REPORT ON DUCTING IN AIR CONDITIONING

V.MAHESH KUMAR
158R1A03L0
INTRODUCTION:

Ducts are conduits or passages used in heating ,ventilation, and air conditioning
to deliver and remove air.Ducts commonly also deliver ventilation air as part of
the supply air .As such, air ducts are one method of ensuring acceptable indoor
air quality as well as thermal comfort
Generally at the time of designing an air conditioning duct system, the
required airflow rates are known from load calculations. The location of fans
and air outlets are fixed initially. The duct layout is then made taking into
account the space available and ease of construction. In principle, required
amount of air can be conveyed through the air conditioning ducts by a number
of combinations. However, for a given system, only one set results in the
optimum design. Hence, it is essential to identify the relevant design
parameters and then optimize the design.

Classification of duct systems:

Ducts are classified based on the load on the duct due to air pressure and
turbulence . The classification varies from application to application such as for
residences, commercial systems, industrial system.
They are classified as follows:
1.Low pressure system: Velocity ≤ 10 m/s, static pressure ≤ 5 cm H2O (g)
2.Medium pressure systems: Velocity ≤ 10 m/s, static pressure ≤ 15 cm H2O (g)
3. High pressure systems: Velocity > 10 m/s, static pressure 15
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 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
1. DUCT DESIGN METHODS:
Equal Friction Method
Duct sizer (mcquay software)
Ductulator method
2.Velocity reduction method
Equal friction method
The Equal Friction design method sizes the ducts for a constant “Friction Rate”, which
describes the average pressure drop per 100 feet of duct in a system. A
very common friction rate for a reasonably well designed system is 0.1 in.-wc per 100 ft. of
duct length. Reduced friction rates of 0.05 in.-wc per 100 ft. increases the duct size and costs
by 15%, but cuts the portion of the total pressure drop in ductwork by 50%, resulting in fan
energy savings of 15% to 20%.
PROCEDURE:
Equal friction method uses a duct slide rule, duct calculator, or friction rate chart to determine
the relationship between duct size and air flow, i.e. how much air will come out of a given
size duct. The methodology is as follows:
a. Select maximum air velocity in main duct after fan outlet.

b. Enter the friction chart knowing the cfm and velocity to determine the friction rate per 100
feet of duct length and select the diameter (refer to chart below).

c. From the friction rate obtained in the previous step, use the same value to determine
diameters for all other sections of the ductwork.

d. The total friction loss in the duct system is then calculated by multiplying the friction loss
per 100-foot of length to the equivalent length of the most critical path of the ductwork
having maximum resistance.
Advantages of the Equal Friction Method:
1. The method is straightforward and simple.

2. It automatically reduces air velocities in the direction of air flow, which in turn reduces the
potential air flow generated noise.

3. It is the most appropriate method for constant air volume (CAV) systems.
Limitations of the Equal Friction Method:
1. There is no equalization of pressure drops in duct branches unless the system has a
symmetrical layout. Balancing dampers must be installed to balance the system.

2. It is not recommended for VAV systems. If used for VAV supply duct design, the terminal
units require pressure independent (Pi) control capability to avoid excessive flow rates when
duct pressure is high.

3. It is not flexible and adaptable to future layout changes.

Velocity Reduction Method:
This method sizes the duct by varying the velocity in the main and branch ducts. The various
steps involved are:
a. Select suitable velocities in the main and branch ducts. The table below indicates
commonly used velocity limits
b. Find the diameters of the main and branch ducts from air flow rates and velocities.
The velocity in duct can be expressed as:

A = Q / v Where,
• A = duct area (ft2)
• Q = air flow rate (cfm)
• v = air speed (fpm) c. From the velocities and duct dimensions obtained in the previous
step, find the frictional pressure drop for the main and branch ducts using the friction chart
above.
The velocity method is one of the simplest ways of designing the duct system for both supply
and return air. However, the application of this method requires selection of suitable
velocities in different duct runs, which requires experience. Wrong selection of velocities can
lead to very large ducts, which, occupy large building space and increases the cost, or very
small ducts which lead to large pressure drop and hence necessitates the selection of a large
fan leading to higher fan cost and running cost. In addition, the method is not very efficient as
it requires partial closing of all the dampers except the one in the index run, so that the total
pressure drop in each run will be same.
Velocity changes according to type of duct

