30 November 1998
(2) Examples of the use of modulating dampers are:
(a) Air plenum temperature control by mixing outside air and return air.
(b) Space temperature control by mixing warm air and cool air.
(c) Space temperature control by varying the flow of cool air.
(3) Examples of the use of 2-position dampers are:
(a) Closing outside air dampers or building exhaust dampers when fans are stopped.
(b) Isolating sections of ductwork for smoke control purposes.
(4) Dampers are classified by the action of their blades, which connect to a common shaft that is
rotated to open or to close the damper. Opposed blade dampers provide better flow characteristic in
throttling applications. A throttling application is one where the damper is installed in series with the path of
flow and the damper is used to add pressure drop to reduce air flow. Parallel blade dampers are used to
provide better flow characteristics in mixing applications. A mixing application is one where more than one
flow path exists in parallel. Usually, two or more dampers are installed in parallel to each other and the
dampers divert flow rather than increase total system pressure drop.
(5) The control action of dampers (NC or NO) depends on the direction of their blade rotation
caused by the spring return stroke of an actuator connected to the damper's drive shaft, when the control
signal or power is removed.
(6) When a control system application requires that a damper be open prior to the start of a fan, an
adjustable switch is connected to the damper; this device is called an end switch or limit switch. The end
switch operates a set of contacts in the fan starter control circuit when the damper is fully open, to allow the
fan to start; the end switch opens the circuit to prevent the fan from continuing to operate if the damper
begins to close.
(1) Actuators are used to operate valves and dampers. Pneumatic actuators are powered by air
pressure, and are controlled directly by a pneumatic control signal and indirectly by an electric or electronic
signal. An electro-pneumatic device converts an electric or electronic signal to a pneumatic signal to stroke
the actuator. Electric and electronic actuators are electrically powered and are controlled directly from an
electric or electronic signal to stroke the actuator. While all pneumatic actuators have a spring-return
feature, some electric/ electronic actuators are not equipped with aspring to move the valve or damper to a
fail-safe position upon loss of power or control signal.
(2) Modulating control of actuators requires either the use of a 4 to 20 milliampere control signal
directly to an electronic actuator or the conversion of the signal to a pneumatic control signal of 21 to 103
kPa (3 to 15 psig). The pneumatic signal can be directly or inversely proportional to the electronic signal.
The signal conversion values are shown in figure 2-2.
Figure 2-2. Conversion of an electronic signal to a pneumatic signal.
(3) Two-position control of electric actuators requires the closing and opening of a contact to
operate an electric actuator. Two-position control of pneumatic actuators requires an electric/pneumatic
device to pass 140 kPa (20 psig) main air to the actuator, or to exhaust air from the actuator.
(4) Sequencing occurs when actuators are modulated from a common signal by using a portion of
the 4 to 20 milliampere signal or the converted 21 to 103 kPa (3 to 15 psig) signal. The actuator stroke is
adjusted to move its connected valve or damper from fully closed to fully open over the assigned portion of
the common control signal. Deadbands between the movement of valves and dampers are achieved by