n14 duct design
DESCRIPTION
Duct Design fundamentals and Example(Equal friction method)TRANSCRIPT
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DUCT DESIGN
DUCT DESIGN FOR “SUPER” SUPERMARKET
Group N14
THIAN CHIK HOW B040710008
SAZALI BIN ABD RAZAK B040810065
ZULKARNAIN BIN OTHMAN B040810112
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FUNDAMENTALS
WHAT IS AIR DUCT?
-An enclosed conduit through which convey conditioned air from HVAC units to desired space and may also bring along fresh and recirculated air to the units. (Veavarz, Kumar, & Hussain, 2006)
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FUNDAMENTALSDUCT TERMS
supply or return duct that connects to the air source specially air handling unit, rooftop unit or fan coil and distributes the air around
the building
extend outward from the trunk duct, forming a tree pattern
across the floor
connect VAV (variable air volume) and CV(constant
volume) terminals to a branch duct
or directly to trunk duct.
( as a fitting) either connects a runouts duct to a branch or trunk duct in order to distribute air to a terminal , or connects (as a duct) the header duct to the room air distribution devices ( diffuser , registers or grillers )
distributes zone air from terminals
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FUNDAMENTALS
WHY AIR DUCT NEED TO BE PROPERLY DESIGNED?
TO AVOID UNDER SUPPLY AND OVER SUPPLY FOR AREAS
TO AVOID EXCESSIVE NOISE, VIBRATION, DISCOMFORT, AND ADVERSE HEALTH EFFECTS
TO SIZE THE DUCT WITH THE PURPOSE OF MINIMIZE PRESSURE FROP THROUGH DUCT WHILE KEEPING MINIMUM SIZE AND COST OF THE DUCTWORK.
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FUNDAMENTALS
DUCT SYSTEMS
Extended plenum system
Individual Round Pipe System
Reducing trunk system
Combination (of two or more
systems)
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CLASSIFICATION OF DUCT SYSTEM1. Low pressure systems Velocity ≤ 10 m/s, static pressure ≤ 5 cm H2O (g)
2. Medium pressure systems Velocity ≤ 10 m/s static pressure ≤ 15 cm H2 O (g)
3. High pressure systems Velocity > 10 m/s, static pressure 15<p ≤ 25 cm H2 O (g)
FUNDAMENTALS
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High velocities in the ducts:
1. Smaller ducts
Lower initial cost and lower space requirement
2. Higher pressure drop
Larger fan power consumption
3. Increased noise
Requires noise attenuation
FUNDAMENTALS
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Air velocities depend mainly on: 1. application 2.noise criteria
Typical recommended velocities: Residences : 3 m/s to 5 m/s Theatres : 4 m/s to 6.5 m/s Restaurants : 7.5 m/s to 10 m/s
If nothing is specified: Velocity of 5 to 8 m/s is used for main ducts Velocity of 4 to 6 m/s is used for the branches.
The allowable air velocities can be as high as 30 m/s in ships and aircrafts to reduce the space requirement.
FUNDAMENTALS
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DUCT FITTINGS
Fitting Function
Geometry Category Sequential Number
S: Supply D : Round (Diameter)
1. Entries2. Exits
1,2,3,…n
E: Exhaust/Return
R: Rectangular 3. Elbows4. Transitions
C: Common (Supply and return)
F : Flat oval 5. Justions6. Obstructions7. Fan and
system interactions
8. Duct-mounted equipment
9. Dampers10.Hoods
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WHAT AIR DUCT MADE OF?
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FUNDAMENTALS
HOW TO DESIGN DUCT SYSTEM?
