HVAC - Ducting system by Chidanand

AIR DUCTING SYSTEM
CHIDANANDA C
TKM08589
P1-Paint
Process Engg. Service Source : Internet & eBook
Engineer self study
TOYOTA KIRLOSKAR MOTORS Pvt.Ltd.
Prepared by,

Content:
1. What is Duct & Duct design
> Typical illustration of duct work
2. Ducting Materials
> Galvanized steel
> Aluminium (Al)
> Polyurethane and phenolic insulation
panels (pre-insulated air ducts)
> Fiberglass duct board (preinsulated
non-metallic ductwork)
> Flexible ducting
> Fabric ducting
> PVC low-profile ducting
> Waterproofing
3. Components of Duct System
> Vibration isolators
> Take-offs
> Stack boots and heads
> Volume control dampers
> Smoke and fire dampers
> Turning vanes
> Plenums
> Terminal units
> Air terminals & Louvers
> Duct joints
Prepared by CHIDANAND 2

Content:
4. Air leakage in duct
> Why duct leak test required
> Where Does Duct Leakage Occurs
> Methods to test Duct Leakage
5. Duct sealing
> Duct sealing methods – Duct tape,
Water based sealant, Mastic resin-
based sealant, Metallic tapes
6. Duct maintenance
> Methods - Contact vacuuming, Air
sweeping, Power brushing, Sealing of
air leak.

1. What is Duct & Duct design
• Ducts are tube like structure or conduits or passages used
in heating, ventilation and air conditioning (HVAC) to
deliver and remove air from the designated area. The
needed airflows include supply air, return air, and exhaust
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.
• A duct system is also called ductwork. Planning (laying out),
sizing, optimizing, detailing, and finding the pressure losses
through a duct system is called duct design.

➢ Typical illustration of duct work

2. Ducting Materials
A. Galvanized steel
B. Aluminium (Al)
C. Polyurethane and phenolic insulation panels (pre-insulated air ducts)
D. Fiberglass duct board (preinsulated non-metallic ductwork)
E. Flexible ducting
F. Fabric ducting
G. PVC low-profile ducting
H. Waterproofing

A. Galvanized steel
• Galvanized mild steel is the standard and most common
material used in fabricating ductwork because the zinc
coating of this metal prevents rusting and avoids cost of
painting. For insulation purposes, metal ducts are typically
lined with faced fiberglass blankets (duct liner) or
wrapped externally with fiberglass blankets (duct wrap).
• Rectangular ductwork commonly is fabricated to suit by
specialized metal shops. For ease of handling, it most
often comes in 4' sections (or joints). Round duct is made
using a continuous spiral forming machine which can
make round duct in nearly any diameter when using the
right forming die and to any length to suit, but the most
common stock sizes range evenly from 4" to 24" with 6"-
12" being most commonly used.

B. Aluminium (Al)
• Aluminium ductwork is lightweight and quick to install.
Also, custom or special shapes of ducts can be easily
fabricated in the shop or on site because of it’s ductile
property.
• The ductwork construction starts with the tracing of the
duct outline onto the aluminium preinsulated panel. The
parts are then typically cut at 45°, bent if required to
obtain the different fittings (i.e. elbows, tapers) and finally
assembled with glue. Aluminium tape is applied to all
seams where the external surface of the aluminium foil
has been cut. A variety of flanges are available to suit
various installation requirements. All internal joints are
sealed with sealant.
• Aluminum can also used to make round spiral duct.

C. Polyurethane and phenolic insulation panels
(pre-insulated air ducts)
• Traditionally, air ductwork is made of sheet metal which was installed first and then
lagged with insulation. Today, a sheet metal fabrication shop would commonly
fabricate with galvanized steel duct and insulate with duct wrap prior to installation.
However, ductwork manufactured from rigid insulation panels does not need any
further insulation and can be installed in a single step. Both polyurethane and phenolic
foam panels are manufactured with factory applied aluminium facings on both sides.
• The thickness of the aluminium foil can vary from 25
micrometers for indoor use to 200 micrometers for
external use or for higher mechanical characteristics.
• Rigid polyurethane foam panels are made up off
including a water formulated panel, foaming process is
obtained through the use of water and CO2 instead
of CFC, HCFC, HFC and HC gasses.

