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Step
1
Not applicable
2
Free area ratio (parallel blades)
3
Pressure drop at 15.08 m/s
4
Approach velocity
5
6
Pressure drop across damper
Had the duct size been 1.50 m
DAMPER APPLICATIONS
The Table 8 indicates the damper types typically used in
common control applications.
Table 8. Damper Applications.
Control Application
Return Air
Outdoor Air or Exhaust Air
(with Weather Louver or Bird Screen)
(without Weather Louver or Bird Screen)
Coil Face
Bypass
(with Perforated Baffle)
(without Perforated Baffle)
Two-Position (all applications)
MIXED AIR CONTROL
Figure 34 shows a mixed air control system. All three dampers
(outdoor, exhaust, and return air) are the primary source of
pressure drop in their individual system so parallel blade
dampers are selected to obtain linear control.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Table 7. Pressure Drop Calculation Example.
= (0.0798 x 1.50 m
=
0.8075 x 0.8876
= 0.717
= –3.114 x (1 – 0.717
= –3.114 x –3.1449
= 9.783 Pa
9.45 m 3 /s
=
1.69 m 2
= 5.59 m/s
25.8
Correction factor
=
5.59 2
= 0.826
9.783 Pa
=
0.826
= 11.86 Pa
2
, the same size as the damper, the pressure drop would have been lower (7.25 Pa).
Damper
Type
Parallel
Opposed
Parallel
Opposed
Opposed
Parallel
Parallel
DAMPER SELECTION AND SIZING
Example
1.50 m 2
) 0.1007 x
2
1.69 m 2
–4.274
)
V
EA
V
OA
POINTS OF CONSTANT PRESSURE
PATHS OF VARIABLE FLOW
V
Fig. 34. Mixed Air Control System
(Parallel Blade Dampers).
When a weather louver or bird screen is used in series with
the outdoor air and exhaust dampers (Fig. 35), the static pressure
drop shifts from the louvers/screens to the dampers as they go
from open to closed. Opposed blade dampers for outdoor air
and exhaust air provide a more linear characteristic for these
systems. The return damper is still the primary source of
pressure drop in its system so a parallel blade damper is used
to minimize pressure drop yet maintain a linear characteristic.
463
RA
V
C2393
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