AC Ducting

Table of Contents

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.
Recommended Air Velocity for Exchanges: (in m/min and bracket figures are in fpm)
RoomMain DuctBranch DuctReturn Duct
Technical room350–500
(1150–1650)
250–300
(820–1000)
280–400
(900–1300)
Non-technical300–350
(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: 

  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.

Thickness of Duct Sheets
Longer side in mmThickness (min) in mmSWG
Upto 7500.6324
751 – 15000.8022
1501 – 2250 1.0020
2251 and above1.2518

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: 

LengthInsulation ThicknessInsulation from
Supply Duct
Up to 18 m-
18 – 45 m25 mmHalf from end.
45 – 60 m25 mmFull
above 60 m50 mmFull
Return Duct
In conditioned space-
AHU walls/ Non-AC area50 mmFull

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: 

  1. Variation in temperature 2–3 °F.
  2. Idle velocity

    v in fpmcommentapplication
    0–16stagnant-
    25human comfortcommercial
    25–5050 is on higher side
    65light papers are blown
    75–300factory 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.

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 14020

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.
Duct Tables

Various tables in duct designing are as follows:

GSS Duct Specifications
Longer side in mmThickness (SWG)Type of connection Bracing 
Up to 7502424 SWG GSS flanges with 6 mm GI bolts and nuts at 125 mm centersCross Bracing
751–100022----do--------do----
1001–15002232 mm × 5 mm angle iron flanges with 8 mm bolts and nuts at 125 mm centers----do----
1501–22502040 mm × 5 mm angle iron flanges with 10mm bolts and nuts at 125 mm centersTo be braced diagonally with 40 mm × 3 mm angle iron, if the piece is more than 1200 mm long.
Above 22501850 mm × 6 mm angle iron flanges with 20 mm bolts and nuts at 125 mm centers or panel ducting----do---- 

Note

  1. All the angle iron flanges to be welded vertically and holes to be drilled.
  2. All the angle iron flanges to be connected to the GSS duct by rivets at 100 mm centers.
  3. 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.
  4. The GSS duct should be lapped 6mm across the flanges.
  5. The duct should be supported by approved type supports at a distance not exceeding 2400 mm.
Recommended Maximum Duct Velocity For Low Velocity System (in fpm)
ApplicationControlling Factor: Noise Generation (Main Duct)Controlling Factor: Duct Friction
Main DuctBranch Duct
SupplyReturnSupplyReturn
Residence6001000800600600
Apartment, hospital bedrooms, hotel bedrooms10001500130012001000
Private office, directors room, libraries12002000150016001200
Theaters, auditoriums800130011001000800
General office, high class restaurant, high class stores, banks15002000150016001200
Average stores, cafeterias18002000150016001200
Industrial25003000180022001500
Ventilation Standards
ApplicationSmokingcfm per personcfm per ft² of floor (minimum)
RecommendedMinimum
Average apartmentSome2015--
Deluxe apartmentSome30250.33
Banking spaceOccasional107.5--
Barber shopConsiderable1510--
Beauty parlorOccasion107.5--
     
Broker’s board roomVery heavy5030--
Cocktail barHeavy3025--
Corridors----------
Departmental storeNone7.550.25
Director’s roomExtreme5030--
     
Drug store ~Considerable107.5--
Factories */None107.510
General storeNone7.55--
Funeral parlorsNone107.5--
Garage *--------1
     
Operating room *#None----2
Patient’s roomNone30250.33
General wardNone2015--
Hotel roomHeavy30250.33
Restaurant kitchen--------4
Residence kitchen--------2
Laboratories ~Some2015--
     
Meeting roomVery heavy50301.25
General officeSome1510--
Private officeNone25150.25
Private officeConsiderable30250.25
Cafeterias ~Considerable1210--
Restaurant Dining ~Considerable1512--
     
School room *None------
Retail ShopNone107.5--
Theater *None7.55--
TheaterSome1510--
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 Pressures
Velocity pressureVelocityVelocity pressureVelocityVelocity pressureVelocityVelocity pressureVelocity
(inH₂O)(fpm)(inH₂O)(fpm)(inH₂O)(fpm)(inH₂O)(fpm)
0.014000.2921500.5830501.284530
0.025650.3021900.6031001.324600
0.036950.3122300.6231501.364670
0.048000.3222600.6432001.404730
0.058950.3323000.6632501.444800
0.069800.3423300.6833001.484870
0.0710600.3523700.7033501.524930
0.0811300.3624000.7233901.565000
0.0912000.3724400.7434401.605060
0.1012700.3824700.7634901.645120
0.1113300.3925000.7835301.685190
0.1213900.4025300.8035801.725250
0.1314400.4125600.8236201.765310
0.1415000.4225900.8436701.805370
0.1515500.4326200.8637101.845430
0.1616000.4426500.8837501.885490
0.1716500.4526800.9037901.925550
0.1817000.4627100.9238401.965600
0.1917400.4727400.9438802.005660
0.2017900.4827700.9639202.045710
0.2118300.4928000.9839602.085770
0.2218800.5028301.0040002.125830
0.2319200.5128601.0440802.165880
0.2419600.5228801.0841602.205940
0.2520000.5329101.1242302.245990
0.2620400.5429401.1643102.286040
0.2720800.5529701.204380  
0.2821200.5629901.244460  
Wind Velocity and Pressures at Various Exposed Heights
 Height of exposed surface above mean retarding surface (m)Horizontal wind velocity (km/h)Horizontal pressure (kg/m²)
08040
39658
610873
911585
1212398
15128105
18133112
21137120
24141127
27144133
30147141
38155151
46160166
53165175
61169185
76175200
92181210
107186224
122191234

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