Face And Bypass Control; Throttling Control - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

DAMPER SELECTION AND SIZING
LOUVERS
OPPOSED BLADE
DAMPER
EA
SERIES RESISTANCE
IN VARIABLE
FLOW PATH
OA
POINTS OF CONSTANT PRESSURE
HIGH PRESSURE
LOW PRESSURE
Fig. 35. Mixed Air System with Louvers.
Figure 36 shows correct orientation of parallel blade dampers
for effective mixing. Proper orientation helps avoid cold areas
in the mixed air stream which could freeze coils.
OUTDOOR
AIR
Fig. 36. Parallel Blade Dampers
Oriented for Effective Mixing.
RA
PARALLEL
BLADE
DAMPER
C2394
RETURN
AIR
MIXED
AIR
C2392
464

FACE AND BYPASS CONTROL

Figure 37 shows a face and bypass damper application. The
system pressure drop is relatively constant across the bypass
damper so a parallel blade damper is used for minimum pressure
drop at full flow. The system pressure drop across the face
damper shifts from the coil to the damper as the damper closes
so an opposed blade damper is used for more linear control.
The face damper should be equal to the coil size to prevent
stratification (hot and cold spots) across the coil.
BYPASS
AIR FLOW
FACE
Fig. 37. Face and Bypass Damper Application.

THROTTLING CONTROL

Either parallel or opposed blade dampers can be used for
throttling applications. If the primary resistance of the system
is the damper, parallel blade dampers are preferred. However,
if significant series resistance exists, like a reheat coil, opposed
blade dampers should be used.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
C2395