N14 Duct Design

DUCT DESIGN

DUCT DESIGN FOR “SUPER” SUPERMARKET

Group N14

THIAN CHIK HOW B040710008

SAZALI BIN ABD RAZAK B040810065

ZULKARNAIN BIN OTHMAN B040810112

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)

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

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.

FUNDAMENTALS

DUCT SYSTEMS

Extended plenum system

Individual Round Pipe System

Reducing trunk system

Combination (of two or more

systems)

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

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

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

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

WHAT AIR DUCT MADE OF?

FUNDAMENTALS

HOW TO DESIGN DUCT SYSTEM?

1. DESIGN CRITERIA

2. FACTOR OF PRESSURE AND VELOCITY DROP

DESIGN CRITERIA

Space availability Installation Cost Air Friction Loss Noise levels Duct heat transfer and airflow leakage Codes and standards requirements

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)

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)

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.

Noise level

Undersize duct: Higher Velocities creates noise

Poor selection or installation of fittings: Turbulence (creates additional noise and air

pressure drops)

How to limit noise?

•Use Damper•Better Design•Better Installation

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

How to minimize energy wasting?

ASHRAE 90.1 Energy Code -Insulation and joint seal

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

Pressure Changes

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)

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

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

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

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.


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.


Equal Friction Method

an "initial guess" for duct sizing by establishing a constant pressure loss per unit of duct length.


T-Method

Calculation intensive (use software) Considers current building costs, energy costs

and future costs.

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.

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

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

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

Supermarket layout

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

AHU

1875 m3/h

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

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

0.48Pa/m

For 30000 m3/h and11.18m/s, the friction rate is 0.48Pa/m (within the shadowed area)

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

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)

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.

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

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

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)

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.

N14 Duct Design

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