Handbook of air conditioning and refrigeration / Shan K

The movement of the split damper from one end to the other modulates the volume of air flowing
into the two legs or branches. A split damper is usually modulated only during air balancing after
installation or during periodic air balancing.
Opposed-Blade Dampers. An opposed-blade damper is a type of multiblade damper that is often
rectangular, as shown in Fig. 5.18d. It is usually used for a flow passage of large cross-sectional
area. The damper blades may be made of galvanized steel, aluminum alloy, or stainless-steel sheets,
usually not exceeding 10 in. (25.4 cm) in width. Rubber or spring seals can be provided at the fully
closed position to control the air leakage rating, which often does not exceed 6 cfm/ft 2 (30 L/s�m 2 )
at a pressure drop across the damper of 4 in. WC (1000 Pa). The bearing used for supporting the
blade axle should be made of a corrosion-resistant material such as copper alloy or nylon. Tefloncoated
bearings may also be used to ensure smooth operation of the damper. Lever linkages are
used to open and close the damper blades. The characteristics of the opposed-blade dampers are
covered later in this section.
The maximum static pressure drop across closed opposed-blade dampers is 6 in. WC (1500 Pa)
for a 36-in.- (914-mm-) long damper (the length of the damper blade) and 4 in. WC for a 48-in.-
(1219-mm-) long damper.
Parallel-Blade Dampers. A parallel-blade damper is also a type of multiblade damper used
mainly for large cross-sectional areas, as shown in Fig. 5.18e. The blade material and the requirement
for the seals and bearings are the same as those for opposed-blade dampers.
Damper Actuators (Motors)
Damper actuators, also called damper motors, are used to position dampers according to a signal
from the controller. As with valve actuators, damper motors can be classified as either electric or
pneumatic.
Electric Damper Motors. These either are driven by electric motors in reversible directions or are
unidirectional and spring-returned. A reversible electric motor is used often for more precise control.
It has two sets of motor windings. When one set is energized, the motor’s shaft turns in a
clockwise direction; and when the other set is energized, the motor’s shaft turns in a counterclockwise
direction. If neither motor winding is energized, the shaft remains in its current position. Such
an electric motor can provide the simplest floating control mode, as well as other modes if required.
Pneumatic Damper Motors. Their construction is similar to that of pneumatic valve actuators,
but the stroke of a pneumatic damper motor is longer. They also have lever linkages and crank arms
to open and close the dampers.
Volume Flow Control between Various Airflow Paths
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.33
For air conditioning control systems, most of the dampers are often installed in parallel connected
airflow paths to control their flow volume, as shown in Fig. 5.19. The types of airflow volume
control are as follows:
Mixed-Air Control. In Fig. 5.19a, there are two parallel airflow paths: the recirculating path um
in which a recirculating air damper is installed and the exhaust and intake path uom, in which exhaust
and outdoor air dampers are installed. The outdoor air and the recirculating air are mixed together
before entering the coil. Both the outdoor damper and the recirculating damper located just
before the mixing box (mixed plenum) are often called mixing dampers. The openings of the outdoor
and recirculating dampers can be arranged in a certain relationship to each other. When the
outdoor damper is at minimum opening for minimum outdoor air ventilation, the recirculating