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FLUID POWER
AND ADVANCED
FLUID
MECHANICS Assignment 2: REDESIGN A VENTILATION SYSTEM
Submitted By
Sreeshob Sindhu Anand 19485180
Muhammed Fajir TK 19485649
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Contents
1. Aim...................................................................................................................................... 5
2. Introduction ....................................................................................................................... 6
3. Method ............................................................................................................................... 7
3.1 Formulas Used: ................................................................................................................. 7
3.2 Velocity Reduction Method ............................................................................................. 8
3.3 Constant Friction Gradient Method ................................................................................. 8
3.4 Current Design and Issues ................................................................................................ 9
3.4.1 Description of current Design .................................................................................... 9
3.4.2 Issues in Current design ............................................................................................. 9
4. Results .............................................................................................................................. 11
4.1 Initial Calculations without Dampers ............................................................................. 11
4.2 With Dampers ................................................................................................................ 15
4.3 Velocity Reduction Method ........................................................................................... 16
4.4 Constant Friction Gradient Method ............................................................................... 19
5. Improved Design ............................................................................................................... 23
5.1 Velocity Reduction Method ...................................................................................... 23
5.2 Constant Friction Factor Method ................................................................................... 24
6. Discussion ......................................................................................................................... 25
6.1 Limitations of the current and modified design ............................................................ 26
7. Reference ......................................................................................................................... 27
8. Appendix .......................................................................................................................... 28
9. Student Declaration ......................................................................................................... 30
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List of figures
Figure 1 Initial Design ................................................................................................................. 5
Figure 2 Improved design using Velocity Reduction Method .................................................. 23
Figure 3 Improved Design (Constant Friction Factor Method) ................................................ 24
Figure 4 Chart to calculate friction loss, velocity and size of ducts. ........................................ 28
Figure 5 Rectangular duct dimensions ..................................................................................... 29
Figure 6 Dynamic loss cpffecient for fittings ........................................................................... 29
List of Tables
Number Table name Page number
1 Velocity in different systems 8
2 Initial design flow rate and dimension in each duct 9
3 Calculations for the initial design without Damper 11
4 T section Calculations 13
5 Elbow calculations 13
6 Grill calculations 13
7 Intake louvers Calculation 14
8 Path 1 total loss calculation 14
9 Path 2 total loss calculation 14
10 Path 3 total loss calculation 14
11 Path 4 total loss calculation 14
12 Path 1 total loss calculations 15
13 Path 2 total loss calculations 15
14 Path 3 total loss calculations 15
15 Path 4 total loss calculations 15
16 Losses in each duct (Velocity reduction method) 16
17 Path 1 total loss 18
18 Path 2 total loss 18
19 Path 3 total loss 19
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20 Path 4 total loss 19
21 Losses in each duct (Constant friction method) 19
22 Losses in damper (Positions changed) 21
23 Path 1 total loss 21
24 Path 2 total loss 21
25 Path 3 total loss 22
26 Path 4 total loss 22
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1. Aim The objective of this assignment is to redesign the ventilation system shown in figure 1 to
achieve a balanced air flow by choosing appropriate duct sizes for each section.
Figure 1 Initial Design
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2. Introduction
The use of ventilation systems in buildings are to supply continuous fresh air supply from
outside, for maintaining humidity and temperature at comfortable levels. The critical part in
the ventilation systems are the ductworks. The duct acts as the pathway to deliver or remove
the air in the buildings. The planning and designing are the important part in the installation
of ventilation systems.
Sheet Metal and Air Conditioning Contractors National Association (SMACNA) is the
international association which provides various standards for duct fabrication, balancing of
air for industrial, commercial, and residential purpose. The different types of materials used
for ducting includes galvanised mild steel, aluminium, polyurethane, flexible ducts. The
material and its thickness are chosen depending upon the area to be ventilated.
The ventilation in buildings are achieved by mechanical, natural and mixed means. The
ventilation in Sustainable building make use of natural method where the outdoor air directed
to the required area by physical phenomenon like wind pressure or stack pressure and the
flow can be controlled through openings like window or doors. Using supply and exhaust fans,
the air flow is directed into the building in mechanical ventilation systems.
