Field Fabricated Ductwork
On bottom vertical duct connections, secure all ducts to
the roofcurb and building structure prior to unit installation.
See Roofcurbs Supplied by Venmar CES (External Applica-
tions Only). Do not secure ductwork to the unit.
On other duct connections, make connections to the
casing by applying caulking around the connection and
screwing flanged ducts directly to the casing and/or flange
with self-drilling sheet metal screws. It is important to seal
all duct connections to prevent air leakage and system
performance problems. Ductwork must be supported by
the building structure.
For unit and duct opening sizes see submittal drawings.
Insulate and weatherproof all external ductwork, joints and
roof openings with counter-flashing and mastic in accor-
dance with applicable codes. Ductwork running through
roof decks must comply with local fire codes. Ducts pass-
ing through unconditioned spaces must be insulated and
covered with a vapor barrier. Flexible connectors should
be installed close to the unit in the duct leading to occu-
pied spaces to minimize noise transmission.
Duct Design Considerations
The discharge ductwork immediately downstream from
the double width double inlet fan (excluding units with gas
furnace or plenum fan options) is critical for successful
applications. Poorly designed ductwork can degrade fan
performance and contributes to excessive pressure drop
and noise.
When designing ductwork in the field, it is important to
use a straight discharge duct of the correct dimensions to
obtain maximum fan performance. The straight section of
ductwork helps the airflow to develop a uniform velocity
profile as it exits the fan and allows the velocity pressure
to recover into static pressure. See Figure 1.
For 100% recovery of velocity pressure into static pres-
sure, the straight portion of the discharge duct must be at
least 2.5 times the ‘equivalent duct diameter’ of the rect-
angular duct connection up to 2,500 fpm outlet velocity
or less. Add one duct diameter for each additional 1,000
fpm outlet velocity to the length of the straight portion of
discharge duct. The equivalent duct diameter is equal to
square root of (4ab/Pi) where ‘a’ and ‘b’ are the rectangu-
lar dimensions of the fan discharge opening.
Refer to Appendix E to determine the effect of undersized
straight sections of duct with or without duct elbows
within the 100% minimum effective duct length (reference
AMCA publication 201, Fans and Systems).
Example:
For a VHC-50 with a supply flow rate of 5,500 cfm with a
bottom supply fan discharge connection with rectangular
connection (from submittal documents) of ‘a’ = 17.4” and
‘b’ = 18.8” the length of straight duct is 24” before enter-
ing a 90 degree elbow is reached in position A as shown
in Appendix E, Figure E1:
• The equivalent duct diameter is equal to square root
of (4 x 17.4 x 18.8/Pi) = 20.41”
• The outlet velocity is equal to
5,500 cfm/(17.4 x 18.8/144) = 2,421 fpm
• Therefore, the minimum length of straight duct for
100% effective recovery needed would be equal to
20.41 x 2.5 = 52” rounded up to the next whole inch
• The effective straight duct was 24” long or
24/52 x 100 = 46% effective
• Using Table E1, with elbow position A and 46% ef-
fective duct length, would be using a curve value,
(50−46)/(50−25) = 0.16 of the way between the ‘U’
curve at 50% and ‘R–S’ curve at 25% for SWSI fans
• All VHC units with forward curved fans are DWDI,
therefore following note ‘C’ below Figure E2 would
use a 1.0 multiplier for the DWDI fans on the pres-
sure drop curves in Figure E2
• From Figure E2, at 2,421 fpm and
1.0 x (0.16 x ((P−Q)−S)+S) curve values added pres-
sure drop is 1.0 x (0.16 x (0.38−0.15)+0.15) = 0.19”
w.c. which must be added to the external pressure
drop for the unit
The minimum straight duct length on the discharge of the
fan for 100% recovery of velocity pressure into static pres-
sure at maximum cfm for the different models are in Table 2.
Table 2: Length of Straight Duct on Fan Discharge for
100% Recovery
Model Max CFM
Minimum Straight Duct Length
Supply Exhaust
VHC-36 2,500 42” [1,066.8 mm] 42” [1,066.8 mm]
VHC-42 3,500 52” [1,320.8 mm] 52” [1,320.8 mm]
VHC-50 5,500 52” [1,320.8 mm] 62” [1,574.8 mm]
Centrifugal fan
Cutoff
Discharge duct
100% effective duct length
2½ diameters at 2,500 FPM
Figure 1: Duct design
VCES-VHC-IOM-1F – VHC-36, -42 & -50
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