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LECTURE-10

FAN AND DUCT SYSTEM

Introduction: -

The conditioned air (cooled or heated) from the air conditioning equipment

must be properly distributed to rooms or spaces to be conditioned in order to provide

comfort conditions. When the conditioned air cannot be supplied directly from the air

conditioning equipment to the spaces to be conditioned, then the ducts are installed. It

may be noted that the duct system for proper distribution of conditioned air cost

nearly 20 to 30 percent of the total cost of equipments required and power required

by fans forms the substation part of the running cost.

Classification of duct: -

The duct may be classified as follows:

1. Supply air duct

2. Return air duct

3. Fresh air duct

4. Low pressure duct: when the static pressure in the duct is less than 50 mm of

the water gage.

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5. Medium pressure duct: when the static pressure in the duct is up to 150 mm

of the water gage.

6. High pressure duct: when the static pressure in the duct is from 150 to 250

mm of the water gage.

7. Low velocity duct: when the velocity of air in the duct is up to 600 m/min.

8. High velocity duct: when the velocity of air in the duct is more than 600

m/min.

Duct Material:

The ducts are usually made from galvanized iron sheet metal, aluminum sheet

metal or black sheet. The most commonly used duct material in air conditioning

systems is galvanized sheet metal, because the zinc coating of this metal prevents

rusting and avoids the cost of painting. The sheet thickness of galvanized iron (G.I.)

duct varies from 26 gauge (0.55 mm) to 16 gauge (1.6 mm).

Now a day the use of non-metal ducts has increased. The resin bonded glass

fiber ducts are used because they are quite strong and easy to manufacture according

to the desired shape and size.

Duct Shape:

The duct may be made in circular, rectangular or square shapes. From an

economical point of view, the circular ducts are performed because the circular shape

can carry more air in less space. This means that less duct material, less duct surface,

less duct surface friction and less insulation is needed.

Pressure in Duct:

The schematic diagram for in air conditioning system figure (1). The flow of air

within a duct system is produced by the pressure difference existing between the

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different locations. The greater the pressure difference, the faster the air will flow.

The following are the three types of pressure involved in a duct syst

1. Static pressure(Ps):

The static or stationary pressure always exists in a duct system. Since it is not

dependent upon the air movement.

2. Dynamic or velocity pressure (Pv):

The dynamic or velocity pressure is equal to the drop in static pressure

necessary to produce a given velocity of flow.

3. Total pressure(Pt):

The total pressure is the algebraic sum of the static pressure and dynamic

pressure.

�� � �� �Note:

Static and total pressure may either be positive or negative.

pressure is always positive.

Figure (1) air conditioning system

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different locations. The greater the pressure difference, the faster the air will flow.

The following are the three types of pressure involved in a duct system:

The static or stationary pressure always exists in a duct system. Since it is not

dependent upon the air movement.

Dynamic or velocity pressure (Pv):

The dynamic or velocity pressure is equal to the drop in static pressure

necessary to produce a given velocity of flow.

The total pressure is the algebraic sum of the static pressure and dynamic

��……………………………….(1)

Static and total pressure may either be positive or negative.

Figure (1) air conditioning system

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

different locations. The greater the pressure difference, the faster the air will flow.

em:

The static or stationary pressure always exists in a duct system. Since it is not

The dynamic or velocity pressure is equal to the drop in static pressure

The total pressure is the algebraic sum of the static pressure and dynamic

The dynamic

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Continuity equation for ducts:

Consider the flow of air through a duct between the two section

shown in figure 2.

Let Q1=discharge of air through section 1

m1=mass flow rate of air through section 1

A1=cross section area of duct at section 1

V1=velocity of air at section 1

P1=pressure of air at section 1

Q2, m2, A2, V2 and P2 corresponding values at section 2.

Mass flow rate of air through

�� � �� � �����…………………………………….(2)

Where:

� � ����.

Mass flow rate of air through

� � � � � …………………………………….(3)

Where:

� � � .

Figure (2) flow of air

2

3

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Continuity equation for ducts:

Consider the flow of air through a duct between the two section 1

discharge of air through section 1

mass flow rate of air through section 1

cross section area of duct at section 1

velocity of air at section 1

pressure of air at section 1

corresponding values at section 2.

Mass flow rate of air through section 1

…………………………………….(2)

Mass flow rate of air through section 2

…………………………………….(3)

Figure (2) flow of air through duct

1

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1 and 2as

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From equation 2 & 3, get

����� � � � …………………………………….(4)

Assume the air density is constant (1.2 kg/m3), therefore continuity equation

for air through duct is

���� � � � …………………………………….(5)

When the duct 1branches into ducts 2 and 3 as shown in figure 2, then

�� � � ���…………………………………….(6)

Or

� � ��…………………………………….(7)

Bernoulli's equation for duct:

The Bernoulli's equation for frictionless, incompressible and steady flow is

��

� ������������…………………………..(8)

Applying equation 8 at section 1 and 2 of a duct, get

����

��� ������

��

�� �� � …………………..(9)

Since � � and �� � � , therefore equation 9 may be written as:

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���������� ��� …………………………..…..(10)

Where:

Ps: - static pressure

Pv: - velocity pressure

Pressure losses in Duct:

A little consideration will show that the pressure is lost due to friction between

the moving particle of the fluid and the interior surfaces of a duct. When the pressure

loss occur in a straight duct (friction loss). The pressure is also lost dynamically at

the changes of direction such as bends, elbows etc. and at the changes of cross

section area of duct (dynamic loss).

Pressure loss due to friction in duct:

The pressure loss due to friction in ducts may be obtained using Darcy's

formula as:

�� � ����

� ………………………..…………..11

Where:

Pf: pressure loss due to friction in N/m2

F : friction factor

L: length of the duct in meter

V: mean velocity

m: hydraulic mean depth in meter

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Note:

1. For circular duct of diameter D, the hydraulic mean depth is:

� � ���

���

�� � �

�…………………………12

2. For rectangular duct of sides a and b, the hydraulic mean depth is:

� � ���

�� ����� …..………………………13

3. The pressure in the duct are usually expressed in mm of water

�� � �

� .�"

…………………………..14