The amount of air flow inside the ducts depends upon the area of cross section and
velocity of air. Lesser velocity inside the duct will cause settling and accumulation of dust
particles and thereby eventually causing clog in the duct. As the velocity increases, it causes
more power loss and noise problems. So, the duct systems should be designed by reducing
the possibility of arising resistance and considering recommended speed. This report details
the design of duct system by choosing the appropriate size of duct for each section and thus
achieving a balanced system.
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3. Method
There are several ways for designing a duct. The main three methods used to simplify the
steps are:
• Equal Friction Factor (Constant Friction Gradient) Method
• Velocity Reduction Method
• Static pressure regains
In some cases, a combination of these methods is also used. The design of the duct
varies with the requirements of the place where it is designed for. For instance, there are
comfort systems which is used in offices and residential buildings and there are industrial
and high-speed systems used in factories and laboratories. The main difference between
these systems are the noise levels, the velocity of air flow will be high in the industrial
systems which results in high noise levels.
To design the duct system for the given requirements (Figure 1) we are using the
velocity reduction method and constant friction factor method.
3.1 Formulas Used:
The total energy loss in a duct:
𝐻𝐿 = ℎ𝐿 ∗ 𝐿
Equivalent Diameter of rectangular ducts:
𝐷𝑒 =1.3(𝑎𝑏)5/8
(𝑎 + 𝑏)1/4
Dynamic Losses
𝐻𝑣 =𝛾𝑎𝑖𝑟𝑣
2
2𝑔𝛾𝑤𝑎𝑡𝑒𝑟=
𝛾𝑎𝑖𝑟𝑣2
2𝑔 ➔ 𝐻𝑣 = (
𝑣
1.289)2
𝐻𝐿 = 𝐶(𝐻𝑣)
C= Dynamic loss coefficient
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3.2 Velocity Reduction Method
In this method the flow velocity is assumed according to the type of system and type of
ducts, Table 1 shows the different velocities for different ducts.
Table 1 Velocity in different systems
Type of Duct Velocity (m/s)
Comfort Systems Industrial/High speed
Systems
Main Ducts 4-8 8-18
Main Branch Ducts 3-5 5-12
Branch Ducts 1-3 3-8
• Suitable duct velocities are selected as per the table 1 for each type of duct.
• As the flow rate in each section is given, we can use the chart (Figure 4) to determine
the values of friction factor and equivalent diameter of the duct.
• The diameter from the chart is the equivalent circular diameter, the rectangular duct
dimension can be obtained from the table from textbook (Figure 5).
• The next step is to calculate the total loss and the losses due to fittings. Then total
losses in each path is calculated.
• The pressure difference is equalised by increasing or decreasing the velocities in
each ducts and smaller difference is adjusted by changing the position of dampers.
3.3 Constant Friction Gradient Method
Constant friction factor method is used by most of the HVAC design companies. This
method reduces the iterations and steps in the design of ducts. In this method friction factor
will be always constant throughout the design.
Velocity in each duct is obtained from the chart by using the flow rate and friction
factor values. By using this method most of the similar branches will have same dimensions
and loss, so the design becomes much simple and easy to install.
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For comfort system the friction factor value is taken to be 1.25 in most of the
designs. And for Industrial and high-speed systems the friction factor value can be much
higher. The total loss is then balanced by the introducing dampers in ducts and adjusting the
position of the dampers.
3.4 Current Design and Issues
3.4.1 Description of current Design
For the given duct shown in figure 1, the total flow rate in the system is 2.28 m3/s. The
system equipped with four grills for the outlet of air and each of the outlet have a flow of 0.57
m3/s. The flow rate and length for each duct is shown in table x. The fan is fixed in between
duct A and B and is connected by using a plenum duct. The connection between the duct and
the grill is achieved using 90-degree elbow. A total of four number of tee and eight elbows
are used in the duct. There are four path of air flow in the system namely B to D, B to F, B to
I, B to K.