D. Fiberglass duct board (preinsulated non-metallic
ductwork)
• Duct board is a pressed fiberglass sheet with a binder that
is commonly used in heating and cooling systems. ... These
sheets are cut and formed into squares that are
connected and provide a path for airflow in the heating
and cooling system.
• Fiberglass duct board panels provide built-in thermal
insulation and the interior surface absorbs [sound],
helping to provide quiet operation of the HVAC system.
• The duct board can be folded along the groove to produce
90° folds, making the rectangular duct shape in the
fabricator's desired size. The duct is then closed with
outward-clinching staples and special aluminum or similar
metal-backed tape.

E. Flexible ducting
• Flexible ducts are typically made of flexible plastic
over a metal wire coil to shape a tube. They have
a variety of configurations. Glass wool and
polyester fiber for thermal insulation. A protective
layer surrounds the insulation, and is usually
composed of polyethylene or metalized PET.
• Flexible duct is very convenient for attaching
supply air outlets to the rigid ductwork. It is
commonly attached with long zip ties or metal
band claps. However, the pressure loss 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 try to minimize turns.
• Kinks (sharp twist or curve) in flexible ducting
must be avoided. however flexible duct can
tolerate moderate negative pressures.

F. Fabric ducting
• This is actually an air distribution device and is not intended as a conduit for conditioned air.
Usually made of polyester material, fabric ducts can provide a more even distribution and
blending of the conditioned air in a given space than a conventional duct system. They may
also be manufactured with vents or orifices.
• Fabric ducts are available in various colors. The determination which fabric is appropriate
(i.e. air-permeable or not) can be made by considering if the application would require an
insulated metal duct. If so, an air-permeable fabric is recommended because it will not
commonly create condensation on its surface and can therefore be used where air is
supplied below the dew point. Material that eliminates moisture may be healthier for the
occupants. It can also be treated with an anti-microbial agent to inhibit bacterial growth.
Porous material also tends to require less maintenance as it repels dust and other airborne
contaminants.
• Fabric made of more than 50% recycled material is also available. The material can also
be fire retardant, which means that the fabric can still burn, but will extinguish when the
heat source is removed.
• Fabric ducts are not rated for use in ceilings or concealed attic spaces. Fabric ducting usually
weighs less than other conventional ducting and will therefore put less stress on the
building's structure. The lower weight allows for easier installation.

Fabric ducting

G. PVC low-profile ducting
• It has a compact design that makes it ideal for
applications where space is limited. It also features
a smooth inner surface which minimizes pressure
loss and therefore improves air flow.
• Supaflo Flatpack duct is a low profile alternative to
round PVC pipe commonly used in ventilation
systems.
• PVC low-profile ducting has been developed as a
cost-effective alternative to steel low-profile
ducting. Low-profile ducting has been used
extensively in non-industrial areas. The growth of
low-profile ducting has grown significantly due to
the reduction of available space in ceiling cavities in
an effort to reduce cost.

H. Waterproofing
• The finish for external ductwork exposed to the weather can be sheet steel
coated with aluminium or an aluminium/zinc alloy, a multilayer laminate, a
fiber reinforced polymer or other waterproof coating

3. Components of Duct System
A. Vibration isolators
B. Take-offs
C. Stack boots and heads
D. Volume control dampers
E. Smoke and fire dampers
F. Turning vanes
G. Plenums
H. Terminal units
I. Air terminals & Louvers
J. Duct joints

A. Vibration isolators
• A duct system often begins at an air handler.
The blowers in the air handler can create
substantial vibration, and the large area of the duct
system would transmit this noise and vibration to
the inhabitants 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 is used, these
sections allows the air handler to vibrate without
transmitting much vibration to the attached ducts.
The same flexible section can reduce the noise that
can occur when the blower engages and positive air
pressure is introduced to the ductwork.

B. Take-offs
• When the system is designed with a main duct
branching into many subsidiary branch ducts, fittings
called take-offs allow a small portion of the flow in the
main duct to be diverted into each branch duct.
• Downstream of the air handler, the supply air trunk
duct will provide air to many individual air outlets such
as diffusers, grilles, and registers.
• 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 attach the take-off to 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 for improved
sealing. The outlet of the take-off then connects to the rectangular, oval, or
round branch duct Prepared by CHIDANAND 18

C. Stack boots and heads
• Ducts which oftenely allow air to travel
vertically within relatively thin walls. These
vertical ducts are called stacks and are
formed with either very wide and
relatively thin rectangular sections or oval
sections at the bottom of the stack.
• A stack boot provides a transition from an
ordinary large round or rectangular duct to
the thin wall-mounted duct. At the top,
a stack head can provide a transition back
to ordinary ducting while a register
head allows the transition to a wall-
mounted air register.

