Torque Requirements; Velocity Ratings; Temperature Ratings; Pressure Ratings - Honeywell AUTOMATIC CONTROL Engineering Manual

DAMPER SELECTION AND SIZING

TORQUE REQUIREMENTS

Operating and close-off torque requirements of dampers and
their actuator sizing guidelines are typically shown in manufacturer
specifications. Occasionally a brief explanation of the theory or
basis for the actuator torque ratings accompanies this data.
Two conditions must be considered when establishing
minimum torque requirements of a damper. One is closing
torque which is the torque required to force the blades together
sufficiently to achieve minimum possible leakage. The other is
the dynamic torque required to overcome the effect of high
velocity airflow over the blades. The maximum dynamic torque
will occur somewhere in the middle two-thirds of the blade
rotation depending on the damper design.

VELOCITY RATINGS

Approach velocity of the air is an important physical
limitation that applies to all dampers and should be considered
when sizing dampers. Generally, the maximum velocity rating
increases as the overall performance rating of a damper
increases. In practical terms, a higher velocity rating of one
damper compared to another indicates the former damper has
stiffer blade and linkage design and that the bearings may also
be capable of higher loads. The velocity rating of control
dampers is usually a statement of the maximum value allowed
for the particular design under conditions of normal (not
excessive) turbulence. Velocity ratings must be severely reduced
under excessively turbulent conditions.
Uneven velocity profiles due to locations near fan discharges,
duct transitions, and elbows require derating of the velocity value.

TEMPERATURE RATINGS

The maximum operating temperature of control dampers is
the maximum temperature at which they will function normally.
Increased temperature ratings on dampers indicate that bearings
and seals are constructed of heat resistant materials. Stated
temperature limits apply to the operating life that would be
expected under normal ambient conditions.

PRESSURE RATINGS

The pressure rating of a control damper is the maximum static
pressure differential which may be applied across the assembly
when the blades are closed. Excessive leakage (caused by
deflection of blades) and abnormally high operating torque (due
to forces on blades, and loads on bearings and linkages) can
result from high differential pressure. In extreme cases, physical
damage to the dampers could occur. Typical ratings are stated
in Table 2.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Table 2. Maximum Static Pressure Differential
Damper Type
Standard Damper
Standard and High Temperature, Low
Leakage Damper
Low Static, Low Leakage Damper
The ratings are probably conservative for smaller sizes since
damper blades tend to deflect more as their length increases.
An alternative method is a listing of differential static capability
as a function of damper blade lengths, such as Table 3.
Table 3. Maximum Static Pressure Differential Capability
Damper Length (in.)
12
24
36
48

UL CLASSIFICATION (FIRE/SMOKE)

Performance criteria for fire or smoke dampers bearing the
UL rating are contained in specifications UL 555 (Standard for
Safety/Fire Dampers and Ceiling Dampers) and UL 555S
(Standard for Leakage Rated Dampers for Use in Smoke Control
Systems). These two specifications govern the design,
performance, installation, and marking (labeling) of the devices
bearing the UL classification.

APPLICATION ENVIRONMENT

Most HVAC system requirements can be met using standard
dampers from major manufacturers. Many manufacturers also
build custom dampers with special features to satisfy particular
application requirements. Custom features or modifications
include blade reinforcement, heavy duty linkage, bearings, axles
and frames, special seals, and construction or finish of corrosion
resistant materials. The application must be carefully examined
to ensure job requirements are met. Some of these special
features add significantly to the cost of the damper so they
should be furnished only when needed.

VELOCITY

As the velocity in a system increases, dampers in the system
encounter higher forces. The impact pressure of the air
movement increases the bending force on the damper blades
and the airflow over the damper blades may cause a torque or
460
Pressure
Differential
in. wc
3
6
2
Max Close-Off
Static (in. wc)
8
8
6
4