Present Ducting Group 1

8/3/2019 Present Ducting Group 1

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GROUP 1

Distribution of air in an air conditioning system

NASRUL ALIF BIN AMALUDDIN

&

SURIANI BINTI SHAFIE


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DUCTING NETWORK &

DISTRIBUTION OF AIR

INTRODUCTION

Duct are use in heating, ventilation & airconditioning (HVAC) to deliver and remove air

Consist of supply air, return air & exhaust air

Duct system also call ductwork


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PRESSURE DROP

Pressure drop is cause by:

1. Friction

2. Vertical pipe difference or elevation

3. Change of kinetic energy


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to Determine of pressure drop, 1st find the Reynolds

number

Where,

Re = Reynolds number

= Velocity of flow

D = diameter of pipe

V =Kinematics viscosity


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If the Reynolds number < 2000, than you have laminarflow.

Laminar flow is characterized by the gliding of concentriccylindrical layers past one another in orderly fashion. Thevelocity of the fluid is at its maximum at the pipe axis anddecreases sharply to zero at the wall. The pressure drop causedby friction of laminar flow does not depend of the roughness ofpipe.

If the Reynolds number > 4000, you have turbulent flow.

There is an irregular motion of fluid particles in directionstransverse to the direction of the main flow. The velocitydistribution of turbulent flow is more uniform across the pipediameter than in laminar flow. The pressure drop caused byfriction of turbulent flow depends on the roughness of pipe.


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2nd ) Select Absolute Pipe Roughness :

Included here is a sampling of

absolute pipe roughness edata taken

from Binder (1973). These values are

for new pipes; aged pipes typically

exhibit in rise in apparent roughness.

In some cases this rise can be very

significant. Relative pipe roughness is computed

by dividing the pipe diameter D, by

the absolute roughness e

Relative roughness = D

e

Pipe Material

Absolute Roughness, e

x 10-6 feetmicron

(unlessnoted)

drawn brass 5 1.5

drawn copper 5 1.5

commercial steel 150 45

wrought iron 150 45

asphalted cast iron 400 120

galvanized iron 500 150

cast iron 850 260

wood stave 600 to 3000 0.2 to 0.9 mm

Concrete 1000 to 10,000 0.3 to 3 mm

riveted steel 3000 to 30,000 0.9 to 9 mm
http://www.efunda.com/formulae/bibliography.cfm?ref=binderhttp://www.efunda.com/formulae/bibliography.cfm?ref=binder


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The solutions to this calculation is plotted vs. the Reynolds number to create a Moody Chart.
http://www.engineersedge.com/fluid_flow/pressure_drop/moody_chart.htmhttp://www.engineersedge.com/fluid_flow/pressure_drop/moody_chart.htm


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example Air at 20o C is flowing at 27.69 m/s through 24 inch GI .Is the

flow laminar or turbulent ?

velocity = 27.69 m/s

V(kinematic viscosity)= 1.51 x 10-5 m2 /s

Diameter= 0.610 m

Re = 27.69 m/s (0.610 m )

1.51 x 10-5

m2

/s= 1.118602 x 106

so the flow is turbulent


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Determine Relative pipe roughness :

Relative roughness = De

e Galvanized iron = 500 x 10-6

ftDiameter = 0.610 m

Relative roughness = 0.610 m500 x 10 -6 ft

= 1220


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Apply to moody diagram


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Determine Pressure drop in circular pipes:

Where:

= Pressure Drop

= Pipe Friction Coefficient

L = Length of Pipe

D = Pipe Diameter

p = Density

= Flow Velocity


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HYDRAULIC DIAMETER

The hydraulic diameter - dh- is used to calculate the

dimensionless Reynolds Number to determine if a flow is

turbulent or laminar. A flow is

laminar if Re < 2000

transient for 2000 < Re < 4000

turbulent if Re > 4000

The hydraulic diameter is also used to calculate the pressure lossin a ducts or pipe.


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The hydraulic diameter is not the same as the

geometrical diameter in a non-circular duct or pipe andcan be calculated with the generic equation

dh = 4 A / p (1)where:

dh = hydraulic diameter (m, ft)

A = area section of the duct (m2, ft2) p = wetted perimeter of the duct (m, ft)

H d li Di f Ci l


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Hydraulic Diameter of a Circular

Tube or Duct

Based on equation (1) the hydraulic diameter of a

circular duct can be expressed as:

dh = 4 r2 / 2 r

= 2 r (2)

where

r = pipe or duct radius (m, ft) As we could expect the hydraulic diameter of a

standard circular tube or duct is two times the radius

H d li Di f Ci l


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Hydraulic Diameter of a Circular