D. Volume control dampers
• The rotating plates which is placed inside the duct which help to adjust the air
flow volume to the other parts of this duct system. These dampers can be fixed
within the duct itself and it can be controlled by automatic or manual.
• Volume control dampers besides the regulation provided at the registers or
diffusers that spread air into individual rooms, dampers can be fitted within the
ducts itself.
• (VCDs; not to be confused with smoke/fire dampers).

E. Smoke and fire dampers
• Fire dampers are passive fire protection products used in heating, ventilation, and air
conditioning (HVAC) ducts to prevent the spread of fire inside the ductwork through fire-
resistance rated walls and floors.
• Smoke dampers are driven by a motor, referred to as an actuator. A probe connected to the
motor is installed in the running duct and detects smoke, either in the air which has been
extracted from or is being supplied to a room, or elsewhere within the run of the duct. Once
smoke is detected, the actuator will automatically close the smoke damper until it is
manually re-opened.
• Fire dampers can be found in the same places as smoke dampers, depending on the
application of the area after the firewall. Unlike smoke dampers, they are not triggered by
any electrical system (which is an advantage in case of an electrical failure where the smoke
dampers would fail to close). Vertically mounted fire dampers are gravity operated, while
horizontal fire dampers are spring powered. A fire damper's most important feature is a
mechanical fusible link which is a piece of metal that will melt or break at a specified
temperature. This allows the damper to close (either from gravity or spring power),
effectively sealing the duct, containing the fire, and blocking the necessary air to burn.

➢ Smoke and Fire dampers

F. Turning vanes
• Turning vanes are sheet metal pieces inside of mechanical ductwork used to
smoothly direct air inside a duct (e.g. at 90° turns) where there is a change
in direction, by reducing resistance and turbulence.
• Shape and size of the vanes determines the effective directional change of
the air pressure.

➢ Turning vanes best practices

G. Plenums
• Plenums are the central distribution chamber and collection units for an HVAC (Heating,
ventilation & Air Conditioning) system.
• The return plenum carries the air from several large return grilles (vents) or bell mouths to
a central air handler. The supply plenum directs air from the central unit to the rooms which
the system is designed to heat or cool.
• The ductwork that distributes the heated or cooled air to individual rooms of the house
connects to the plenum

H. Terminal units
• Terminal unit. A small component that contains a heating coil, cooling
coil, automatic damper, or some combination of the three. Used to
control the temperature of a single room.
• Two types of Terminal units are available on different requirements are
constant air volume (CAV) and variable air volume unit (VAV).

I. Air terminals & Louvers
• 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.
• Louvers are a type of damper consisting of several blades mounted parallel across
a duct at air terminals. They are versatile and are a lightweight, compact, and have
a low leakage to the outside environment.
• Louvers are used to divert the air flow in different direction at the air terminals or
at the run duct.
Louvers

J. Duct Shape & Joints
• Ducts commonly used for carrying air are of round, square, or rectangular
shape. All have advantages and disadvantages, and find applications where
one is definitely superior to the other.
a. Round Ducts - The duct shape that is the most efficient (offers the least
resistance) in conveying moving air is a round duct, because it has the
greatest cross-sectional area and a minimum contact surface. In other
words, it uses less material compared to other shape ducts for the same
volume of air handled.
b. Rectangular Ducts - Square or rectangular ducts fit better to building
construction. They fit above ceilings and into walls, and they are much
easier to install between joists and studs.
c. Oval Ducts - Flat oval ducts have smaller height requirements than round
ducts and retain most of the advantages of the round ducts. However,
fittings for flat oval ducts are difficult to fabricate or modify in the field.

➢ Shapes of duct
1. Round shape 2. Square/Rectangular
shape
3. Oval shape

➢ Duct joints
• Connection between one duct to the other is called duct joint.
• Elbows and other connections, such as branches, are designed using geometric
principles. Types of duct connections used in constructing duct systems are shown
in Figure.

