DUCT WORKS Selection Tips DETAILED TECHNICAL SPECIFICATION

الحمد لله والصلاة والسلام على رسول الله وعلى آله وصحبه أجمعين 

DUCT WORKS Selection Tips DETAILED TECHNICAL SPECIFICATION

A. All ducting must be made of galvanized steel sheet to ASTM A525
lock-forming quality, having zinc coating of 275gm/M² for both sides
in accordance with ASTM A90 and of tensile strength 28/33 tons per square inch

B. All ducting must be made as indicated in the installation drawings and 
successful tenderer must check the installation and building before 
hand and ensure that the existing structures, etc., must not come in 
the way of ducting at time of installation
Dimensions of ducting can be altered by the contractor with the prior 
approval of concerned Engineer without altering permissible airflow rates

C. All ducting must be suitably stiffened to avoid vibration, sagging and 
running effects due to airflow. All ducting must be made airtight

D. Gauges of G.I sheeting, type of joints, bracing, etc., used for the 
manufacture ducting must be as shown below

E. The throat radius of bend must not exceed half the width of associated 
side of the duct and where it is not possible to comply with this duct turning 
vane must be used along with the right angle bends
The duct turning vanes must be spaced in such a way that the speed
ratio of each of the individual elbows must be around 6:1 

Maximum Side inches US Standard Gauge 
Transverse Joints and Bracing 154BUp to 12″ 24 
¾” pocket or slip joint at 8 ft. centres 13 – 18” 24 
1” pocket joint at 4 Ft. centers with cross breaking 19 – 30” 22 
1” pocket joint at 4 Ft. centers with cross breaking 31 – 42” 22
1½” pocket joint at 4 Ft. centers with cross breaking and ½ x ½ x 1/8 
angle with girth at 8 ft. centres bolted to the pocket joint 43 – 54” 20
1½” pocket joint at 4 Ft. centers with cross breaking and 1½ x 1½ x 3/16 
angle with girth at 8 ft. centres bolted to the pocket joints 55 – 72” 20
1½” pocket joint at 4 Ft. centers with cross breaking and 1½ x 1½ x 3/16 
angle with girth at 4 ft. centres bolted to the pocket joints 73 to Up 18
2½” x 1½” x 3/16 angles at 4 Ft. centers bolted to the duct with 3/8 
bolts at 1½ centers and fastened together with 3/8” bolts at 2 centres
with rubber gasket between the 2 angles and cross breaking 

Joints 

50 mm Slip Joint or Draw Band is acceptable through 1500 mm size 
Slip or Draw Band Joint Flanged Joint 

All tees, Bends and Elbows must be constructed with radius 
of not less than 1½ times diameter of duct on centerline
Where not possible and where rectangular elbows are used
aerofoil-turning vanes must be provided 
Transformation of duct sizes must be gradual, not exceeding 
15 degrees divergence and 30 degrees convergence 
Round duct fittings must be of welded type construction
For ducts of diameter up to and including 300-mm. diameter
one-piece smooth 90 deg. elbows must be used. Over 300-mm dia
they must be 5 piece welded

Turning Vanes

must be fabricated in accordance with SMACNA Standards 
Turning vanes must be as follows 
Factory or site-fabricated using factory-fabricated vane spacing rails 
Turning vanes must be provided in all square elbows, take-off or 
extension collars to supply outlets and tap-in branch take-off connections 
Turning vanes must be reinforced by rods or of sectional construction 
Turning vane arrangements and thickness must be as shown 
153 B SINGLE 152 B VANE 151 B SCHEDULE Type Space between Vanes
Radius Gauge Small Large
1½” 3” 2” 4½” 24 22

Volume Control Damper

Volume control dampers must be provided in ductwork to regulate and 
balance air systems, since dampers on the air inlet and outlet must be 
used only for fine secondary regulating or balancing
The damper must be fabricated in accordance with 
SMACNA Duct Construction Standards 

Single blade dampers must be used for damper widths up to 300 mm
provided that there is sufficient length of straight duct after the 
damper to ensure equal flow before the next take-off or outlet
Where the damper width is over 300 mm. multi -blade dampers must be used
Multi blade dampers styles must be generally parallel blade for two-position
operation and opposed blades for modulating position
The blades must be constructed of galvanized sheet, suitably 
stiffened by forming or other method, and assembled in prime coated 
or galvanized channel frame with suitable hardware
The bearings must be of bronze or oil impregnated nylon