Tube with an inside Circular Tube

Based on equation (1) the hydraulic diameter of a

circular duct or tube with an inside duct or tube can be

expressed as

dh = 4 ( ro2- ri2) / (2 ro + 2 ri)

= 2 (ro - ri) (3)

where ro = inside radius of the outside tube (m, ft)

ri = outside radius of the inside tube (m, ft)

H d li Di f R l


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Hydraulic Diameter of Rectangular

Tubes or Ducts

Based on equation (1) the hydraulic diameter of

a rectangular duct or pipe can be calculated as

dh = 2 a b / (a + b) (4)

where

a = width/height of the duct (m, ft) b = height/width of the duct (m, ft)


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EQUIVALENT DIAMETER

The hydraulic diameter is not the same as

the equivalent diameter. The equivalent

diameter is the diameter of a circular duct or

pipe that gives the same pressure loss as arectangular duct or pipe.


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The equivalent diameter of a rectangular tube or

duct can be calculated as (Huebscher)

de = 1.30 x ((a x b) ^0.825) / (a + b)0.25)(1)

where

de = equivalent diameter (mm, inches)

a= length of major or minor side (mm, inches)

b= length of minor or major side (mm, inches)


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Circular equivalent diameter - de (mm)

Duct

side - a

mm

Duct side - b (mm)

100 150 200 250 300 400 500 600 800 1000 1200 1400 1600 1800 2000

100 109 133 152 168 183 207 227150 133 164 189 210 229 261 287 310200 152 189 219 244 266 305 337 365250 168 210 246 273 299 343 381 414 470300 183 229 266 299 328 378 420 457 520 574400 207 260 305 343 378 437 488 531 609 674 731500 227 287 337 381 420 488 547 598 687 762 827 886600 310 365 414 457 531 598 656 755 840 914 980 1041800 414 470 520 609 687 755 875 976 1066 1146 1219 1286

1000

517

574

674

762

840

976

1093

1196

1289

1373

1451

1523

1200 620 731 827 914 1066 1196 1312 1416 1511 1598 16801400 781 886 980 1146 1289 1416 1530 1635 1732 18221600 939 1041 1219 1373 1511 1635 1749 1854 19521800 1096 1286 1451 1598 1732 1854 1968 20732000 1523 1680 1822 1952 2073 2186

Equivalent diameters for some common mm rectangular dimensions are shown in

the table below:


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Equivalent diameters of some common inches rectangular ducts are shown in the

table below:

Equivalent diameter

(inches) Length - a - (inches)

Length - b - (inches) 4 5 6 8 10 12 16

4 4.4 4.9 5.3 6.15 4.9 5.5 6 6.9 7.6

6 5.3 6 6.6 7.6 8.4 9.1

8 6.1 6.9 7.6 8.6 9.8 10.7 12.2

10 7.6 8.4 9.8 10.9 12 13.7

12 9.1 10.7 12 13.1 15.1

16 12.2 13.7 15.1 17.5


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Oval Equivalent Diameter

The equivalent diameter of a oval duct or tube

can be calculated as (Heyt & Diaz)

de = 1.55 A0.625/P0.2 (2)

where

A = cross-sectional area oval duct (m2, in2) P = perimeter oval duct (m, inches)


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The cross-sectional area of an oval duct can beexpressed as

A = ( b2/4) + b(a- b) (2a)

where

a = major dimension of the flat oval duct (m, in) b = minor dimension of the flat oval duct (m, in)

The perimeter of an oval duct can be expressed as

P = b + 2(a- b) (2b)


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FABRICATION OF DUCTING

Duct materials There are several type of ducting material :1. Galvanized steel2. Polyurethane duct board (Preinsulated aluminum

ducts)

3. Fiberglass duct board (Preinsulated non metallic

ductwork)4. Flexible tubing


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Galvanized steel

Various fittings allow transitioning between the various shapes and sizes

can easily be cut and bent to form additional shapes when required.

commonly wrapped or lined with fiberglass thermal insulation, both to reduceheat loss or gain through the duct walls and water vapor from condensing on

the exterior of the duct when the duct is carrying cooled air.

Insulation, particularly duct liner, also reduces duct-borne noise. Both typesof insulation reduce 'breakout' noise through the ducts' sidewalls.

P l h d b d (P i l d


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Polyurethane duct board (Preinsulated

aluminum ducts)

Rectangular ducts are more being manufactured from duct board

no need any further insulation.

Among the various types of rigid polyurethane foam panels available, a new

water formulated panel stands out

the foaming process is obtained through the use of water instead of the CFC,

HCFC, HFC and HC gasses The foam panels are then coated with aluminum sheets on either side, with

thicknesses that can vary from 50 micrometers for indoor use to 200

micrometer for external use in order to guarantee the high mechanical

characteristics of the duct.

