duct system in large commercial building

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Duct system in large Commercial Buildings
Major assignment
of
Refrigeration &Air conditioning
Submitted To:- Submitted By:-
Mr.Aditya Mishra Nilesh Prajapati
B.tech 6th Sem [Me]

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Introduction:-
THE DUCT SYSTEM, used in air heating and air cooling your home, and large
commercial buildings is a collection of tubes that distributes the heated or
cooled air to the various rooms. This system can make a big difference in both
the cost and the effectiveness of heating and cooling the home. The duct
system can have an important effect on health of the occupants through the
distribution of indoor air pollution. Changes and repairs to a duct system
should always be performed by a qualified professional. This brochure is meant
to help you understand the problems that can affect the duct system and how
you can:
• Save money
• Improve comfort
• Protect health

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General rules for duct design:-
1. Air should be conveyed as directly as possible to save space, power
and material
2. Sudden changes in directions should be avoided. When not possible
to avoid sudden changes, turning vanes should be used to reduce
pressure loss
3. Diverging sections should be gradual. Angle of divergence ≤ 20o
4. Aspect ratio should be as close to 1.0 as possible. Normally, it should
not exceed 4
5. Air velocities should be within permissible limits to reduce noise and
vibration
6. Duct material should be as smooth as possible to reduce frictional
losses

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Lay Out

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Objectives
1 to characterize energy losses in the duct systems. within
large commercial buildings, along with the pertinent physical
features of the ducts
2 develop and evaluate methods of measuring duct system
energy losses. Our broader program of research on large
commercial ducts includes modeling to quantify the
influence of these losses on HVAC system energy use 8 and
an investigation of the feasibility of sealing ducts in large
commercial buildings by injecting sticky aerosols into the
ducts.

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Duct Design and Fan Performance
Commercial and residential and, industrial air duct system design must consider
1) Space air diffusion
2) Space availability
3) Noise level
4) Duct leakage
5) Duct heat gain and losses
6) Balancing
7) Fire and smoke control
8) Initial investment
9) System operating cost

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DUCT MATERIALS
•Ductwork must meet local codes
•For years, galvanized sheet metal was used exclusively
Other ductwork materials
Aluminum
Fiber glass duct
Board Spiral metal duct
Flexible duct

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Benefits of Properly Designed Installed Duct System
Adequate air flow
•Less conditioned air leakage
•Smaller, less expensive equipment
•Longer equipment service life
•Healthier, safer indoor air quality
•Reduced house air leakage
•Fewer call backs
•No Potential for Back drafting Appliances Higher energy efficiency
Greater comfort for occupants

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AIR-MEASURING INSTRUMENTS FOR DUCT SYSTEMS
Velometer – Measures actual air velocity (how fast the
air is actually moving in the duct)
Air volume in cfm can be calculated by multiplying the
air velocity by the cross-sectional area of the duct in
square feet
Pitot tube – Used with special manometers for
checking duct pressure

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DUCT INSULATION
A 15°F temperature difference from the inside of
the duct to the outside of the duct is considered
the maximum difference allowed before insulation
is necessary
Metal duct can be insulated on the outside and on
the inside
The insulation is joined by lapping it, stapling it,
and taping it

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SUMMARY
Duct systems can be made of galvanized metal, aluminum,
fiberglass duct board, spiral metal, flexible duct or a
combination of different materials
Branch ducts deliver the proper amount of air to remote
locations in the structures
Balancing dampers are used to help ensure proper airflow
to the remote locations
The return air system can be configured as a central or
individual return air system

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Friction in the duct slows the air flowing in it
Slower air experiences less friction
Air balancing ensures the proper amount of air
is delivered to each supply register
CFM = velocity x cross sectional area
The friction chart is used to properly size duct
systems

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