Table 2 Initial design flow rate and dimension in each duct
Duct Flow Rate (m3/s) Length (m)
A 2.28 15.2
B 2.28 10.7
C 1.14 6.1
D 0.57 5.5
E 0.57 9.1
F 0.57 5.5
G 1.14 12.2
H 1.14 6.1
I 0.57 5.5
J 0.57 9.1
K 0.57 5.5
3.4.2 Issues in Current design
The major issues with the given design shown in figure 1 are:
• The dimensions for the ducts are not specified
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• The calculations carried out shows that system is not balanced
• There is no volume control dampers (VCD) included in the system. So, the flow of air
through each branch cannot be controlled manually.
• The dimensions for the plenum duct used for fixing the fan are not given in the
design
• The grill size used are not mentioned.
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4. Results
4.1 Initial Calculations without Dampers
Table 3 Calculations for the initial design without Damper
Section Length
(m) Velocity
(m/s) Flow Rate
(m3/s) hl
(Pa/m) Hv (Pa) HI (Pa) De
(mm) Possible rectangular
dimensions(mm/inch)
A 15.2 4.3 2.28 0.23 11.12835769 3.496 795
• 775 (30.5) → (30in X 26in)
• 805 (31.7) → (30in X 28in)
• 777 (30.6) → (28in X 28in)
• 833 (32.8) → (30in X 30in)
B 10.7 6.1 2.28 0.58 22.39514276 6.206 688
• 676 (26.6) → (30in X 20in)
• 688 (27.1) → (28in X 22in)
• 693 (27.3) → (26in X 24in)
C 6.1 4.9 1.14 0.55 14.45061483 3.355 525
• 526 (20.7) → (30in X 12in)
• 526 (20.7) → (20in X 18in)
• 523 (20.6) → (26in X 14in)
• 518 (20.4) → (22in X 16in)
D 5.5 4 0.57 0.45 9.629730831 2.475 420
• 409 (16.1) → (30in X 8in)
• 419 (16.5) → (24in X 10in)
• 426 (16.8) → (20in X 12in)
• 417 (16.4) → (16in X 14in)
E 9.1 4.9 0.57 0.78 14.45061483 7.098 375
• 384 (15.1) → (26in X 8in)
• 371 (14.6) → (24in X 8in)
• 368 (14.5) → (18in X 10in)
• 386 (15.2) → (20in X 10in)
• 361 (14.2) → (14in X 12in)
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• 384 (15.1) → (16in X 12in)
• 389 (15.3) → (14in X 14in)
F 5.5 4 0.57 0.45 9.629730831 2.475 420
• 409 (16.1) → (30in X 8in)
• 419 (16.5) → (24in X 10in)
• 426 (16.8) → (20in X 12in)
• 417 (16.4) → (16in X 14in)
G 12.2 6.1 1.14 0.93 22.39514276 11.346 465
• 465 (18.3) → (30in X 10in)
• 465 (18.3) → (24in X 12in)
• 462 (18.2) → (20in X 14in)
• 470 (18.5) → (18in X 16in)
H 6.1 4.9 1.14 0.55 14.45061483 3.355 525
• 526 (20.7) → (30in X 12in)
• 526 (20.7) → (20in X 18in)
• 523 (20.6) → (26in X 14in)
• 518 (20.4) → (22in X 16in)
I 5.5 4 0.57 0.45 9.629730831 2.475 420
• 409 (16.1) → (30in X 8in)
• 419 (16.5) → (24in X 10in)
• 426 (16.8) → (20in X 12in)
• 417 (16.4) → (16in X 14in)
J 9.1 4.9 0.57 0.78 14.45061483 7.098 375
• 384 (15.1) → (26in X 8in)
• 371 (14.6) → (24in X 8in)
• 368 (14.5) → (18in X 10in)
• 386 (15.2) → (20in X 10in)
• 361 (14.2) → (14in X 12in)
• 384 (15.1) → (16in X 12in)
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• 389 (15.3) → (14in X 14in)
K 5.5 4 0.57 0.45 9.629730831 2.475 420
• 409 (16.1) → (30in X 8in)
• 419 (16.5) → (24in X 10in)