Duct type FPM

Main duct 1000-1500 fpm
Branch duct 1000-1200 fpm
Sub- branch duct 600fpm

Constructional shapes of duct:

1.Round duct
2. Square duct
3.rectangular duct
4 flat oval duct
1. Round duct:

2. Square duct:
3.Rectangular duct:

4.Flat oval duct:

DUCT SIZING:
It can be done by two methods
1.Duct lator
2.Duct sizer software
1.Duct lator:

MANUAL METHOD:
Q=AxV
Where
Q=cfm (cubic feet per minute)
A=area
V=velocity in fpm (feet per minute)
EQUIVALENT DIAMETER:
Equivalent Diameter =1.30XA^0.625/P^0.250
Where
P= perimeter
=2*(w+h)
FRICTION :
Friction ΔP=0.03XF(L/d^1.22)(V/1000)^1.82

F= Internal surface roughness of duct (0.9)

L= length of duct
V=velocity
d= equivalent diameter
 ROOM 1
Section A-B
Q=AxV
6643.34=Ax1500
A=6643.34/1500 area =wxh
2
W x h = 4.42ft
h=1feet =12 inch, w=4.42feet =53.04inch
Perimeter p=2xW+H
=2x53.04+12
=132
Equivalent dia d=1.30XA^0.625/P^0.250
=1.30X53.04x12^0.625/132^0.250
=21.9
Friction ΔP=0.03XF (L/d^1.22)(V/1000)^1.82
=0.03X0.9 (153.04/21.9^1.22)(1500/1000)^1.82
=2.0
Section B-C
Q=Ax V
5694.3=Ax1500
A=5694.3/1500 area =w x h
2
W x h = 3.79ft
h=1feet =12 inch, w=3.79feet =45.55inch
Perimeter p=2xW+H
=2x45.55+12
=115.10
Equivalent dia d=1.30XA^0.625/P^0.250
=1.30X45.55x12^0.625/115.10^0.250
=20.40
Friction ΔP=0.03XF (L/d^1.22)(V/1000)^1.82
=0.03X0.9 (237.45/20.40^1.22)(1500/1000)^1.82
=0.33
Section C-C1
Q=AxV
949.04=Ax1500
A=949.04/1100 area =wxh
2
W x h = 0.86ft
h=0.86feet =10.35 inch, w=1feet =12inch
Perimeter p=2xW+H
=2x10.35+12
=44.70
Equivalent dia d=1.30XA^0.625/P^0.250
=1.30X10.35x12^0.625/44.70^0.250
MANUAL METHOD:

Q=AxV
Where
Q=cfm (cubic feet per minute)
A=area
V=velocity in fpm (feet per minute)
EQUIVALENT DIAMETER:
Equivalent Diameter =1.30XA^0.625/P^0.250
Where
P= perimeter
=2*(w+h)
FRICTION :
Friction ΔP=0.03XF(L/d^1.22)(V/1000)^1.82

F= Internal surface roughness of duct (0.9)

L= length of duct
V=velocity
d= equivalent diameter

ROOM 1
Section A-B
Q=AxV
6643.34=Ax1500
A=6643.34/1500 area =wxh
2
W x h = 4.42ft
h=1feet =12 inch, w=4.42feet =53.04inch
Perimeter p=2xW+H
=2x53.04+12
=132
Equivalent dia d=1.30XA^0.625/P^0.250
=1.30X53.04x12^0.625/132^0.250
=21.9
Friction ΔP=0.03XF (L/d^1.22)(V/1000)^1.82
=0.03X0.9 (153.04/21.9^1.22)(1500/1000)^1.82
=2.0
Section B-C
Q=Ax V
5694.3=Ax1500
A=5694.3/1500 area =w x h
2
W x h = 3.79ft
h=1feet =12 inch, w=3.79feet =45.55inch
Perimeter p=2xW+H
=2x45.55+12
 =115.10
Equivalent dia d=1.30XA^0.625/P^0.250
=1.30X45.55x12^0.625/115.10^0.250
=20.40
Friction ΔP=0.03XF (L/d^1.22)(V/1000)^1.82
=0.03X0.9 (237.45/20.40^1.22)(1500/1000)^1.82
=0.33
Section C-C1
Q=AxV
949.04=Ax1500
A=949.04/1100 area =wxh
2
W x h = 0.86ft
h=0.86feet =10.35 inch, w=1feet =12inch
Perimeter p=2xW+H
=2x10.35+12
=44.70
Equivalent dia d=1.30XA^0.625/P^0.250

DUCT SIZING BY SOFTWARE

DRAFTING & DUCTING :

Ducting and done by using two methods they are

Concentric method
Eccentric method
In the ducting single line diagram is considered as the centre line of the duct in concentric
method by offsetting the distance on either side ducting is done. It is difficult for design
engineer to draw but fabrication is simple and execution is easy while they are installing duct
size always reduces gradually by decreasing cfm.