1. DESIGN CRITERIA
2. FACTOR OF PRESSURE AND VELOCITY DROP
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DESIGN CRITERIA
Space availability Installation Cost Air Friction Loss Noise levels Duct heat transfer and airflow leakage Codes and standards requirements
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DESIGN CRITERIA
SPACE AVAILABILITY-Ceiling plenums, duct chases and
obstructions(walls and beams) often dictate that a certain size duct be used
-Coordination is required to avoid sprinkler piping, power and communication conduit , light fixtures and audio speakers
-Larger trunk and branch ducts require greater coordination (to be located in the core areas of the building)
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Installation CostFACTOR 1. Size of ducts 2. Type of ductwork material 3. Number and complexity of duct fittings 4. Height or complexity of site conditions
Ductwork : 15% of total cost (Carrier,2011)
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Air Friction Loss
Affected by : 1. Duct Size 2. Duct Shape 3. Material 4. Fittings
Economical: use smooth, efficient fittings with low
turbulence to reduce the duct system air pressure drop, and use as few fittings as possible to lower the installation cost.
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Noise level
Undersize duct: Higher Velocities creates noise
Poor selection or installation of fittings: Turbulence (creates additional noise and air
pressure drops)
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How to limit noise?
•Use Damper•Better Design•Better Installation
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Heat Transfer And Leakage
Heat Gain/Loss : Ductwork that runs through very warm or very cold
areas Effect: reduces the capacity of the cooling and
heating equipment.
Leaky ducts: Same energy wasting effect May create odors Stained ceiling tiles formation of condensation at the
leak points
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How to minimize energy wasting?
ASHRAE 90.1 Energy Code -Insulation and joint seal
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FACTOR OF PRESSURE AND VELOCITY DROP
TOTAL PRESSURE = STATIC PRESSURE + VELOCITY PRESSURE
Static pressure and velocity pressure increase and decrease as the air proceeds through the ductwork, depending on the cross-sectional area of the flow.
The total pressure of the airstream decreases as the air proceeds through the ductwork due to the conversion of mechanical energy to heat caused by friction
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Pressure Changes
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VELOCITY PRESSURE
V Pv1 0.602
2 2.408
0
0
0
0
0
0
0
2602.0 VPv
For air at standard conditions (1.204 kg/m3)
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Static Pressure Losses Frictional Losses
Friction losses can be found by using the frictional chart for round duct.
Due to fluid viscosity and turbulence in the flow through the
ductwork, and occur along the entire length of the ductwork
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Equivalent Duct Size
Height Width De10 20 15.23375
#DIV/0!#DIV/0!#DIV/0!#DIV/0!#DIV/0!#DIV/0!#DIV/0!#DIV/0!#DIV/0!#DIV/0!#DIV/0!
For Non-circular ducts, hydraulic diameter
Page 797
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Static Pressure Losses
Dynamic LossesResult from flow disturbances caused by fittings that change
the airflow direction or area
hvo Co hLd0
For Co value, refer ASHRAE Handbook Page 816-820.
velocity head at section o
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Frictional Loss Calculation
Velocity Reduction Method
Velocity at the fan discharge is pre-selected. The duct system is designed to provide progressively lower duct velocities as the air proceeds from the main duct to the branches.
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Frictional Loss Calculation
Static Regain Method
The ducts are sized so the increase in static pressure at each take-off offsets the pressure loss of the succeeding section of ductwork.
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Frictional Loss Calculation
Equal Friction Method
an "initial guess" for duct sizing by establishing a constant pressure loss per unit of duct length.
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Frictional Loss Calculation
T-Method
Calculation intensive (use software) Considers current building costs, energy costs
and future costs.
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Equal Friction Method1. Volume of air required to each space within
building (heat losses, heat gains and/or ventilation load)
2. Sizes and location of all supply air outlet and return inlets
3. Building plan and sketch duct system
4. Lay out a return air duct system.
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4. Size the supply and return air duct system.
