Static Pressure; Temperature; Corrosion; Turbulence - Honeywell AUTOMATIC CONTROL Engineering Manual

twist on the blades. Because the blade profile of conventional
sheet metal dampers is not streamlined, the stresses imposed
on the damper blades due to air movement are dynamic in nature
rather than static. To strengthen the damper blades, the gage of
metal and the number and depth of longitudinal bends can be
increased or reinforcing plates can be spot welded to the blade
to increase the blade cross-section. Strengthened dampers also
require heavy duty linkage, bearings, and frame. The dynamic
and static stresses and linkage and bearing loads all mean that
large actuators are needed.

STATIC PRESSURE

The maximum static pressure that an air handling system can
develop across a damper occurs when the damper is fully closed.
As the damper opens, system airflow increases and a portion of
the total pressure is converted to velocity pressure and the forces
on the damper become more dynamic than static. It is important
to determine the maximum possible static pressure in normal
operation and to consider this when selecting dampers.

TEMPERATURE

Some dampers are capable of satisfactory operation in the
temperature range of –40 to 400F, primarily to meet high
temperature requirements. A maximum temperature rating of
200F is usually satisfactory for HVAC use. The specific
temperature range for a given damper can be found in the
manufacturer specifications.

CORROSION

Dampers used in conventional HVAC systems typically
require galvanizing or zinc plating for corrosion protection.
Damper applications that may encounter corrosive elements
and require additional protection include:
— Buildings in immediate coastal areas where salt spray
can enter with the outdoor air.
— Outdoor air applications where the outdoor air damper is
located very close to the outdoor air intake, when the
outdoor air intake is not protected from rain or snow by
fixed louvers, or when the velocity of the outdoor air
intake is in the range of 750 to 1000 fpm or more.
— Face dampers near spray coils.
— Dampers near electronic air cleaners with in-place
washers.
— Dampers near spray humidifiers.
— Dampers used in cooling tower applications.
— Dampers in exhaust ducts that carry corrosive fumes or
vapors.
DAMPER SELECTION AND SIZING
All aluminum or all stainless steel construction is preferred
in many cases. Optionally, protective finishes are available. The
requirement for corrosion resistant dampers usually necessitates
a custom built damper.

TURBULENCE

The flow of air in an air handling system is turbulent.
Excessive turbulence or pulsations can have the same effects
on dampers as increasing air velocity. There is a direct
relationship between air velocity and the turbulence caused by
airflow through a damper. The effects of moderate turbulence
can be noticed on dampers located near abrupt duct transitions
or near elbows without turning vanes. Effects of severe
turbulence, capable of destroying a damper, can be noticed on
dampers located in close proximity to a fan. A damper located
near the discharge of a fan should be inspected during actual
operation over a full range of positions (from full open to full
closed) to be certain no severe vibration occurs (due to the
damper being in resonance with a frequency generated by the
fan blades). If the damper encounters severe vibration, the
vibration may be decreased by adding stiffening members to
the damper blades, extra damper linkage, or additional actuators.
The preferred method for preventing these damper problems is
initial selection of a location with minimal turbulence. However,
if high turbulence cannot be avoided, a custom heavy duty
damper may be required.

NUCLEAR/SEISMIC APPLICATIONS

Damper applications in nuclear power plants and other similar
facilities must be fully compatible with safety system designs
and meet all applicable regulations. Some dampers in nuclear
facilities are required to operate during and after an earthquake.
Seismic or earthquake susceptibility requirements vary and are
specific for each individual job or geographic location. Seismic
certification involves verification (usually through testing) that
the control device can withstand specified levels of vibration.
Test procedures include low-frequency, high-amplitude,
multiaxial vibration. The tests vary in intensity, not only with
different geographic locations but also with the physical
elevation within the building. Therefore, test requirements for
nuclear facilities must be carefully reviewed to accommodate
all applications.

ACTUATORS AND LINKAGES

NORMALLY OPEN/NORMALLY CLOSED

Actuators open and close dampers according to an electric,
electronic, or pneumatic signal from a controller. Actuators
provide normally open or normally closed damper operation.
In a normally closed application, the damper blades are closed
461 ENGINEERING MANUAL OF AUTOMATIC CONTROL