Damper Characteristics; Inherent Characteristic; Installed Characteristic - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

DAMPER SELECTION AND SIZING
Stability of space temperature is important in providing a
comfortable, energy-efficient environment. The most significant
factor in achieving stability of a control loop is the gain of the
system elements. The gain of a damper system is the ratio of
the change in airflow to the change in signal to the actuator.
In an ideal damper system, the gain is linear over the entire
operating range (e.g., a small increase in space temperature
results in a small increase in cooling airflow). The more linear
the system, the more constant the system gain and the more
stable the system operation over its entire range.

DAMPER CHARACTERISTICS

INHERENT CHARACTERISTIC

The relationship between damper blade position and airflow
through the damper is defined as the inherent characteristic.
The inherent characteristic is defined at a constant pressure drop
with no series resistance (coils, filters, louvers, diffusers, or
other items).
Figure 25 shows the inherent airflow characteristic curves of
parallel and opposed blade dampers. This difference in airflow
is important when selecting the proper damper for a system.
100
INHERENT
80
PARALLEL BLADE
DAMPER CURVE
60
40
20
0
0 20 40
% DAMPER OPENING
Fig. 25. Parallel versus Opposed Blade Damper Inherent
Airflow Characteristic Curves at Constant Pressure Drop.

INSTALLED CHARACTERISTIC

The inherent characteristic is based on a constant pressure
drop across the damper. This is frequently not the case in
practical applications. Series resistance elements such as duct
resistance, coils, and louvers, cause the pressure drop to vary
as the damper changes position (Fig. 26 and 27). The resulting
installed characteristic (Fig. 28) is determined by the ratio of
series resistance elements to damper resistance and will vary
for parallel and opposed blade damper.
INHERENT
OPPOSED BLADE
DAMPER CURVE
60 80 100
C1491
DAMPER
SYSTEM FLOW RESISTANCE
Fig. 26. Resistance to Airflow in Actual System.
RAIN
LOUVER
COIL
FACE AND
BYPASS
DAMPER
Fig. 27. Examples of Resistance to Airflow.
100
IDEAL LINEAR SYSTEM
CHARACTERISTIC
INSTALLED
CHARACTERISTIC
MODIFIED BY
SERIES
RESISTANCE
0
0
% DAMPER OPENING
Fig. 28. Installed versus Inherent Airflow
Characteristics for a Damper.
Series resistance modifies the damper airflow characteristic.
The greater the series resistance, the greater the modification. The
ratio of series resistance to damper resistance is called the
characteristic ratio. Figures 29 and 30 show modified
characteristics for parallel and opposed blade dampers based on
various ratios of series resistance to full open damper resistance.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
458
SERIES
RESISTANCE
ELEMENTS
C1500
BIRD SCREEN
OUTDOOR AIR
DAMPER
VOLUME
DAMPER
ELBOW
DIFFUSER
C1495
INHERENT
DAMPER
CHARACTERISTIC
100
C1492