Table of Contents
- Ducting
- Duct Tables
Ducting
Function:
- To supply conditioned air.
- To provide return path to air.
Economics:
- Initial.
- Operating.
Factors affecting ducting:
- Shortest path to reduce power, material and space.
- Avoid short bends, high velocity, small diameter duct.
- Smooth material for duct.
- Dampers at branch outlet for balancing.
- Rectangular duct section should be as close as possible to square.
- 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.
Room | Main Duct | Branch Duct | Return Duct |
---|---|---|---|
Technical room | 350–500 (1150–1650) | 250–300 (820–1000) | 280–400 (900–1300) |
Non-technical | 300–350 (1000–1150) | 200–250 (650 – 820) | 250–300 (820–1000) |
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:
- Low Pressure Duct: up to 3.75 inH₂O
- Medium Pressure Duct: up to 6.75 inH₂O
- 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.
Longer side in mm | Thickness (min) in mm | SWG |
---|---|---|
Upto 750 | 0.63 | 24 |
751 – 1500 | 0.80 | 22 |
1501 – 2250 | 1.00 | 20 |
2251 and above | 1.25 | 18 |
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:
- Proportional to aspect ratio.
- Proportional to smaller air quantity at low speed.
- Inversely proportional to insulation thickness.
Insulation for Ducts:
Length | Insulation Thickness | Insulation from |
---|---|---|
Supply Duct | ||
Up to 18 m | - | |
18 – 45 m | 25 mm | Half from end. |
45 – 60 m | 25 mm | Full |
above 60 m | 50 mm | Full |
Return Duct | ||
In conditioned space | - | |
AHU walls/ Non-AC area | 50 mm | Full |
Duct Friction: Δp = 0.03 f (L / d1.22) (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:
- Variation in temperature 2–3 °F.
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 -
Air direction
Blow and drop: Blow should be at least ¾ th of the distance between grill and opposite walls.
- 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.
Note
As per fire bylaws, duct should not cross floors.
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
Duct size | cfm | diameter (inch) | velocity (fpm) | Rectangular size |
---|---|---|---|---|
O–A | 8800 | 34 | 1400 | 46 × 22 |
A–B | 1800 | 19 | 950 | 22 × 14 |
B–C | 1200 | 16 | 850 | 16 × 14 |
C–12 | 600 | 12.5 | 700 | 14 × 10 |
12–13 | 300 | 9.5 | 600 | 10 × 8 |
Equivalent Length
Duct portion | Item | Length | Additional length |
---|---|---|---|
O–A | duct elbow | 60 | 15 |
A–B | duct | 20 | |
B–C | duct | 20 | |
C–12 | duct elbow | 30 | 5 |
12–13 | duct | 10 | |
TOTAL | 140 | 20 |
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:
- 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.
- 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.
Duct Tables
Various tables in duct designing are as follows:
Longer side in mm | Thickness (SWG) | Type of connection | Bracing |
---|---|---|---|
Up to 750 | 24 | 24 SWG GSS flanges with 6 mm GI bolts and nuts at 125 mm centers | Cross Bracing |
751–1000 | 22 | ----do---- | ----do---- |
1001–1500 | 22 | 32 mm × 5 mm angle iron flanges with 8 mm bolts and nuts at 125 mm centers | ----do---- |
1501–2250 | 20 | 40 mm × 5 mm angle iron flanges with 10mm bolts and nuts at 125 mm centers | To be braced diagonally with 40 mm × 3 mm angle iron, if the piece is more than 1200 mm long. |
Above 2250 | 18 | 50 mm × 6 mm angle iron flanges with 20 mm bolts and nuts at 125 mm centers or panel ducting | ----do---- |
Note
- All the angle iron flanges to be welded vertically and holes to be drilled.
- All the angle iron flanges to be connected to the GSS duct by rivets at 100 mm centers.
- All the flanged joints to have 6mm thick felt packing stuck to the flanges with shellac varnish. The holes in the felt packing to be burnt through.
- The GSS duct should be lapped 6mm across the flanges.
- The duct should be supported by approved type supports at a distance not exceeding 2400 mm.
