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

What is ducting?

• Ducts are passages used in heating, ventilation, and air conditioning to deliver and remove air.

                                                                        or

• 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)

Note:

• 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