1.
SMACNA (Sheet
Metal and Air Conditioning Contractors National Association) standards for
vanes in L-bow fitting for duct work:-
1.1 Up
to size 400 mm – no vanes
1.2 401-800
mm – 1 vane
1.3 801-
1200 mm – 2 vanes
1.4 Above
1200 mm – 3 vanes
2. CLASS –A TYPE DUCT SYSTEM
Shoe, Collar, Reducers etc.
3. CLASS - B TYPE DUCT SYSTEM
Y pieces only used
4.
VCD of two types of blade construction
(Simple blade type and aero foil blade type)
4.1 Parallel
blade type
4.2 Opposed
blade type.
5.
DUCT SYSTEMS CLASSIFICATIONS
5.1 ACCORDING
TO VELOCITY
5.1.1
Low velocity (1200-2200 FPM or 6 - 11 M/S) or High velocity ( 2500- 3000 FPM or 12.5 - 15 M/S) system.
5.1.2
Low pressure (Upto 3 ¾” WG) or High pressure (6 ¾” – 12 ¾” WG) system.
For return air ducts only low
velocity is considered and 20 % less than supply air ducts.
5.2 ACCORDING TO PRESSURE (TOTAL P = SP+VP)
5.2.1. Low Pressure – Upto 3
¾” WG.
5.2.2. Medium Pressure 3 ¾” –
6 ¾” WG.
5.2.3. High Pressure 6 ¾” –
12 ¾” WG.
1” OF WATER GAUGE = 250 PASCALS
6. FAN AND DUCTING RELATION
6.1. Low Pressure ducting –
Class 1 Fan
6.2. Medium Pressure ducting-
Class 2 Fan
6.3. High Pressure Ducting –
Class 3 Fan
7.
VELOCITY PRESSUR
√VP = V/4005
VP = (V/4005)2
8.
STATIC PRESSURE (PRESSURE LOSS) :
Ps = 0.03F (L/d1.22) (V/1000)0.85
in.wg
F=
roughness of duct material
L=
length of duct in feet
D=
Equivalent dia of duct in inches
V=
air velocity in FPM
Roughness of GI Sheet-0.9
Roughness of SS Sheet – 1
Roughness of MS Sheet – 0.7
9. DUCT CLASS VARIES FROM 1-6 AND IT SHOULD BE DESIGNED
BY CONSIDERING ASPECT RATION (1:4). LOWER THE ASPECT RATIO, LOWER WILL BE THE
CLASS AND DESIGN WILL BE ECONOMIC.
10.
LINEAR EQUATION
FOR DUCT DESIGN
Q = AV
11.
Noise Criteria
for different spaces (ASHRAE Standard)
11.1 Bed room – 20-25 (db)
11.2 Hall /lobby/office – 30 (db)
11.3 Toilet kitchen – 35-40 (db)
11.4 Workshop/ factory etc- 50 (db)
12.
Friction chart is
taken into consideration for selection of duct sizes, friction loss, equv.dia
and velocity.
13.
DUCT SIZES TO
THICKNESS RELATION
Gauge
|
Thickness(mm)
|
Size (Width)
|
26 g
|
0.6
|
0-400
|
24g
|
0.7
|
401-750
|
22g
|
0.8
|
751-1050
|
20g
|
1.0
|
1100-1500
|
18g
|
1.2
|
Above 1500
|
14. SMACNA STANDARDS FOR VANES
Upto 400 width
|
No vanes
|
Short radius L-bow
90’
|
401-800
|
1 vane
|
|
801-1200
|
2 vanes
|
|
Above 1200
|
3 vanes
|
15. Factors Influencing initial and operating
cost :-
15.1
Aspect ratio of duct (AR-1:4 as per ASHRAE Standard).
15.2
Types of fittings (Class A or Class B).
15.3
Duct friction rate (from friction chart).
15.4
Heat gain or loss from duct.