MIL-HDBK-1003/3
dictate use of mitered elbows, such elbows shall have single
thickness turning vanes. Using double thickness turning vanes
instead of single thickness vanes increases the pressure loss of
elbows by as much as 300 percent. Use the circular equivalents
table in ASHRAE Handbook, Fundamentals instead of matching areas
when you change aspect ratios. The aspect ratio is the ratio of
larger to smaller rectangular duct dimension. Try to use an
aspect ratio of 3 to 1 with a maximum aspect ratio of 6 to 1 or
less.
6.1.3.3
Access Doors. Show access doors or panels in ductwork
for apparatus and devices for maintenance, inspection, and
servicing.
6.1.3.4
Flexible Ducts. To save construction expense, flexible
duct may be used to connect ceiling outlets. Limit the length of
flexible ducts to straight runs of 5 feet. Seek self-balancing
by having equal lengths of flexible ducts instead of long and
short lengths on the same branch. Do not use flexible ducts for
elbows, including connection to diffusers; provide elbows at
ceiling diffusers. Do not use flexible ducts in industrial
ventilation systems.
6.1.3.5
Rooftop Ductwork. Rooftop ducts exposed to the weather
can leak rain water. Exterior insulation tends to have a short
life. One way to avoid such problems is to put insulation inside
the duct, and then use galvanized steel ductwork with soldered
joints and seams. Exterior insulation shall have weatherized
coating and wrapping throughout, where it must be used; such as
on kitchen exhaust hoods containing grease.
6.1.3.6
Glass Fiber Ductwork. Investigate the bidding climate
in your local area before deciding that ductwork made from glass
fiber panels will always be less expensive than galvanized steel
ductwork. Fiberglass ductwork should be coated inside to avoid
bacteria growth. In some parts of the country the sheet metal
subcontractor can make or buy metal ducts made on an automatic
machine at competitive prices.
6.1.3.7
Balancing Dampers for HVAC. Provide balancing dampers
on duct branches and show dampers on drawings. See Figure 3 for
damper installation. Use extractors or volume dampers instead of
splitter dampers at branch connections. Do not use splitter
dampers since they make ductwork more difficult to balance than a
job with volume dampers. Provide access in the ceiling and
clamping quadrants for dampers or use a type with a remote
control that extends through the ceiling. Outdoor air dampers
should be located away from the intake louver and after the duct
transition to minimize exposure to weather and oversizing of
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