medium pressure classification as a minimum. Secondary
air ductwork (runouts/branches from main to terminal
Table 5-4
terminals and distribution devices) shall be low pressure
Recommended Duct Velocities
classification as a minimum.
Pressure loss in ductwork shall be designed to comply
Controlling Factor Noise Generation
with the criteria stated above. This can be accomplished
(Main Duct Velocities)
by using smooth transitions and elbows with a radius of
Application
m/s
(fpm)
at least 1.5 times the radius of the duct. Where mitered
elbows have to be used, double foil sound attenuating
Private Offices
6
(1,200)
turning vanes shall be provided. Mitered elbows are not
Conference Rooms
permitted where duct velocity exceeds 10.2 m/s (2,000
Libraries
fpm).
Theaters
4
(800)
Sizing of Ductwork: Supply and return ductwork shall be
Auditoriums
sized using the equal friction method except for ductwork
upstream of VAV terminals. Duct systems designed using
General Offices
7.5
(1,500)
the equal friction method place enough static pressure
capacity in the supply and return fans to compensate
Cafeterias
9
(1,800)
for improper field installation and changes made to the
system layout in the future. In buildings with large areas
of open plan space, the main duct size shall be increased
for revisions in the future. Air flow diversity shall also be
material with field applied mastic. Use of pressure
a sizing criterion. Eighty percent diversity can be taken
sensitive tape is not permitted.
at the air-handling unit and decreased the farther the
Air Delivery Devices: Terminal ceiling diffusers or booted-
ductwork is from the source until air flow diversity is
plenum slots shall be specifically designed for VAV air
reduced to zero for the final portion of the system.
distribution. Booted plenum slots shall not exceed 1.2 m
Ductwork Construction: Ductwork shall be fabricated from
(4 ft) in length unless more than one source of supply is
galvanized steel, aluminum or stainless steel sheet metal
provided. "Dumping" action at reduced air volume and
depending on applications. Flex duct may be used for low
sound power levels at maximum delivery shall be
pressure ductwork downstream of the terminal box in
minimized. For VAV systems, the diffuser spacing
office spaces. The length of the flex duct shall not exceed
selection shall not be based on the maximum or design
the distance between the low pressure supply air duct and
air volumes but rather on the air volume range where
the diffuser plus 20 percent, to permit relocation of
the system is expected to operate most of the time. The
diffusers in the future while minimizing replacement or
designer shall consider the expected variation in range
modification of the hard ductwork distribution system.
in the outlet air volume to ensure the air diffusion
Flex duct runs shall not exceed 3.0 m (10 ft) nor contain
performance index (ADPI) values remain above a
more than two bends. Joint sealing tape for all
specified minimum. This is achieved by low temperature
connections shall be of reinforced fiberglass backed
137
MECHANICAL
ENGINEERING
5.8
HVAC Components
Revised March 2005 PBS-P100
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