5. Determine friction loss of supply air duct(critical path)
6. Analyze return air system with same method.
7. Select supply fan for the system
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DUCT DESIGN PROBLEM BASED GUIDELINE
50 m
40 m
12.5m
10 m
125 m2
PROBLEM INFORMATION Supermarket ( 2000m2) height=3m Air change rate : Supermarkets 4 – 10 1/hr (5 1/hr) Duct maintain at 1500mm width
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Air flow (m3/h) = room volume x air change= 2000 x 3 x 5= 30000 m3/h
Air flow for each section = 125 x 3 x 5=1875 m3/h
Assume r/W for elbows = 1.0
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Supermarket layout
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
AHU
1875 m3/h
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15m
5m 10m
15m15m5m
2
1
3 4 5
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
AHU
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Design Steps Step 1: Uniform design friction rate
Main duct velocity is up to 2200 fpm(11.18m/s),up to 900 fpm (4.57m/s)for branch for public building.
Based on supermarket layout, Total capacity = 1875 x 16 = 30000 m3/h
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0.48Pa/m
For 30000 m3/h and11.18m/s, the friction rate is 0.48Pa/m (within the shadowed area)
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Duct section capacity Design Friction rate Round duct Rect Duct Rect Duct Rect duct Velocity Velocity Pressure
m3/h Pa/m mm W(mm) H(mm) V (m/s) =Q/A Pa12 30000 0.48 2000 2300 1500 1.811594203 1.975687881
2B/2C 1875 0.48 630 275 1500 3.006253006 5.4406093972A/2D 1875 0.48 630 275 1500 3.006253006 5.44060939723 22500 0.48 1700 1700 1500 2.162629758 2.815534416
3F/3G 1875 0.48 630 275 1500 3.006253006 5.4406093973E/3H 1875 0.48 630 275 1500 3.006253006 5.44060939734 15000 0.48 1450 1200 1500 2.394636015 3.452037551
4J/4K 1875 0.48 630 275 1500 3.006253006 5.4406093974I/4L 1875 0.48 630 275 1500 3.006253006 5.44060939745 7500 0.48 1100 700 1500 2.705627706 4.406893611
5N/5O 1875 0.48 630 275 1500 3.006253006 5.4406093975M/5P 1875 0.48 630 275 1500 3.006253006 5.440609397
Step 2: Result Table
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Step 3: Flow quantities and design friction rate
Step 4: Find round duct diameter (air flow rate and constant design friction rate chart)
Step 5: Convert round duct to rectangular equivalent sizes
Step 6: Find airflow velocity in each section (V=Q/A)
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Step 7 : Critical Path
Determine the greatest pressure loss for the system
By inspection, it is found that greatest loss is from 1 to outlet P.
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Loss 1 to Outlet PElbow between 5 and P
assume r/W= 1 Aspect ratio H/W= 5.714
From Fitting Loss Coefficients data, Co = 0.21Therefore elbow loss,
(hL)d = Co x hv = 0.21 x 4.687 = 0.984 Pa
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
AHU
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Straight duct loss, (hL)1-5 =0.48 Pa/m x 66 m
= 31.68 Pa
Total loss A to outlet 5 = 0.984+ 31.68 = 32.664 Pa
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Step 8 : Total static pressure
Total pressure at A = duct lost A - 4 + diffuser loss + velocity head in section 5P= 0.984+ 31.68 + 5.44
= 38.104 Pa
Step 9 : Select Fan (based on catalogue)
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Special thanks to…
Ashrae Handbook Committee. (2001). ASHRAE HVAC 2001 Fundamentals Handbook.
Carrier Corporation.(2005). Duct Design. Level 1: Fundamentals. Technical Development Program. New York: Carrier.
Haines & Wilson.(2003). HVAC system design handbook. New York: McGraw-Hill
Kharagpur. (2011). Slides Lesson 38: Design of Air Conditioning Ducts.
Mark Terzigni.(2009). SMACNA Presentation Slides.
Razali. (2011). Slides M13 & M14 : Introduction to Duct Design and Duct Sizing Example.
Vedavarz, Kumar, & Hussain.(2006). HVAC: Handbook of heating, ventilation, and air conditioning for design and implementation.