Looking and indicating quadrant regulator must be provided for each damper
The indicating quadrant must be clearly marked to indicate 
close, ¼, ½ , ¾ and open positions
On insulated ducts, quadrant regulators must be mounted 
on stand off mounting brackets or bases

For motor operated damper, the motor-operator must be electric type 

Fire Dampers

must be fabricated in accordance with NFPA 90A 
and UL 555 or equivalent and must be 
1• To be provided in ducts and openings in shafts, floor
firewalls, fire partitions and the rated ceilings

2• In horizontal ducts, fire dampers may be single or of a multi 
blade type. In vertical ducts only single blade type must be used

3• Galvanized sheet metal housing to be provided with angle flanges on both sides

4• The damper must be of 1.6 mm thick sheet steel or steel/mineral 
fiber/steel sandwich of equivalent fire resistance, thermally released

5• Fire dampers must be equipped with fuse plug, positioning levers
fittings and position indicator accessible from outside

Grilles and Registers 

They must be furnished with sponge rubber gaskets or foam
tape behind the frame to eliminate air leakage and with nylon
bushings or PVC at the blades connection to the frame to eliminate
corrosion and vibration. Free cross-sectional area must be at least 65 percent

1- Supply air registers and grilles must have individually adjustable
horizontal front blades and vertical rear blades

2- Supply air register must be equipped with key 
operated, opposed blade volume control damper

3- Return and exhaust air registers and grilles must have 
fixed single deflection blades set at an angle of 35 degrees

4- Return and exhaust air registers must be equipped with 
key operated, opposed blade volume control damper

5- All opposed blade volume control dampers must be polyester 
powder coated ovens taken with black colour finish

Jet Diffusers

must have manual discharge pattern adjustment from the 
front or back of diffuser, the outer flange of the jet diffuser 
to be made of aluminium, the inner core and rings to be made 
of mild steel construction, to be coloured by 
electrostatic polyester powder coating finish

Access Doors

Airtight access doors with rubber linings must be installed in all ducts
at fans, filters, heaters, fire dampers and volume dampers for inspection
cleaning and maintenance purposes

The opening in the ductwork must be properly stiffened at the edges
with frames riveted or welded to the duct
The access door must be 
All Access Doors mounted on insulated ducts must be 
of sandwich type, having the same insulation thickness 
as that of the duct, with Cam Lock handles

Maximum Size(mm) No. of Hinges No. of Handles Door Gauge Back Frame
12” x 12” 2 1 26 26 24 16” x 20” 2 2 24 26 22 24” x 24” 3 2 22 26 22

Flexible Connections

Flexible connections must be fitted on all suction and discharge 
connections of fans and air conditioning units, for preventing 
transmission of vibration through the ducts to occupied spaces

Flexible connections must be made from flame proof, factory 
fabricated from chemically impregnated canvas
Connections must fit closely and be secured in an airtight 
fashion to duct work, fans and apparatus by means of angle
iron or flat iron frames. The unclamped section of the flexible 
connection between the apparatus and the ductwork must be
not less than 150 mm. in length. Flexible connections must not be painted

Supports and Hangers

Installation of supports and hangers must be 

Supports and hangers must be attached only to structural 
framing members and concrete slabs. They must not be anchored 
to metal decking unless a means is provided and approved for 
preventing the anchor from puncturing the metal decking

Supports must generally be made from galvanized steel sections
and where heavy items of equipment occur within ductwork 
additional support must be provided as necessary

Duct work hung from inserts or from clip angles must 
be secured with expansion bolts in shear.

Supports for horizontal ducts must be

RECTANGULAR DUCT Length of Longer Side (mm) Drop Rod Diameter (mm)
Bearing Member (mm) 155BMaximum Spacing ( inch) 156 B up to 12” 8
20 x 3 Flat 120” 157B Over 12” up to 24” 8 25 x 25 x 3 angle 120”
Over 24” up to 40” 10 40 x 40 x 4 angle 100” Over 45” up to 64” 10
50 x 50 x 5 angle 100” Over 64” 15 75 x 75 x 6 angle 100”

Design of supports for vertical ducts will be dictated by site 
conditions and spacing may be greater than for horizontal ducts

Supports for vertical ducts must be angles and 
channels made of galvanized or painted black steel

Vertical duct must be fastened with a minimum 
of 2 supports at each floor

في أمان الله