Pr. loss = [ 1-A1/A2 ]² * [ V1/4.04 ]²

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A1 = Cross section area of the duct at section 1-1
A2 = Cross section area of the duct at section 2-2
V1 = Velocity of air at section 1-1
C1 = [ 1-A1/A2 ]² = Enlargement Coefficient
 Pr. Loss = Cr*C1* [V1/4.04]²
= Cr*C2* [V2/4.04]²

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2 )Pressure loss due to contraction
Pr. Loss = [ 1-A1/A2 ]² * [ V1/4.04 ]²
= C1 [ V1/4.04 ]²
= C2 [ V2/4.04 ]²

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Requirement of air distribution system
1) There should be enough entrainment of room
air with the supply air , so that upon reaching
the occupied zone, the air stream attains
desired temperature.
2) The temperature throughout the occupied
zone of the room should be within ± 1ºC of the
design temperature.
3) Only minor horizontal or vertical temperature
variation should be there in occupied zone.
4) Noise level should be below the objectionable
level.
5) Effect of natural convection and radiation

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Terms used in air distribution
1 ) Outlet :-
It is an opening through which air is
supplied to the conditioned space.
2 ) Intake :-
It is an opening through which air is
return from the space.
3 ) Grills :-
Grills provide decorative covering
for an outlet or inlet.

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4 ) Diffuser :-
It is an outlet grille designed to
guide the direction of the air.
5 ) Register :-
It is a grille provided with a damper
or control valve.
6 ) Drop :-
It is the vertical distance that the
lower edge of the horizontally projected air
stream drops between the outlet and the end of
its throw.

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Types of air outlets
1 ) Grill outlet :-
These outlets have adjustable
bar grills which are the most common types
with vertical and horizontal vanes.

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2 ) Slot diffuser :- It is an elongated outlet with
an aspect ratio 25 : 1 and maximum height of
7.5 cm. They are used in side walls but at a
higher height of the floor.
3 ) Ceiling outlets :- They are mounted in the
ceiling. Multi-passage round, square or
rectangular are most common type.

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4 ) Perforated ceiling :-
In this case, confined space
above the ceiling is used as supply plenum.
 The air from the plenum is supplied to the room
through small holes or slots.
 The air is supplied at the rate of 0.3 to 4.5
m³/m² of the floor area.
 They are specially suited to large zones.