What is Duct ?

 What is Duct?

Ducts are pipe or passages used in  HVAC to deliver and remove air , For example, supply air, return air, and exhaust .As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort.

The duct is mostly classified on the basis of velocity, pressure, and shape.

Types of Duct 

Based On Shape    

•Rectangular duct

•Round duct

•Oval duct

•Squared duct

 Based on Pressure

•High Pressure

•Medium Pressure

•Low Pressure

Based on Velocity

•High Velocity

•Low Velocity

Types of Joint Used In-Duct 

1.Slip Joint

2.Drive slip

3.Flange joint

Material Used for Ducting

1. GI

           GI (Galvanized iron) is a standard and most commonly used material used in fabrication. This being cheaper, It is acceptable in most of the applications. All the shapes of ducting can be fabricated with GI material.

2. Aluminum

         It is noticed that bacterial grow at the rusted spot. So the GI sheet might rust over the days giving rise to bacterial growth. This is not of much concern as a normal human being has adequate resistance to bacteria. In the hospital the patient in the operation theatre, ICU, etc. is in a most vulnerable condition. Therefor at these place AL ducting are used. It is light in weight and rustproof. But this is most uneconomical and superfluous in normal conditions.

3. PI (Pre Insulated Material)

               Made of Polyurethane & phenolic foam panel is manufactured with a factory applied Aluminum facing on both sides which thickness varies from 25 micrometers to 200 micrometers. The main advantage of PI material is that it is light in weight and its fabrication takes very little time.

4. Fabric material

In principle, a fabric duct is a round, semi-round, or quarter round duct made of a light-weight fabric material instead of e.g. galvanized steel, stainless steel or aluminum, and designed for delivery and distribution of cooled or heated air.
Now days it is used because of its flexibility, mounting, even air distribution and no insulation required.

    

Duct Design

Sizing of duct-works in HVAC systems can be done by the

1.Velocity Method

2.Equal Friction Method

3. The Static Pressure Recovery Method

Other factor Taken in consideration during Duct Design

Friction the rate can be kept constant at 0.08 inch/100Ft of duct length.

- Air flow to be taken from the load calculation of that area.

- Restrict aspect ration to 4:1 maximum.

  Velocity to be kept as per the application

ØComfort AC

a.Low velocity-1200 -1500 FPM

b.High Velocity –above 2500 FPM

ØIndustrial Application

a.Low velocity-1200 -1500 FPM

b.High Velocity –above 2500 FPM

Thickness  Of GI sheet

   

Max Size	GI	Al	Joint used
Up to 300 mm	0.63 mm	0.8 mm	S Slip Joint
300 to 600 mm	0.63 mm	0.8 mm	S Slip Joint
			Drive Slip Joint
600 to 750 mm
750 to 1000 mm	0.8 mm	1 mm	Drive Slip Joint
1001 to 1500			40x40 Angle joint
1501 to 2250	1.0 mm	1.5 mm	40x40 Angle joint
2251 to above	1.25 mm	1.8 mm	50x50 Angle joint

Gauge
0.63 mm	24 G
0.8 mm	22G
1.0 mm	20 G
1.25 mm	18 G
1.5 mm	16 G
1.8 mm	14 G

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