The ducts construction starts with the plotting of the single pieces on thepanel. The pieces are then cut from the panel (with a 45 cut as explainedbelow), bent if necessary in order to obtain the different fittings, and finally

closed through an operation of gluing, pressing and taping.


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Fiberglass duct board (Preinsulated non metallic

ductwork)

Also the fiberglass panels provide built-in thermal insulation and the

interior surface absorbs sound, helping to provide quiet operation of the

HVAC system.

The duct board is formed by sliding a specially-designed knife along the

board using a straightedge as a guide; the knife automatically trims out a"valley" with 45 sides; the valley does not quite penetrate the entire depthof the duct board, providing a thin section that acts as a hinge.

The duct board can then be folded along the valleys to produce 90

folds, making the rectangular duct shape in the fabricator's desired size.

The duct is then closed with staples and special aluminum or similar'metal-backed' tape.

Commonly available duct tape should not be used on air ducts, metal,

fiberglass, or otherwise, that are intended for long-term use; the adhesive

on so called 'duct tape' dries and releases with time.


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Fiberglass duct board


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Flexible tubing

known as flex, have a variety of configurations, but for HVAC applications,

they are typically flexible plastic over a metal wire coil to make round, flexible

duct.

Most often a layer of fiberglass insulation covers the duct, and then a thin

plastic layer protects the insulation.

Flexible duct is very convenient for attaching supply air outlets to the rigid

ductwork.

However, thepressure loss through flex is higher than for most other types

of ducts. As such, designers and installers attempt to keep their installed

lengths (runs) short, e.g., less than 15 feet or so, and to minimize turns. Kinks in flex must be avoided. Flexible duct is normally not used on the

negative pressureportions of HVAC duct systems.


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Flexible tubing


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Duct system components

Vibration isolation A duct system often begins at an air handler.

The blowers in the air handlers can create substantial vibration and the large

area of the duct system would transmit this noise and vibration to theinhabitants of the building.

To avoid this, vibration isolators(flexible sections) are normally inserted into the

duct immediately before and after the air handler.

The rubberized canvas-like material of these sections allow the air handler to

vibrate without transmitting much vibration to the attached ducts.


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Take-offs Downstream of the air handler, the supply air trunk duct will commonly

fork, providing air to many individual air outlets such as diffusers, grilles, and

registers. When the system is designed with a main duct branching into many subsidiary

branch ducts, fittings called take-offsallow a small portion of the flow in the

main duct to be diverted into each branch duct.

Take-offs may be fitted into round or rectangular openings cut into the wall

of the main duct.

The take-off commonly has many small metal tabs that are then bent to retainthe take-off on the main duct; round versions are called spin-in fittings.

Other take-off designs use a snap-in attachment method, sometimes coupled

with an adhesive foam gasket to provide improved sealing.

The outlet of the take-off then connects to the rectangular, oval, or round

branch duct.


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Terminal units While single-zone constant air volume systems typically don't have them,

other types of air distribution systems often have terminal units in the

branch ducts.

Usually there is one terminal unit per thermal zone. Some types of terminal

units are VAV 'boxes' of either single or dual duct, fan-powered mixing boxes

of either parallel or series arrangement, and induction terminal units.

Terminal units may also include either, or both, a heating or cooling coil.
http://rds.yahoo.com/_ylt=A0S020lK96BHOgQB.l2JzbkF;_ylu=X3oDMTBpc2VvdmQ2BHBvcwM3BHNlYwNzcgR2dGlkAw--/SIG=1gr3ah2hf/EXP=1201817802/**http%3A//images.search.yahoo.com/search/images/view%3Fback=http%253A%252F%252Fimages.search.yahoo.com%252Fsearch%252Fimages%253Fp%253DTerminal%252Bunits%2526y%253DSearch%2526ei%253DUTF-8%2526fr%253Dyfp%2526x%253Dwrt%2526js%253D0%2526ni%253D21%26w=175%26h=136%26imgurl=www.advanced-air.co.uk%252Fnew%252Fimages%252Funits_vav.jpg%26rurl=http%253A%252F%252Fwww.advanced-air.co.uk%252Fnew%252Funits_vav.htm%26size=3.6kB%26name=units_vav.jpg%26p=Terminal%2Bunits%26type=jpeg%26no=7%26tt=3,265%26oid=7bf4793bb16c8892%26ei=UTF-8


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Air terminals 'Air terminals' are the supply air outlets and 'return' or 'exhaust air inlets'.

For supply, diffusers are most common, but grilles, and for very small HVAC

systems such as in residences, 'registers' are also used widely.

Return or 'exhaust grilles' are used primarily for appearance reasons, but some

also incorporate an air filter and are known as 'filter returns'.

Documents

Present Ducting Group 1