Application | Controlling Factor: Noise Generation (Main Duct) | Controlling Factor: Duct Friction | |||
---|---|---|---|---|---|
Main Duct | Branch Duct | ||||
Supply | Return | Supply | Return | ||
Residence | 600 | 1000 | 800 | 600 | 600 |
Apartment, hospital bedrooms, hotel bedrooms | 1000 | 1500 | 1300 | 1200 | 1000 |
Private office, directors room, libraries | 1200 | 2000 | 1500 | 1600 | 1200 |
Theaters, auditoriums | 800 | 1300 | 1100 | 1000 | 800 |
General office, high class restaurant, high class stores, banks | 1500 | 2000 | 1500 | 1600 | 1200 |
Average stores, cafeterias | 1800 | 2000 | 1500 | 1600 | 1200 |
Industrial | 2500 | 3000 | 1800 | 2200 | 1500 |
Application | Smoking | cfm per person | cfm per ft² of floor (minimum) | |
---|---|---|---|---|
Recommended | Minimum | |||
Average apartment | Some | 20 | 15 | -- |
Deluxe apartment | Some | 30 | 25 | 0.33 |
Banking space | Occasional | 10 | 7.5 | -- |
Barber shop | Considerable | 15 | 10 | -- |
Beauty parlor | Occasion | 10 | 7.5 | -- |
Broker’s board room | Very heavy | 50 | 30 | -- |
Cocktail bar | Heavy | 30 | 25 | -- |
Corridors | ---- | -- | -- | -- |
Departmental store | None | 7.5 | 5 | 0.25 |
Director’s room | Extreme | 50 | 30 | -- |
Drug store ~ | Considerable | 10 | 7.5 | -- |
Factories */ | None | 10 | 7.5 | 10 |
General store | None | 7.5 | 5 | -- |
Funeral parlors | None | 10 | 7.5 | -- |
Garage * | ---- | -- | -- | 1 |
Operating room *# | None | -- | -- | 2 |
Patient’s room | None | 30 | 25 | 0.33 |
General ward | None | 20 | 15 | -- |
Hotel room | Heavy | 30 | 25 | 0.33 |
Restaurant kitchen | ---- | -- | -- | 4 |
Residence kitchen | ---- | -- | -- | 2 |
Laboratories ~ | Some | 20 | 15 | -- |
Meeting room | Very heavy | 50 | 30 | 1.25 |
General office | Some | 15 | 10 | -- |
Private office | None | 25 | 15 | 0.25 |
Private office | Considerable | 30 | 25 | 0.25 |
Cafeterias ~ | Considerable | 12 | 10 | -- |
Restaurant Dining ~ | Considerable | 15 | 12 | -- |
School room * | None | -- | -- | -- |
Retail Shop | None | 10 | 7.5 | -- |
Theater * | None | 7.5 | 5 | -- |
Theater | Some | 15 | 10 | -- |
Toilets * | ---- | -- | -- | 2 |
Note
* Local bodies code may be seen
~ May be governed by exhaust
/ Use these values unless governed by local codes
# All outside air is recommended to overcome explosion hazard of anaesthetics
Velocity pressure | Velocity | Velocity pressure | Velocity | Velocity pressure | Velocity | Velocity pressure | Velocity |
---|---|---|---|---|---|---|---|
(inH₂O) | (fpm) | (inH₂O) | (fpm) | (inH₂O) | (fpm) | (inH₂O) | (fpm) |
0.01 | 400 | 0.29 | 2150 | 0.58 | 3050 | 1.28 | 4530 |
0.02 | 565 | 0.30 | 2190 | 0.60 | 3100 | 1.32 | 4600 |
0.03 | 695 | 0.31 | 2230 | 0.62 | 3150 | 1.36 | 4670 |
0.04 | 800 | 0.32 | 2260 | 0.64 | 3200 | 1.40 | 4730 |
0.05 | 895 | 0.33 | 2300 | 0.66 | 3250 | 1.44 | 4800 |
0.06 | 980 | 0.34 | 2330 | 0.68 | 3300 | 1.48 | 4870 |
0.07 | 1060 | 0.35 | 2370 | 0.70 | 3350 | 1.52 | 4930 |
0.08 | 1130 | 0.36 | 2400 | 0.72 | 3390 | 1.56 | 5000 |
0.09 | 1200 | 0.37 | 2440 | 0.74 | 3440 | 1.60 | 5060 |
0.10 | 1270 | 0.38 | 2470 | 0.76 | 3490 | 1.64 | 5120 |
0.11 | 1330 | 0.39 | 2500 | 0.78 | 3530 | 1.68 | 5190 |
0.12 | 1390 | 0.40 | 2530 | 0.80 | 3580 | 1.72 | 5250 |
0.13 | 1440 | 0.41 | 2560 | 0.82 | 3620 | 1.76 | 5310 |
0.14 | 1500 | 0.42 | 2590 | 0.84 | 3670 | 1.80 | 5370 |
0.15 | 1550 | 0.43 | 2620 | 0.86 | 3710 | 1.84 | 5430 |
0.16 | 1600 | 0.44 | 2650 | 0.88 | 3750 | 1.88 | 5490 |
0.17 | 1650 | 0.45 | 2680 | 0.90 | 3790 | 1.92 | 5550 |
0.18 | 1700 | 0.46 | 2710 | 0.92 | 3840 | 1.96 | 5600 |
0.19 | 1740 | 0.47 | 2740 | 0.94 | 3880 | 2.00 | 5660 |
0.20 | 1790 | 0.48 | 2770 | 0.96 | 3920 | 2.04 | 5710 |
0.21 | 1830 | 0.49 | 2800 | 0.98 | 3960 | 2.08 | 5770 |
0.22 | 1880 | 0.50 | 2830 | 1.00 | 4000 | 2.12 | 5830 |
0.23 | 1920 | 0.51 | 2860 | 1.04 | 4080 | 2.16 | 5880 |
0.24 | 1960 | 0.52 | 2880 | 1.08 | 4160 | 2.20 | 5940 |
0.25 | 2000 | 0.53 | 2910 | 1.12 | 4230 | 2.24 | 5990 |
0.26 | 2040 | 0.54 | 2940 | 1.16 | 4310 | 2.28 | 6040 |
0.27 | 2080 | 0.55 | 2970 | 1.20 | 4380 | ||
0.28 | 2120 | 0.56 | 2990 | 1.24 | 4460 |
Height of exposed surface above mean retarding surface (m) | Horizontal wind velocity (km/h) | Horizontal pressure (kg/m²) |
---|---|---|
0 | 80 | 40 |
3 | 96 | 58 |
6 | 108 | 73 |
9 | 115 | 85 |
12 | 123 | 98 |
15 | 128 | 105 |
18 | 133 | 112 |
21 | 137 | 120 |
24 | 141 | 127 |
27 | 144 | 133 |
30 | 147 | 141 |
38 | 155 | 151 |
46 | 160 | 166 |
53 | 165 | 175 |
61 | 169 | 185 |
76 | 175 | 200 |
92 | 181 | 210 |
107 | 186 | 224 |
122 | 191 | 234 |
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