4. Air leakage in duct
• Duct leakage refers to the leakage of air from the air distribution system
ductwork. The leakage can occur at joints, seams and penetrations. This
leakage causes two problems: 1) some areas at the end of the run may be
short of air flow, and 2) the fan energy increases as the cube of the air leakage
increases. For example, if a system has 10% leakage and the fan design was
originally 20 Horsepower (HP), the new motor required to overcome the 10%
loss will be 20 HP x (1.1)3 = 26.6 HP. In this case, the owner pays the equivalent
of 6.6 HP in additional fan energy for the life of the building.
• The air duct leakage can be represented as a function of the duct surface
area, leakage class, and static pressure. Leakage class is determined by the
construction methods employed in duct fabrication in accordance with the
HVAC duct construction standards.

➢ Why duct leak test required
• Conserve energy
a. 10-30% of heated/cooled air lost through
ductwork.
b. Leaky supply ducts don’t delivery air where
needed.
c. Leaky return ducts add load.
• Affects Indoor Air Quality
a. Leaky returns can pull in air from
uncontrolled spaces, causing Humidity
problems and Contaminants

➢ Where Does Duct Leakage Occurs
• Transverse Joint
Duct-to-duct, branch, -tap, etc.
• Longitudinal Seam
Joining of 2 edges in direction
of airflow.
• Penetration
Rod, wire, tubing, etc.
• Seal points
Spiral seams, Screws &
fasteners, Damper rods

➢ Methods to test Duct Leakage
a. Anemometer: The anemometer is a device used to measure the velocity of
air travelling through a duct or grille. Readings are measured in feet per
minute (fpm). Air flow is measured in cfm, therefore: Air Flow (cfm) =
Velocity (fpm) x Area (ft2) For example, if the mean velocity in a rectangular
duct 12” x 24” is 1,000 fpm, the volume flow rate is 12 x 24 x 1,000/144 =
2,000 cfm.
b. Manometer: The manometer is a device used to measure relatively low
pressures or more commonly, pressure devices. Manometers are made in
two (2) different styles. Both are used to measure the pressure difference
across an object, e.g. filter, coil and fan:
i. U-Tube Manometer
ii. Inclined Manometer
And many other modern techniques.

5. Duct sealing
• Air pressure combined with air duct
leakage can lead to a loss of energy in
a HVAC system. Sealing leaks in air ducts
reduces air leakage, optimizes energy
efficiency, and controls the entry of
pollutants into the building. Before
sealing ducts it is imperative to ensure
the total external static pressure of the
duct work, and if equipment will fall
within the equipment manufacturer's
specifications. If not, higher energy usage
and reduced equipment performance
may result.

➢ Duct sealing methods
• Air leakage can be arrested by
a. Duct tape (made up of polythene or plastic or fiber) can be used on
air ducts, but that are not intended for long-term use. The adhesive
on so called duct tape dries and releases with time.
b. A more common type of duct sealant is a water-based paste that is
brushed or sometimes sprayed on the seams when the duct is built.

C. Mastic is a resin-based sealant that widely used
in all kinds of duct repair. It creates a flexible yet
solid joint and to form airtight seals around ducts.
It’s also a waterproof, resists temperature
extremes that affect ventilation systems, and will
not cause corrosion over time to metal surfaces.
D. Metallic tapes are the more proficient “duct
tapes” used in professional duct sealing, and some
technicians prefer them to mastic because they
create an actual mechanical seal and tend to have
greater longevity than resin-based sealant. Most of
these tapes are made from foil because its
lightweight. Metallic tapes have high metal
resistance and have proven their durability in other
industries such as aerospace, transportation, and
automotive. Prepared by CHIDANAND 37

6. Duct Maintenance
The process of preserving a condition of duct in
its most efficient form to support HVAC system.
• When there’s a air leak in the system.
• When there is substantial evidence of visible
mold growth inside the hard surface of ducts
or on other components of the heating and
cooling system.
• Ducts are infested with vermin, e.g. (rodents or
insects).

➢ Duct Maintenance methods
• Methods of duct cleaning vary, although standards have been
established by industry associations concerned with air duct
cleaning.
• Typically, a service provider will use specialized tools to dislodge
dirt and other debris in ducts.
• Common duct cleaning methods include:
(1) Contact vacuuming.
(2) Air sweeping and.
(3) Power brushing.
(4) Sealing of air leak.

➢ Duct Maintenance methods
3. Power
Brushing
1. Contact
vacuum cleaning
2. Air sweeping

THANK YOU

HVAC - Ducting system by Chidanand

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