Honeywell AUTOMATIC CONTROL SI Edition User Manual

ENGINEERING MANUAL OF AUTOMATIC CONTROL
DAMPER SELECTION AND SIZING
458
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.
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
PARALLEL BLADE
DAMPER CURVE
INHERENT
OPPOSED BLADE
DAMPER CURVE
100
60
40
20
0
80
0 20406080100
% DAMPER OPENING
% FULL AIRFLOW
C1491
Fig. 26. Resistance to Airflow in Actual System.
Fig. 27. Examples of Resistance to Airflow.
SYSTEM FLOW RESISTANCE
DAMPER
SERIES
RESISTANCE
ELEMENTS
C1500
BIRD SCREEN
OUTDOOR AIR
DAMPER
RAIN
LOUVER
FACE AND
BYPASS
DAMPER
COIL
ELBOW
DIFFUSER
VOLUME
DAMPER
C1495
INSTALLED
CHARACTERISTIC
MODIFIED BY
SERIES
RESISTANCE
INHERENT
DAMPER
CHARACTERISTIC
100
0
0
% DAMPER OPENING
% FULL AIRFLOW
C1492
IDEAL LINEAR SYSTEM
CHARACTERISTIC
100
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.

Honeywell AUTOMATIC CONTROL SI Edition Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL SI Edition
Category	Control Systems
Language	English