• 426 (16.8) → (20in X 12in)
• 417 (16.4) → (16in X 14in)
Table 4 T section Calculations
T Section Velocity C Hv HL
T to duct C from B (Branch) 6.1 1 22.39514276 22.39514276
T to duct D from C (branch) 4.9 1 14.45061483 14.45061483
T to duct G from B (flow through Mains) 6.1 0.1 22.39514276 2.239514276
T to duct I from H (Branch) 4.9 1 14.45061483 14.45061483
Table 5 Elbow calculations
Elbow Velocity C Hv HL
Elbow in C 4.9 0.18 14.45061483 2.601110669
Elbow in F 4 0.18 9.629730831 1.73335155
Elbow in H 4.9 0.18 14.45061483 2.601110669
Elbow in K 4 0.18 9.629730831 1.73335155
Elbow to grille (D) 4 0.18 9.629730831 1.73335155
Elbow to grille (F) 4 0.18 9.629730831 1.73335155
Elbow to grille (I) 4 0.18 9.629730831 1.73335155
Elbow to grille (K) 4 0.18 9.629730831 1.73335155
Table 6 Grill calculations
Grill HL
Duct D 15
Duct F 15
Duct I 15
Duct K 15
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Table 7 Intake louvers Calculation
Table 8 Path 1 total loss calculation Table 9 Path 2 total loss calculation
Table 10 Path 3 total loss calculations Table 11 Path 4 total loss calculations
Intake Size (m) K (Well Rounded) Flow Rate
Velocity (m/s)
HL (Pressure drop) (Kpa) hl
Gradual Contraction 1 x 1 0.04 2.28 4.3 17 0.037696
Path 1
Section Loss (Pa)
B 6.206
Elbow in C 2.601110669
T to duct C from B (Branch) 22.39514276
C 3.355
T to duct D from C (branch) 14.45061483
Elbow to grille (D) 1.73335155
D 2.475
Grill in D 15
Total Loss 68.21621981
Path 2
Section Loss (Pa)
B 6.206
Elbow in C 2.601110669
C 3.355
T to duct C from B (Branch) 22.39514276
E 7.098
Elbow to grille (F) 1.73335155
Elbow in F 1.73335155
Grill in F 15
F 2.475
Total Loss 62.59695653
Path 3
Section Loss (Pa)
B 6.206
T to duct G from B (flow through Mains) 2.239514276
G 11.346
Elbow in H 2.601110669
H 3.355
T to duct I from H (Branch) 14.45061483
Elbow to grille (I) 1.73335155
I 2.475
Grill in I 15
Total Loss 59.40659132
Path 4
Section Loss
B 6.206
T to duct G from B (flow through Mains) 2.239514276
G 11.346
Elbow in H 2.601110669
H 3.355
J 7.098
Elbow to grille (K) 1.73335155
Elbow in K 1.73335155
K 2.475
Grill in K 15
Total Loss 53.78732804
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4.2 With Dampers
Table 11 Losses due to damper
Damper Velocity C Hv HL
Damper in duct A 4.3 0.2 11.12835769 2.225671538
Damper in duct D 4 0.2 9.629730831 1.925946166
Damper in duct F 4 0.2 9.629730831 1.925946166
Damper in duct I 4 0.2 9.629730831 1.925946166
Damper in duct K 4 0.2 9.629730831 1.925946166
Table 12 Path 1 total loss calculations Table 13 Path 2 total loss calculations
Table 14 Path 3 total loss Table 15 Path 4 total loss
Path 1
Section Loss (Pa)
B 6.206
Elbow in C 2.601110669
T to duct C from B (Branch) 22.39514276
C 3.355
T to duct D from C (branch) 14.45061483
D 2.475
Damper in duct D 1.925946166
Elbow to grille (D) 1.73335155
Grill in D 15
Total Loss 70.14216598
Path 2
Section Loss (Pa)
B 6.206
Elbow in C 2.601110669
