What is ducting/Types of ducting/ thickness of GI duct sheet/How to calculate the area of ducting?

What is ducting?
Ducting image

  • Ducts are passages used in heating, ventilation, and air conditioning to deliver and remove air.
  • The Duct system carries air conditioning air from air conditioning equipment to air distribution points and carries return air from the room to air conditioning equipment for reconditioning and recirculation.
  • Example: Supply air duct, return air duct, exhaust air duct, and fresh air duct.
  • Supply air duct: It is used to supply air into the room.
  • Return air duct: It is used to remove the air from the room to maintain pressure inside the room.
  • Exhaust air duct: It is used to remove smoke, odor from the kitchen, and toilets.
  • Fresh air duct: It carries the outside air,

Types of duct:

 Based on shape:
  • Rectangular duct
  • Round duct
  • Square duct
  • Oval duct 
 Based on pressure:
  • High pressure(static pressure is in between 150mm to 250mm of water)
  • medium pressure(static pressure is in between 50mm to 150mm of water)
  • low pressure(static pressure is less than 50mm of water)
 Based on Velocity:
  • High velocity(velocity > 750m/min)
  • Low velocity(velocity up to 750 m/min)
  • For industrial we use high-velocity duct 
  • for residential we use low-velocity duct
Types of Joint used in duct system:
  • Slip joint
  • Drive slip
  • Flange joint
The material used for ducting:

    1. GI:
  • It stands for Galvanized iron.
  • This material is most commonly used in fabrication.
  • It is cheaper.
  • All the shapes of ducting can be fabricated with GI material.
    2. Aluminum:
  • It is rustproof.
  • Its weight is very less.
  • Hence it is used mostly in the hospital and ICU rooms.
    3. PI(Pre-insulated duct)
  • It has a long life.
  • It is light in weight.
  • Made of polyurethane and phenolic foam panel are manufactured with factory applied aluminum facing on both sides.
    4. Fabric material:
  • It is designed for the delivery and distribution of cooled or heated air.
  • Its shape is round, quarter round and semi-round type.
  • No insulation required.
  • It is used for its flexibility.
Duct design method:
  • Velocity method
  •  Equal friction method
  • Static pressure recovery method
Pressure losses in duct:
  • The pressure is loss due to friction between the moving particle of air and the interior surface of the duct.
  • When pressure loss occurs in the straight duct, it is termed as friction loss.
  • When pressure loss in elbows, bends, etc, it is termed as dynamic loss.
The thickness of the GI duct sheet:
  •  As per SMACNA, the gauges of the duct sheet depend upon the width of the duct and type of application of a particular project.
  • SMACNA stands for sheet metal and air conditioning contractor's national association.
  • Here are the few gauge system as per SMACNA.
  • 26 gauge is used for the duct size below 300 mm and thickness is 0.5mm.
  • 24 gauge is used for the duct size above 300mm to 750 mm and thickness is 0.6mm.
  • 22 gauge is used for the duct size above 750 mm to 1200 mm and thickness is 0.8mm.
  • 20 gauge is used for the duct size above 1200 mm to 3000 mm and thickness is 1mm.
  • 18 gauge is used for the duct size above 3000 mm and thickness is 1.2mm.
  • 16 gauge is used for the machine cabinet and thickness is 1.6mm.
Formula to find the area of ducting:

 Circular duct area = 2 π r L
    where r= radius of the circular duct
               L= length of the circular duct
Square duct area= 4 a L
        Where a= sides of the square( all sides are equal)
                   L= Lenght of the square duct
Rectangular duct:
Area of straight piece= 2(W+H)L

Elbow duct area=2(W+H) L2+2(w1+H1) L1

Transition duct area= 2(W+H) L- {(W+H)-(πr)} L

Offset duct area= 2(W+H) (L1+L2)

Reducer duct area=2(W+H) L-{(W+H)-(W1+H1)}L

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