Ducting

Function: 

1. To supply conditioned air.
2. To provide return path to air.

Economics: 

1. Initial.
2. Operating.

Factors affecting ducting: 

1. Shortest path to reduce power, material and space.
2. Avoid short bends, high velocity, small diameter duct.
3. Smooth material for duct.
4. Dampers at branch outlet for balancing.
5. Rectangular duct section should be as close as possible to square.
6. Noise and pressure should be minimum.

Shape of Duct: 

• Circular: Least material and space, more fabrication cost, branching is difficult.
• Square: Comparatively more material and space.
• Rectangular: Material and space requirements are highest.

Note: For diffuser of size 375 mm × 375 mm, velocity is taken as about 650 cfm as free area is considered 60 %.

Pressure Category of Ducts: 

1. Low Pressure Duct: up to 3.75 inH₂O
2. Medium Pressure Duct: up to 6.75 inH₂O
3. High Pressure Duct: up to 12.75 inH₂O

Low pressure duct is recommended for telephone exchanges, go for medium and high pressure duct only if space limitation is there.

Duct Aspect Ratio: It is the ratio of the longer and shorter side of the duct.

Heat Loss: Heat loss due to air flow in ducting is:

1. Proportional to aspect ratio.
2. Proportional to smaller air quantity at low speed.
3. Inversely proportional to insulation thickness.

Insulation for Ducts: 

Duct Friction: Δp = 0.03 f (L / d^1.22) (v / 1000)^1.82

where

Δp friction loss in inH₂O.

f interior surface roughness (0.9 for GI)

v velocity in fpm.

d diameter in inch.

L length in feet.

Static regain: It is a measure of pressure gain due to decrease in velocity and is calculated as follows:

Regain in iwg = 0.75 [ (v₁ / 4000)² - (v₂ / 4000)² ]

where,

v₁ is initial velocity in fpm.

v₂ is end point velocity in fpm.

(Relation between pressure and velocity is p = (v₁ / 4005)²)

Room air distribution: 

1. Variation in temperature 2–3 °F.

2. Idle velocity

   v in fpm	comment	application
   0–16	stagnant	-
   25	human comfort	commercial
   25–50	50 is on higher side
   65	light papers are blown
   75–300	factory	industrial

3. Air direction

   

4. Blow and drop: Blow should be at least ¾ th of the distance between grill and opposite walls.

   
5. Induction: Total air is divided into primary and secondary air and the induction ratio is the ratio of total air to primary air. Higher the IR (Induction Ratio), lower the blow.

Example: Duct designing on the basis of equal friction method:

Assumption: 

• Friction loss throughout the length of the duct is constant.
• Velocity reduces with reduction in cfm.
• Initial velocity is assumed within the range.
• Min duct size 10” × 8”.
• Min reduction/increase in size is 2”.
• At one time only one side is changed.

For 8800 cfm and 1400 fpm, we have –

Friction loss = 0.067 inH₂O per 100 ft

Diameter of duct = 34”, so we select

Equivalent Length

So equivalent length = 140 + 20 = 160 feet.

Friction loss in duct = 160 × 0.065 / 100 = 0.104 inH₂O.

Static regain with v₁ = 1400 fpm and v₂ = 610 fpm = 0.073 inH₂O.

So total pressure required = Terminal + Friction + Regain = 0.15 + 0.104 + 0.073 = 0.181 inH₂O.

Other Methods: 

1. Velocity Reduction: Selection of starting velocity at the fan discharge depending on the requirement of air quantity and reduction is made in the velocity down the duct run. It is applicable for very simple layouts.
2. Static Regain: To size the duct so that the increase in the static pressure (regain due to reduction in velocity) at each branch just offsets the friction loss in the succeeding section of the duct.