C 3.355
T to duct C from B (Branch) 22.39514276
E 7.098
Elbow in F 1.73335155
Damper in duct F 1.925946166
Elbow to grille (F) 1.73335155
Grill in F 15
F 2.475
Total Loss 64.5229027
Path 3
Section Loss (Pa)
B 6.206
T to duct G from B (flow through Mains) 2.239514276
G 11.346
Elbow in H 2.601110669
H 3.355
T to duct I from H (Branch) 14.45061483
Damper in duct I 1.925946166
Elbow to grille (I) 1.73335155
I 2.475
Grill in I 15
Total Loss 61.33253749
Path 4
Section Loss
B 6.206
T to duct G from B (flow through Mains) 2.239514276
G 11.346
Elbow in H 2.601110669
H 3.355
J 7.098
Damper in duct K 1.925946166
Elbow to grille (K) 1.73335155
Elbow in K 1.73335155
K 2.475
Grill in K 15
Total Loss 55.71327421
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4.3 Velocity Reduction Method
Table 16 Losses in each duct (Velocity reduction method)
Section Length
(m) Velocity
(m/s) Flow Rate
(m3/s) hl (Pa/m) Hv (Pa) HI (Pa) De
(mm) Available duct size
A 15.2 4.3 2.28 0.23 11.12835769 3.496 800
• 805 (31.7) → (30in X 28in)
• 833 (32.8) → (30in X 30in)
• 777 (30.6) → (28in X 28in)
B 10.7 8 2.28 1.25 38.51892332 13.375 600
• 600 (23.7) → (30in X 16in)
• 597 (23.5) → (26in X 18in)
• 620 (24.4) → (28in X 18in)
• 607 (23.4) → (24in X 20in)
• 610 (24) → (22in X 22in)
C 6.1 4 1.14 0.29 9.629730831 1.769 580
• 582 (22.9) → (28in X 16in)
• 577 (22.7) → (24in X 18in)
• 562 (22.9) → (22in X 20in)
• 561 (22.1) → (26in X 16in)
D 5.5 3.3 0.57 0.28 6.554235547 1.54 475
• 465 (18.3) → (30in X 10in)
• 465 (18.3) → (24in X 12in)
• 483 (19) → (26in X 12in)
• 462 (18.2) → (20in X 14in)
• 485 (19.1) → (22in X 14in)
• 470 (18.5) → (18in X 16in)
E 9.1 3.9 0.57 0.41 9.154262871 3.731 430
• 434 (17.1) → (26in X 10in)
• 426 (16.8) → (20in X 12in)
• 417 (16.4) →
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(16in X 14in) • 439 (17.3) → (18in X 14in)
F 5.5 3.9 0.57 0.41 9.154262871 2.255 430
• 434 (17.1) → (26in X 10in)
• 426 (16.8) → (20in X 12in)
• 417 (16.4) → (16in X 14in)
• 439 (17.3) → (18in X 14in)
G 12.2 6.1 1.14 0.89 22.39514276 10.858 580
• 582 (22.9) → (28in X 16in)
• 577 (22.7) → (24in X 18in)
• 562 (22.9) → (22in X 20in)
• 561 (22.1) → (26in X 16in)
H 6.1 6.1 1.14 0.89 22.39514276 5.429 580
• 582 (22.9) → (28in X 16in)
• 577 (22.7) → (24in X 18in)
• 562 (22.9) → (22in X 20in)
• 561 (22.1) → (26in X 16in)
I 5.5 5 0.57 0.75 15.04645442 4.125 375
• 371 (14.6) → (24in X 8in)
• 384 (15.1) → (26in X 8in)
• 368 (14.5) → (18in X 10in)
• 386 (15.2) → (20in X 10in)
• 361 (14.2) → (14in X 12in)
• 384 (15.1) → (16in X 12in)
• 389 (15.3) → (14in X 14in)
J 9.1 5.4 0.57 1.1 17.55018444 10.01 360
• 358 (14.1) → (22in X 8in)
• 371 (14.6) → (24in X 8in)
• 368 (14.5) → (18in X 10in)
• 361 (14.2) → (20in X 14in)
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• 361 (14.2) → (14in X 12in)
K 5.5 5.4 0.57 1.1 17.55018444 6.05 360
• 358 (14.1) → (22in X 8in)
• 371 (14.6) → (24in X 8in)
• 368 (14.5) → (18in X 10in)
