PHYSICAL FACILITIES
2017 Consultant’s Handbook
Division 23 HVAC
3100 DUCTWORK, DAMPERS, AND LOUVERS
1
3
General HVAC Systems
All main HVAC ductwork should be
galvanized steel.
1.1
1.2
When replacing ductwork in existing
systems, the new duct should be designed to be
similar to the original.
1.3
The last five (5) feet of supply ducting to
an air diffusion product may be flexible duct
pulled taut with no elbows or changes of
direction.
Note: Research indicates that flexible duct has a
much greater pressure loss that steel, especially if
the flex changes direction. Thus, keep flex duct
usage to a minimum and only when pulled taut.
1.4
The last take-off(s) at the end of a run of
ductwork must be drawn with an elbow or will
leave two duct diameters of main duct after the
take-off.
Note: If the take-off at the end of a straight run of
duct simply comes off the side of the duct, close to
the end, there will be significant turbulence causing
a static pressure loss.
1.5
All sheet metal ductwork on the finished
drawings should be drawn double line.
2
Dampers
2.1
All duct run-outs to diffusers need to have
a manual volume damper at the branch take-off.
2.1.1 To reduce turbulence noise the MVD
should be as far as reasonably possible from the
air diffusion devise.
2.2
On air distribution ductwork systems or
segments of systems in which volume airflow is
pressure dependent, install multi-blade manual
volume dampers at all main branches to assist in
balancing.
2.3
If the blade length on a motorized damper
is more than 36” the damper should be
segmented.
Note: Dampers wider than 36” will twist on the
shaft affect the damper operation.
2.4
To avoid a cold length of duct that could
condense causing a water problem, OA and
relief dampers should be placed as close to the
louver plenum as possible.
Louvers
3.1
All louvers should be stationary blade type
constructed of extruded aluminum and have
integral exterior bird screen.
Note: Combination damper/louvers have proven
to be unreliable.
Note: External bird screens, though a special
order, prevent birds from building a nest in the
louver blades.
3.2
All louvers should be designed with a
resistance to airflow less than 0.10" w.g.
3.3
Intake louvers need to be “drainable” with
a gutter at the lip of each blade to prevent water
from running down the face of the louver.
3.4
If the bird-screen is on the outside face the
intake louver gross velocities should be 157 FPM
or free area velocity should be 350 FPM.
3.5
If the bird-screen is on the inside face the
intake louver gross velocities should be 100 FPM
or free area velocity should be 222 FPM.
Note: The velocities in 3.4 & 3.5 are to minimize
snow entrainment.
3.6
Smooth transitions (30° angle with respect
to longitudinal axis) shall be used to reduce duct
from louver to general distribution duct size.
Note: “Plenum boxes” do not create uniform air
velocity at louver face. The plenum-box-backed
louver is susceptible to “plug-holing” even if
selected at a 222 FPM free area velocity (this is an
average velocity over the face area). The localized
area of active intake will have a much greater
velocity.
4
Snow Entrainment:
4.1
The
first
defense
against
snow
entrainment is shielding the louver face using a
hood on the exterior of the louver.
4.1.1 The hood should be enclosed on the
top and sides forcing air to enter vertically up.
4.1.2 Air velocity through the opening must
be <400 FPM.
4.1.3 Air velocity behind the hood must be
<200 FPM
Page 1 of 3
PHYSICAL FACILITIES
2017 Consultant’s Handbook
Division 23 HVAC
3100 DUCTWORK, DAMPERS, AND LOUVERS
Note: See ASHRAE Fundamentals 2009:
21.7 for explanation of hydraulic duct
diameters
4.2
The second defense against snow
entrainment is an appropriated designed inlet
plenum with a snow drop-out section that is
sloped to drain, either out through the louver or a
floor drain.
4.2.1 To calculate D (drop-out distance)
use the snow free-fall velocity of 197 fpm. With
VG as the louver gross area face velocity and
the variable H as the louver height, then the
drop out distance D is calculated by the
equation:
D (feet) = H * (V G / 197)
4.3
Following the drop out section the plenum
transitions to the duct size. Transition can be no
more than a 30° angle.
4.4
The total distance from the louver to the
duct can be no less than two duct diameters. If
rectangular ductwork is used then the total
distance from the louver to the duct can be no less
than two hydraulic duct diameters.
Page 2 of 3
PHYSICAL FACILITIES
2017 Consultant’s Handbook
Division 23 HVAC
3100 DUCTWORK, DAMPERS, AND LOUVERS
5
5
Low pressure duct sizing table
Type
Use
Velocity
(FPM)
SP Loss
per 100’
SA
After VAV box
≤ 1000
≤ 0.08”
From riser to
VAV box
≤ 1800
≤ 0.20”
Vertical risers
and main trunks
≤ 2000
≤ 0.20”
Mechanical
rooms
≤ 2000
≤ 0.20”
Plenum inlet
≤ 600
≤ 0.04”
Grille connection
≤ 600
≤ 0.04”
Above occupied
rooms
≤ 1000
≤ 0.08”
Vertical risers
≤ 1500
≤ 0.1”
Mechanical
rooms
≤ 1800
≤ 0.1”
OA
Penthouse or
mechanical room
≤ 1000
≤ 0.1”
overall
Relief
Penthouse or
mechanical room
≤ 1000
≤ 0.1”
overall
Transfer
≤ 500
≤ 0.04”
General
exhaust
≤ 1500
≤ 0.08”
Grease &
Kitchen
hoods
Between
1500 &
2000
DNA
Wet
exhaust
≤ 1500
≤ 0.08”
Fume
hoods
Between
1500 &
2000
DNA
RA
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