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DR.A.M.S
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The main purpose of a duct is to supply conditioned air from the
central cooling coil to required buildings
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And to return the warm air from these buildings to the cooling coil.
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Duct may comprise a single line or with several branch lines
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Duct between the blower and the branch lines is often called Main
duct.
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Duct are usually rectangular/square/ circular
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Fabricated from G.I., black steel, Aluminum, S.S. and Copper.
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Duct system should transport a given volume of air with minimum
pressure drop and heat loss having less noise and vibration.
PRESSURE DROPS IN DUCTS
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As flow continues in a duct, the static pressure of air drops.
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This is due to
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1. Duct friction
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2. Change of direction /velocity.
METHODS OF DUCT DESIGN
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Three common methods:
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i) The equal friction or pressure drop method.
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Pressure drop/unit length Δp/ΔL is to be taken to be the
same for whole system.
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It is quite suitable when the ducts are symmetrical since
pressures are balanced, eliminating the use of dampers
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ii) static regain method :
The drop in pressure during a given length is made up by reducing
velocity at the end of the section.

iii) the velocity reduction method
This a very simple method of duct design.
In this case as the flow proceeds, the velocity decreases.
The velocity in each duct including main duct is adjusted in
order to avoid noise.
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