• 361 (14.2) → (20in X 14in)
• 361 (14.2) → (14in X 12in)
Table 17 Path 1 total loss Table 18 Path2 total loss
Path 1
Section Loss (Pa)
B 13.375
Elbow in C 1.73335155
T to duct C from B (Branch) 38.51892332
C 1.769
T to duct D from C (branch) 9.629730831
D 1.54
Elbow to grille (D) 1.179762398
Damper in duct D 1.310847109
Grill in D 15
Total Loss 84.05661521
Path 2
Section Loss (Pa)
B 13.375
Elbow in C 1.73335155
C 1.769
T to duct C from B (Branch) 38.51892332
E 3.731
Elbow in F 1.647767317
Elbow to grille (F) 1.647767317
Damper in duct F 4.760216693
Grill in F 15
F 2.255
Total Loss 84.4380262
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Table 19 Path 3 total loss Table 20 Path 4 total loss
4.4 Constant Friction Gradient Method
Table 21 Losses in each ducts (Constant friction method)
Section Length
(m) Velocity
(m/s)
Flow Rate
(m3/s) hl (Pa/m) Hv (Pa) HI (Pa) De
(mm) Available Duct
Size
A 15.2 8 2.28 1.25 38.51892332 19 600
• 600 (23.7) → (30in X 16in)
• 597 (23.5) → (26in X 18in)
• 620 (24.4) → (28in X 18in)
• 607 (23.4) → (24in X 20in)
• 610 (24) → (22in X 22in)
B 10.7 8 2.28 1.25 38.51892332 13.375 600
• 600 (23.7) → (30in X 16in)
• 597 (23.5) → (26in X 18in)
• 620 (24.4) → (28in X 18in)
• 607 (23.4) → (24in X 20in)
• 610 (24) → (22in X 22in)
Path 3
Section Loss (Pa)
B 13.375
T to duct G from B (flow through Mains) 3.851892332
G 10.858
Elbow in H 4.031125697
H 5.429
T to duct I from H (Branch) 22.39514276
Damper in duct I 3.009290885
Elbow to grille (I) 2.708361796
I 4.125
Grill in I 15
Total Loss 84.78281347
Path 4
Section Loss
B 13.375
T to duct G from B (flow through Mains) 3.851892332
G 10.858
Elbow in H 4.031125697
H 5.429
J 10.01
Damper in duct K 9.126095909
Elbow to grille (K) 3.159033199
Elbow in K 3.159033199
K 6.05
Grill in K 15
Total Loss 84.04918034
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C 6.1 7 1.14 1.25 29.49105067 7.625 450
• 450 (17.7) → (28in X 10in)
• 447 (17.6) → (22in X 12in)
• 445 (17.5) → (16in X 16in)
D 5.5 5.8 0.57 1.25 20.24650907 6.875 350
• 345 (13.6) → (30in X 6in)
• 342 (13.5) → (20in X 8in)
• 358 (14.1) → (22in X 8in)
• 348 (13.7) → (16in X 10in)
E 9.1 5.6 0.57 1.1 18.87427243 10.01 350
• 345 (13.6) → (30in X 6in)
• 342 (13.5) → (20in X 8in)
• 358 (14.1) → (22in X 8in)
• 348 (13.7) → (16in X 10in)
F 5.5 5.6 0.57 1.1 18.87427243 6.05 350
• 345 (13.6) → (30in X 6in)
• 342 (13.5) → (20in X 8in)
• 358 (14.1) → (22in X 8in)
• 348 (13.7) → (16in X 10in)
G 12.2 7 1.14 1.25 29.49105067 15.25 450
• 450 (17.7) → (28in X 10in)
• 447 (17.6) → (22in X 12in)
• 445 (17.5) → (16in X 16in)
H 6.1 7 1.14 1.25 29.49105067 7.625 450
• 450 (17.7) → (28in X 10in)
• 447 (17.6) → (22in X 12in)
• 445 (17.5) → (16in X 16in)
I 5.5 5.8 0.57 1.25 20.24650907 6.875 350
• 345 (13.6) → (30in X 6in)
• 342 (13.5) → (20in X 8in)
• 358 (14.1) → (22in X 8in)
• 348 (13.7) → (16in X 10in)
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J 9.1 6 0.57 1.3 21.66689437 11.83 350
