Download - Duct Design 2007
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11ME 425 - Air Distribution & ASHRAE Outlet Selection
ME 425Keith E. Elder, P.E.
Duct Design
Duct DesignThe purpose of air conditioning ductwork is to deliver air fromthe fan to the diffusers which distribute the air to the room.Air Moves Through the Ductwork in Response to a PressureDifference Created by the FanThe necessary pressure difference will be a function of the waythe ductwork is laid out and sized.The objective of duct design is to size the duct so as tominimize the pressure drop through the duct, while keeping thesize (and cost) of the ductwork to a minimum.Proper duct design requires a knowledge of the factors thateffect pressure drop and velocity in the duct.
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22ME 425 - Air Distribution & ASHRAE Outlet Selection
Duct System Pressure
Total Pressure, TP, is Related to the Energy in the AirStream, and is equal to:TP = Static Pressure + Velocity Pressure
Static pressure and velocity pressure increase anddecrease as the air proceeds through the ductwork,depending on the cross-sectional area of the flow.The total pressure of the airstream decreases as theair proceeds through the the ductwork due to theconversion of mechanical energy to heat caused byfriction
Duct System Pressure
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33ME 425 - Air Distribution & ASHRAE Outlet Selection
Velocity Pressure
Where:VP = Velocity Pressure, inches H2OV = Velocity, feet per minute
VP V
4005
2
Static Pressure Losses
Frictional Losses Due to fluid viscosity and turbulence in the flow through the
ductwork, and occur along the entire length of the ductwork
Dynamic Losses Result from flow disturbances caused by fittings that change
the airflow direction or area.
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44ME 425 - Air Distribution & ASHRAE Outlet Selection
Frictional Loss CalculationVelocity Reduction Method Velocity at the fan discharge is pre-selected. The duct
system is designed to provide progressively lower ductvelocities as the air proceeds from the main duct to thebranches.
Static Regain Method The ducts are sized so the increase in static pressure at
each take-off offsets the pressure loss of the succeedingsection of ductwork.
Equal Friction Method The duct is sized to result in a constant pressure loss per
unit length of duct.
Equal Friction Method
Frictional Losses, p, can be approximated:
Rectangular Ductwork is converted to round:
p f LdV
0 03
10001 221 82
..
.
D ab
a be
130
0 625
0 25..
.
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55ME 425 - Air Distribution & ASHRAE Outlet Selection
Equal Friction Chart
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66ME 425 - Air Distribution & ASHRAE Outlet Selection
Maximum Duct VelocityControlling Factor Controlling Factor - Duct Friction
Application Noise Generation Main Ducts Branch DuctsMain Ducts Supply Return Supply Return
Residences 600 1000 800 600 600ApartmentsHotel Bedrooms 1000 1500 1300 1200 1000Hospital BedroomsPrivate OfficesDirectors Rooms 1200 2000 1500 1600 1200LibrariesTheatres 800 1300 1100 1000 800AuditoriumsGeneral OfficesHigh Class Restaurants 1500 2000 1500 1600 1200High Class StoresBanksAverage Stores 1800 2000 1500 1600 1200CafeteriasIndustrial 2500 3000 1800 2200 1500
* From Carrier Air System Design Manual
Approximate Ductwork Cost
Dimension Gauge* lb/SFUp to 12 26 0.90613 to 30 24 1.15631 TO 54 22 1.40655 TO 84 20 1.65684 and Over 18 2.156
Galvanized Steel = $8.25 per lb**
* Source: Uniform Mechanical Code**Source: Recent bids includes fittings & everything
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77ME 425 - Air Distribution & ASHRAE Outlet Selection
Economical Duct DesignMinimize aspect ratio (a/b)Minimize total pounds of sheet metal andMinimize number of reductions (2-inch rule)
Duct Sizing ExerciseAn office space has a sensible peak cooling load of29,400 Btu/hr. The inside design temperature is 78 Fand the supply air temperature is 60F. The duct mustbe lined and yet fit under the beam shown below andnot exceed a friction loss of 0.10 inches per 100 feet.
Determine themost economicalduct size.
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88ME 425 - Air Distribution & ASHRAE Outlet Selection
Dynamic Loss Calculation
pf= C
op
v,o
Fitting Loss Exercise
1000 CFM is flowing througha 16 x 12 rectangular duct.
400 CFM flows through an8 x 12 branch to another room, and the remaining600 CFM continues along the 12 x 12 main duct.
What is the pressure drop, in inches of water,through the branch?
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99ME 425 - Air Distribution & ASHRAE Outlet Selection
Homework
Read 05F35.1-25The ductwork shown below will serve several openoffice spaces. Size the ductwork and calculate themaximum pressure drop. The maximum verticalspace is 12 inches and the branch fittings are of thesame type as the example in the handout.
1200 CFM
900 CFM750 CFM
FROMFAN 1200 FPM
70 ft
30 ft
40 ft
80 ft
40 ft
30 ft
30 ft