Duct Design

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ME 425 - Air Distribution & ASHRAE Outlet Selection

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Duct Design

ME 425
Keith E. Elder, P.E.

Duct Design
The purpose of air conditioning ductwork is to deliver air from the fan to the diffusers which distribute the air to the room. Air Moves Through the Ductwork in Response to a Pressure
Difference Created by the Fan
The necessary pressure difference will be a function of the way the ductwork is laid out and sized.
The objective of duct design is to size the duct so as to
minimize the pressure drop through the duct, while keeping the size (and cost) of the ductwork to a minimum.
Proper duct design requires a knowledge of the factors that
effect pressure drop and velocity in the duct.

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ME 425 - Air Distribution & ASHRAE Outlet Selection

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Duct System Pressure
Total Pressure, TP, is Related to the Energy in the Air
Stream, and is equal to:
TP = Static Pressure + Velocity Pressure
Static pressure and velocity pressure increase and
decrease as the air proceeds through the ductwork,
depending on the cross-sectional area of the flow.
The total pressure of the airstream decreases as the
air proceeds through the the ductwork due to the
conversion of mechanical energy to heat caused by
friction

Duct System Pressure

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ME 425 - Air Distribution & ASHRAE Outlet Selection

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Velocity Pressure

V
VP  


 4005 

2

Where:
VP = Velocity Pressure, inches H2O
V = Velocity, feet per minute

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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ME 425 - Air Distribution & ASHRAE Outlet Selection

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Frictional Loss Calculation
Velocity Reduction Method
 Velocity at the fan discharge is pre-selected. The duct
system is designed to provide progressively lower duct
velocities as the air proceeds from the main duct to the
branches.
Static Regain Method
 The ducts are sized so the increase in static pressure at each take-off offsets the pressure loss of the succeeding
section 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:

 L   V  1.82
p  0.03 f  1. 22  

 d   1000 
Rectangular Ductwork is converted to round:

 ab 0.625
De  1.30
 a  b 0.25

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ME 425 - Air Distribution & ASHRAE Outlet Selection

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Equal Friction Chart

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ME 425 - Air Distribution & ASHRAE Outlet Selection

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Maximum Duct Velocity
Application
Residences
Apartments
Hotel Bedrooms
Hospital Bedrooms
Private Offices
Directors Rooms
Libraries
Theatres
Auditoriums
General Offices
High Class Restaurants
High Class Stores
Banks
Average Stores
Cafeterias
Industrial

Controlling Factor
Noise Generation
Main Ducts
600

Controlling Factor - Duct Friction
Main Ducts
Branch Ducts
Supply
Return
Supply
Return
1000
800
600
600

1000

1500

1300

1200

1000

1200

2000

1500

1600

1200

800

1300

1100

1000

800

1500

2000

1500

1600

1200

1800

2000

1500

1600

1200

2500

3000

1800

2200

1500

* From Carrier Air System Design Manual

Approximate Ductwork Cost
Dimension
Up to 12”
13” to 30”
31” TO 54”
55” TO 84”
84” and Over

Gauge*
26
24
22
20
18

lb/SF
0.906
1.156
1.406
1.656
2.156

Galvanized Steel = $8.25 per lb**
* Source: Uniform Mechanical Code
**Source: Recent bids – includes fittings & everything

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ME 425 - Air Distribution & ASHRAE Outlet Selection

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Economical Duct Design
Minimize aspect ratio (a/b)
Minimize total pounds of sheet metal and
Minimize number of reductions (2-inch rule)

Duct Sizing Exercise
An office space...
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