• 345 (13.6) → (30in X 6in)
• 342 (13.5) → (20in X 8in)
• 358 (14.1) → (22in X 8in)
• 348 (13.7) → (16in X 10in)
K 5.5 6 0.57 1.3 21.66689437 7.15 350
• 345 (13.6) → (30in X 6in)
• 342 (13.5) → (20in X 8in)
• 358 (14.1) → (22in X 8in)
• 348 (13.7) → (16in X 10in)
Table 22 Losses in damper (Positions changed)
Table 23 Path 1 total loss Table 24 Path 2 total loss
Damper Velocity C Hv HL
Damper in duct A 8 0.2 38.51892332 7.703784665
Damper in duct D 5.8 0.52 20.24650907 10.52818472
Damper in duct F 5.6 1.5 18.87427243 28.31140864
Damper in duct I 5.8 1.5 20.24650907 30.36976361
Damper in duct K 6 1.5 21.66689437 32.50034155
Damper in duct J 6 0.52 21.66689437 11.26678507
Path 1
Section Loss (Pa)
B 13.375
Elbow in C 5.308389121
T to duct C from B (Branch) 38.51892332
C 7.625
T to duct D from C (branch) 29.49105067
D 6.875
Damper in Duct D 10.52818472
Elbow to grille (D) 3.644371633
Grill in D 15
Total Loss 130.3659195
Path 2
Section Loss (Pa)
B 13.375
Elbow in C 5.308389121
C 7.625
T to duct C from B (Branch) 38.51892332
E 10.01
Elbow in F 3.397369037
Damper in duct F 28.31140864
Elbow to grille (F) 3.397369037
Grill in F 15
F 6.05
Total Loss 130.9934592
Page | 22
Table 25 Path 3 total loss Table 26 Path 4 total loss
Path 3
Section Loss (Pa)
B 13.375
T to duct G from B (flow through Mains) 3.851892332
G 15.25
Elbow in H 5.308389121
H 7.625
T to duct I from H (Branch) 29.49105067
Damper in duct I 30.36976361
I 6.875
Elbow to grille (I) 3.644371633
Grill in I 15
Total Loss 130.7904674
Path 4
Section Loss
B 13.375
T to duct G from B (flow through Mains) 3.851892332
G 15.25
Elbow in H 5.308389121
H 7.625
J 11.83
Damper in duct K 32.50034155
Elbow in K 3.900040987
Elbow to grille (K) 3.900040987
K 7.15
Grill in K 15
Damper in Duct J 11.26678507
Total Loss 130.9574901
Page | 23
5. Improved Design
After all the calculations and determining the dimensions of the ducts two designs are
proposed by using two different methods. In both the cases the pressure drop is balanced
which increases the comfort.
5.1 Velocity Reduction Method
The designing of duct done in such a way that the velocity of air flow inside the duct
decreased as the flow proceeds. The velocity in the improved design varied from 8m/s to 4
m/s.
Figure 2 Improved design using Velocity Reduction Method
Page | 24
5.2 Constant Friction Factor Method
Usually in his method the friction factor value is constant, but in this case for most of
the cases we took the friction factor value as 1.25. But in four ducts the friction factor value
is different (1.1 and 1.3). In normal case the pressure drop in this method is balanced by
adjusting the dampers. But as we have only 0,10,20,30,40 and 50, it’s not possible to balance
the pressure drop. So, we have taken a slightly lesser friction factor to balance the air flow.
The damper can be set to 15 degrees, but the value of dynamic coefficient is not available, so
the only way was to reduce the velocity.