In eccentric method of ducting only one side of the duct is offset is done and foe design
engineer it is easy to draw and for fabrication it is difficult. In eccentric method single line
diagram is considered as one of the side of the duct

FITTINGS USED IN DUCTING

Fittings used in ducting are
1. Reducer
2. Elbow
3. Shoe piece
4. Wye/ Trouser
5. 2-way fitting
6. 3 way
1. Reducer: It is connection between two different widths of duct. It is of two types:

 Single slide reducer

 Double slide reducer
 Fig 3.3- Single side reducer

Single slide reducer: To find reducer length

𝑤2
Lr = (𝑤1 − )×5
2

Where ,Lr = Length of reducer

W1=Width of 1st duct and

W2=Width of 2nd duct

Fig 3.4-Two side reducer

Double slide reducer: To find reduce length

𝑤2
,Lr = (𝑤1 − )×6
2

ELBOW: Elbow is a standard fitting which generally have radius of curvature between one to
twice the diameter of pipe or duct .It is classified into two types:

-Long radius elbow

Long radius elbow: To find the radius of the elbow

R1 = 1.5 W

R2 = offset with R1 with width of duct

 Short radius elbow

Short radius: short radius elbow is used where we have less space .It produces more friction
when compared to long radius elbow.

R1 = 0.5 W or W/2

R2 = offset with R1 with width

Short radius elbow with the reducer :-

R1 = W1/2 = 0.5 w

R2 = offset of R1 multiplied by width of duct

Lr =𝑤1 − 𝑤2
2X5

Where Lr= Length of the reducer

short radius elbow with reducer

Long radius elbow with reducer:-

R1 = w1/2 = 0.5 W
R2 = offset of R1 multiplied by width of duct
Lr =𝑤1 − 𝑤2
 2X5

Where, Lr = Length of the reducer

Shoe fitting:-To throw the air into the branch ducts with required velocity thesed type of
fittings are used.

Fig 3.8 – Shoe fitting

Commonly used shoe fittings are 30 degree and 45 degree

Two way Trouser: A two way trouser is also used to to connect two branch ducts with main
duct.These ducts may be same or different in size

Fig 3.11- Two way trouser

For connecting main duct with first duct we have,

 We have X = W/W1+W2
 We1= XW1
 R1= 0.5 X We1
 R2= Offset R1 with width of duct
 Lr = (w1-we1) X 5
2

For connecting main duct with second duct we have,

 We2 = W-We1
 Lr= (W2-We2) X 5
2

3 Way Trouser:A two way trouser is also used to toconnect more than two i.e.., three ducts
with main duct.These ducts may be same or different in size
For connecting main duct with first duct we have,

 We have X = W/W1+W2
 We1= XW1
 R1= 0.5 X We1
 R2= Offset R1 with width of duct
 Lr = (w1-we1) X 5
2

Fig 3.12- Three way trouser

For connecting main duct with second duct we have,

 We2= X W2
 R2 = 0.5 X We2
 R2= Offset R1 with width of duct
 Lr = (W2-We2) X 5
2

Now width of third duct, We = W-(We1+We2)

 Length of reducer Lr = (W3-We) x 5

2

Here Lr =Length of the reducer

W = Width of the main duct

We1,We2,We3 = Width of branch ducts respectively

CONCLUSION:

The cooling load calculation of an bulding is done and duct design is carried out by equal
friction method .the calculated value of frictional pressure drop is less compared to existing
value ,due to less value duct diameter is increased but loss in static pressure velocity pressure
can be avoided ,a smaller diameter of duct would increase noise level .

Due to proper branching of ducts loss is minimized in this design but in existing plan there is
a straight branching in so many locations which may increase pressure loss .

REFERENCES:

The fundamental volume of the ASHRAE hand book

HVAC systems – duct design 3rd ED SMACNA 1990 .

Designer duide to seling based room air diffusion .

NADCA 2013 ACR