Air Duct Design

Air Duct DesignThis post will talk about for the Design of Air Duct (Size of Duct ) also will talk about for Air Outlet with example .

Air Duct Design
Air Duct Design

Air Duct design

How to Calculat Air Duct Design

Method of designing and calculating that weigh of the Air Duct.

This post will talk about for the Design of Air Duct(Size of Duct ) with example .
Air duct uses Air Conditioning Systems to transfer cold air from the internal unit to the place to be conditioning .

The duct is divided into 

  • Supply Air Duct ( S.A.D)
  • Return Air Duct ( R.A.D)
  • Fresh Air Duct (F.A.D)
  • Exhaust Air Duct (EX.A.D).

The Material from which the duct is made

  • Galvanized steel 
  • stainless steel 
  • textile (Fabric )
  • Aluminum Cladding 

Several factors must be considered when designing a Duct system . Generally in order of importance , they are as follows :

  • Space availability 
  • Installation cost
  • Air Friction loss
  • Noise level
  • Duct heat transfer and Air flow leakage 
  • Codes and standards requirements 

Suppose That 
1 T.R Take from (300 CFM TO 400 CFM ) but these number are appreciated .


Assume that after calculation ( space area ) its needed to :

  • 3 T.R 
  • 1200 CFM ( Assume 1 T.R take  400 CFM )
  • Velocity = 600 fpm this value from velocity tables ( SMACNA Code )
  • Assume that ( duct thickness 1 mm )

Velocity Table
Velocity Table

This part is pass it air flow rate (1200 CFM) . and the velocity it 600 fpm
Enter the program 

  • Enter the velocity = 600 fpm

Click here ( on calculation ) to know friction

Aspect Ratio ( Width / high less than or equal 1 : 4)

Aspect Ratio
Aspect Ratio

Choose any value or according to Aspect Ratio 
I choose 550 * 350 mm 

1200 CFM divided to 600 cfm in first branch and 600 CFM in second branch 

Enter the program the new value 

  • Air Flow Rate = 600 CFM
  • Air Velocity = 600 fpm

Choose 550 * 200 mm

Vibration isolators (canves )

Its used to prevent vibrations resulting from the indoor unit  to the related duct with indoor unit ( FCU or AHU 

Volume control dampers (VCD )

Volume Control Dampers
Volume Control Dampers (VCD )

Volume damper is used to control the amount of air outside from the outlet air 

Type of Volume damper : 

  • Manual Volume damper .
  • Motorized Volume damper .

Volume Control Dampers (VCD )
Volume  Dampers

Fir damper (F.D)

Fire Damper is installed on return Duct . to prevent smoke from entering the AHU and passing smoke from place to place during Fire .

Fir damper
Fire Damper

Plenum box
Plenum box

Flexible duct

Its a flexible duct used to connected the outlet air and the duct and easy move the outlet air 
flexible duct
Flexible Duct

Type of flexible duct

  • Air connector 
  • Air duct 

Duct leakage Test

  • Smoke Test .
  • Light Test. 
  • Pressure Test .

Duct Wight

Wight = 2 * ( W + H ) * L * T * density 

  • W : duct wight 
  • H : duct depth 
  • L : duct Length 
  • Duct thickness 
  • Density = 7850 kg / kg3

Air Terminal Outlet
Air Terminal Outlet 

    Advantages of uses Air Terminal outlet:

    • Distribute the air place well
    • Give an aesthetic shape to the place 

    Type and shape of Air Terminal outlet : 

    • Square diffuser 
    • Round diffuser 
    • Swirl diffuser 
    • Slot diffuser 
    •  Jet diffuser
    • Perforated diffuser 
    • Air grille and register
    • Fresh air lover 
    • Disk Valve
    • Door grille and under cut 

    Factors that influence the selection of air outlet:

    • Ceiling Type and Decoration 
    • Ceiling high 
    • Construction 

    Square Diffuser
    Square Diffuser
    Square Diffuser

    Square diffuser : two size ( neck size and face size )
    Square diffuser : two size ( neck size and face size ) 

    Factors influencing the determination of output size:

    • Air flow Rate ( CFM)
    • Noise Criteria (NC)
    • Throw
    To determine the nick size (from catalogue or equation) .
    Q = A * V
    Q : air flow rate cfm
    A :diffuser area ( X * Y )
    V: velocity 

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