Figure 3 Improved Design (Constant Friction Factor Method)
Page | 25
6. Discussion
In the initial design there is higher possibility of power loss and the system is not efficient.
The dimensions for the duct are not specified in the design. Based on the calculations carried
out using standard values taken from table shown in appendix, it shows that the system is not
balanced. The specification of fittings like elbows, tees, dampers, reducers used in the design
are not mentioned in the drawing. Thus, using improper connections increases the resistance
in the path of air flow. Even though the design shown in figure 1 shows each outlet has a flow
rate of 0.57 m3/sec, since there are no dampers for controlling the air flow, the system cannot
provide the equal outlet flow rate as shown in the design.
The calculations done based on the given parameters in the initial design shows that the
system is not balanced. The air flow path from duct B to outlet in duct D shows a higher
friction loss value of 68 Pa while in path B to K shows a value of 53Pa. This difference in value
shows that outlet flow through the grill is not the same in every branch in the duct. In the
modified design, this imbalance is corrected by using excel calculations and the friction loss
occurring in four paths are made equal and hence the outlet flow through each grill will be
the same.
The type of elbows and tees used in the design are not mentioned in the drawing. There
are several types of elbows and tees used in the ventilations systems. Choosing the right type
of fittings depends upon the type of air flow and design. In the initial drawing there is no such
details are not mentioned. So, there will be errors in the calculation of friction loss in the
system. In the modified design developed using AutoCAD software, the type of fittings is
chosen based in the standards used in the industry. So, while calculating the various
parameters related with those fittings and duct, the chances of error is reduced.
In order to regulate the flow of air through each branch and balance the system, volume
control dampers are required. There are no dampers included in the drawing and thus the
system is not efficient. In the improved designed a total of five dampers are included in the
drawing in order to regulate the flow. To avoid errors, the positions of each dampers are also
specified. The balancing of the modified drawing is achieved by adjusting the position of
damper.
Page | 26
6.1 Limitations of the current and modified design
The main design limitation for this design is the application of this design. Normally when
a HVAC duct system is designed, the main parameters determining the final design are:
• Application (Comfort /Industrial systems): The flow velocity is determined
• Location/Plan of the place the system is installed:
The proposed design is just focused on a balanced air flow. There will be several issues like:
• Clogging of the ducts
Depending on the area where the duct is erected, there is chances of dust particles
entering inside the duct. The long-term usage will cause the particles to settle down
and eventually leads to block in the ducts. There should be proper filters used in the
systems to avoid this condition.
• Installation
The ceiling height in the buildings need to be considered while designing a ventilation
system. There are other utility services passing through the same area where the duct
needed to be hanged. So before planning and designing the dimensions of duct, the
clearance should the made clear. This factor is not considered in the modified design.
Normally the selection of the shape of the ducts depends on the architecture and ceiling
height of the building where the duct is installed. In the proposed design all the ducts are
rectangular. This is because rectangular ducts are much cheaper and easy to install. But the
design can only be finalised by the ceiling height of the place where it is installed.
Page | 27
7. Reference
Mott, R. L., & Untener, J. A. (2015). Applied fluid mechanics : Global edition.
ProQuest Ebook Central https://ebookcentral.proquest.com
Engineering ToolBox, (2003). Ducts Sizing - Velocity Reduction Method. [online]
Available at: https://www.engineeringtoolbox.com/sizing-ducts-d_207.html
Engineering ToolBox, (2008). Duct Sizing - Equal Friction Method. [online] Available
at: https://www.engineeringtoolbox.com/equal-friction-method-d_1028.html
Page | 28
8. Appendix
Figure 4 Chart to calculate friction loss, velocity and size of ducts.
Page | 29
Figure 5 Rectangular duct dimensions
Figure 6 Dynamic loss coefficient for fittings
Page | 30
9. Student Declaration
I have not copied any part of this report from any other person's work, except as correctly
referenced. No other person has written any part of this report for me.
1. Student Name: Sreeshob Sindhu Anand
Student declaration of the above ___________________________________ signed.
2. Student Name: Muhammed Fajir TK
Student declaration of the above ___________ ________________________ signed.